Removing Water From Fuel Tank Cavity Drain

[As06655]

I think [installation of a] drain tube [in the fuel tank cavity of a classic-era Boston Whaler boat with below deck fuel tanks] is a great idea, but

Q1: how do you get the water out [of the boat after it flows out of the fuel tank cavity via the new drain tube]?

Q2: [can a] Shopvac [be used to remove water that drains out of a new drain installed in the fuel tank cavity of classic-ear Boston Whaler boats with a below deck fuel tank]?

[jimh]

Q1: how do you get the water out [of the boat after it flows out of the fuel tank cavity via the new drain tube]?

Water is never going to flow uphill by itself, so a pump must be employed to lift the water to some place where the water can be moved off the boat.

Q2: [can a] Shop-type [WET-DRY] vacuum [be used to remove water that drains out of a new drain installed in the fuel tank cavity of classic-ear Boston Whaler boats with a below deck fuel tank]?

Yes. If you use a shop-type canister vacuum the water will flow into the canister of the vacuum.

Most shop-type canister vacuums run from 120-VAC power. On smaller classic-era Boston Whaler boats with integral below deck fuel tanks, there is generally no 120-VAC power available on the boat, often even if tied to a dock with shore power. Your proposal to use a shop-type canister vacuum would only be practical if there were plenty of 120-VAC power available. Many shop vacuum electric motors draw 10-Amperes or more current when operating.

A vacuum pump can only lift water a finite amount, depending on how much reduction in air pressure it can create on the suction side of its filter media. A vacuum pump would need to be tested to see if it can lift water out of the bottom of whatever part of the boat you propose to have the fuel tank cavity drain into.

Typically in a marine application a pump that will be used to lift water into the pump itself will be a bellow-action pump often called a diaphragm pump. Diaphragm pumps are much more expensive than small centrifugal pumps, but a good diaphragm pump can self prime by lifting water a foot or more, and can tolerate a much higher height of head in the exhaust path.

A centrifugal pump generally has zero lift and a very limited range of head on the pump at which height it can produce any significant flow of water above the pump intake. A centrifugal pump inlet must always be completely submerged by water for the pump to produce any flow at all. This results in there always being residual water left in the sump.

[Phil T]

For all models that include a below deck fuel tank, the bottom of the tank cavity is below the level of the bilge. The only model I can think of where it is practical to install a drain tube in the aft wall of the fuel cavity is the 1984-1989 Revenge 22 and 25 with the aft fish well.

Several members have installed a bilge hose vertically down the aft tank cavity wall and ran the hose aft so it could be pumped out using a prime-less pump or a manual bilge pump. Reports I have seen have shown limited success and a few that noted it was a total waste.

For addressing water ingress into the fuel cavity:

• Keep the deck cavity cover seams and the amidship fuel hose covers sealed with marine sealant.
• Do not foam in the tank, rather use foam in the corners to secure the tank and allow the moisture to evaporate via opening the inspection ports in the cover.
• Foam in the tank and fiberglass the top of the tank completely therefore preventing any water to make contact with the foam or tank surface.

I would not be interested or recommend piercing the fiberglass wall of the fuel tank cavity for a drain tube and opening the possibility of exposing the inner hull foam to potential water ingress via a drain tube failure.