Replacing a Holding Tank Plywood Top - SailNet Community
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post #1 of 1 Old 08-16-2009 Thread Starter
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Replacing a Holding Tank Plywood Top

My daughter bought me a Scad Solo level gauge for Christmas to keep track of the contents of the holding tank on my Crealock 34. I installed the gauge then calibrated it by filling the empty holding tank with water measured in a bucket and poured into the holding tank through the deck pump out fitting. I was surprised that the tank contents ran out the vent after adding only 10 gallons of water. Either I was not pumping all the liquid out of the tank or there was something solid in there. I tried adding detergent to the tank every time we were expecting rough weather during our trip to and from the Bahamas, but that did not help.

It was time to remove the plywood top and have a look inside. I expected that I would at least maim if not destroy the top prying it from the tank, so I called Pacific Seacraft and got a quote for a vinyl ester replacement before I started. After removing all the screws, it took an hour of careful cutting of the silicone caulking and prying on the plywood before the top popped off. The top remained in relatively good shape although there was some damage to the layer of fiberglass on its bottom, a few screwdriver notches dug into its edges, and a splintered area at the forward end.

Inside the tank was a nearly uniform one to one and a half inch thick layer of mushy brown stuff with something like sand or salt crystals in it. The smell was overpowering. It was easily scraped out and into a bucket before journeying to the marina’s portapotti dumping station. Each trip was a delight both for me and anyone downwind. Underneath the brown stuff was one superficial tank wall blister that looked like it had been previously repaired. Several water flushes, elbow grease, and a bit of Clorox removed the last of the brown material and most of the smell from the tank. A wire brush cleaned off the last of the caulking. The plywood top was another matter. It went into a trash bag tied tightly shut to await a few days in the sun outside our house.

I live in the east Tennessee mountains and the boat is on the North Carolina coast. That made the logistics of getting and installing a factory tank top difficult, so I decided to follow Jim Lehr’s lead (see the list archives Jan 10, 2008) and make a new tank top from solid PVC. I bought a 2 x 4 foot piece of half inch PVC from McMaster Carr. It was enough plastic to give me two tries at making the top. I also bought a 4 inch inspection plate with a clear center to put in the new top.

Making the new top was dead simple. I c-clamped the old plywood top to the PVC, drew a line around it with a marker, removed the old top, cut the plastic on the outside of the line with a jig saw, re-clamped the two tops together, used a router with a following bit to trim the PVC top to the exact size of the wooden one, and with a drill press drilled the new screw holes through the old ones to insure they would be both straight and in the right places. After again separating the two tops, I rounded off the bottom edge of the new top with the router. The hardest parts were moving the c-clamps around as I worked and doing a little freehand router work in the areas where the plywood had been damaged. I also sawed out a hole in the top for the inspection plate and drilled holes for the bolts to hold it in the PVC top. I roughed up the edges of the PVC where caulking would go with 80 grit sandpaper to get better adhesion.

I sanded the blister down, ground out the loose bits with a Dremel tool, filled the resulting low spot with epoxy resin and silica, then I covered the area with fiberglass and three coats of epoxy.

After checking the fit of the new top (perfect first time), I installed it using a GE silicone caulking in red squeeze tubes from Lowes that said it had good adhesion to both fiberglass and PVC. I replaced the original flat head sheet metal screws around the edges of the top with pan head screws and used pan head bolts, nuts, and washers around the inspection plate. After allowing 24 hours for everything to set up, I filled the tank to overflowing with water to check for leaks… none.

After a week underway and several days at the dock, I’m happy with my now 17 gallon holding tank. The Scad level gauge works well, and if need be I can lift the vee berth cushion and look into the tank through the clear inspection plate. If the tank needs cleaning again, I should be able to do it through the inspection plate.

Bill Murdoch
Irish Eyes
1988 PSC 34

Last edited by wsmurdoch; 08-16-2009 at 06:24 PM. Reason: removed extra word
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