As previously descibed I went for wooden hatches. This picture shows the fabricated wooden screw in day hatch cover. Construction is prety obvious:

• All the material is 4mm ply, the same as the rest of the kayak.
• The lid turns just 45 degrees to screw up.
• The lid is the circle cut from the deck, to the bottom of which is glued a smaller diameter 'spacer' circle, to the bottom of which again is glued the 'castellated circle' with the four sticking out sectors for engaging with the 4 sticking in sectors on the hatch lip. Glueing this up is actuall the final step in the contruction - see later.
• These sticking out sectors on the lid and sticking in sectors on the lip are sanded to have 'ramps' where they first engage, to provide the 'screwing down' action.
• Do think about how long the ramps should be though: if the ramp is full length along the sector, with the sector at 0mm thick at one end and the full 4mm thick at the other end, then screwing the lid on 1/8 turn unil the sectors are fully ovelapping each other only pulls the lid down half as far as if the ramps only extend along the first third of the sector, with the remaining 2/3 being the full 4mm thick. Either would work, but would need different spacing to end up with the hatch lid flush to the deck.
• It is not obvious in the picture, but one sector in the lid and cut out in the lip is bigger than the others to ensure the lid only fits in one orientation.
• Also invisible on the underside of a sticking in sector on the lip is a small wooden 'stop' to limit the rotation of the lid. These two points mean the lid always ends up with the grain matching how it was cut from the deck.
• The lip is also construted from 4mm ply, with a ring under the deck of the same inside diameter as the hole in the deck (pretty much invisible in the picture.
• Under that is another ring with a smaller inside diameter. This forms the bottom of the channel into which the hatch gasket will late be stuck to fom the seal. I should have chosen a better bit of wood for this - a blackene knot is clearly visible bottom left. I planned to paint it when I chose the wood.
• On top of this is the inner 'inside castellated ring' which mates with the lid. Again, I should have chosen better wood, with a defect visible just to the ight of the hatch lid in the picture. (Scrimping on materials in an unfortunate habit of mine I must break!)
• Everything is epoxy coated and bonded. The final step, after the lip was fully installed, was bonding the hatch lid together. The hole in the middle of the lid is there as I first screwed the layers of the lid together from below, into the wooden handle on top (visible in late pics). This was without glue, and allowed me to fine tune the thickness of the invisible disk between the lid and the 'castellated circle' bottom piece. Tuning this, and careful sanding of the engaging sectors, allowed me to get the lid to end up absolutely flush with the deck with the required level of tightness or looseness of the lid when full screwed on.
• A note on tightness vs looseness: after experiments I ended up with the lid just very slightly loose and rattly when fully screwed on, and relied on the pressure of the gasket to make it a tight fit which would not wok loose. It's worked well so far.
• You can't see in the picture, but one side of the mating sector is covered in epoxy mixed with carbon powder for smooth sliding.

I haven't been using the kayak for long, so only time will tell whether this poves to be a good design or whether sand gets embedded in the threads and causes trouble.

The other hatches have very similar lips, with the channel for a sealing strip, just without the sticking in 'screw hread sectors'.

The other hatch lids are just the cut outs from the deck with 1/4" ply cross ribs standing up near the front and back. These ribs serve two purposes.

• Holding the lid to the required shape. This is flat for the rear hatch, but curve for the front hatch.
• having hooks on the end to accept the elastic hold-downs.

The rear hatch was very easy to make. The deck is flat, so the whole hatch rim can be assembled off the kayak and glued in place fully formed. The hatch lid stays nice and flat when cut from the deck, so the two cross ribs have flat tops - easy.

The front hatch was harder. Recall that the plywood at this point is rather tortured (bent to a shape it doesn't really want to adopt). This shape is only formed when the deck is attached to the hull - which was why I had attached it before cutting the hatches. But, cutting a hole for the hatch will change the stresses in both the remaining deck and the hatch lid, so both will change shape and the lid will no longer match the hole. But the lid matching the hole is the whole point of flush wooden hatches. The following scheme worked very well, starting with the deck already attached to the hull, fully glassed etc.

• Mark out the location of the front hatch (on masking tape on the deck)
• Mark out 4 lines across the deck: two inside the hatch, a little (1"?) in from the front an back of the hatch, and two outside the hatch, about 1/2" infront of and behind the hatch.
• The two lines inside the hatch mark where the under hatch ribs will be, the two outside mark where under deck ribs will be. These will run the full width of the kyak.
• These ribs under the deck and the hatch will hold each in the exact shape they are in now before the hatch is cut, so they will match.
• Make cardboard templates of the curve of the deck along each line. You'll end up with four cardboard templates each with a slightly different curve. Mark them so you know which is which!
• Cut a rib from 1/4" ply to match each template. Mine were about 1" deep in the middle for the hatch ribs, and 1.5" deep for the under deck ribs. The under deck ribs are longer since they run right from side to side of the kayak. The under hatch ribs are as long as they can be while fitting in the hatch rim. My rim has a 10mm wide channel for the seal then a 10mm wide wooden ledge for the hatch lid to sit on, so the ribs have to end 20mm away from each side of the hatch.
• Cut the hatch lid from the deck. (Both deck and hatch lid do indeed change shape as predicted - I've not been doing this work in vain - hurrah!)
• Glue the ribs into place under deck an under hatch lid.
• Fabricate the hatch lip in situ: First glue on the ring around the opening, then the channel bottom piece then the 10mm wide ledge piece. Lots of clamps. This could not be made off the kayak and added in one piece later as the rear hatch lip was owing to the complex curve of the deck.
• Try the hatch lid in place. Breathe sigh of satisfaction that it lines up to within about 0.5mm height-wise all round the edge.

That's more than I expected to write about hatches, but they did take up a bunch of time and thought. If I wanted flush wooden hatches I would do exactly the same again next time for front and rear hatches - unless I got tempted to try magnetic hold-downs - but I'd need to do some more research / thinking about how to let the magnets into the hatch lids before trying that. I won't do another screw wooden hatch until enough time has passed to test the current one thoroughly, just because of the extra time involved.

[Sorry about the mixed units above, it's just the way I think. MM make sense to me much better than inches up to about 1/2", then inches are better!]