Liner oil-canning?

[Bruce Barber]

For several years now, I've noticed a knocking sound when my 1982 25D pitches in waves. It sounds like a small hammer hitting wood, with a frequency matching the pitching period. Some days it'll be obvious, some not. This year it seems to occur more frequently than in the past.

The noise emanates from the head area, which is well forward on the 25D. For those of you with this boat, it’s on the port side just aft of the locker bulkhead and outboard of the head supply seacock. I'm certain it's liner-related – I can feel the vibration on the liner, coincident with the noise. I long ago ruled out something striking the hull or some object rolling around or shifting inside the boat.

This week I noticed it at anchor for the first time, and it was bad enough to cause loss of sleep. First, because it was loud, and second, because it's become something I'm starting to lose sleep over.

My theories are two: First, it's caused by relative movement between the hull and the liner, perhaps due to insufficient use of adhesive or whatever gunk they used between the two; second, it's local oil-canning of the liner when it's stressed during the pitching motion.

I think theory 2 makes more sense, and I’ve tried a fix (in the middle of that night!) by drilling
thru the liner (I found a liner-to-hull void of about a third of an inch; the liner in that area is about a quarter-inch thick), threading a #10 screw into the liner and tightening it against the hull laminate, in hopes of stabilizing the liner so it wouldn't "pop." That wasn't successful, though; maybe a pattern of screws over a large area would fare better.

I'm also thinking of drilling through the liner in a pattern and injecting thickened epoxy or construction adhesive, to fill the void and hopefully “glue” together the liner and the hull laminate.

Obviously I'd like to bring other minds to bear on this. Any similar experiences out there? Any other thoughts of what the problem might be, or possible solutions?