Hi w.mass boat build, 

the deck side of the hatches involves three layers - the deck, the spacer and the sill.  there is a fourth piece, a very thin rim that sits on the sill and creates a bump up  on the inside edge of the deck side of the hatch hole.  the purpose of this last piece is to press-up tightly into the foam which is mounted on the deck.

the hatches themselves, along the perimeter, is only one layer thick.  and there is a doubler on the hatch that make it two layers thick  but that second layer does not oppose the sill....so you can think of the hatch as one layer thick. 

the foam sits on the hatch and has for almost all of it, one layer (4mm) of space for the foam to compress into.

so it will work..... and the hatches on the shearwater series is extremely water-tight and can require some pretty aggresive pressure to compress the foam and then lock into shape with the toggle. 

once you take the time to compress it, you will find it will become a bit easier as the foam 'learns' its shape.

while it is tight, having use a lot of kayaks, having a really water-tight hatch is a real blessing.  particularly if you want to learn to roll, go offshore or otherwise play in very wet conditions where the deck can become awash or is underwater.

hope that helps.

h

https://www.clcboats.com/shop/kit_options/shearwater-sport/1081.html

i just wanted to add a point on this which is subtle.

absent the foam, as you mentioned, it will sit flush, but there is a space for the foam to occupy between the bottom of the hatch, the sill spacer, the hatch sides (deck/space) the sill and the rim spacer (so the foam in this design is actually compressed from four sides (the outer hatch rim, the rim spacer on the sides of the foam and the hatch bottom and the sill top on the top/bottom of the foam),

anyway, that is why it is such a solid/water tight hatch but you have to take care with the details and the foam and the space it fits into....and it will take pressure to push compress the foam.

but it worked on my shearwater...which while not a sport, has the same hatch design.

h

Just some pictures (ckick to enlarge) to augment Howard's descriptions which are such that there's no need to add anything else.

The hatch spacer, normally mounted against the underside of the deck but being used here as a template for cutting the hatch opening.

The completed hatch opening - the spacer mounted under the deck and invisible, the sill mounted under the spacer extending into the opening and the rim mounted on top of the spacer.

The hatch cover (upside down) with the doubler mounted on its inside surface. Don't clamp/glue it like this, this is just for illustration. It gets a curve that keeps it from lying flat. Use a flat surface.

Final hatch assembly. Depending on the tension in your hold-downs and the type of foam you use, it may not lay as flat as this.

Laszlo

One more.

Underside of the deck/hatch showing the sill sitting on the spacer (covered by sill) sitting on the deck with a nice fillet to smmoth the force transfer and avoid stress concentrations.

Laszlo

Thanks for the explaination, just to be clear the foam tape goes over the rim right? Or next to it. And I also see there is a forum question about putting the tape on the sill or on the hatch. Which do you prefer?

Tom   

good follow-up question...

usually you have a little part of the foam tape hitting the rim....but most of it should be on the sill (e.g. next to it).  i can't recall the exact widths.  if all of it was on the rim, you would not be able to make the hatch sit flush.

my personal preference is to have the foam on the boat and not the hatch....however, i don't have the manual in front of me to know what the default is.....but you can make some variations like this to your personal preference.

the reason i prefer to fix it to the hull is i just find it has less tendency to get dirty or torn or damaged.  when it is attached to the hatch, it  can hit the rocks or sand when the hatch is removed/laying by the side of the boat.

the key thing for me is water-tight and flush.  so you have to have good compression but not so hard that you can't get the hatch to sit in it flush position....and that element is seperate from whether the foam is fixed to the hatch or the hull.

h