: The gasket material is most likely to form a water tight seal if the gasket
: material is on the hatch cover, postioned to straddle the lip edge the
: hatch cover rests on. This will allow half the gasket to compress and the
: other half to remaim uncompressed forming a water tight seal.

i think upper gasket is best as less likely to get damaged and easier to clean out the sand, easier to clean the yak side, easier to work on,etc.

the gasket overhang idea is neat and i bet better achieved if silicone(careful with bare wood and finishes) or polyyrethane sealant is used. if the flange edge is rounded and the gasket area is stopped say 1/4-5/16in inside the flange, when smooshed down the sealant will partially gush around the round portion giving almost a snap seal.

If the inside perimeter of the gasket is saraned so doesn't adhere to the hatch, I bet you'd even get 1/4 in or more of upward motion before snapping off. like the gasket'll be in tension against the flange.

YOu could carry this idea to extremes w/ a tunnel downturn gasketed the whole way to get sealed motion of as far as you wanted the tunnel to be.

Another way to get sealed and yet motion w/ a slight redundancy is to use say a lipped hollow extrusion. for eg look at #50449 on http://www.cleanseal.com/Products_01.htm
for motion, the gasket seals in comression mode for the thickness of the air.

as well for motion the lip'll seal like a wiper against the 1/4in thickness of the hull, even jam in the corner. would probably work even better if set down say 1/2in.

but i've always liked the formed silicone approach. get a perfect perfect seal , absolutely boneass flush, to no matter what inconsistencies and humps and bumps, and with the rounded inset approach, get whatever inset motion or snap that you want to design in to it.

-mick