How much weight will a flush hatch bear?

I am thinking of installing a flush hatch on the top of the aft tank of the Chester Yawl.  Unlike the kayak applications I've been reading about on the forum, the top is flat, and is usable as a seat or a place to put a cooler or something heavyish. 

Assuming the dimensions of the hatch around12"X18" and epoxying the spacers and rim under the seat, how much weight would it support?

I see from Jamestown distributors that MAS FLAG epoxy has a tensile strength of 7,500 psi, but can't find the Low Viscosity number.  But I'd guess it's on the same order of magnitude.  If a particular cousin of mine were to step onto the hatch, a pretty extreme non-accident incident, that would only be 300 lbs spread over several square inches, if I'm calculating correctly, far less than the tensile strength of the epoxy.

The top itself is plenty strong 8mm ply, and I could use that thickness for the rim as well.

Appreciate any thoughts you all might have.

2 replies:

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RE: How much weight will a flush hatch bear?

Before I started painting the deck of my Waterlust expedition canoe build I was careful to assemble its flush hatch, set between the two main mast pockets ahead of the cockpit.

It's not quite as long as yours while maybe a bit wider, more squarish than 1:1.5.

Rather than have to worry whether it'd support a misplaced foot I added a couple of 1/2" thick pine ribs to the undersides, that were carefully fitted to complement the camber of the deck and the piece cut out of it that would become the hatch.

Too, before the deck was bonded in place I added a couple of crosswise stringers, again made of pine, to add a bit of stiffness to the deck both in front of and behind the hatch opening. Those stringers are fillet-bonded to the deck stringers running under the deck, alongside the sides of that hatch, and of course the deck too.

I'm pretty confident I could stand on that hatch w/o issue, but I'm also well shy of 300#'s... even when soaking wet....

RE: How much weight will a flush hatch bear?

   Thanks sp!  Some good ideas to spread the loads without cutting up the space in the tank.  In my Passagemaker, built 12 years ago, I added a big commercial hinged hatch on the forward tank, for stowing stuff, and put in a vertical support to reinforce the aft edge of the opening.  No sign of weakness, but larger items going into the tank have to be pushed forward.

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