Building Cobra

Has anybody tried on yet? I think I will try to build one, and I’m wondering if I should use the ammonia spray to soften the 1/16 balsa I will be using? I have a bottle of generic ammonia cleaner- will that do? After you spray the ammonia cleaner on, do you need to remove/wipe it or just let the wood air dry before you apply masking tape? How well does the masking tape stick after the treatment?

since the bottom is curved, should you have a curved block to support it, or just pre-bend it some ( about 5-8mm it looks) before you attach the sides.

It looks like it might help to bend the side pieces to fit together almost exactly before you even tape them together?


Hi Tomohawk,
I have built two Cobra hulls using the patterns printed from the web. The first one I didn’t prebend the 1/16 balsa panels, and it was a real handful trying to put it all together. The second one was built by soaking the panels in ammonia and taping them to a large cooking pot and letting them dry overnite. I made a folded aluminum foil pouch just a bit bigger than the largest panel, and soaked the panels for a 1/2 hour or so. The pot I used was the biggest one in our kitchen, but should have been a bit bigger, since I actually bent them a bit too much, and this boat was also a handful to assemble. I do not like this building method because it is too hard to keep the hull square and true, without building in a twist. I was pleasantly surprised that the panels were pretty accurately shaped, and if I was to try this method again, I might try to use pre-bent 1/32 plywood panels, which wouldn’t need as much taper at the joints as the 1/16 material. It probably would be less likely to cup in mid panel, too. I really would prefer to build upside-down on the traditional board with vertical stations that keeps things straight & true, but I couldn’t get the rest of the plans to print clear enough and in the right size, in order to make the stations. The line color seemed to be a blue-green that my scanner had a hard time seeing. If I had a black-only laser printer it probably would have worked out better. Some pictures of the first hull are in the Photo Album titled SoFlaFooty in the photos section over on the FootyUSA Yahoo group. I’m presently making a new keel that mounts farther aft, so I can move the sailplan aft also, in an effort to reduce nosediving on a run (a problem shared by most Footys, due to too much sail area and a plumb bow that maximizes waterline length). I would also suggest mounting the rudder aft of the transom to take full advantage of the rule allowing the rudder to extend 51mm beyond the measurement box. Setting up the boat to sit stern low in the water when it’s not moving, should also help, since once the boat is underway it’s bow tends to go down somewhat. Keep us informed of your progress, photos would be good!

Bill Nielsen
Oakland Park, FL USA

So I will conclude that Windex won’t work for bending the balsa, and I’ll try some ammonia cleaner tomorrow. Then, it will take probably several hours for the balsa to dry enough for the masking tape to stick.

I think that if you made a simple jig of a flat 12" board with 1" blocks on the ends (and one shorter block in the middle) to curve the bottom panel to about the right shape with the belly about 45% from the bow about 10mm deep, then you could keep things untwisted or at least close enough to work with.

If you think it was tough to build without pre-bending, try building the boat one-handed, like I do.

If anyone can get a Cobra built correctly, it would help to get these measurements.


Hi guys,

Tomo, I can understand that it would be difficult to build a boat this way one-handed. Other methods might be more suitable.

For those with 2 hands, the taped-panel method is actually a lot easier than building over forms, but people sometimes get in trouble by hurrying the process or not following the advise from my posts and articles, so here’s a reminder.

The thicker the material, the more it requires prebending. There’s no getting around this point. If you don’t want to prebend, use 1/64 ply…we’ve done this on a couple of hulls and it worked rather well. 1/32 or 1/16 balsa is fine…though 1/32 can be fragile…thicker than that is overkill and difficult to work. All balsa requires prebending. Use a form that approximates the bend you want…I use a large lobster pot. Use an ammonia product to wet the panels, tape them to the form (I use tape over the entire panel to make sure it stays flat against the form) and let dry overnight.

Tape ALL the panels together before you glue any. Use small pieces of tape, but don’t leave gaps between them…the entire joint should be taped. Get the joints nice and tight and the hull will naturally take the right shape. Be patient and get it right. You will find the bow needs special attention…take your time. When you are happy with the shape, then glue the stern piece in carefully. Work forward from there to glue all the joints.

Dozens of hulls have been built this way…it’s quick and easy but needs to be done carefully. Do each step with quality and you’ll have a happy ending.

Bill H

I got a gallon of generic ammonia cleaner and I can say that it does soak into the balsa wood better than Formula 409 ( or any cleaner that “foams” when you use it, meaning it has oils in it.) I put a test piece in a shallow pan with weights on it to see how it bends & keeps the bend when dry. I might even try a little warm air to accellerate the drying.

On the Cobra, would it help to put the grain on the bottom panel across or lengthwise, to get the bottom curved? Maybe it would be better to do ALL the side panels with a vertical grain? I was thinking of using 1/8" balsa with a cross-grain for the bottom only; the rest 1/16". That’s if I have any 1/8" stuff.