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Monday, December 3, 2012

Engine Selection

I've been accused of writing a "guy" blog.  Well, yah, have you looked at me lately?  I'm a guy.  So to all my guy-friends, read on, this one's for you....

For a while I expect to be writing about the thought process and decisions that go into building a boat.  We have accumulated a bunch of experience over the years and have become clearer and clearer on what we like and don't like, but we've never built a boat before, so much of this is new to us.  Even on a "production" boat, there are still hundreds of decisions to be made.  Nordhavns are production boats, but they are built-to-order which means all boats are built for a specific owner, customized to their wishes.  I think it's safe to say that no two boats are the same.  I won't go through all the detailed decisions, but I think it will be both interesting and useful to others looking at different boats with different systems to hear about the tradeoffs and selections we are making.  But keep in mind that our decisions are only right for us (hopefully), and others will make different decisions.  Boats are an amazing exercise in trade-offs.  Nothing is free, either figuratively or literally (especially on a boat).  The trick is to find the right balance for your needs.

The first installment will be engines.  The actual choice of engines is easy because there is only one.  A large percentage of Nordhavns are powered by John Deere engines.  Many carry the name Lugger, but it's still a Deere at its core.  Lugger is a company that adapts them to marine use, and more recently Deere has started doing it themselves.  Deeres are widely used in commercial fishing boats and have a great reputation.  You don't hear a lot about them because they just keep on working.

The big question is whether to go with a single engine or twins?  Most Nordhavns have singles.  They yield a bit better fuel economy (order 10% in theory), represent one engine to maintain rather than two, and costs less.  The argument for twins is two-fold; better maneuvering in tight quarters like docking, and redundancy in case on engine fails.  The penalty is two engines to maintain and slightly worse mileage.

All the boats I've had prior to our Grand Banks have been singles, and I have to admit that I really like the meneuverability of the twins.  If you have been following this blog, you will recall this little incident where twins really saved us st-anne-de-bellevue.

But priorities on this boat are different.  Range matters, which means mileage matters, and for some reason the penalty for twins on this boat is much more than 10%.  It's more like 25%-30%, if not more.  That's a big price to pay, and has a big impact on range.  You can always slow down to regain range (fuel mileage is all about how fast you go), but the reverse is true too - the better mileage you get, the faster you can run on a long passage and still have the range to make it.  That last part I like!

But what about redundancy?  If an engine craps out, with twins you have another one to keep you going and get you home.  That seems pretty important, right?  Yes, it is important, but it's not that simple, and there are other ways to gain redundancy.  I like to break engine failures into three groups:

1) Fuel problems:  Many, maybe even most experts will tell you that fuel problems are the most common cause of engine failure.  You can take on a load of contaminated fuel, and a little time after you get underway, your engine dies.  Or you can be in rough seas and the inevitable accumulation of crud that has settled to the bottom of your tanks get stirred up and sucked into your engines and kills them.  There is a sound argument that a fuel-related failure will take out both engines, not just one, so having twins doesn't buy you any protection against this class of failures.

2) Accessory failures:  These are things like drive belts, alternators, pumps, impellers, etc.  If one fails, then it will indeed stop on engine, and only one engine.  A second engine will get you home.  Your other choice is to repair the problem at sea, which for many failures is very doable, and for others it's more difficult.

3) Core engine problems.  Here we are talking about major problem.  A shot injection pump, burned valve, broken mechanisms, etc.  Even McGiver isn't going to fix these problems at sea, so they make that second engine look pretty good.  But this sort of problem is VERY rare.

These are pretty compelling reasons for twin engines, until you consider another way to solve the problem; a wing engine.  A wing engine is a small, second engine who's sole purpose is to keep the boat moving when the main engine is off.  They can only move the boat at 5kts or so, but that's enough to keep the boat stable and heading in the right direction while you carry out a repair, and it can get you home if the main engine is unrepairable.  For this reason, wing engines are often referred to as get-home engines.  In addition, to protect against fuel-related problem, a wing engine is typically equipped with its own dedicated fuel tank that is filled through a filter to prevent contamination.  If you pick up a load of bad fuel, it gets filtered before going into the wing tank, and filtered again two more times before being consumed by the engine.  Wings are also equipped with folding propellers typically used on sail boats to minimize drag when not in use.  A wing adds cost, but not nearly as much as twin mains.

There is also a little gotcha with twins running on one engine.  If you lock the prop shaft on the failed engine, the stationary prop creates a lot of drag.  And if you let it freewheel, some transmissions will overheat and risk damage, and some shaft seals will overheat and risk failure.  On many boats you must lock the shaft, and how the heck are you going to do that?

But lets not forget maneuverability.  Twins give you quite a bit, especially after you get the hang of them.  You can spin the boat around in a circle in place, and with a little practice and the right rudder action, you can walk the stern to one side or the other.  Add in the bow thruster, and you can move the boat sideways in either direction.

With a single and the right rudder action, you can kick the stern to either side when powering forward.  But in reverse, the stern always pulls to one, and only one side - the rudder does nothing.  This is known as prop walk, and the direction it pulls depends on the rotation of the prop.  On Nordhavns, it pulls to starboard.  Our last boat was a single engine with bow and stern thrusters, which serves to compensate for the reduced maneuverability of a single.  But after I got used to it, I seldom used the thrusters at all - so little that by the end of the season they were completely ineffective due to marine growth on the props.  The key, though, is that thrusters can make a single just as maneuverable as twins as long as they stay clean and are powerful enough.  Stay tuned until another post for a discussion about what "powerful enough" means.

Can you guess what configuration we picked?  We are going with a single 325hp main engine, an 80hp wing engine, and both bow and stern thrusters.  It's worth noting that a bow thruster is standard equipment on pretty much all boats these days, including the N60.

Coming up next - stabilizers and other hydraulics


2 comments:

Anonymous said...

I'm not a guy, but I love the boat-centric techie blog entries :-)
Keep them up!
Kristina
N5708

Tanglewood II said...

Welcome, Kristina. I'm glad you are enjoying the techie parts of the blog, and hope you didn't take any offense to the "guy-blog" comment. It's a running joke between my wife, me, and a few other people.