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Sunday, January 6, 2013

Fuel Management

You'd think this would be a simple topic, right?  Fill your tanks and off you go, keeping an eye on the gauges until it's time to fill up again, right?  Unfortunately it gets a little more complicated on the N60 for a variety of reasons.

First, there are 4 tanks to keep track of.  Having multiple tanks is not unusual in a boat, but I think 4 is on the higher side.  Our current boat, for comparison, has 2 tanks.  The 4 tanks on the N60 are arranged as two 1000 gal main tanks, and two 170 auxiliary tanks for a total of 2340 gal.

Oh, but there is a 5th tank called a day-tank, or perhaps more accurately a run-tank.  The engine and generator draw their fuel from this run-tank, and the run-tank gets filled from any combination of the 4 storage tanks.  Why, you might ask?  Reliability is the answer.  Contaminated fuel is reported to be the most common cause of engine failure, and the idea behind the run-tank is to create a known-good supply of fuel, even if the storage tanks have questionable fuel in them.

When everything is working properly, the run-tank is kept full via gravity feed from any one of the storage tanks.  But if your stored fuel is in questions, you can instead route all fuel through a filter on its way to the run-tank, thereby filtering out any crud  before it gets in the run-tank.  Ideally you would run all fuel through the filter before it goes in the run-tank, thereby ensuring good fuel at all times, but running through the filter is a manual process.  You have to operate a pump to move fuel through the filter.  That means that every few hours you need to go down in the engine room and run the pump long enough to top up the run-tank.  And if you forget, you can easily run out of fuel and find yourself dead in the water.  It doesn't seem like a very good solution overall, but I haven't figured out how to best use it, or how to improve it.  Maybe in time I'll come up with something.

Going back to the 4 storage tanks, there is another complication.  As equipped from PAE, the smaller tanks have no gauge at all, and the big tanks have a site gauge that only reads the bottom 600 gal in each tank.  This strikes me as one of the most brain dead things I've encountered on this boat.  I don't understand how anyone can set out on an ocean crossing that's expected to consume 90% of the boats full fuel capacity, and not have tank gauges.  Yes, fuel burn is pretty predictable, and yes, you can calculate how much you will use.  But I'd want to be constantly cross checking the expected fuel level against the actual level.  A crossing calculated to consume 90% of the boats fuel means you have a 10% margin of error before you find yourself in very deep trouble.  Rough seas, opposing winds, opposing currents, and route variations to avoid weather will all impact fuel consumption and can pretty easily consume that 10% of margin.  The way the boat is equipped, you have no confirmation of how much fuel you have burned until you have used about half of it.  Only then do you get your first confirmation of how much fuel you actually have used, and how much you actually have left.  And only then can you compensate if external factors are negatively impacting your consumption.  I just don't get how a boat designed for ocean crossings can be equipped like this.  It's like crossing an ocean with your GPS turned off until your reach the half-way point, relying solely on dead reckoning until then.  At the midway point, you get to turn on your GPS for the first time to see where you actually are vs where you calculated you would be.  And the solution is so simple - just put gauges on the tanks like every other boat on the planet.  That's what we did, but why on earth it's not standard equipment is beyond me.  For all that PAE does right - and that's most everything - they have these odd blind spots here and there that leave me scratching my head.

7 comments:

Bob and Vivian said...

Hi
Really enjoying your blog and look forward to following your build progress and decisions along the way.
On the fuel subject, Ken williams (Sans Souci N68) recently had a write up on the Nordhavn Dreamers forum explaining how he used one of the main tanks as a "day" or "run" tank to simplify the process and avoid having to open and close valves in the engineroom to keep filling the standard "day" tank.
Keep the blogs coming
Regards

Bob

Tanglewood II said...

Thanks. I've read most of Ken's stuff, but must have missed the one your are referring to so I'll go back and hunt it down.

Cedric Rhoads said...

Have you checked out the Maretron N2K monitoring system? Very thorough, bootstraps approach to vessel monitoring. They have a great solution for tank monitoring as well. I also love the current-sense switches that can tell you if a circuit is drawing current (like a bilge pump) or not (like your navigation lights). I'm a Dreamer now, but I still spent hours architecting a solution for my future Nordy. Keep up the great work. We Dreamers (and others, no doubt) are loving the decision process of your new build. Well done. Cedric

Cedric Rhoads said...

Oh, at the very least, I'd put in the back-bone, step-to-stern, and ER-to-PH-to-Tower now, while it's cheap. You can always add it the system later.

Tanglewood II said...

I'll do a future post with more detail, but I plan on installing N2K along with a variety of Maretron and other N2K devices. I am using their stuff on my current boat, including pressure sensors for water tank gauges. It works quite well. Stay tuned for more detail....

Tanglewood II said...

I found the post. I'm not sure how the tanks are arranged in an N68, but on the N60 all the tanks are on one side of the boat or the other. None (other than the day tank) are center mounted. That means that running off one tank will pretty quickly lead to imbalance and listing to one side of the boat.

I'm still pondering how to best manage fuel consumption and storage.

Eliza Sanchez said...

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Fuel management system