View Full Version : Building a curved coaming for a catboat

Norm Bernstein
11-14-2003, 11:58 AM
Hi, all... I'm about 40 hours into building a 15' x 6'6" catboat, sort of my own design (I say 'sort of', because I started with the outside dimensions of a double chine plywood design, and converted it to round bottom by drawing elliptical frames to match the original outside dimensions).

I'm thinking ahead to the 'trimming out' process, and would like very much to do a classic curved coaming. However, I'm having a hard time coming up with a plan for doing it... and a source of suitable materials.

The best coaming would be a curved teak one, built from solid lumber, and probably steam-bent. However, I'm working in severely restricted space, and worry about the cost, the thickness, the ability of teak to take the relatively small radius bends, etc.

Alternatively, I could laminate a curved coaming from teak, or any other wood, using thin stock like 1/8" or 1/16". Unfortunately, I haven't found a source for thin dimensional stock that isn't outrageously expensive.

I thought of kerf-bending (two layers of thin teak plywood, cut with shallow kerfs to facilitate the bending, and bonded, kerfs-to-kerfs) but that leaves an ugly upper edge which would need ot be covered somehow, leaving me back with the same problem.

Can anyone suggest a method, AND a source of reasonably priced materials, for doing a curved coaming?

Thanks, Norm Bernstein

11-14-2003, 12:14 PM
Let me start by saying my 15 foot Marsh Cat's coaming is laminated with epoxy in four layers of 1/8" mahogany veneer. So that works but veneer can be expensive, and if you look closely, since there is no rail cap (yet) you can see it's not solid timber. Actually with the exception of the laminated ash bowspriit, its the only "plys" visible on the entire cold molded boat, so it does bug me sorta.
I assume you have a half decked daysailor going together. The way these were "really done" was with two lengths of mahogany or white oak 1/2 to 3/4 inch planks steamed and bent into place, with an overlapping butt block at the centerline forward. Often other similar doublers were put into position at the beam to reinforce oar locks as well.
Bending two of these would not be much of a project in a homebuilt temporary steam box. You could bend them, even over bend them, over a mockup jig, let them cool, and then epoxy/screw fasten them in place in your boat. That way you could use blind fasteners from underneath your deck through the deck framing headed inboard into the wood and not have any fastener bungs (which I do any that also sorta bugs me. Its amazing how important a piece this is to do to suit you because it's always close up in your field of view and often your arm and hand are often in comntact with it. Its the most salient feature from in the cockpit and sets the tone for your whole effort.

Norm Bernstein
11-14-2003, 12:25 PM
Thanks for the advice. Interestingly, I was originally going to build a Marsh Cat, and actually bought the plans, but decided to try my own design idea instead.

I thought of steam bent oak, but I don't care for oak as a finished wood, and I'd rather not have to go through the process of assembling a steam box, building a form, etc. I could live iwth the 'laminated edge' appearance on the coaming... where did you buy your thin mahogany, and do you remember what it cost?

11-14-2003, 12:41 PM
My boat was built by others in 1989 so I have no idea. I have spent quite a bit of time rebuilding, refairing, relassing, rerigging, and redoing the interior to suit me. As originaling done the coaming was put on with glue and finishing washers with SS screws. UGH!. It had delaminated spots in the plys and was wavy as to trim and curve so I did a lot of work straightening ( to a fair curve) that out and putting in bungs. I know somebody can tell you other sources, but look in the back of a WoodenBoat and you'll see guys who ship it advertised. Read your MarshCat plans ( I have a set at home) but as I remember, it as for 100 extra square feetod 1/8" veneer if you were not using solid timbers.
If you go the two kerfed layers of wood back to back route, you could use a round over router bit on both sides of a plank, then rip to produce a bullnose molding half round ( yop might be doing this anyway for a rubrail) that you could easily bend over the trimed edge and glue and brd into postion. That would give you the apperance of a solid plank.
I really think going with a steambox of building foam/foil insulation, a old gas can and a gas grill fire would be surprisingly less time and money consuming, you'd learn a new skill, and your boat would have one more IMPORTANT traditional feature.
Another two cents worth.
So waht does this baby look like. Can you please post us some pictures.
I have always loved catboats, the half decked ones particularly.
I'm leaving the office now to sail mine. Sunny,55 degrees, about 10 under average, and blowing ten to twelve mph.
I'll check back later.

Rich VanValkenburg
11-14-2003, 12:55 PM
Here's a link to some photos, including our combing laminate-in-progress. We had a 1 1/4" x 16" x 16' piece of mahogany cut down by a guy with a WoodMizer into 1/4" rough veneers. Those were sent through the thickness planer to 1/8"+ and then laminated on a form over several days one layer at a time.

If you can't get into this site, holler and I'll email it to you.



Norm Bernstein
11-14-2003, 01:03 PM
Covering a kerfed 'sandwich' with a bullnose trim is not a bad idea. I'd still have to bend that bullnose, and I'm not sure if bending teak in those dimensions will be all that easy. I've done steambending before (and still have the 'boiler' for the steambox) but it was flat ash laminations, which were easy to bend.


Hope the photo works. This pic shows the frames, sawn from 3/4 plywood and partially prefinished, set up on the strongback... a couple of test battens lying on top, to 'prove' the fairness. The planking stock, machined from Eastern white cedar, is on order and arrives late next week. After planking, it will get a HEAVY fiberglass coat (two layers of 10 oz cloth set in epoxy, plus one layer inside, along with tape and fillets at the frame joints). The boat will feature a gaff rig, a partial deck (daysailer style), and slatted floorboards and seats. Shallow draft is not a requirement where I'll sail her, so it will have a deep wheighted centerboard and transom-hung rudder. The 'workshop', by the way, is a tiny garage, barely enough room to walk on either side.

Norm Bernstein
11-14-2003, 01:15 PM
I found the pictures, Rich... that would be a great solution... if only I knew a local guy with a WoodMizer and a thickness planer. Years ago, Constantines used to sell thick veneers... but they apparently don't, anymore. I can't find a source for thick veneers on the web, either, except for Rockler, whose prices are utterly ludicrous.

11-14-2003, 01:42 PM
I would suppose almost any cabinet shop would have a bandsaw and planer to get veneers out of resawing ,what, an 8" plank? You'd need two 12 footers of 1" thick or maybe just 3/4". Going with teak at $17 bf that's $275 for the coaming you want. What's this boat's bill of materials gonna cost you with sail, trailer and outboard? Say $7500 to $10000?. So is three percent so much to spend to get what you really want, a teak coaming? That's why you are DESIGNING and building your own in the first place I bet.
Looks like a deep bellied rascal- what are you figuring for the displacement?

Norm Bernstein
11-14-2003, 01:54 PM
Well, Buddy, you've got a point. Actually, you reminded me that I do know a guy with a cabinet shop, perhaps he might be able to help me out... or direct me to a source of thin lumber, thick veneers, etc.

As for total cost of this project, I am admittedly trying to keep it cheap. The eastern white cedar planking has been the biggest single expense so far..

The shape of this boat is admittedly experimental... and it strikes me as being kind of shallow, and narrower than a classic catboat shape. The elliptical cross section should mean good speed, but I suspect it will be tender without a deep weighted centerboard and perhaps some ballast along the keel plank. I really don't care about performance, per se, as long as it has no bad habits... the purpose of the boat is liesurely daysailing in Narragansett Bay, RI. I never race: I wouldn't want to spill my beer!

John Blazy
11-14-2003, 02:25 PM
You guys are on the right track with Kerf bending and then capping. I capped all my bent laminated coaming with segments of 1/8" mahogony, then flush trimmed them to the curve. They look great to me, and like you, I didn't want to go through all the work of steaming, just to risk one crack and scrap the whole thing.

In this shot you can see five areas of curved coaming.
The seat was bent lauan plywood (cheap method to make it bend by planing off one of the face veneers) over the curved barrel of my shop vac as a form. The other curves were done with quick MDF forms.
This photo should be a better closeup of the segmented capping if you ignore the "slick".

I don't know why everyone on this forum is so afraid of making their own 1/8" veneer. Just put a sharp, thin kerf blade on the tablesaw and resaw 1-1/2" into the edge of a 5" wide board, flip over, run the kerf again, then raise the blade to cut the rest of the way. Safe and fast, - been doing it for years. The T-saw cuts smoother, faster and truer than a bandsaw. If you need wider like 7", make a higher fence extension, cut as deep as you can in multiple passes, and then hand saw the central 2" and hand plane down.

Norm Bernstein
11-14-2003, 02:46 PM
Wow, John, that boat of yours is just incredible! It really illustrates the distinction between craftsman (you) and woodbutcher (me). The innovative shape of your coaming is bound to be an amazing eyecatcher!

Well, I very much like your idea of capping a laminated coaming with a pieced veneer layer... I really don't trust my ability to bend teak without breaking it, and the veneer solution is obviously simple and easy.

I'm a bit less certain about your method of cutting your own 1/8" veneer, though. First, my table saw is a cheap one from a big box store, and I don't think it has the stability or accuracy to do that... second, I don't think *I* have the skills to do that.

However, if I can manage to somehow find a source of 1/8" teak veneer, I think I've got the plan now: laminate the coaming in place, and cap it with the same teak veneer. OK, the cap won't have a bullnose shape, but it will look good, I think.

Bruce Hooke
11-14-2003, 02:51 PM
For the quantity of 1/8" material you need I would probably saw the stock myself on a bandsaw. Depending on how thick you can go without making it too hard to bend you might be able to surface the cut pieces using a planer. Otherwise a thickness sander would do the trick, or if you are careful with your sawing no surfacing might be necessary.

John Blazy
11-14-2003, 04:39 PM
I have to apologize - my resawing idea presupposes that one has a saw with some power. Mine is 3 hp.
If you were to buy 1/4" teak plywood, you can rip to width, and then router / hand plane off the back face veneer like I did and the material bends like a wet noodle (assuming 3 layer plywood) because the core layer's grain runs perpendicular to the face veneers. I forgot to mention that cutting the cap pcs is very fast if one has a bandsaw. I used tape to hold the caps in place while the epoxy cured, or just let 'em sit with gravity.

Norm Bernstein
11-14-2003, 04:46 PM
Good point about removing one face of a teak veneered plywood panel... in 1/4" sizes, they're undoubtedly made from just three plies. I might not have to kerf them at all, depending on the curvature. I'd like to do a complete racetrack-oval coaming, which I guess is the most traditional shape.

It occurs to me that I can probably cut thin veneer 'strips' from the edge of a teak board for the coaming caps... even if they're a bit thick, I can sand them to a suitable thickness for capping purposes.. and they might even be coaxed to bend laterally a bit.

ken mcclure
11-14-2003, 04:50 PM
Norm, if you call WoodMizer, they can give you a list of owners in your area.

11-14-2003, 05:27 PM
For a 15' catboat a 1/2" thick oak coaming steam bent would be the classic way to go, two pieces with butt blocks outside especially if you are going all the way around. In big cabin cats that bent oak would be bigger and have a vertical stave facing on the inside to the cockpit sole with cap on the top of the lot.

Nicholas Carey
11-14-2003, 07:22 PM
If you do make your own veneer to use as plys in a glued laminated coaming...

1. Strike up the planks prior to sawing with witness marks, so you can put the stack of cut veneer back together in the same configuration it started from. If you do that, and use a suitable glue, the resulting lamination should, since the grain is contiguous, have virtually invisible glue lines—you may not need a cap on the coaming.

2. Springback. Using thinner plys may actually be better. According to the Gougeon Brothers' "Shop Tip #1:
1. Computing Springback in a Curved Laminate

When a strip of wood, a ply, is bent to a curve and released, it springs back to its original shape. A stack of loose plies will do the same. When these plies are glued together while clamped to a curve, they tend to stay curved, but will straighten out or spring back a little. When building the mold or form for the laminate its common to make the curve a little tighter to compensate for the springback. The amount of springback depends on the number of plies in the laminate, not in the ply material or the ply thickness. A simple formula can help predict the amount of springback you can expect

[b]Y = X / N&sup2;.</font> Y is the amount of springback</font> X = the amount of deflection</font> N is the number of plies</font>http://www.westsystem.com/webpages/images/springback.jpg
Thus with four plies (N), the springback (Y) will be 1/16" if the amount of deflection (X) is 1". By using plies half the thickness you double the number of plies for a given thickness of laminate and reduce springback four times-in this case to 1/64".