Subject: 21' Inboard '85 Supra Ski Boat Rot
Date: Tue, 19 May 1998

My 21', mid-engine fiberglass ski boat has a rotten deck and sub-structure. The 3/4" plywood deck was covered with a felt-like carpet and a thin layer of chopped glass and resin. The keel-line bilge trough of the boat is about 20" wide, and the V-8 engine is bolted right in the middle. The 12" high walls of the bilge are of glass cloth and resin coated wood, and the wooden stiffeners and deck-support framework to either side of the bilge trough have had no place to drain for many years. And, to make the drainage problem worse, all the space around the framework to either side of the bilge is packed with hosed-in foam. Being trapped beneath the plywood deck, water that has leaked into this area could not evaporate and the wood has become quite soft and rotten. The 3/4" plywood deck is captive around the edges by the top half of the fiberglass hull. I have cut out the deck with a saber saw in some areas to expose the rotten wood beneath, and intend to remove the upper part of the boat so I can completely replace the deck.

Right now, I am trying to decide if I should dig out all of the foam and cut out the rotten wood 2X4's, 2X6's and plywood, and glass in new members, or perhaps just glass on doublers. All of the cross members are uncoated 2X's, and the longitudinal members are cloth and resin covered 3/4" plywood. Most of the 2X cross members are spongy and black while the resin and fabric seem to have protected the longitudinal plywood members to a greater extent. I have not yet dug out the foam to see what the joints between the fiberglass hull and the wood pieces looks like. Much of the wood, particularly that forms the walls of the bilge trough is covered with glass and resin, so it is not so easy to determine if there is rot beneath. Out-of-site and out-of-mind is a tempting approach to the problem, but the invisible wood is structural in nature, and I'd hate for my engine to come loose and go bouncing around!

Perhaps you could make some suggestions as to how I should go about inspecting for hidden rot and repairing the damage. Also, should I drill some drainage holes through the bilge trough walls to allow drainage of the areas to either side and sleeve them with glued-in pvc pipe? What do you think of using that green, treated, "Womanized" lumber and plywood for replacing the deck and other wood? Should I put carpet on top of the new plywood deck? Have you ever heard of using bathroom construction "glass-mesh-mortar units" as boat decking? These come in 4'X8' sheets like plywood, but they are made of glass cloth, resin, and perhaps some portland cement. They are typically nailed to studs and then ceramic tile is mortared to them for waterproof shower enclosure construction. They are only 3/8" thick, so I were to use this material, I might need to double them up to get my 3/4" thick deck. I don't think I would have to worry about a deck made of this material ever rotting.

Thanks for your help!

David S. in San Antonio, Texas


The amount of work you have ahead of you takes my breath away!
(webmaster: it can't be much worse than the rubrail/plank project that we took on at Delta's last haulout, Doc! Look for details of THAT project in the next issue of Rottenboat.)

It's really hard to inspect for rotten wood when the rot is as pervasive as you suggest. I think you need to tear everything away (that you can) and look. With a V-8 engine, you need your structure to be strong. I would replace all the rotten wood possible, and especially be sure to dry the remaining wood and apply the Clear Penetrating Epoxy Sealer (CPES) to the area. This will deter any rot in the old wood and reinforce the strength. You might also want to think about applying the CPES to the new wood for protection.

I don't like foam. What they use adds very little structural strength, and it traps water and forms the perfect environment for rot. If you gotta use foam for filler, try and first coat the wood around it with CPES and go look for one of the good industrial structural foams.

Glassing in doublers (sisters) will work, as long as you treat the bad wood next to them with the CPES. Otherwise, the rot fungi in the bad wood will just move over to the new wood and you have rot again.

Yes, I would put drainage holes in the bilge bulkheads. At least that will let the water come aft where you can get at it.

I don't like carpet on the decks. It traps water, and there goes the rot again. Wolmanized lumber is fine. If you're using ply, be sure and treat the edges especially with the CPES to water proof it. I would cover the decks with glass cloth (heavy) or mat, and then epoxy resin if you can afford it, and if not, then polyester. It's as easy to use as anything and has a known structural strength when resin-coated. I don't know anything about the bathroom glass-mesh-mortar pieces, but they've got to be more expensive than the plain glass cloth or mat, and they were not designed for structural strength. Maybe they'd work -- I just don't know.

As you progress with this project -- and it is a project! -- get back to me on specific situations and I will be as helpful as I can. Right now you're deep in the middle, and it's hard to be really specific.