Whaler transom

I'm helping a buddy with a little work on his 20 year old 17' center console whaler. We are attaching a transducer on the transom. I am always uneasy about sealing holes in cored fiberglass. I started to do my standard thing, which is to drill a oversized hole, fill it solid with glass, and then drill into that.

As soon as I drilled my pilot hole it was obvious that the plywood was wet. I cut a 2" hole with a holesaw, the plywood was saturated but sound. There was no softness or rot where I made my penetration. I dried it out as best I could, cleaned it up and wiped with acetone then filled most of the cavity up with marine tex. Tomorrow I plan on completing the repair with normal fiberglass and mat, fair and finish, then drill and install my mounting block for the transducer.

What does the assembled brain trust think about all this?

20 year old whaler stored wet.

plywood soaking wet but not rotting

my below waterline patch job.

Did the water migrate through the fiberglass or is this the result of a faulty penetration somewhere else?

 

Drill another test hole low and on the centerline (near the drain). Moisture will collect here and have the first signs of rot. I suspect your core has sheered away from the internal and external 'glass skins. Some broken down bedding or loose fasteners have permitted moisture to get into the transom. The usually culprits are the lower engine mount through bolts.

Perform a good "thumping" on your transom to see how much separation you have, possibly locating rot as well. Lift the engine while it's locked in the up position and closely examine the mounting bolts. Is there any movement, distortions, dents, puckers, etc.? These are signs your core needs to be replaced.

Considering the age and model, it very probable that you may need a new transom core.