PS Advisor January 15, 2001 Issue

PS Advisor 01/15/01

Rigid PVC Plumbing
I purchased a 1975 Islander 36 with a 6-gallon holding tank designed to be discharged when beyond the 3-mile limit. On Lake Superior the discharge thing doesn’t work so I’m in the process of upgrading the size of the tank to 23 gallons. The new location of the tank will be about 4 feet from the head.

My question is this: Would it work to use rigid PVC to connect the new holding tank as it is clear from your September test that everything else will fail given enough time? I’m aware that the PVC must be supported very well to avoid breaking in rough seas. What problems might one have?

Dale Mackereth
Via email

We used rigid PVC to replumb much of our Tartan’s holding tank/toilet system and never had a problem, but the runs were short. No odors, period. Some marine sanitation experts warn against too much rigid PVC due to possibility of joints cracking with boat working in a seaway, but if you support it well, hopefully there will be no problems. Rigid PVC is tough stuff, including the glue joints, so we would proceed using only common sense in laying out the pipes.

Loose Hull/Deck Thru-bolts
My 1976 Scampi 30 had some minor leaks at the hull/deck joint. When I checked the bolts securing the deck I found about 25% of them were very loose at various points around the boat. Most of the rest needed at least some tightening.

The hull has an inward-turning flange that the deck rests on and there is an aluminum toe rail. The joint is secured with a non-setting compound and bolts with nuts and washers on 6" centers.

When I tightened the first bolt the fiberglass began to crack before the nut was very tight. After that I just snugged up the rest of the bolts.

My questions are how tight should these bolts be and how often should they be checked?

Also, should I be concerned that this is indicative of a larger problem?

Neil Donovan
Chicago, Illinois

Perhaps some of the compound used between the hull and deck has squeezed out over the years, eliminating the “cushion” for the bolt and nut. Now when you tighten, the bolt head and nut exert all the load directly on the gelcoat/glass and cause it to crack—not good. We wouldn’t overtighten any of the others…you were right not to.

But we don’t like the idea of the bolts being loose either. Getting new compound (like 3M 5200 or a rubber gasket material) into the joint would require lifting the deck—a lot of work. So, what you might try instead would be to seat the bolt head and/or nut on soft washers—nylon, rubber, etc. with the hope that you can tighten the nut without cracking the glass. (If you still can’t tighten the nuts without cracking the glass, use Nylock nuts that won’t unthread.) This doesn’t solve the problem of the two hard glass surfaces on the inside joint touching, but it may be the best you can do.

We do not think the loose bolts are indicative of a larger problem, but you should address the situation because the hull-deck joint is critical.

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