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
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
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
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.