Replacing Powerboat Fuel Tank (part 1)

Most permanently installed powerboat fuel tanks do not get proper treatment. In most cases they are placed below a leaky deck and encased in water-retaining foam. On top of that, they’re fed bad gas with water in it and then left to sit all winter. It’s no wonder they give up, corrode, and develop pinhole leaks. At least you hope it’s a pinhole leak.  In all seriousness, that describes some of the conditions in which many of our fuel tanks live. A small powerboat’s fuel tank will often be found near amidships beneath the plywood deck. It is likely to be foamed in place between plywood or foam-cored stringers, and will be further secured by floors running from side to side at its forward and after ends.

The main choices for fuel tank material are fiberglass, plastic, or aluminum. Plastic tanks are the best of the lot. Properly supported, a rotomolded polyethylene tank will last a long time.  Fiberglass tanks, on the other hand, are doomed to a short and ugly life in the time of  ethanol-laced gasoline. The ethanol in gas will dissolve most fiberglass, and it will send that dissolved gunk down the fuel line into your fuel injectors or carburetor.  Aluminum tanks are a little better but can develop pinhole leaks from internal or external corrosion or can develop cracks if not properly supported. Many permanently installed tanks
are, in fact, aluminum.

Unfortunately, most tank leaks are discovered by traces of fuel in the bilge water. It will usually start out as a light sheen on the water. Pay attention, because this is extremely dangerous, and it’s bad for the environment if any gets pumped overboard. It’s also illegal, and the Coast Guard will levy a hefty fine against you if you’re caught (and the sheen is a dead giveaway). Before assuming the leak is from the tank, make sure there are no leaks in the fuel lines. Nothing could be more frustrating (not to say expensive) than to replace a tank only to find that the problem was a carburetor leak or a loose fuel fill hose.

Some boats have saddle or wing tanks mounted on both sides. These are usually easy to find and are removable for replacement. They may be behind some sort of paneling, but that isn’t normally a big deal. Take the tanks out, find replacements that match the old ones as closely as possible, and reinstall them. While you’re at it, replace all the hoses associated with the tank: the fill hose, the vent hose, and
the fuel line. Chances are if the tank is old enough to leak, it’s time to replace the hoses. Besides, the stress on old fuel lines during their removal is often enough to crack or damage hoses. Replacing them is cheap insurance.

Under-deck tank installations are more work. The deck covering the tank must be removed. If you don’t want to remove and replace the entire deck, you’ll need to determine the dimensions of the tank and its location under the deck. Good luck. Unless you can find somebody with a similar boat who has done the job or get the information from the boatbuilder, it’s exploratory surgery time.In most cases, the tank is located between the main stringers and near the fore-and-aft center of the boat. This is to keep the boat in trim as the fuel is used up. The tank will have been placed in a compartment formed by the two stringers on the sides and floors forward and aft. It probably had foam poured around and under it to keep it firmly in place.

The foam is soft enough to be dug out with hand tools like screwdrivers or putty knives. Once the tank is exposed, you can measure it and search for a replacement. If you are extremely lucky, you will be able to find an after-market tank that is an exact match from a company like Moeller or Todd. Tempo was formerly a major supplier but has gone out of business. These tanks will be neutral, dark gray, or black rotomolded polyethylene. Note that red tanks are for above-deck use, not for permanent installation belowdecks. The under-deck tanks come with fuel level sending units installed. Don’t worry about compatibility: they work with all electric fuel gauges. If you can’t find an exact replacement tank, find one that’s close and, before you buy it, replicate it in a simple cardboard mock-up. Place this in the space occupied by the old tank to confirm the fit and to ensure that the tank fill, vent, and drain fitting locations work with the hose locations in your boat.(continued)