This is a photo of the deck of a kayak just aft of the rear hatch. The damage is visible as specks of raised fiberglass.This is a result of stress caused by contraction of the air inside.

I used this kayak last weekend on a 100 degree day -- after rolling the boat a few times I happened to look back at the rear deck and noticed how the deck had suddenly collapsed about 1/2".

This was caused by the air in the rear chamber contracting. Remember, the outside air temperature was over 100 degrees and the air inside the chamber was at least 20 degrees warmer than that (I suspect it may have closer to 30 degrees warmer). After the hull was cooled by the cold water, and the temperature inside dropped considerably, the vacuum that was created was powerful enough to cause the deck to collapse slightly.

The lesson learned from this event was --- "Don't forget to drill a relief hole in your bulkheads dummy!"

If my hatches were not airtight or if I hadn't paddled in such heat, this collapse may not have occurred. Luckily, the damage sustained is very easily repaired -- sand the damaged fiberglass off, reglass, and revarnish.

Another positive note is the fact that no damage to the wood or the interior glass occurred -- strip-built boats are tough!