Fuel tank elevation

[lat 64]

Today I actually have a question instead of just being a smart-alec.

I am replacing my old fuel tank with a new plastic one from Moeller.
The old one is under the quarter berth and molded in to the hull liner. It is fiberglass. This one will be abandoned and turned into tool storage.
The top of this tank is about even elevation with the carburetor.

The new tank will be in the lazarette. This will be perhaps 20-inches higher.
The bottom of the new tank will about the same elevation as the carb.

Do I have a pressure problem now?

Russ

[FelicityRebuild]

Not unless the new tank uses a gravity feed through the bottom of the tank. I don't think this is very common or recommended though. Most tanks feed through a connector in the top of the tank with an internal fuel line feeding about 1/4 to 1/2 inch off the bottom. Gravity does not play a role and therefore all fuel is fed properly via the pump. There shouldn't be any pressure problem.

[Jesse Delanoy]

You never want fuel to be gravity-fed out of the bottom of the fuel tank. If the fitting should ever give way, you'll have a bilge full of gasoline. Drawing fuel out of the top of the tank is much safer. The bottom of the tank should not have any penetrations.

[FelicityRebuild]

Not to mention feeding sludge from the bottom of the tank directly into your carb. yuck.

[Baltimore Sailor]

The only possible downside I can see is that if the tank is higher than the pump and the fuel line somehow came off at the pump, your tank could siphon itself into your bilge.

[hanleyclifford]

Russ, I have such a tank installed 1964 when the boat was new, about 14 inches bottom of tank to carb vertical drop. The fuel is brought up thru the top of tank (ss). My tank is old enough to have a bottom drain (no longer up to code I believe). Pressure is almost nil but if your fuel pump fails, the engine can limp along about 1200 rpm or so.

[lat 64]

The lines come from the top of tank

There is also a check valve in the fuel pump.
I'll be doing this in a little over a week. I 'll post a story maybe.

Thanks all,
Russ

Nice to know about gravity being an ad hoc pump. Have to bypass the pump for the limp home.

[marthur]

A quick calculation shows that with gasoline, a 20 inch height difference would give only about a half pound per square inch of pressure. With the tank less than full, you would have even less pressure. The ordinary A-4 fuel pump puts 3 or 4 lbs per square inch. An extra half pound of pressure shouldn't be an issue.

It will syphon though, so it is probably a good idea to shut off the fuel at the tank when the boat is not in use.

Fair Winds,

Mike

[ndutton]

In agreement with Mike, if somehow the carburetor float valve doesn't close properly (damaged or worn seat, debris) the fuel will siphon out of the tank unchecked, through the fuel pump (mine's a Facet), overflow the carb and come to rest in the bilge where only a cup of evaporated gasoline has an explosive potential of about 3 sticks of dynamite. An easily accessible manual shut off valve is imperative. I learned this in a heart stopping episode with far less height differential between the tank and the carb. Walking down the dock I could smell the gasoline from 2 slips away.

And,
to be certified for gasoline, the only allowable position for fittings is on the top of the tank for the reasons mentioned here by others.

[hanleyclifford]

Russ - I use a Carter automotive type pulse pump, and if the pump stops running fuel will continue to flow at a reduced rate. For reasons others have already mentioned it is imperative that a shut off at the tank be closed every time the engine is stopped.

[Dave Neptune]

Russ, the tank in my ole' E-35 sits under the cockpit floor and the bottom of the 15/18" deep tank sits just about 6" above my carb. I use a mechanical pump and I have a shut off at both ends. I would suggest that if you are doing all this work on the fuel system that mounting a cheap static preassure gage just befor the carb can eliminate a lot of guess work when something goes amiss. I don't think you will have a preassure related problems, as 3~6 PSI is more than sufficient to feed the beastie.

Fair winds and an easy instalation to you!
Dave Neptune

[lat 64]

Thanks all,
Some of this already on the list. Especially the shutoff valve. I definitely believe in them. I too have had a gas leak in the bilge. Kinda get your heart goin' until you get it aired out.

Russ