hi PS,

i did my shearwater 17 from a kit with the specified spacer/foam - which is the 4mm and 7/16th thick foam.  it was water tight.   i routinely rolled with the boat and never had a problem. the hatches were flush and looked flush and i used the 4 hatch toggles per hatch as the mechanism to hold the hatch down.   

it is one of the few hatch kits i have worked with that i would describe as truly water-tight when built as specified.

i would note that the hatch design for the stitch and glue kit is a flat hatch cover and the the kit also included hatch rims and hatch cover doublers which i think is the key to its great performance.  i am not sure if you are building from plans if these construction details are part of it.   the hatch doubler ensures the hatch itself is stiff, the hatch rims ensure a proper engagement of the foam/hull to ensure water-tightness.   i think these hatches are also sensitive to the right foam....and CLC has sorted that out and is consistent in their supplier.

i would describe the hatches as taking a bit of push to get them in place...the foam is quite compressible....but takes a bit of effort and a bit of time, especially when new, to get the foam to compress.  after it was set up, the on-off process was much easier as long as you did not leave the foam uncompressed for a long period.

i hope this commentary helps but i would really recommend checking to ensure the hatch details on the plans capture the same level of detail in the kit.  like i said, these have been the only hatch i have been truly happy with and water-proof i have had on handmade boat like this...and i have been doing this for a long time.

h.  

h

   I did not see in the assembly manual what size the gasket should be. You said 7/16 foam which works out to about 11mm thick. That means with a 4mm spacer it has to compress the gasket about 7mm or over a quater of an inch. That seems like a lot of compression for the bungies to pull down. Maybe that is why I have seen hatches that sit too high.  

The plans call for sill rims and a doubler only on the back hatch which is almost flat. They say it is for in the event of "wet re-entry." The front deck is convex which probably give the cedar strip hatch more strength.

PS

ok....i see now that you are making the hybrid version...not the pure stitch and glue....so yes, your forward hatch will be a rounded some what.

fwiw, i have never had a flush hatch system like on these home-built kayaks be really 'water proof' f it is based on straps.....straps simply cannot provide the downforce required to properly seal the hatch.    the toggles, however, are another story.  since the toggle is flush with the top of the deck, the hatch gasket has no choice but to be compressed and can be no higher than the top of the deck if you are going to be able to swing that toggle back into place....hence it should be flush.  but like i said, it takes a bit of muscle until the gasket is compressed.

if you take a piece of the gasket material and pinch it between your fingers, you will see it has no problem compressing 7 mm.   it's just challenging to create that kind of pressure uniformly around an entire hatch perimeter.

i am currently finishing up a night-heron hybrid and for the first time will be applying the toggle technology to the flush hatch strip deck (just doing varnish and paint this week).  so i am happy in a couple weeks to report out its performance.     i have flush hatches held down by straps on all my other strip builts but i wanted my newest boat to really have a hatch that performs like the shearwater i described above.   the ones with straps are not terrible, but in long passages with sloppy seas i can often get a half cup/cup of water......and i would really prefer bone dry.

picture of nigh heron with straps is below:

https://photos.google.com/share/AF1QipOhm0ErK_yaxyHkfGidUkJRW8y_6QR6-v0Y7M1PYTBB1YgBEvSZINtbGfVmcDb9sg/photo/AF1QipNdbcfY1DJzbZPN6bo7WLChPO1ZA13-yTNiELp5?key=ZnBDNS0ySjJkUlFHZ2d5RmNGWWtqUWhyUzkzVlZn

picture of shearwater with toggles is below:

https://photos.google.com/share/AF1QipOhm0ErK_yaxyHkfGidUkJRW8y_6QR6-v0Y7M1PYTBB1YgBEvSZINtbGfVmcDb9sg/photo/AF1QipOVYCmCb5WgJIEceEnOcXV0taBGDG5NlaHdJgJQ?key=ZnBDNS0ySjJkUlFHZ2d5RmNGWWtqUWhyUzkzVlZn

so the new boat will look like the nigh heron but with the toggle system of the shearwater.

I've been meaning to write up some 'lessons learned' from my Shearwater Sport Hybrid kit build, one of which concerns the hatch doubler.

If you choose to use it, be **very careful** that it does not flatten out the curve of your hatch.  This happened to me, and I ended up sanding away the doubler because the curve of the hatch no longer matched the curve of the deck at all.  Luckily I only did one at a time, and when the doubler was sanded away the original curve was restored.

hi jwm....thanks for pointing that out.

i have had the same experience and, to your point, it's important that any work on hatches and/or deck is secured in a way to keep the proper shape while any doublers or hatch spacers/sills etc. are glued up.

to prevent the deck itself from flattening when sills and spacers are glued up i always have the deck secured in place....even if temparily with glass reinforced packing tape.

h   

Ironic this thread appears now; I just last week cut & finished the flush hatch on my Waterlust's deck.

I was apprehensive about the cut-out holding its camber once freed from the surrounding deck material.

Also about its ability to resist deformation once finished & in place.

So I made up two ribs from 1/2" thick pine, scribed to match the deck's camber, carefully band-sawed out then planed.

Once the hatch had been cut out the benefit of 'glassing the deck's top surface first was proven: the epoxy + fiberglassed upper surface kept the cut-out hatch cover's camber nicely.

To ensure it remained that way I used those two curved ribs to hold the hatch sill & spacer to the same camber when I bonded them together. Once this sub-assembly was complete I bonded it to the underside of the deck, then those curved ribs were bonded to the underside of the hatch cover.

Doing it this way left no chance the deck would have been affected by having a flat hatch sill & spacer clamped up against it, or that the deck camber wouldn't match that of the hatch crover.

Without the toggles fitted yet the finished hatch is twisted less than 1/16" on the diagonal, but once the foam sealer and toggles are in place it should fit nicely flush with the deck from which it was cut.

   JWM, thanks for pointing out about the hatches deforming on your SW Sport Hybrid. I am surprised the hatches deformed when you put on the doubler because the strips are glued together and should be fiberglassed on both sides. I will watch for that when the time comes. My plans only call for a rear doubler and the deck is fairly flat there. 

The kayaks I have built in the past had hatches and gaskets that sat on top of the deck and were held down with straps and buckles. On those I made a frame out of lattice and glued it to the bottom of the hatch cover for the proper curve. I may have to do something similar to that with the frame on this build, like what spclark said. 

As far as the spacer thickness, I am thinking that I will cut it from 6mm ply because I will be using the invisible style internal bungie hatch hold down that pull down from below the deck. I am concerned that they will not pull the hatch flush with the deck with 7/16" foam and if I use 4mm spacers.

Pete