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Saturday, April 13, 2013

A Plan for Fuel Management

A few posts ago I talked about fuel management, mostly focused on the need to have fuel gauges on all 4 tanks.  The other issue I was trying to sort out at the time was the best strategy around which tanks to draw down when, and when to move fuel around between tanks.  Now I have a plan, subject to change of course, and unverifiable until I actually have the boat.  But here it is anyway.  It should allow someone, somewhere to look back and say "I told you so"....

As a recap, the N60 has 4 storage tanks and 1 "day tank".  The two main storage tanks are 1000 gal each, with two forward tanks of 150 gal each.  The day tank is about 30 gal.  All operating fuel is drawn from the day tank, and excess is returned to the day tank.  Fuel can get from the storage tanks to the day tank in one of two ways; via a filtered fuel transfer pump, or via gravity flow.

As I understand the concept of a day tank, it is supposed to be an isolated tank filled with enough filtered and clean "known-good" fuel to run the boat for a day, hence the name "day tank".  You fill up the day tank once a day, and do it through a filtered transfer pump, thereby ensuring that only clean fuel is ever in there.  It's a great concept, and a good way to ensure that stirred up crap in the bottom of a storage tank doesn't find its way to your engine.  I think every book that I've read about a power boat disaster involves heavy seas that stir up crap in an old fuel tank, plugs filters, and kills the engine.  Once the engine is lost, bad turns to worse, and there is something to write a book about.

The trouble with the N60 (and a bunch of other models) is that the day tank only holds enough fuel for roughly 2-5 hrs depending on how fast you are going.  Rather than filling daily, you need to fill it multiple times per day.  And, despite its many benefits, day tanks have a down side to them.  If you let them run dry, the engine will stop, and restarting a diesel that has run dry is a royal pain in the butt because all the air needs to be bled out of the system.  This can easily be a 30-60 minute process.  I love the concept of a day tank, but it's just too small on the Nordhavn to accomplish its goal.

There is another issue with the small day tank on the Nordhavn.  Heat.  And the problem is worse with the transition to modern, high pressure diesel engines.  Diesels draw fuel from the tank, pump it up to some pressure, inject what's needed into the cylinders to power the load, and return the excess back to the tank.  This circular flow is not only a technique to maintain a predictable injection pressure, but serves to cool the pump and injection system by carrying heat away in the excess fuel flow.  Common rail engines (high pressure fuel systems) generate a lot of heat when the they pump the fuel up to that high pressure.  The return fuel can be well over 150 deg.  All that hot fuel goes into the day tank and circulates back to the engine, reheating it over and over again, and radiating heat into the engine room.  A larger body of fuel would heat more slowly, and a larger body of fuel has more surface area to dissipate the heat, preferably through the hull to the surrounding sea water.  The small day tank accomplishes neither of these things.

I've talked to a number of owners, and the most common approach to fuel management seems to be:

- Keep the small forward tanks empty unless you are planning a passage.

- Draw fuel from the large tanks via gravity feed, alternating port and starboard at some interval to keep the boat trim.

- Fill the forward tanks in advance of a planned passage, and once you are certain the main tanks are down enough, pump the entire contents of the forward tanks into the main tanks.

This works, but in my opinion completely defeats the goals of a day tank, and leaves the boat exposed to stirring up of crap in the main tanks.

So what's a boater to do?  This is what left me puzzling after my last post on the topic of fuel management, but I think I've found a solution.

- Rather than use the small forward tanks only for extra storage for long passages, I'm planning to use them constantly as a big day tank.  Instead of a 30 gal day tank, I'll have 300 gal, and there will be a gauge on it so I know when I'm getting low.

- The forward tanks will flow via gravity to the real day tank, and be drawn off from there for the engine and generator.

- The forward tanks will ONLY be filled via the filtered transfer pump.  They will never be filled via the deck fillers.  Prior to fueling, I'll transfer fuel from the main tanks to top up the forward tanks.  Then I'll fill the main tanks from dockside.  If I happen to be down so low that I can't pre-fill the forward tanks, I'll just transfer fuel while I'm filling.

- While underway, I'll always move fuel from the main tanks into the forward tanks for use, running through the transfer filter on the way.  That will ensure isolation of any crud that develops in the main tanks.

- To mitigate the heat build up issue, I can return the excess fuel to the forward tanks rather than the day tank, slowing the heat build up, and providing better dissipation through the bigger forward tanks.

- Right now I'm thinking that I will just open the valves for both forward tanks in parallel, allowing fuel draw from both.  This could lead to some listing, but I figure that I'll just compensate for trim by controlling fuel in the main tanks.

I think this will work, and restore many of the virtues of a day tank, and eliminate the problems associated with a day tank that's just too small.  Time will either confirm or contradict this theory, and hopefully the unknowns won't bite me in the ass.  For example, I'm assuming that keeping the forward tanks full won't negatively impact the fore-aft trim of the boat, or it's ride in various seas, but I won't know until I actually try it.

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