The recent quest to solve the leaking hatches problem in one of my kayaks prompted a reader from Estonia to suggest venting the hatches.
As the air inside the hatches contracts and expands with the difference in temperatures from ambient air to water temperature while rolling a kayak, a non vented hatch will try to draw (or expel) air via the hatch cover. I have observed hatch covers bow alarmingly on a hot day and I have heard of damaged kayaks in some extreme cases. I sometimes manually pop the seal of the tight cover and "burp" the air to equalize the pressure that builds inside.
The same would happen when the hot air inside the hatch will suddenly meet cold water and contract: the hatch cover would flex inwards, possibly sucking water inside.
A simple solution is to drill a tiny hole in the bulkhead and let the air pressure exchange through that hole.
The problem with the tiny hole is that often is not large enough to exchange the air pressure swiftly enough.
If I enlarge the hole water would seep into the hatch compartment (even though not much) when the cockpit is flooded, like in a re-enter and roll.
As I tend to take the occasional swim while surfing (oh yeah, my rolls aren't that "bombproof" at times) the flooded cockpit would seep water into the day hatch and front compartment.
Toomas suggested to install Gore-Tex plugs on the bulkheads.
Intrigued by his suggestion I researched the plugs and found that the air permeation is rather high, at least three times that of of some factory drilled bulkhead went holes.
I purchased some plugs and proceeded to installed them.
I selected a location in the rear bulkhead (cockpit) that was a midpoint from hull to deck.
I used a high speed drill to make a hole in the carbon/Kevlar laminate (small size drill bit) to prevent fraying the Kevlar. I enlarged the hole to a size large enough to insert a round file.
I finished the edges with smooth long strokes of the file to minimize the fraying.
The Gore-Tex vent plug has an "O" ring that seals against the surface. A matching plastic nut was sourced (M12 -1.25 pitch). I used the nut of an electrical gland that fitted perfectly.
I found that the plug seals against water intrusion but allows ample venting of the bulkhead preventing the hatch covers from bowing. Installation was dead easy (20 minutes).
I also found that venting the hatches was only partially the problem; the real one is the dual density hatches expanding in the heat of summer to the point of becoming so loose to literally wobble on the rim.
Needless to say I am now sourcing "old school" hatch covers, made out of rubber, that stay put no matter what temperature.