Two meter troll wrote:

his fuel polishing system disallows this if he wants a proper fuel
line pressure in all cases. his decision, his boat. pointing out
options was my idea.

I appreciate the suggestions. However, for sake of discussion and anyone
looking at it for ideas on their sailboat:

(Schematic of my system he
http://home.roadrunner.com/~rlma/StriderFuelSystem.htm)

My system is a very tolerant, low pressure system. It runs from the main
tank quite well with gravity feed alone up to the mechanical fuel pump. The
mechanical fuel pump draws quite well from the low tank with the electric
fuel pump turned off but the vacuum is higher than I would like to subject
the pump to for long periods.

The pressure head at the mechanical pump inlet is the same when running off
the bypass as it is when gravity feeding from the main tank. If this should
be insufficient due to the secondary engine mounted filter starting to load
up or a fault in the mechanical fuel pump, I can gradually close the
isolation valve to force fuel to the engine at up to the full pressure that
the polishing pump can develop. This pump BTW does not have the pressure
cut off and regulation feature of a normal FO lift pump.

In a single tank installation with a bilge tank significantly below the
engines and fuel system, there might not be sufficient fuel pressure and
partial closing of the isolation valve might be necessary. In my case
though, I get normal fuel line pressure in all cases as well as the option
of increasing it.

It's worth repeating Rich H's brilliant idea for further redundancy:

The fuel polishing line runs to a small gravity feed tank that contains 1 -
2 hours of fuel. The line goes in the bottom of the tank from the polishing
system and exits the top on its way back to the main tank. A line runs from
this tank directly to the engine bypassing the engine driven fuel pump and
the vent has a valve on it. The fuel line could also bypass the engine
mounted fuel pump in some installations but not on mine since the injector
return line goes back to the filter instead of the tank, a Yanmar oddity.
This tank is constantly being filled, flushed, and renewed with just
filtered fuel that doesn't sit in the tank when the engine is running. If
all else fails, just open the supply and valves and a supply of clean fuel
sufficient to get the vessel out of trouble or change filters goes directly
to the engine.

In any event, I can change filters alternately on my system until I run out
of elements so I'm pretty well covered for the worst case situation. I've
also got those sails

--
Roger Long