Mirror, Mirror, on the Wall, What's the Fairest Engine of All

I received a lot of questions about our Scania main engine and the thinking behind the decision, so here goes.

First, none of the engine choices we had would have been bad.  In fact, all are excellent engines with great reputation, and I wouldn't reject any boat just because it had one of these engines.  Quite the opposite.  But when faced with a blank (or almost blank) sheet of paper, the engine choice is worth some thought and consideration.

We have two engines, but not in the traditional sense of a twin engine boat where both engines are identical.  We have a main engine that is the primary propulsion engine, plus an auxiliary engine for hydraulic power for thrusters, windless, pumps, etc..  It also acts as backup propulsion in case of a main engine failure, using it's own drive shaft and a folding sailboat propeller to minimize drag when not in use.  It's location is offset to one side of the center line of the boat, and for that reason is typically referred to as a "wing" engine.

All this means we need two engines with different specs.  The wing engine needs to be around 150 hp, and the main engine around 400 hp, both with continuous duty rating.

Duty ratings are worth a little sidebar.  All engines are rated with a maximum horsepower or kw output power.  That's always part of the headline.  But elsewhere in the fine print are limits on how long you can run the engine at full power, or more specifically what portion of the run time can be at full power vs something less than full power.  Think of your car which might have an engine rated for 200 hp.  How long do you think it would last if you ran it at full power, peddle to the floor, all the time, never letting up for even a second?  Not very long.  But that's fine because that's not how we drive.

With industrial engines things are different because there are many application where the engine needs to operate at full power for sustained lengths of time.  Think about an irrigation pump that might need to pump water continuously, or a ferry that is underway at full power for 3hrs, then at dock for 2 hrs, then underway again...  To deal with this, industrial engines all have service ratings to help match them to the application.  Every manufacturer uses different terms with different definitions, but conceptually they are all the same.  By way of example, here are Deere's so-called "M ratings".

M1 - Unlimited full power hours, unlimited hours per year.  This is a "continuous duty" rating.  You can start the engine, load it up to full power, and leave it there for it's entire life, save maintenance time.  You can put almost 9000 hrs on it and still be under warranty, and continue to clock hours at 9000 per year.

M2 - Full power is allowed for 16 out of every 24hrs, and annual hrs are expected to be between 3000-5000.  This could run a ferry with two hrs underway at full power, 1 hrs at dock, then repeat for 12hs every day for the life of the engine.

M3 - Full power for 4hr out of every 12hrs, and annual hours of between 2000-4000.

M4 - Full power for 1hr out of every 12hrs, and 1000 to 3000 annual hours.

For many pleasure boats, any of these ratings would be fine, and very few can't be served just fine by an M2 rating.  In fact, many Nordhavn's are specified with M2 rated engines.  But others, especially if expected to operate wide and far, are specified with M1 rated engines.

All this brings us to requirement #1 for the engines:  They must be continuous duty rated.

Requirement #2 is that the engine needs to meet US, EU, and IMO emissions requirements.  For those in the US, that means Tier 3 which does NOT include special after treatment or catalysts, but is a very clean burning engine.

Requirement #3 is that the engine needs to provide the desired power output, 400 hp for the main, and about 150 hp for the wing.

Then there is a long list of stuff that I just assume is a given like a reputable manufacturer, parts availability, excellent reputation and service record in marine applications, etc. etc.

Wing Engine

Starting with the wing, the choices were:

1) Deere 4045
2) -----

It's a short list.  Caterpiller's smallest propulsion engine is 7 liters and 280 hp, which is just way too big.  Similarly, Cummins' smallest engine is 6.7 liters and 250 hp.  Yanmar has a much smaller 2.7 liter engine that operates at around 3500 rpm that puts out the right hp, but they caution about continuous full power operation.  They are really meant for lighter duty applications.  Someone asked about a "baby Scania', but the smallest Scania is a 5 cylinder, 9 liter engine, and it's not Tier 3 certified.  The bottom line is that there really isn't much choice.

Deere 4045AFM85

But only having the Deere isn't a bad thing because it's a really good engine.  The 4045 (4.5 liters) has been around for a very long time and has an excellent reputation, so I'm very happy with this engine.  It's continuous  duty rated (M1) producing 160 hp at 2300 rpm.  I'm told it will move the boat at about 6 kts which is excellent for a wing engine.

Deere, front view with clutched power take off

Main Engine

The wing engine was the easy decision, because there was no decision to make.  The main engine was a little tougher.  I was considering three, excellent engines, any one of which I expect would provide excellent service.  They were, in my ultimate ranked order:

1) Scania DI-13 (400 hp @1800 rpm)

2) Cummins QSM11 (400 hp @ 1800 rpm

3) Deere 6135SFM85 (425 hp @ 1800 rpm)

Absent from the list is Caterpillar.  They just don't have an offering in this power range and duty rating.  Also absent is Lugger, but they have been absent for a number of years after exiting the propulsion engine business a number of years ago.

Engine Room Heat

One particular technical aspect of all three engines ended up playing a major role in the decision;  heat, and engine room cooling.  I delved into this quite a bit on our 60, and am working hard to head is all off early in the 68.  For reasons that I can only guess, there is a huge difference in the amount of heat these three engines dump into the surrounding engine room.  That heat needs to be removed by ventilation, and that can be a challenge.  The difference isn't due to engine efficiency - they are all about the same.  The difference is WHERE the waste heat goes.  It exits the engine through one of three paths;

1) Out through the cooling system and into the surrounding sea water, either through a heat exchanger or a keel cooler.  This is where 90% of the heat goes.

2) Out the exhaust pipe in the hot exhaust gas.

3) Radiated into the surrounding environment, in our case the engine room.

Whatever heat doesn't go out one path, must go out another, and when it comes to engine room cooling, #3 is what we care about, and these three engines have radically different performance in this respect.

The Scania dumps a freakishly small amount of heat into the engine room.  14kw, to be exact.  In contrast, the Cummins dumps 23kw which is more normal, but still nearly twice the Scania.  Then the Deere dumps a whopping 40kw, which is freakishly large, and almost three times the Scania.  The result is a huge difference in the back flips required to vent the space, and the resulting temperature rise in the ER.  This alone was probably the dominant factor in the ranking.  Beyond this, each engine had characteristics that bumped it up a little, or bumped it down a little in the ranking.  It ended up being a very close call between Scania and Cummins, with Deere a distinct third.

Here's some random narrative on each.

Scania DI-13

Scania has very little presence in the US, but is a huge, $12B Swedish company.  They primarily build and sell trucks and buses, but build their own engines and have a separate business selling those engines into marine and other industrial application, much like Caterpillar and Deere.  And as of a few years ago, they became part of Volkswagon AG.

Scania DI-13

I have always favored Swedish engineering.  Older Volvos, Electrolux vacuums, Husqvarna chain saws are all brilliant in their simplicity, effectiveness, longevity, and serviceability.  Scania trucks and buses have a similar reputation, and that carries into the marine engines.  I spoke with a number of distributors, dealers, and owners of Scania marine engine powered equipment, and not a single one had anything but praise for the product.   And all the dealers and distributors deal in other brand engines as well, so had no incentive to favor Scania over anyone else.

Just looking it over, it's a very well thought out design with very well integrated marine accessories.  On many engines, the so-called "marinization" becomes it's Achilles heel, with heat exchangers and water pumps bolted on in ways that block service access to other parts, and creates a more complex package.  The Scania is very compact and streamlined, with excellent access to all servicable parts.

One thing I was concerned about was cost and availability of parts.  Volvo, another Swedish marine engine manufacturer, has a reputation for extremely expensive parts.  I owned one a while back and found that if you imagined an incredibly high price for a part, then doubled or tripled it, you would be in the right ball park.  So I made up a list of typical spares and service parts, and got a quote for price and availability.  Everything on the list was available either at the dealer or a day away in a central warehouse, and prices were all in line with other vendors.  So they passed that test.

The product checked out with flying colors, as did parts cost and availability, but what about service.  If there is any down side to Scania, that's it.  Pretty much anywhere you can find a Deere or Cummins service guy, but Scania?

For me, I'll be doing most if not all work on the engine myself, so in many ways access to techs is not an issue.  But in the event one is required, they do have a network, including Hatton Marine in Seattle who I have used in the past.  So worst case I'll be flying someone in from Hatton.  But in all honesty, if I need to get someone in to help, it would likely be serious enough that I'd be flying one of their guys in anyway, so not really a problem.

So I am very confident in the product, and am confident I can get parts.  The one risk is Scania's longevity in the North American engine business.  They have completely forfeited the North American truck and bus business, and it's not because it's a small market.  They made a go of it back in the 80s, but only lasted a few years before pulling out.  But really the bigger concern is the size of the engine business compared to their overall business, and the engine business's size in NA compared to their costs.  Unlike other regions where they have a large truck and bus business to carry the cost of spares and a dealer network, in NA it has to be carried entirely by the engine business.

Let's look at it via the numbers.  Scania reports that their world wide engine business is 1% of overall revenue.  Then, when they break down the engine business by geography, North America isn't even in the pie chart, and the smallest sliver in the chart is 3%, so we know NA is less than that.  So the NA engine business is less than .03% (3% of 1%) of Scania's business.  This was my only hesitation with Scania, and why Cummins was such a close second.  In contrast, Cummins is also a $12B company, and their whole business IS engines - 100%.  Or at least very close to 100%.  There's just no question where they will be tomorrow.

Sounds pretty scary, right?  Well, not really.  There have already been two Nordhavn't built with Scania engines, all running well.  And there are three of us currently building 68s with Scania engines.  And of the fleet of 70 or so Scania-powered commercial boats operating in Bristol Bay, Alaska, the only reported problem is a broken oil spinner.  The engine was installed with poor access and it got man-handled and broken.  So I really do think we are getting a superior product.  And even if Scania departs the NA market, there are plenty of ways to get parts, most of which we keep on board anyway.  And if it turns out to be a total, unmitigated disaster, I can always pull it out and install a Cummins for a small cost relative to the value of the boat.  So ahead we go.

Scania DI-13

Cummins

The Cummins candidate was the QSM11, continuous duty rated at 400hp, 1800 RPM.  Most of the QSM11s out there are higher HP rated, pleasure duty engines.  They have a good reputation, but a well known issue with the exhaust manifold to head joint leaks.  But that issue is only in the higher output rated models, and not the commercial duty version.  It's a well proven engine, with an excellent commercial duty track record.  Parts and service are available on nearly every street corner, so no problem there.  It was a very strong candidate, and frankly lower risk than Scania, but that was offset by the superior heat rejection of the Scania.  Had Cummins had similar heat performance, I probably never would have looked further.

Cummins QSM11

Deere

The Deere under consideration was the 6135SFM85 which is the raw water cooled, heat exchanger version of the 6135.  Nordhavn specs the 6135AFM85 as standard equipment, which is the keel cooled version with an M2 (16/24 hrs full power operation), 425hp at 1900 rpm rating.  I wanted wet exhaust and cooling, hence the SFM version, which also has the benefit of an M1 (continuous duty) 425hp at 1800 rpm rating.  It's a touch more HP than the Scania and Cummins, but not enough to make any difference.

Deere 6135AFM85

The real killer was the heat rejection.  It makes for a challenging engine room venting design vs an easy and breezy one.  Plus there were some pesky issues I had with the Deere 6090 on my 60 that made me less than confident in the marinization of the 6135.  I think they are stellar base engines, but pesky problems can add up to a real annoyance and erode confidence in the over all package.  Here are some of the issue that I encountered.

1) Excessive belt dusting and shedding.  All the 6090's seem to have this issue shedding profuse amounts of dust and shrapnel.  I checked alignment of all the pulleys, tension, belt wrap, belt loading, and consulted everyone and anyone I could find, and never figured it out.  What I do know is that all the  other 6090s I know of in boat service experienced the same thing.  It does seem to reduce considerably somewhere in the 800-2000 hrs range, with ours finally subsiding at close to 2000 hrs.

2) Oil seepage from various engine joints and seems.  I was always getting oil seeping from valve cover seems, the oil pan seem, and other joints.  I just don't understand why this should be.  Oil just shouldn't ooze out of an engine.

3) Turbo gasket coolant leaks.  For the first couple of years I had a very slow loss of coolant, with no trace of where it was going.  I had heard of issues on the 6090's predecessor, the 6081, where coolant would leak past the turbo gasket and disappear out the exhaust in unnoticeable quantities, other than long term.  The master of all things Diesel, Bob Senter, suggested a sealing procedure on the turbo gasket, so I set about to do it.  The gasket alone suggests that the joint is difficult to seal.  It's a layered gasket, riveted together in a sandwich. The center layer is a typical gasket material, and the two outer sandwiching layers are thin steel each with a molded in rubber seal, kind of like a built-in o-ring.  The procedure was to further seal using Permatex Red high temp sealant.  I actually drilled out one of the rivets and fanned opened the layers like you would fan out a hand of cards.  That allowed me to get a film of the sealant on all the mating contact surfaces, then carefully fold it back up.  I put it all back together and presto, no more slow coolant leak.  Problem solved, but not a confidence builder.

Special turbo gasket with riveted layers

Unfolded gasket layers

Magic Permatex sealant

4) Coolant pump leak.  The cooling pump, like many, has a weep hole to detect leaks.  If the coolant seal is leaking, you will get cooling out of the weep hole, and if the drive side of the pump is leaking, you will get oil out of the weep hole.  It's a gear driven pump off the timing gears, hence the side exposed to oil.  Early in the engine's life I was getting drips of coolant from the weep hole, so the pump was replaced under warranty. 

Coolant drips from coolant pump

That sort of thing happens, but it exposed another issue which is accessibility due to the marinization of the engine.  To change the coolant pump, something that would be a couple of hours max on the tractor or industrial version of the engine, was a solid two day job for two guys.  Endless amounts of stuff had to be removed from the front of the engine to gain access, all of it added as part of the marinization package.  It just wasn't well thought out. 

Stripped engine to access coolant pump

Large pile of removed parts to access coolant pump

Frustrating, but that was only the beginning.  About 20 hrs into the new pump, and I was getting drips of coolant again.  So after the season was done, I pulled it all apart again, this time doing the work myself because it seemed logical that the guys doing the first replacement had done something wrong.  After I got the pump out, there was no sign of poor workmanship by the other guys, so I went to get another pump under warranty.  Well, warranty only provides for a rebuilt pump, so I decided to take the rebuilt pump and keep it as a spare, and sprung for a new pump on my own dime, just in case.  After two days of solid work it was all back together and ready to go.  But to my horror, after about 20 hrs (same as the last two times), it was dripping again.   Escalation of the issue through the distributor to Deere just went in circles and kept pointing back to an inapplicable tech bulletin that had the words "coolant pump" in it.  I finally gave up and decide to just live with it, like the oil seeps.  I went for well over a thousand hours and it never changed or progressed, so I just kept on going.  There was no way I was going to tear the front of the engine apart again unless someone had a very convincing explanation for the problem, and an equally convincing solution, and nobody ever did.

So the engine never missed a beat in 2000 hours, but these little things bugged me the whole time, and made me less than thrilled about another Deere engine.

There you have it. Scania is the fairest of them all.

Scania DI-13

Propane tank level gauge

One problem I expect other cruisers encounter is knowing when your propane tank is getting low.  It's much harder than you might imagine.  Liquid Propane, or LP gas, is stored in a liquid form in your tank.  If you lift the tank and shake it, you can feel the liquid slosh it around.  The gas that is drawn out to fuel your stove or grill is the vapor that accumulates above the liquid fuel.  As vapor is drawn out, more boils out from the surface of the liquid to replace it.  As it boils out from its liquid form into vapor, the liquid level drops until there is none left, and which point your grill unexpectedly shuts down.

Because most tanks are aluminum or steel, you can't see the liquid level, so you can't visually assess how full the tank is.  There are a few companies that make fiberglass tanks where you can see the level, but they have been problematic with a number of recalls, and some refill stations won't fill them at all.  I've also heard that some countries won't allow them to be filled.

A pressure gauge doesn't help assess how full the tank is either.  Because of the whole vaporization process in the tank, the pressure remains pretty much constant right up until the tank is empty.  So a pressure drop is kind of a sudden-death indication that the tank is empty, just like seeing your grill flame go out.

Home grills often indicate the tank level by means of a weight scale.  The tank is held by a sprung mechanism with a pointer indicating the tank level as its weight changes.  But on a boat, the tank is secured in one way or another, so a scale won't work.  And for the same reason you can't easily pick them up to feel how heavy they are.

But recently, a few companies have started selling tanks with an actual float gauge built into the tank valve.  U-Haul carries them, and I tracked one down at the local U-Haul in Seattle and decided to give one a try.  It was a standard gas grill size tank, and I watched the gauge go from empty to full as the attendant filled it up.  I'd say the gauge was not exactly a proportional representation of how full the tank was, but it was a whole lot better than sudden-death.

I figured I had this problem solved - at least until I got back to the boat.  It turns out my tanks on the boat are a different shape than the standard grill tanks, standing a little taller and little narrower.   I couldn't fit two of the new tanks in my propane locker, and they wouldn't work with the lock-down brackets in the boat.  And so began my unexpected education in propane tank valves, valve manufacturers, dip tubes, and dip tube lengths.

What made the most sense was to replace the tank valves on my tanks with valves that included the float gauge.

For obvious reasons, tanks and valves are highly regulated with requirements to meet a variety of standards, be stamped with the approval numbers, inspection dates, etc.  Valves are all equipped with a dip tube that is used to prevent over filling.  There is a little bleeder screw on the side of the valve that opens the dip tube to the atmosphere.  This is opened while the tank is being filled, and vapor will come out of the bleeder while the tank is being filled.  But as soon as the liquid level reaches the bottom of the dip tube, liquid will start spitting out, and you know the tank is full.  Tanks are all stamped with the required dip tube length.  For example, a 4" dip tube will start to spit when the liquid is 4" below the valve neck.

To retrofit my tanks, I needed to find the gauge valve vendor, confirm I could get them with the correct dip tube length, then get a certified propane shop to order the valves, swap them onto my tanks, and re-certify the tanks.  I had been working with Sure Marine in Seattle on some other projects, and asked them if they could do the work if I could find the valve.  They said sure (no pun intended).

Rochester Gauges in Texas makes the tank valves with float gauges.  But the more common gas grill tanks use 4.0" dip tubes, where my tanks require 4.6" dip tubes.  That posed a complication.  Rochester did indeed offer a 4.0" valve with gauge, but the only other one was 4.7" designed for a 30lb tank (mine are 20lb).  A quick check with the folks at Sure Marine confirmed that using a LONGER dip tube is ok, but not a smaller one.  With a longer tube, the tank will just read full a little bit sooner, so you lose a bit of capacity.  But it's perfectly safe.  If you used a shorter tube you would be over filling the tank, and that's not good.

So I decided to proceed with the 4.7" dip tube valve, and had Sure Marine order two for me.  When they came in, I dropped my tanks off and they swapped the valves and re-certified and filled my tanks.  And now, finally, I have gauges on my propane tanks that at least give an idea when they are starting to get low.  I don't remember the exact cost, but I think it was about $50 per tank, much less than replacing the nice aluminum tanks that I had.

Float gauge indicating actual (approximate) fuel level

One interesting thing is that there is no mechanical coupling between the float mechanism in the tank, and the gauge on the outside.  This makes sense from a fuel/leak containment perspective.  The gauge just clips onto the valve body, and I expect uses some sort of magnetic coupling between the indicator needle and the float.

Gauge just snaps onto valve body, presumably with magnetic coupling to float

Broken Turbo Studs - One Last Time

Back in this article I mentioned that on our way north last spring we had another occurrence of broken turbo studs.  It had happened about a year before up in Juneau, and we thought we had the problem licked, but apparently not.

When the studs crack, the seal between the exhaust manifold and turbo is compromised, and because both the manifold and turbo are cooled by engine coolant, it all starts leaking out.  Sooner or later this completely disables the engine because without enough coolant, it will overheat.  Fortunately both times it happened we were on our way to port, and made it in without overheating.

Below you can see the exterior evidence of the failure.  The yellow stuff is coolant.  You can also see the vertical brace installed last year to help take the weight of the exhaust off the turbo.   It may have helped, but clearly wasn't a complete solution.

Leaking coolant due to turbo stud failures

In the picture below, once the turbo has been removed you can see the two upper studs have parted.  The one on the left broke leaving a good stub to grab onto to remove the stud.  But the one on the right broke off about 1/2 thread inside the manifold, so I had to drill it and use a bolt extractor to get it out.  You can also see the coolant passages.  The center opening is the exhaust passage, and the oblong openings above and below are the coolant passages.

Cracked turbo mounting studs

Since the exact same thing happened again despite installation of a support brace after the first occurrence, further investigation was clearly warranted.  One common factor is that leading up to both failures, we were running the boat pretty hard - on the order of 2000 RPM out of a max of 2200.  This is absolutely fine - the engine is rated for 24x7x365 operation at 2100 RPM, and up to full 2200 RPM 16 out of every 24 hrs. - but it's different from our normal cruising at 1600-1900 RPM.

To make a long story short, the picture below will show what's happening.   The tall structure circled in red is rigidly attached to the turbo.  Because it's rigidly attached, when the engine vibrates, the exhaust structure has to vibrate with it.  But the structure has a very long lever arm, so requires lots of force to make it move along with the engine.  It's like holding a baseball bat and trying to shake it around.  If you hold it in the middle where it's center of mass is located, it's not real hard to shake it back and forth.  But if you hold it from the handle end, now the center of mass is out maybe 2' from your hand and it's much harder to shake it back and forth.  That's what's happening with this exhaust structure, except instead of weighing 2 lbs like a baseball bat, this structure weights over 30 lbs.  Trying to move it in time with the engine vibrations simply overwhelms the turbo mount and breaks the studs.

Excessively tall and heavy exhaust riser

After talking with a number of people, I decided to pursue a two part solution.   The first, and most important part, was to install flex piping in the vertical exhaust section.  Doing so would reduce the mass directly attached to the turbo, and decouple any engine vibrations from the rest of the exhaust structure.  The second part was to add a new set of spring hangers over the big flange to carry the weight of the exhaust over the turbo.  Ideally the exhaust structure should mate with the turbo elbow, but exert no pressure on it in any direction.  It should just float above it.

I probably could have ordered parts and gotten all this done in Port McNeill, but we didn't want to lose any more time than necessary, so I decided to haul everything down to Seattle in a rental car, get the work done there where I knew the people and places, then come back and install it.  So I removed all the insulation blankets, that exhaust riser, and the turbo.  Got a rental car, and drove for 10 hrs including a ferry crossing and boarder crossing to get back to Seattle.  I had lined up Hatton Marine to do the exhaust modification, and they started working on it before I even left Port McNeill.  They also pulled and ordered all the parts that I would need for reassembly.  And I lined up Ballard Insulation to modify and/or make new insulation blankets as needed to fit around the modified exhaust, and around the new mounting brackets for the hangers.

The next morning I was at Hatton at 7:00, dropped off the exhaust structure, picked up the parts, then went to visit some friends while I waited.  Hatton finished early in the afternoon, and I hauled everything over to Ballard Insulation for them to work on.  By 3:00, everything was done and I was back in the car on my way north with plans to stay the night in Nanaimo, and continue on to Port McNeill in the AM.

Below is the modified exhaust riser with attached mounting brackets to connect to the hangers.  And note the buckets of coolant.  It's about 12-13 gallons that has to be drained out to do this job.

Modified exhaust with flex section and new hanger brackets

After arriving back in Port McNeill, I spent the next two days putting everything back together again.  First, the turbo goes back on along with the oil deliver and return pipes, the compressed air pipe, coolant return, and the vent pipe.  Oh, and don't forget new mounting studs and nuts.

Turbo reinstalled

Then the turbo elbow gets installed.  It's easier to attach the elbow to the turbo and ensure a good fit, then align the rest of the exhaust to just float over and kiss the square flange that you see below.

Exhaust elbow attached to turbo

And fast forward to installed hangers and fitted exhaust section.  Once everything is in place, the spring hangers can be adjusted and the various joints rotated to get everything perfectly aligned, and with no weight on the turbo flange.

Modified exhaust installed, with flex section and spring hangers

Here's a video overview of the modification, also showing how the mating flange should float freely over the turbo elbow.

One obvious question is why, after building 60 of these boats, am I now having this failure?  I think the two pictures below say it all.  The first picture is of a friends earlier vintage N60.  His boat, and pretty much the entire N55/N60 fleet were built with the Deere 6081 engine.  The engine and turbo stand taller, with a resulting shorter exhaust riser.

Compare that to the next picture of my boat with the newer Deere 6090, introduced to meet EPA Tier II regulations.  The engine sits noticeably lower, resulting in a significantly taller exhaust riser.  This larger structure apparently is just enough to overwhelm the turbo, where the older arrangement was fine.

Shorter riser on Deere 6081 engine

Taller riser on Deere 6090 engine

So that was our major repair last spring.  But to be sure you don't go away thinking that I do nothing other than fix things on this boat, I have come up with a new benchmark for measuring a boat's condition.  You judge based on how quickly you run to the hardware store or marine supply store when you arrive in port.  The faster you run, and the more you buy, the more trouble you are having with your boat.

I'm very pleased to report that after this repair, and for the rest of the summer, I did not make a single purchase at a hardware store or marine supply store.  Based on that, I'd say the boat is running pretty darn well.

Alternator Reconfiguration for Improved Power

Like many boats, our main engine is equipped with two alternators.  One smaller 85A unit charges the starter battery, and one larger 190A unit charges the house battery.  Ours also came equipped with Balmar smart regulators controlling each alternator to provide 3-stage battery charging for each battery bank.  This is a good setup, and worked fine, but for a handful of reasons I wanted to make some changes.

Dual alternators

Dual Balmar regulators

The first motivation to make changes was because the 85A start battery alternator was getting very little use.  Under normal conditions, an engine start battery doesn't get used very much.  When you run the starter, it draws a huge amount of current, but the engine typically starts within a few seconds, so the total amount of power draw out of the battery is very small.  As a result, it takes very little time to recharge it, after which the alternator is just spinning and producing little to no power.  That's 85A of potential charge power sitting idle, and meanwhile the house alternator is running full tilt for hours on end recharging the house battery after a day and night at anchor.  So I wanted to use that alternator to help re-charge my house battery faster while under way.

The second motivation is that I also had plans to add 240V inverter service on the boat so we could do laundry while under way.  This will be the subject of another article.  Up until now we had to run the generator, which works, but with only the washer and drier running, it was a very light load for our 20kw generator.  So running off the main engine alternator(s) via an inverter makes sense in the long run.

All this led to the same conclusion; combine both alternators and direct them to the house battery, and charge the start battery some other way.  The 190A alternator puts out a total of about 5kw, and the 85A unit puts out about 2.2kw.  By combining them it's almost a 50% increase in charging capacity, and is a close match to my 7kw of inverter capacity.

Problem 1: How to charge the start battery?

If I take away the dedicated alternator charging the start battery and use it to charge the house battery, it's just a matter of time before the start battery would go dead.  So I needed to come up with a new way to charge the start battery.  The good news is that there are lots of ways to so it.  The bad news is that you need to pick one and live with the trade-offs.  If boats teach you nothing else, they do teach you about trade-offs.  Here are the different approaches and the associated trade-offs as I see them.

Options A: Parallel the start and house batteries while charging the house battery.

There are a number of ways to do this, but they all have the same effect.  When the house battery is getting charged, simply connect the start battery to the house battery and let it go along for the ride.

Advantages:
- It's a nice simple approach
- There are a multitude of devices to automate the operations
- As long as the house and start battery are the same type, i.e. AGM, flooded acid, Gel Cells, the house and start battery will share the charge pretty well.
- When on a shore charger, both batteries are maintained.  This is important when leaving the boat for an extended time to compensate for self-discharge in the battery.

Disadvantages:
- Doesn't work if you have different types of battery for house and start, e.g. flooded for one, and AGM for the other.  They won't share the charge properly.
- Even though the battery will share the charge, the charge cycle will be tailored to the house bank, not the start bank, and may not be all that good for the start battery.  This is probably the biggest issue, and is most prominent with 3-stage chargers.  Here's a good example of how it can be a problem.  Say you are cruising, anchoring every night, and running for 4-6 hours each day.  While at anchor your house battery gets drawn down, so the next day when you are underway your charging system will be operating at higher voltages for most of the cruise as it works to recharge your house battery.  Meanwhile, your start battery only got a slight discharge when you started your engine, but because it's along for the ride as the house battery gets charged, it's going to be charged at a higher voltage all day when all it really needs is a relatively short recharge.  As a result, your start battery ends up getting over charged.  It won't be sudden death, but it likely won't last as long as it would if it were charged according to its needs rather than the house battery's needs.

Lot's of devices are available to do this battery paralleling.  Some support very high charge current, and some support lower current, but they all have the effect of sending the start battery along for the ride as the house battery gets charged according to its needs.

Example products:
Xantrex Echo ChargeBlue Seas ACRs
BEP VSR

Option B: Use a Battery Charger

The other approach is to connect a dedicated battery charger to the start battery.  Ideally it would be DC powered by the house battery, and automatically switch on only when the house battery is being charged.  If your house battery isn't being charged, you probably don't want the start battery charger to drain it down.

Advantages:
- The start battery gets charged according to its needs rather than the needs of the house bank.  This gives max life to the start battery.
- The only power consumed is what's needed to recharge the start battery.
- Start battery can be maintained at full charge while on shore power for extended periods of time.

Disadvantages:
- Dedicated charger are generally more expensive than battery paralleling devices.

Several devices are available, including:

Balmar Duo Charge
Mastervolt  MAC/Magic

I elected to use a battery charger to ensure that our start battery received a charge appropriate to its needs.  Our cruising often involves the exact scenario that leads to over charging if you use some sort of combiner, so this seemed to make sense.

I'm generally not a fan of Mastervolt.  Their products are pretty good, but poorly documented, ignore the needs of North American split phase power (only really applies to their inverters), and have little to no tech support in the US.  But in this case I picked them, and it's entirely because of what I consider to be a major flaw in the Balmar Duo Charger.

The Duo Charger is limited to 30A, which is fine.  But if the battery calls for more than 30A - say because you had to crank your engine a whole lot to get it started after changing the fuel filters - rather than limiting charge current to 30A and running for however long it takes to recharge the battery, the Duo Charger shuts down.  It periodically starts up again and retries, but unless the Battery Charging Fairy came and recharged your battery in the mean time, there will still be a call for more than 30A and it will shut down again, over and over, and never recharge your battery.  They do have a provision for an external bypass relay.  But it's manually operated, so no better than the manual battery parallel switch that I already have.  So I consider this to be a major design flaw, and instead used the Mastervolt Magic 24/24-20 which is a 24V to 24V, 20A 3-stage charger.  It works well, recharges the start battery quickly, then maintains it at a comfortable float voltage for the rest of the time.

Matervolt Magic 24/24-20 charging start battery bank

Now, with an alternate way to charge the start battery, I was free to move on to part 2 of the problem - how to parallel the two alternators.

Problem 2: Paralleling two alternators

How you regulate multiple alternators depends greatly on how the alternators, engines, and batteries are set up.  The alternators can be on the same engine or on different engines.  And the alternators can be connected to the same battery bank, or different battery banks.  Unless they are on the same engine, and connected to the same battery bank, the alternators need to be individually regulated.  This was the case originally on my boat where the two alternators were on the same engine, but connected to different battery banks.  As a result, each had its own Balmar regulator.

But with the reconfiguration, both alternators would now be connected to the same battery bank, making it possible to regulate with a single regulator.  It's still fine to regulate the alternators individually, just not required.  Some friends on MV Dirona elected to regulate them separately as described in this article.  I decided to regulate them off of a single regulator for two simple reasons.  First, I could stash the second now-unused regulator away as a spare in case the other failed.  Second, I wouldn't have to worry about balancing the alternator outputs since they are self-balancing with a single regulator.

You might be wondering how this self-balancing works.  If not, skip ahead.  But it's really pretty simple.  The Field terminal on the alternator controls its output.  If you apply 0 volts to the Field terminal, you get zero output.  If you apply full battery voltage to the field terminal, you get full rated output.  And it's proportional in between.  The regulator controls the alternators by varying the Field voltage.  So if the regulator applies half voltage to the Field terminal, each alternator will put out half of it's rated current.  That means the big alternator will put out 95A, and the small one will put out 42A.  Each is always doing it's proportional amount of work.  It works really well, but does require that they be driven by the same engine, and be connected to the same battery.

Rewiring was actually very easy.  All I did was move the small alternator's output over to the big alternator's output, connecting them together right at the current shunt that measures the big alternator output current.  Wired this way, the pilot house meter shows the combined current of the two alternators.  I also had to up-size a couple of fuses from 200A to 300A to handle the extra charge current.  Fortunately the cables carrying all this current were already oversized (4/0), so capable of carrying the increased power with only about 0.7% voltage drop.

This is probably a good place for a little caution.  It's really important to check the entire wire run from the alternator to the battery to be sure all the intervening wires are sized to handle any increased current.  And this includes checking the ground path as well as the positive path.  Electrical fires are not a good thing.  And also recalculate the voltage drop.  3% would be the max, and even that is a bit much when you are working with charge voltages where 0.1V can be significant.

Next, I removed the now-unused second Balmar regulator and stashed that in the spare parts bin, and jumpered the field connection between the two alternators.

Another lesson in boating it to always expect the unexpected, and guess what.... This was no exception.  Initial testing was not producing the expected current out of the smaller alternator.  But why?

While poking around, I noticed something odd on the small alternator.  In the picture below, the box on the back of the alternator has two wires coming out of it connected to the positive and negative posts.  That's the way built-in fixed voltage regulators are set up.  And you see the two small holes in the box?  That's where you would reach in with a small screw driver to adjust the fixed charged voltage.  This alternator looks like it has an internal regulator rather than using an external regulator.

Alternator with unexpected internal regulator

I sent this picture to Balmar and after a bunch of head scratching they agreed that this alternator was equipped with an internal regulator, even though the alternator part number was for an externally regulated model.  So my start alternator was internally regulated, even though it was wired to an external regulator that was actually doing nothing more than turning on the internal regulator.  How messed up is that?  It's unclear where this mix-up occurred between Balmar (supplier of the alternator and regulator) and Cascade (supplier of the packaged engine and charging system), but at this point it doesn't really matter.  Fortunately Balmar was only about a 45 minute drive away, so I packed up the alternator and took it to them, and they swapped out the internal regulator box for a box wired for external regulation.

Once reinstalled, testing now showed it working exactly as expected with each alternator putting out its portion of the charge current across different loads.  And I now have a whopping 275 of charging current to get our house battery up to full charge between anchorages.  And when the charge loads are more modest, the work is shared between the two alternators rather than concentrated on one while the other loafs around doing nothing.

Wow, that ended up being a much longer article than I expected, but hopefully it will help others who are evaluating alternative charging systems.

Engine Room Cooling

Keeping a boat's engine room cool, relatively speaking, is very important.  When things get too hot, all sorts of wear and deterioration starts to accelerate, including wear on you if you need to spend any time in there.  Spending any length of time in a 130 degree space is not just uncomfortable, it's down right dangerous.  But if something breaks, you gotta go in.

Speaking of 130F, that's one of the magic numbers.  A variety of equipment is specified up to a max ambient temp of 130F.  Go above that, and you are on your own.  Warranties are void, etc, etc.  I say that 130F is one of the magic numbers, because there is another one.  30F delta-t.  Delta is a Greek letter, and is used in science to represent the difference between two numbers.  So 30F delta-t means a temperature difference of 30 degrees Fahrenheit.  And the difference is between the outside air temperature and the temperature in the engine room.  If you want to run your engine room electrical equipment and machinery within the limits allowed by the manufacturers, you need to stay under BOTH numbers.

The effectiveness of an engine room's cooling system will determine the delta-t for the boat.  Cooler is always better, but this means that a cooling system must keep the ER temp at no more than 30F over the outside temp.  This in turn limits the boat to operation in 100F outside temp, because if it gets hotter than that outside, you will start to exceed the 130F absolute temp limit.  If, on the other hand, you can limit delta-t to 20F, your ER runs cooler under all circumstances making it more hospitable to you personally, and it allows for operation in a hotter ambient temp.  So the goal is to keep the delta-t as low as practical.  Everything in a boat's design is a trade-off, so these goals are often hard to meet, or in some cases not met at all.

The key to making this work is to apply a little bit of science and engineering.  You need to look at:

1) What are the sources of heat and how much do they generate.

2) How much air flow is required to remove that heat and maintain a reasonable delta-t

3) How will you get that air into and out of the ER

Sources of Heat

The biggest source of heat in your ER is your engine (or engines).  Additionally there may be a generator and some other gadgets, but the main engine will dominate, so that's the thing to focus on.

Engines work by burning fuel.  Some of that energy is turned into propulsion power. but most of it (about 70%) is waste heat.  The majority of waste heat is removed via the engine's cooling system and ends up in the surrounding sea water.  In the case of a keel cooler, it transfers to the sea water that passes by the outside of the boat.  In the case of a wet exhaust, it's transferred to the sea water that is pumped through the heat exchanger and exhaust system.

But there is still a significant amount of heat that is dissipated into the surrounding engine room.  It's only about one tenth the amount of heat that goes out through the cooling system, but it is still a lot.  One way to look at it is that you have a 1000lb to 2000lb radiator sitting in your ER running at 200F.  That warms things up.  The amount of this "ambient heat rejection" is specified by the manufacturer for every engine so whoever is designing around it can figure out the cooling.  Below is the spec page for my engine, with the heat rejection number highlighted.  The heat rejection is 22kw.  A little electric space heater is about 1kw, so it's like having 22 of them running in your ER.  It's a lot of heat to remove.

Specs for Deere 6090AFM75 engine.  Heat rejection highlighted.

Airflow Required to Remove Heat

Getting that heat out of the ER is usually accomplished by air flow, pulling fresh air in from the outside, passing it through the ER where it heats up, then dumping it back outside.  Physics is everywhere, and there is a formula relating heat, air flow, and delta-t.

CFM = (55.0 * BTU/m)/ DeltaT

CFM is cubic feet per minute of air flow.  BTU/m is British Thermal Units per minute and is a heat rate.  And Delta-t we have talked about.  So with this formula you can figure out the air flow required to maintain a certain delta-t, given the heat rejection rate of your engine.

Or, looking at it another way;

DeltaT=(55.0 * BTU/m)/CFM

This tells you what the Delta-T will be for your engine's heat rejection rate and some given air flow rate.

When you run the numbers on my engine, it says that I need about 3500 CFM of air flow to maintain 20F delta-t.  I think 20F is a good number to strive for.   Under most circumstance it will keep your ER at 90-110F which is tolerable, and it means you can run in 110F ambient if required.  Lower would be wonderful, but as you'll see in the next section, it gets harder and harder to move the requisite amount of air, so you need to find a balance.  So I set 20F as my goal.

How to Move Lots of Air

This is where things get challenging.  Up to this point it's really easy to calculate how much air flow is needed, and it's really easy to say you want more.  But now you have to find a way to actually make it happen and this was the problem on my boat.

Are moves easily in open spaces, and is progressively harder to move as space becomes more confined.  But this is often overlooked.  Take breathing as an example.  You can break in and out easily because your wind pipe and mouth are large enough.  There is no restriction.  Now try breathing through a 1" hose.  You will probably be fine, but will surely feel the restriction as you push and pull air through that hose.  Make the hose longer, and it gets harder.  Now try breathing through a straw.  Good luck with that.  It's just too small to move the requisite air.  This was a major problem on my boat.

As originally built, there were two blowers serving the ER.  One was rated at 220 CFM and connected to a 3" hose, and the other rated at 1200 CFM and connected to a 5" hose.  Each hose was then about 10' long.

When you look at the performance specs for a fan, it lists the CFM rate in free air, i.e. with no restrictions.  That's like you breathing, and is the best you will ever get out of the fan.  Then they list the performance at different levels of back pressure which is the restriction imposed by a duct or hose.  This is like breathing through various size hoses.  Once again, physics is everywhere, and there are formulas for calculating the back pressure when trying to move various quantities of air through various size hoses and ducts.  There are simple back pressure calculators on line, and I made use of this one.  When you run the numbers, the back pressure trying to move 1200 CFM through 10 of 5" duct hose is 2.5" of H2O.

Now take a look at the performance specs for that 1200 CFM fan.  In the chart below, you can see that with 2.5" H2O of back pressure, the fan is completely stalled and not moving any air at all.

Air flow curve for Dayton blower.  It stalls at a little over 2" of H2O

That's not good.  Now in practice, as the duct resistance increases, air flow decreases, and eventually the war between the duct resistance and the fan finds a point of equilibrium along the line in the chart.  As part of this project I bought an inexpensive airflow meter to help see exactly what was going on, so I measured the air flow through the 1200 CFM fan.  It was 370 CFM.  This is the big fan in the ER cooling system, and it's only moving one tenth of the required air.  The situation for the other fan was similar, with actual air flow of 70 CFM vs the free air spec of 220.  The obvious conclusion is that both fans are contributing very little to cooling the ER, and might even be a net negative given their power consumption.  Between the two fans they represent about a 1kw electric load while underway.  That power is generated by the engine alternators which dissipate more heat the more they are loaded.  The moral of the story here is don't try to breath through a straw.

Large blower sucking through a straw.  Hint - it doesn't work very well

As originally build, my boat had another fan about half way up the exhaust stack, and that's another example of what not to do.  Air, water, electricity, and probably lots of other things follow all possible paths, and do so in proportion to the resistance of each path.  A waterfall will have a main channel, and tributary paths, each sized according to how much water can squeeze through each path.  Air does the same thing.  This upper stack fan had a hose that hung down inside the stack cavity about 3 feet, then it blew out into a vented compartment on the boat deck.  I think the idea was to pull air up the stack and push it out through the compartment.  But in actuality it pulled air from down the stack, from up the stack, and from any direction it would flow.  The net result is that is too did pretty much nothing but consume electricity.

What does effectively move a lot of air through the ER is the same thing that generates all the heat - the engine.  It's actually a pretty large and very effective fan, pulling air from inside the ER, and expelling it out the exhaust.  Going back to the engine specs, it tells us that my engine moves 730 CFM of air.  So far, that's the biggest air mover on the boat.

Adding it all up, we have 730 from the engine, 370 from the big fan, and 70 from the little fan.  That's 1170 CFM total.  With that amount of air movement, the predicted delta-t is 52 degrees.  That's obviously not acceptable.  They cooling system as originally built had no chance of succeeding.

Stepping back, the problem is the size of the air passages.  We haven't talked about the intake side and that's just as important.  On my boat there are two intake vents, one on each side of the cockpit.  These lead down through a large tunnel and emerge through apertures in the ER.  Running the back pressure calculator on those passages suggest that they are large enough to carry the desired 3000 or so CFM without creating undue resistance.  So the intake side looks good.  The outflow side of the equation, however, is a big problem for those two meager houses.

The Light Bulbs Lights

After a while the light bulb finally lit, and it became evident that the exhaust stack was a good size cavity leading straight from the ER to the outside.  It's much, much bigger than the hoses.  Running the calculations using it as an air duct shows it can move 3000 CFM without excessive back pressure.  Perfect.   If a way could be devised to pull air up the stack shaft, there would be enough space to actually move the required amount of air.  With this approach, the airflow path would be in the cockpit vents, into the back corners of the ER, forward through the ER, then up the stack right over the main engine.

The best place for fans was in the upper stack at the boat deck level.  Looking at the picture below, this structure is all open inside with storage space below by the door, and open space with vents above.  Then there is a removable panel that runs the full height separating the cabinet from the stack.  So the wall with the port light in it separates the cabinet from the stack, with the removable panel inside the cabinet.

Boat deck cabinet and air vent

The first attempt at this was done in Dana Point when we were commissioning the boat and struggling with high engine room temps.  The idea was to eliminate the silly fan with snorkel hose (it previously lived in this cabinet), and install another 1200 CFM fan in the divider panel.  The fan would pull air from the stack and exhaust it in the cabinet where it could flow out the vents.

Below is the fan mounted on the divider panel, and below that you can see the panel's full height looking up inside the cabinet.  Remember this for later on...  Oh, and ignore that red hose.  It's the exhaust for the shroud blower that cools the exhaust pipe jacket.

Fan pulling directly from stack cavity, exhausting into cabinet

View of divider panel between stack and cabinet.  A perfect fan mount location

The idea was good, and seemed to get ER temps down to the magic 30F delta-t.  But that didn't last, and later I would find that on long runs the temps would still climb over 30F.

This biggest issue with this first step is that it was only a first step, and still suffered the same flaw as the snorkel hose fan.  Air follows all available paths, and this was pulling just as much cool air down the stack as it was pulling hot air up the stack.  What was still needed was some sort of baffle in the stack to seal off the area above the fan and force all air to be pulled up from the ER.  In the spring of 2015 I fabricated a simple baffle made out of galvanized duct material.  Although not a perfect seal, it had the desired effect of forcing the air flow to come up the stack.  For the first time, air flow measurements in the stack were right about 1200CFM, exactly what the specs for the fan stated.

Air baffle to force air flow up the stack from the ER, rather than down the stack

We ran for the summer of of 2015 like this, and although it was a big improvement, we were still hovering around 30-35F delta-t.  Running the calculations with the full 1200CFM of air flow, plus the engine air flow, it predicted 33F delta-t - just what we were seeing.  The problem remained a need for more air.  Something between 2000 and 2500 CFM in fans, with the rest from the engine would get it to the desired 20F delta-t.

One experiment I tried over the summer was shutting down the two engine room fans.  Together they were consuming about 1kw of power, and only moving a small amount of air.  I was curious what would happen if I just shut them off, so I tried.  The answer...nothing.  Turning them off had no impact at all on ER temps, so I left them off permanently, and when we returned to Seattle, I removed them completely and just connected the ducts through to the ER with no fans.  Now they serve as supplemental passive air intakes.

In the fall of 2015 I visited Delta-T Systems, manufacturers of fans and systems for boats, and sat down with one of their applications engineers to review my plans and calculations, and see if we could pick some fans for the job.  Long story short, he agreed with the calculations, and I selected their Small AC Axial Fans.  AC power meant no rewiring since AC fan power was already present in the cabinet.  The 11" model is quiet, consumes only 185W of power, and can move about 1000 CFM.  I bench tested one at home, and the thing took off across the bench from the air blast.  Their size allowed be to fit three of them in a vertical row in the cabinet divider panel, and still have space below in the cabinet.  One down side of the first fan installation is that it consumed the entire cabinet.

I had to fabricate a new divider panel and mount everything up, but was able to do it out on the bench.  Below are the three fans ready for final wiring and then installation as an assembly.

New triple decker fans on new divider panel

Another view of the new fans

The picture below shows the approximate locations of the stacked fans once installed in the boat deck cabinet.

X-ray vision view of fans inside cabinet

In conjunction with installing the new fans, I need to relocate the baffle.   The original baffle was just above the original fan location, and the new triple decker fans were located up towards the top of the cabinet.  So I built a new baffle which is nothing more than two pieced of fiber board that fit into this square recess where the stack transitions from the boat deck to the flybridge.  This picture shows the location, but not the baffle itself.

Looking down stack from fly bridge, and convenient inset for baffle boards

In January of 2016 I finished installing the fans, and even wired up thermostats in the ER to switch the fins on incrementally as the ER temp climbed.  But when I fired them, I was in for a great disappointment.  Instead of measuring 2500 or more CFM as expected, I measured 1300 CFM.  I was crushed and perplexed but had a bunch of other projects that needed attention, so I left it all for later investigation.  After all, it was moving more air than the previous system, and consuming a LOT less power, so it was still a step forward,  Just not as far as I had hoped.

Over this past summer (2016) we observed a pretty consistent 30F delta-T on long runs.  My suspicion was that the cabinet was imposing too much back pressure on the fans, and not allowing them to move their rated air flow.  The cabinet has a few baffles to prevent water entry, and the vent slits are potentially restrictive, so very late this summer I started to experiment on a couple of long runs where there was plenty of time for the ER to heat up.

First I removed the upper grill, and much to my pleasure saw a distinct drop in ER temp - about 5F over a couple of hours.   Then I opened the cabinet door, and the temp dropped again down to a delta-t of 18F.  Wow, all I needed to do was open that silly door.  A bunch of subsequent experimenting, including taking air flow measurements in the stack, showed that opening the door had the biggest impact, followed by opening up the upper part of the cabinet.  With everything opened up, I can get almost 2000 CFM of air flow which is much closer to the 2500 or so that I expected.  And the upper most fan is only 4-6" away from the cabinet with no relief from opening the vent grill or door, so probably explains the missing air flow.

But now I know the system works, I know it can achieve 20F delta-T, and I know what final modifications I need to make.  I have two 12" square stainless grills on order, and plan to install one in the door, and one up towards the top of the blank space directly in front of the uppermost fan.  I also ordered a 4x18" grill to install immediately above the door, but will probably wait to see how things work with the other two before I cut that one in.

Door opened and upper grill removed to reduce back pressure in cabinet

 Alternate Approaches

There are always lots of ways to skin a cat, and ER cooling is no exception.

Many people have upgraded various fans, but kept the basic cooling arrangement.  This helps a bit, but if you go back to the fundamental problem which in adequate ducting, it will never really solve the problem.

Where a number of people have found some level of success by installing large fans over the intake vents in the ER.  I believe this works because it achieves the same air flow as what I've done, just in a different way.  By pressurizing the intakes, you force air into the ER, and it needs to exit somewhere.  The lowest resistance path is through the stack, so that's where most of the air will flow.  The big difference is that you are pushing the air into the ER and out though the stack, rather than pulling it up the stack.  Both work, but the unfortunate side effect of pressurizing the ER is that you also push some amount of air into and through the boat.  That both heats the boat, which sometimes can be good, but it has the undesirable effect of also pushing ER odors into the boat as well.  By sucking air up the stack, the ER runs at a slight vacuum and any fumes or smells get pulled out of the boat.

This has been a full two years of head scratching, measuring, fabricating, measuring again, failing, trying again, and finally tripping over the final piece of the puzzle.  The key to this approach is the stack cavity and it's ability to carry a lot of air.  It may not exist on other boats, or be as easily used as on the 55/60 model Nordhavns.  But hopefully this will help others think through how their ventilation air flows, where it might be restricted, and take a more scientific approach to improving it.  After all, science always improves your luck....