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From: Candy Chapman and Gary Bell (no email)
Date: Mon Mar 03 2008 - 08:36:49 EST
Wayne, Arild, Rob, Mike et. al.
I am enjoying this discussion of fuel tanks and plumbing. I
particularly like the day tank notion, and am already designing one,
incorporating heavy filtration in the day tank fill, gravity feed to
both engines and with fuel returns plumbed to the day tank (plus a
number of other features less germane to this discussion).
I've not yet seen mention of the one consideration that trumps all the
others in my view: Fuel return from the engine. Even an engine system
with a meager fuel return flow can in time overfill (or empty) a tank if
fuel is drawn from one tank and returned to another. I see three
attendant risks: Fuel spill; emptying a tank and drawing air into the
fuel feed line; and as Mike mentioned trim and stability issues. The
varied designs in boats today shift that threesome about, but do not
eliminate the issue, and any of the three can become a significant
problem. Duplicate manifolds for fuel feed and return would present at
least twice as many valves to puzzle out, and several times the
opportunity to mess up. Ganged valves (two valves on the same handle)
would allow two effective manifolds with the absolute certainty of never
messing up the return. Sadly my search for suitable ganged tee and
shutoff valves, at reasonable cost, has turned up nothing so far.
Tank crossover plumbing will indeed resolve these problems, but only for
tanks that are mounted at the same level, and then only when any
isolation valves in these crossover lines are open. As Mike suggested,
a significantly large crossover line could cause an induced list (like
lifting something over one side) to grow ever greater as the fuel
crossed from the now higher side tank to the lower one. Keenly aware
of the KISS principle and the difficulties of idiot proofing anything
(they so quickly come up with a better idiot) I see an additional single
day tank with few, or no valves to operate as the best approach.
Conceptually the day tank (provided the fuel is returned from the
engines to it) becomes an automatic master manifold. If one fuel supply
tank is designated as the main tank, the system could operate without
any valves to manipulate. An adequate fuel transfer pump system could
supplement the main tank from any additional tanks, and these supply
tanks wouldn't need to be at the same level. Fuel polishing and other
capabilities would be easy to incorporate. I'll try to put my design
ideas in a subsequent post.
BTW, have any of you found ganged valves (tee and shutoff) suitable for
simultaneous fuel handling?
Gary Bell
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