Fuel Tank in deep bilge

[Wade]

I am considering new fuel storage. My diesel was replaced with a 4 stroke outboard by the PO. That's fine with me. Coastal cruising is all that is in my plans for this boat (31 Bombay Clipper). It has an integral modified full keel with a very deep spacious bilge. I was considering the space for water storage but started wondering about the possibility for fuel storage. A plastic tank or a custom cradle with a fuel bladder might be a way to go. Would enjoy hearing the con's for such an arrangement. I know there are a few.
                                                                                                      Thanks,  Wade

[CharlieJ]

I would never even consider storing gasoline below decks in a fuel bladder or a plastic tank. A properly constructed, and vented metal tank, perhaps- been lots of those. A bladder? no way.

How is the OB mounted?

[Wade]

Thanks Charlie,
     The OB is mounted on a standard "swingarm" set up. I do not know the real name for it but you see them on sailboats all the time. I am going to change that this summer to a sliding sled that will lift the outboard straight up to deck level- through an aft deck extension. An aluminum fuel tank would be fairly easy to accomplish. The Bilge tapers from about 30 inches at the top and about 9 inches at the bottom. over 3' deep and between 3-4' long. It could be set up a few inches off the bottom to stay out of water and flared to follow the cavity so the weight is distributed. I'd like to leave room for bilge pump access but that makes it easier. I also think a rectangular high box tank could work. Properly secured obviously.Do you think  the bottom of the bilge could support the weight? I like the idea of dispersing the weight across the bottom and sides of the bilge. It is a large empty space and I would prefer the extra weight low and centered. There is room beneath the lazerettes but with the out board hanging from the transom I would like the weight more amidships. My batteries are boxed and secured where the diesel was mounted.    Wade

[Norman]

A couple of thoughts, Wade.

I very much like the idea of a custom tank fitted to the bilge.

If you carefully cut two cardboard bulkheads that fit the shape of the bilge contours, and suitable distance apart, a sheet metal shop can cut matching aluminum pieces, then add the aluminum curved to fit them the desired distance apart.

Don't make the bottom flat, curve it port to stbd, making a relatively small actual sump area.  Then have two outlet pipes welded in, one at the very center of the sump, the other 3 or 4 inces up.  Use the upper for normal fuel to the engine.  Bring the lower one to a convenient place to occasionally pump out the detritus that always seems to find its way into fixed tanks. 

Airplanes have this sump arrangement, for each tank, and they are drained before EVERY flight!

Your vent should go well above wherever you are exiting the cabin, then down and out on a vertical surface.  Any water that splashes in the end will run back out.  Last but not least, cut a small piece of scrubby and fit it in the vent to stop bugs. Leave a little sticking out so you can pull it out and clean it.

i have sailed on two boats with vents on the flat deck next to the filler, both had persistent water in fuel problems.

Installing a tank low is not only a ballast when full, it also breathes in and out less due to more constant temperature, keeping condensation to a minimum.   The only disadvantage is an inclination to loose the prime to the fuel pump on the engine, but if the prime bulb is accessible, that is noroblem.

Norman

[Wade]

Great advice Norman,  I was actually thinking there must be some reason why I have never seen this done. I sure like the idea.

[CharlieJ]

Norm pretty much said what I would, except be sure the tank doesn't sit flush on bottom-  leave room for water to flow under.

[Godot]

Norm pretty much said what I would, except be sure the tank doesn't sit flush on bottom-  leave room for water to flow under.

I just pulled my aluminum diesel tank and had it repaired. It sat in the bottom of the bilge in the water. It generated micro-holes. The welder cut three inches off the bottom, and the tank now has space below it. You don't want it leaking. Especially with gasoline.