I recently completed a skin on frame kayak and I thought I would show how I added waterproof access hatches to the kayak. This needs to be done before the skin is painted, as it will be too stiff after the paint for the method to really work well.
1 plastic 5Gal bucket $5-10
1 Gamma Seal Lid or similar product $7
thread: the same stuff you used for stitching on your coaming should work. I used 25lb test nylon thread (leftover from stitching the skin)
something to brace the bucket top as you hammer on the lid (I used a scrap plank I had in the garage)
Saw: suitable for cutting the top off a plastic bucket
drill: suitable for drilling holes in plastic
Needle: The same on you used on the coaming should work
The Gamma Seal Lid is a product designed to snap onto the top of a 5 Gallon bucket like a normal bucket lid. The centre of the lid can then be screwed out giving access to the bucket more simply than prying off a normal bucket lid. I found these in the local Home Depot, but they are available online if you are willing to pay for the shipping. Both the snap portion and the screw in-out portion have gaskets and should be watertight, although I wouldn't put my trust in them to be absolutely perfect in that respect.
The basic plan is to saw the top off of the bucket and stitch it into the skin of the kayak. Then we can snap the Gamma Seal Lid on and we have a ready made hatch!
One warning: these lids take a lot of force to get on. I tried compressing them on with my hands, with clamps, and finally with a rubber mallet. My hands were too weak (and I'm a Big guy so I'm not just a weakling). The clamps just deformed the plastic without actually seating the snap on portion of the lid. The mallet was the only thing that worked. Sadly you can't hammer against cloth, so I had to run a brace under the area I was pounding on. My kayak was built using a somewhat unconventional design and had lots of heavy plywood bulkheads for me to use as a platform to run a board for bracing. Read though the whole instructable and assess whether you will be able to properly brace a support for hammering BEFORE you go cutting holes in that nice new skin.
Also, you will want to leave gaps in the seam where the hatch will go, otherwise, you will have to do a little fancy work to keep your seam from unravelling when you cut the hole in the skin. This requires you to have in mind where the hatch will go, before you stitch on the skin. If you didn't leave a gap and you really desperately want to try this, I can only suggest that you cut the stitching thread at the centre of where you want your hatch, then carefully unravel the stitches in each direction and use the loose end to tie off the seam so all your work doesn't unravel.
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Step 1: Selecting a Location
There are two primary considerations beyond simply where you think a hatch would be most useful.
First, you need some relatively heavy pieces of frame nearby to act as bracing for pounding the lid on. If you can insert a board between the skin and your gunwales, you may be able to use a board running from gunwale to gunwale as your support. If you choose to do this you need to recognize that the board will sit above the gunwale, and the bucket top protrudes below the skin surface. Consequently the board will push the bucket top up 1-2 inches. This means you will need to have enough fabric surface to stretch up. If you place the hatch in a narrow area near the front or rear of the kayak, there will not be enough fabric to let the lid rise for this operation. If you can use some of the ribs or bulkheads for bracing this becomes less of an issue
Secondly, the process of stitching in the lid can warp the lid if there is not enough skin to stretch. I put one of my hatches at the extreme rear of the kayak, where there is only 1/4 inch between the gunwales and the hatch. This hatch is deformed, and it is VERY difficult to get the lid to screw in.
As a consequence of these two factors I recommend that you place your hatches towards the centre of your kayak so that there is at least 2-3 inches of clean fabric in all directions around the hatch. Placing the hatch between two heavy ribs or bulkheads also provides a handy support base for hammering on the lids.
Step 2: The Lid
I'm using a product called the Gamma Seal Lid. I found this locally at Home Depot, and I have not seen any other products that work similarly.
These lids have two parts: a ring which snaps over the top of a bucket, and an inner lid which screws into the ring. One thing I noticed is that either during manufacture or shipping, the snap on ring portion of these lids can get deformed (see the pics). Once you get the lid on, this isn't a problem, however, it definitely makes getting the lid on more difficult. The best I can suggest is that you inspect the lids before you buy them and get the least dented ones you can find. If you can't find any smooth ones, or you are purchasing online and can't inspect, don't worry. I only noticed this while I was trying to get my hatches installed and you can work around the problem by hammering a little harder to seat the ring.
Step 3: Cut the Top Off Ouf Your Bucket
This step is pretty straight forward. I used a normal hand powered wood saw for the actual cutting and that worked well. Most buckets have several lips up near the top, and the ones I used had three: the primary lip at the top edge, a secondary lip, and then a final heavier lip that the bucket handle inserts into. Since it won't help for this project and I wanted to be able to use the buckets later for other things, I left the third lip on the bucket and cut off the top two. After cutting the top off, drill a series of holes in the lid. You are going to stitch this in very similar to how you stitched in your coaming, so I recommend using the same hole size and spacing that worked for your coaming.
Step 4: Cut the Skin
I used the bucket top as a template and drew a circle to cut out with a sharpie. I originally cut 2inches inside the top and kept removing thin strips of material until the hole was just right. I wanted the top to be able to pop into the hole so that it was held snug, but not so tight that it was deforming the bucket top or wrinkling the kayak skin. I also wanted to leave enough that there would be a 'fold' when I stitched the fabric to the top, as this would help prevent the skin from unravelling.
To initially seat the bucket top, I simply pushed it into the hole. This carries the cut edge of fabric down with the lower lip of the bucket top, so that the fabric lies up against the plastic in the groove between the two lips, and then pulls away from the bucket top just below the upper lip.
For your coaming you probably stitched the skin to the inside surface of the coaming, since this adds an additional twist to the path water needs to take before it gets into the kayak. In this case, the lid will snap over the whole area, and the bucket top is much flimsier than your coaming, so it is much easier to stitch to the outside.
Step 5: Stitch the Bucket Top In
Stitching should go very similar to the coaming, but the fabric will be on the outside of the bucket top.
From the inside of the bucket top I pushed the needle through the layer of skin that was up against the plastic, and then angled up to pierce the top surface of the skin (using a curved needle will help here).
When piercing the top surface of the skin, try to catch just enough extra surface to pull the skin tight. You may have to vary the amount you grab in order to change the shape the skin is pulling the lid into.
Going from outside to inside, just reverse the piercing procedure.
Step 6: Seating the Ring on the Bucket Top
Now things start to get exiting. If you want, you can try to force the ring onto the bucket top with your hands. I actually encourage this as you might succeed, and then you save your poor kayak frame from a hammering.
If that fails we need to firs provide a brace to support the bucket top against our hammering. I ran a piece of scrap wood across the station bulkheads that I used instead of conventional ribs in my kayak. Once you have a solid support place the ring onto the bucket top and press down the edge that sits over your support. I started with the area of the ring that had the worst dents in it, and that worked out pretty well for me.
Once everything is set give it a good smack with the mallet. the ring should snap down about 1/2 inch just under where you hit it. Continue hitting around the edge to progressively seat the ring. Take the time to adjust your support and press down the ring by hand as you go.
Step 7: Screw in the Top
Now you screw in the top. I had one hatch I installed to close to the tail of the kayak and the ring portion is slightly elliptical. This makes it hard to get the lid in. On the plus side the lid seems to be OK with me just pushing in hard and ignoring the initial cross-threading. As I force it in it straightens out and everything works OK. My other two hatches go in nice and easy.
Now you just need to paint the skin and go paddling! (When I painted mine I made sure to get the paint extra thick around the hatches same as you would on the coaming)