T&T: Fuel Manifolds

[Candy Chapman and Gary Bell]

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