I encountered the same problem with my plans-built Ches. 14 and LT-17 (which is to say, I don't know how kits approach the same problem), and dealt with it by sawing numerous kerfs across the *underside* of the coaming rim, cutting down through one ply.  At the ends the kerfs were about an inch apart, ever closer at the very forward end, and maybe two inches apart on the sides.  This made the plywood very limber ... but also fragile while working.  Epoxied in place, there's no compromise in strength, as the kerfs are filled with epoxy squeezed into them as you clamp the coaming layer down.

If you've already cracked your coaming, saturate the cracked area with epoxy and let it cure. If the coaming is already epoxied in place, do it in place obviously; if it's not yet part of the boat, flatten it and weight it down while the epoxy cures. It will work fine ... and even if you varnish the rim (I didn't; mine is painted) I suspect it won't be too noticeable ... and won't be noticeable at all with a spray skirt of cockpit cover on.  One of the many beauties of working with epoxy and plywood is that such fixes are easy and effective!

As a big fan of plywood coamings, I'm surprised yours cracked. If you cut a new one, be sure the exterior grain is running across the coaming and not in a fore and aft direction. You don't need to soak the plywood, just wet out the outer plies. And it needs to be completely dry before you epoxy it.

What kind of boat is it? The coaming is 4mm okoume? I'm kind of surprised it would crack since that stuff is pretty darn flexible. But at any rate, if it isn't anything serious, I'd probably just give it a good dose of expoxy. On my last boat, I had a 3mm sapele coaming. Seemed a little thin so I glassed both sides with 4 oz cloth before installing it. Nothing is going to crack that sucker now:) Perhaps some 4 oz., or lighter, glass would solve the problem?

Good luck!

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Ogata, eric

Minor correction, it's all coming back to me now....

I glassed the bottom of the coaming before installing it. I glassed the top of the coaming, rolling glass down the inside of the coaming and coaming spacers after the coaming had been installed.

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Ogata, eric

When I need to do a hard bend I usually soak the wood with a garden hose ,or under the kitchen sink if it will fit since hot water seems to help, for about 3-5 minutes then let it sit for an equal amount of time to softne up. Wipe it dry and coax it into place with clamps and let it sit for an hour or two before gluing. That lets it dry out and take a "set" which makes the final clamping a bit easier.

               Chris

Thanks, everyone for your help. I should have mentioned that I'm building a Chesapeake 16LT from a kit. The coaming isn't glued to the spacers yet, so I have some options. Also, the crack only appears to be in the top layers of the plywood.

This morning, I was looking at the spacers, which are glued to the boat, and I notice that they aren't perfectly level in the front. I think I may sand them slightly, which will make it so the coaming rim won't have to bend so much. I guess I have to be careful not too sand too much so the spray skirt will still fit!

Eric, do I understand this correctly--You glassed the bottom of the coaming rim before you installed it, let it cure, and you were still able to bend the rim to follow the contour of the spacers? Did you put fiberglass cloth on the bottom of the coaming rim or just resin?

Thanks again,

Mark

After several failed attempts at pending regular wood, I found bending ply easier.  1088 has a grain when it comes to bending so you will find that is way more pliable in direction than the other.  For my SOF boat, I replaced two layes of bent clear wood with one layer of 6MM bent ply, scarfed to complete the ring.  Turned out real nice.  I then cut some thin strips, bent to make the rim... the look is really nice but I would never do this agian too tall. 

My point is yes you can bend ply, I made up a steam box though for the first bend, I managed to get the job done on a really humid day.  And yes, I let everything dry and set before glueing.

Couple pics with the rim:

Another woodworkers trick is to use a heatgun, or blow drier to bend wood (even plywood) and it doesn't require any water.  If that fails, then steam is the only other method I know of.  Using water direct may cause ugly stains.

Kev

Hey David,

That looks like an interesting and unusual kayak. Skin on frame? What's the big rectangular raised opening?

Mark, I built a CH17LT and don't remember having any great difficulty bending the coaming that a lot of clamps didn't solve. And it didn't crack. My guess is you just must have a bum coaming. Possibly it was cut on the wrong bias or was just a bad piece. The coaming on a Shearwater, that I glassed, was 3 mm sapele, so it was pretty flexy. I glassed the bottom, let it cure, glued it on, let that cure, then glassed the top running down the inside of the coaming spacers.

I think Kev's suggestion of the heat gun is the easiest and most practical given the limited amount of curvature you need to encourage. It's nowhere near as extreme as what David is trying to do with his cockpit. But since it is already cracked, you'll either have to repair it, some epoxy, possibly a layer of glass might do that. Or if it is hopelessy bad, I'm sure you could get another one from CLC. Or use the old one for a template and cut another one yourself.

Good luck!

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Ogata, eric

I am have missed it, but there doesn't seem to be a lot of detail when aligning and cutting out the coaming opening on the deck.  How do I determine the exact placing of the coaming?  I know how to center it, but what about front and back? I know it is supposed to go 1/2 in front of the rear bulkhead and flush with the front support, but how do I know were these marks are when the deck is covering both positions?  Also, should I do a dry run on my coaming pieces to make sure they don't crack as well?  I am very nervous about getting everything epoxied up and having it crack on me when I start to bend. Thanks for your responses.

If I remember right, I did the following to line mine up...  Side to side should be pretty straightforward, as you can string a line up and see where it falls on deck.  Since the rough-cut deck is currently overhanging the cockpit opening by quite a bit, measure from the exposed edge of the plywood forward to where the aft edge of the deckbeam is.  Then transfer that measurement up to the top of the deck plywood and draw a line.  This will tell you where the deck beam actually falls in relation to the deck ply.  My coaming is about 4 inches forward of the rear bulkhead, as my bulkhead was moved aft a bit for increased cockpit space, but still perfect in relation to the deck beam.  You just want to make sure that your center of mass falls where it is suppose to.  If you lift the boat from the center/forward edge of the cockpit, it should balance perfectly (a few inches either way isn't going to sink the project though!)

~Chris 

... and yes, do a dry run.  Or two or three.  I layed out all my pieces to double check the measurements, then crossed my fingers and mixed the epoxy.