minicell bulkheads, do they absorb water?

Hello there, in the end i got a perception expression 15, & am quite happy with it.
after an afternoon of capsize/re entry practise (with a cockpit half full of water for an hour )
, at home i noticed the rear bulk head had a cup of water in the bottom, i sponged it up & thought maybe i put the hatch on wrong.
but 2 hours later (still at home) , i found a half a cup of water there! the bulkhead minicell is the only possible source of this water.

is it normal for a minicell bulkhead to soak up water? it still feels wet.

Real minicell will not absorb water to any extent, but the surface will feel and remain damp for a while.

i will do a test it by filling the rear bulkhead with water from a hose & see if any significant water seeps to the cockpit.

there is a hole with no sealant high up the minicell for the skeg cable to pass through, maybe that leaks a little when the kayak is upside down?

The foam bulkheads on RM boats are prone to leaking where they are sealed. The test you mention is good, but a better test is to use a source of low pressure air into the hatch and some soapy water to check for bubbles.

no water seeped through, & after i emptied out the rear bulkhead at the extreme rear i found a deeper ‘well’ beyond the skeg, that can trap a little water, that probably explains the water i saw,after i had moved the kayak at home. the source of the water was probably the hole for the skeg cable, in usual conditions it never sees water, but the kayak spent a lot of time today upside down in the water.

incidentally, why isnt common polyestrene used for bulkheads? it is closed cell & lighter that minicell, so the resulting boat would be lighter.

Polystyrene foam is closed cell but not water impermeable. There are interstitial gaps between the extruded closed cell packets which can be permeated by water much the way in which liquids can completely permeate through Styrofoam cups. Polystyrene is also dissolved by many organic solvents including those used in common adhesives that work for minicell. I knew a fellow who thought he could save a few bucks by making his own canoe pedestal out of 2 or 3 inch thick planks of polystyrene foam. Every adhesive he tried to glue the blocks together dissolved the foam and the entire project was a write off. Polystyrene foam is also flammable and much more heat sensitive than minicell foam. Water that permeates the foam and freezes can cause it to break up.

Polystyrene foam (“Styrofoam”) is much too weak to be an effective bulkhead. It will also crush under impact and will leak as a result. Minicel foam is not that heavy and it’s a very resilient material. If there was something lighter that worked as well, kayak manufacturers would be using it; they’re not stupid.

Minicell bulkheads in a plastic kayak have a tendency to develop small leaks over time because the hull can slightly flex, at some point it’ll flex beyond the ability of the edge sealant to stop it. Normal maintenance on such a boat is to hit the edges of the bulkhead with something like Lexel, a relatively flexible sealant you can get in most hardware stores, once or twice a year. How often depends on use level. But it is no big deal.

That said, it is hardly unlikely that you have a smidge of water coming in thru the hatch cover.

None of this is fatal. It is why most of us put our gear in dry bags even inside the hatches. Less to worry about.