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Replacing a Keel Fuel Tank

Many sailboats are constructed with a fuel tank in the keel. Often these fuel tanks are installed early in the boat’s construction and are surrounded by foam. The Whitby 42 is a typical example, and corrosion in these tanks is a common problem. When this occurs and a tank begins leaking, it becomes unusable. Some owners condemn the tank and simply ignore it if they have adequate alternative tankage to meet their needs. However, one of the reasons we purchased our Whitby 42, Allegria, in 1994 was because of its large fuel capacity, so we embarked on a plan to replace our keel tank when we found it was leaking badly.

We had noticed water intrusion in the aluminum center tank early on, and after finding several small holes in the top of the tank forward of the access plate I glassed them over. For several years after that we had no further problems. Two years ago, however, we experienced another large accumulation of water, and I found a quarter-sized hole in the top of the tank just abaft the glass work I had done earlier. I pumped all the fuel out of the tank, patched the hole and left it empty for a year. It collected no water during that time, so we put 10 gallons of fuel in and sailed from Cape Fear to Beaufort, South Carolina; still no water in the fuel. We then put 50 gallons in and sailed from Beaufort to the St. Mary’s River. Unfortunately, after that trip we tested the tank and found a large amount of water. It was then that I decided to look into replacing the tank.

There are several ways to remove an old keel tank, including taking it out through the engine space if the engine is out of the boat. Another option is to cut a window in the side of the keel, which is the approach we took.

I started by sounding the keel with a mallet to outline the boundaries of the tank, after which I drilled a couple of small holes at the lower corners to release a large amount of foul-smelling water. I then drilled some larger holes with a hole saw so I could insert a screwdriver to further establish the boundaries of the tank, which I ultimately found extended a little past the boundaries I’d delineated by the soundings. After that I cut through the fiberglass from hole to hole with a small circular saw and removed the resulting panel, exposing the foam insulation. Removing this insulation exposed the tank.

I’d had visions of removing the tank in one piece, but soon realized its size made this impossible, so I began to cut it into smaller pieces with a Sawzall. Surprisingly, the tank was intact at the bottom, with all the leaks near where the top met the sides. I eventually discovered the tank had been installed with no covering, which had allowed water to saturate the surrounding foam. As a result, the entire outside of the tank was corroded; however, the wall thickness was such that it remained sound everywhere except at the top. This is very different from what many other owners have reported, which is numerous small holes at the bottom. On my tank, however, the bottom on either side of the baffles was covered with an inch or two of protective asphaltine, presumably because these areas couldn’t be accessed for cleaning.

Once the tank and remaining foam were removed, I closely inspected the cavity and found the top open to the bilge except for where I’d made my earlier fiberglass repairs. In the interest of keeping the exterior of the tank dry, I decided to roof the cavity by glassing in a layer of quarter-inch fiberglass and fashioning an opening for the inspection plate and tank plumbing.

After that I created a floor for the new tank and a sump for any water that did get in. To do this I glassed in two small bulkheads, left a 12in space under the access opening and filled the floor of the cavity with an epoxy-sand mix. To keep costs down, I used some smooth river stones to take up space in the cavity. I also created a flat floor in the sump area using an epoxy/sand mixture. Finally, I plumbed in two strum boxes and hoses so that it will be possible to pump out the sump if necessary. The strum boxes sit in the flat floor of the sump, one as a spare in case the other fails, since the area will be inaccessible. The hoses are led up the port side of the cavity and through the sole into the engine room where they are plumbed into the pumping system.

I took careful measurements of the cavity to determine the dimensions of the new tank. My plan was to leave an air space all around the tank and support it with strips of Coosa board, which is rot-proof and does not absorb moisture. I then bonded the Coosa board to the tanks with 3M 5200 adhesive sealant in a manner that prevents water from getting between the tank and the stringers. After that the stringers were glassed onto the sides of the cavity, leaving space around the tank to prevent corrosion. I also coated the entire tank with an epoxy barrier to keep at bay any moisture that might accumulate.

There were a couple of hiccups positioning the access plate, but once all was done and the plumbing attached, I was ready to glass the panel back in and close the window. This was done using epoxy and some filler, with the joint ground out about 6in on each side. The glass work consisted of 15 layers of alternating woven rovings and mat, with three layers of biaxial cloth, resulting in a very strong repair. The outside was then faired and the bottom painted. After that, we were ready to fuel up and go sailing.
The new tank capacity is around 75 gallons, down from 85 gallons in the original, but with luck I’ll be long gone before it starts leaking again.

Robert and Molly Strickland have been sailing together some 30 years

December 2015

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