Here are a few thoughts on the subject of adding or replacing… Tanks
Tankage is a subject about which there are probably more opinions than there are hard and fast rules. Materials that are generally accepted as suitable for keeping water out of the hull are often considered unsuitable for keeping it in. The differences are not limited to various types of metal and plastic tanks and various types of plastic and metal boats. Much of the controversy centers around the practice of building tanks integral with the hull.
Industry standards and practices change from year to year. The most important rule that I can isolate is the coast guard regulation that prohibits integral tanks for gasoline. The designer of course can excuse himself from the controversy as fast as he can letter the words “all tanks to be custom fabricated of model” across a drawing. But the high cost of custom built tanks of monel or stainless steel or even aluminum often leaves the owner who wishes to add or replace a tank in something of a quandary. Let’s consider some specifics…
The very word gasoline is enough to send a shiver up my spine. Just a cupful of gasoline vaporized in the bilge has the explosive power of several pounds of dynamite. So why I feel the addition or replacement of a gasoline tank can be a do- it-yourself project, the construction of a gasoline tank is best left to the experts. The reason I say this is first for safety and insurance considerations – all gasoline should carry a certification plate – indicating that they have passed pressure, fire and vibration tests required by the US Coast Guard and The American Boat and Yacht Council. And second, the materials most suitable for gasoline tank construction (monel, stainless steel, aluminum) are the least suitable for the do it yourselfer.
FRP (fiberglass reinforced plastic) tanks are in use for gasoline both in marine and industrial applications, and are available in numerous ready-made sizes and shapes. Fiberglass tanks are most often seen installed above deck in open, outboard powered boats. But as they are frowned upon by insurance companies, and no less expensive than aluminum tanks, there seems to be little to recommend them, other than their resistance to corrosion. It should also be noted that fiberglass gas tanks are not constructed of the same ortho resins commonly used for boat hull construction, and for my money, should not be considered for home construction.
Many ultralight moonhulls and racing multihulls have gasoline tanks built of epoxy coated plywood that have been in service for many years. However as epoxy manufacturers seem reluctant to recommend the practice, I would be too.
If it’s not possible to buy a ready made aluminum, stainless steel, or aluminized steel tank to suit the requirements, I believe that the most practical and economical alternative is to have an aluminum tank custom built . Since the expertise required for tank construction is not common to all tank welders, it’s best to have tanks built by a shop that specializes in tank construction, rather than the local welding shop.
One important point to keep in mind with all aluminum tanks is that copper a and brass fittings must be electrically isolated from the tank to prevent leaks due to galvanic corrosion. Also remember that fuel tanks need a good connection to ground (not just to the engine block).
Fuel Oil Tanks
The fact yacht fuel oil is considerably less volatile than gasoline does not mean that it’s dangers can be overlooked or taken lightly. But as damage to, r leakage from, a diesel fuel tank is more likely to be inconvenient, rather than disasterous; this fact makes it easier to suggest that the do-it-yourselfer consider building his own.
The metals most often recommended for fuel oil tanks are the same as those for gasoline, but as integral tanks are acceptable for diesel fuel, mild steel tanks (in steel boats) and FRP tanks (in fiberglass boats) seem to be more readily accepted.
The actual construction of a fiberglass tank will be the subject of a future article in Better Boat. In the meantime, if you’re interested in additional information on the subject; the best treatise that I have seen on the subject of fiberglass tank construction, both integral and free standing, is in Ken Hankinsin’s book, Fiberglass Boatbuilding for Amateurs. The only thing I would add to his chapter on tanks be coated internally with gelcoat (preferably unpigmented). Gelcoats are usually made with iso resins which are significantly more resistant to the migration of fluids. Alternatively, the interior of the tank could also be coated with an 100% solids epoxy resin.