Why are you so resistive to a hatch on the bulkhead? You can buy them cheaply, or make them even cheaper. I made mine.

Remember, the water is not going to be under much pressure trying to get into the compartment. Even if you torpedo the boat under a wave, it will not stay down long, and does not go down far. Water intrusion from pressure is minimal therefore.

Foam is neat stuff, but does have its drawbacks. Namely its failure to actually completely fill a void, and its superb water holding abilities. I've opened up far to many boats to find water in a water tight void. That's bad enough, but with foam there holding the water against a single location, it sets rot off nicely. Which is also why I do not like inaccessable water tight compartments. Bad news, imo.

Foam still does not address the pressure change problems that take place with temperature. It does a bit in that it insulates, and a bit more because it takes up space. But put a sealed compartment out in the sun and it will pressurize. Same with cold. The foam helps, but doesn't change physics. You will still have structural problems as a result, they'll just take longer to happen, and reverse.

Be aware to that some of the foams re-cure as the temperature goes up. So if you build the boat on a cool day, deck it and take it out into the sun to bake, the foam may kick off and start expanding and recuring, potentially blowing your deck off. I've seen it happen before.

A bulkead hatch is therefore good news imo. You can inspect the compartment, drain water, dry it out, and even store things. Personally, I prefer bulkhead hatches to deck hatches because of waves, but that's my own preference.

> Actually though you've touched on my solution so I may as well reveal it
> now. I'm thinking of displacing the air in this compartment with CLOSED
> cell urethane foam. If I fill this compartment with foam, there will be no
> air to expand and contract and this should provide plenty of flotation at
> the same time. Problem solved. The only thing I'm a little nervous about
> is the expanding foam might just decide to blow out the sides of the kayak
> rather than taking the path of least resistance and expand upward as it
> should (the forward deck is not on yet so it should expand upward). I'm
> more than a little nervous about this since once you pore the 2-part
> mixture in this cavity and it expands, it's a done deal.