Rot Doctor


Subject: How to detect transom rot?
Date: Wed, 17 Apr 2002

Dr. Rot,

I have an old chris craft lancer 23′ 1976. After removing some screws from the transom I noticed that there appeared to be some moisture in the transom.

Do you have a means of telling if the whole transom is rotten or if the problem is isolated to screw areas.

There are basically two ways marine surveyors look at transoms: 1) With a moisture meter, which tells them the amount of moisture in a transom. This only measures moisture, and moisture doesn’t necessarily equal rotten wood. It can lead to rotten wood, for sure, but moisture per se doesn’t equal bad wood. 2) By tapping the transom with something like the wood handle of a screwdriver. Substantially different tones will mean that either the wood inside is soft, or that the glass has “de-bonded” from the wood.

Generally it’s best to look at it as a practical problem, as you are with the moisture in the wood around the transom screws. Bad rot will lead to totally deteriorated wood and a lot of flex in the transom when the outdrive unit is under load. If you’re not experiencing that, then your transom overall is probably okay.

You can probe with a piece of stiff wire around the screw holes and see if there is any severe softness. If the wood seems substantially okay (it’ll be a ply so some give to the wood is expected) then I’d just give it the CPES-treatment mentioned below.

Also, what happens to an I/O boat when the wood under the fiberglass gives way. Does this usually lead to instant sinking? I’d imagine that the lower unit might create a large leak at that point?

As noted above, there will be a lot of glass flexing and outdrive alignment problems if the wood is severely rotted. What will happen is that the flexing of the glass will cause leaking, probably, and that will be a clue to take a good look at the inside of the transom. Fiberglass does not suddenly just give way…it will withstand the drive loads for awhile.

I suggest you let the area around the screw holes dry out, and the assuming you’re seeing wood treat it with our CPES (Clear Penetrating Epoxy Sealer) by generously brushing/swabbing it around and allowing the wood to absorb all that it will. Give it a day or so to air-out, and then put your screws back in with a little polyurethane (3M’s 4200) around the screw head bases. The CPES will give the wood around those screws substantial protection against wood deterioration.

Regards, Jim.

Come on back if you have further questions.