It should go without saying that the security and integrity of your vessel's fuel tank or tanks is critically important. Regardless of how trusty the engine may be, it's of little use if the fuel supply is unreliable. And should flammable gas or diesel leak into the bilge, the best you can hope for is a mess to clean up; at worst, there may be an onboard fire that you won't be able to contain.
Nevertheless, I've boarded countless boats with tanks that are improperly designed, poorly installed, or ill maintained. During your routine commissioning, make sure that tank inspection is on your to-do list.
Some tanks are made of steel, but most are made of aluminum or polyethylene. Whatever the material, once installed, the tank shouldn't move. Interestingly, this is contrary to American Boat & Yacht Council guidelines, which allow for up to a quarter of an inch of movement. This is one of the few A.B.Y.C. guidelines with which I disagree, and I believe with good reason. Imagine that you're making an extended ocean passage and that with every wave you encounter, your fuel tank moves back and forth even slightly. This can cause it to chafe and eventually leak.
My preference is for tanks to be fabricated with flanges, or mounting feet, that can be through-bolted to purpose-made shelving. Alternatively, a tank may be cribbed securely in place using stout timbers or prefabricated fiberglass channels. A tank should also be supported across its entire bottom surface; it should rest on a shelf rather than on such hard points as stringers or frames. Resting on the hull is acceptable, provided that the support is even and not concentrated on just a few spots.
A flat resting place becomes more important for larger tanks. The weight of the fuel in a 50-gallon tank is about 350 pounds, and this can cause considerable shock-loading when a boat falls off waves. If the tank isn't uniformly supported across its bottom, it's more likely to deform or work, which often leads to structural failure and leaks, especially in welded metal tanks.
Unfortunately, supporting a metal tank on a shelf causes its own set of problems because doing so can increase the likelihood of corrosion. Horizontal surfaces aboard boats tend to collect and hold water. If an aluminum or steel tank rests on such a shelf, the water trapped between can lead to rapid corrosion. In order to prevent this, strips of nonabsorbent material should be placed between the tank and the shelf. A common approach is to use .25- by 1-inch strips of high-density plastic; King Starboard is one popular brand (www.kingstarboard.com). Place these strips between the tank and the shelf in an athwartships orientation. The strips should be lightly abraded, and after the tank's thoroughly degreased, they should be bedded to the tank bottom using a high-quality polyurethane bedding compound. This prevents water from collecting between the tank and the insulating strips, where moisture can also lead to corrosion.
Finally, if you're designing or installing a tank, make sure that its top won't be horizontal when the vessel is at rest. A little tilt will help water to drain off.
Steve D'Antonio, a regular CW contributor, offers services for vessel owners, boatbuilders, and others in the marine industry through Steve D'Antonio Marine Consulting (www.steve
dmarineconsulting.com). Visit the site to sign up for his just-launched blog in which he addresses a range of marine subjects. Next month, he writes about hardware installation.