Sunday, March 14, 2010
i torture the timbers
Sunday 14 March 2010: Over the last couple of weeks I have been experimenting with various ways to fail at steam bending a cockpit coaming from red oak. The process began in the last week of February when I sketched a coaming outline, 23 inches long by 16 inches wide out of poster board. I transferred this pattern to MDF and cut it out on the bandsaw. After drilling holes to allow for clamping, and hammering it all together I had a jig for steam-bending the cockpit coaming.
This jig is taken mostly from Chris Cunningham's book, Building the Greenland Kayak and uses a strip of aluminum flashing that wraps around the outside of the piece being bent to help support the wood and prevent splits from developing.
Though it is not the ideal material, I so far have nothing better to try than some kiln-dried red oak 2x4 from Home Despot. I ripped it down into strips about 1-1/4 inch tall, 3/8 inch thick. The strips were then cut to length -- the circumference of the form, plus 12 inches -- and then the 12 inches at each end are planed to make a long scarf joint where the ends overlap when wrapped around the jig. My first try almost worked. But I was too slow getting it clamped down, and it was a rather cool windy day, so that it came out somewhat asymmetric. The section near one end where I started the bend held close to the form. The other side and far end of the strip had cooled too much by the time I got it clamped and so didn't bend as easily. This resulted in a somewhat lopsided coaming. I may try to fix it with a heat gun. This is a general problem with the approach of bending by starting at one end of the strip and wrapping it around. You have to work very fast. It seems easier to get symmetry by marking the spot where the strip crosses the center of the cockpit at the front, clamp there first and work both sides back together. But that would make it more difficult to use the aluminum flashing to contain the strip. I'm still pondering whether the ability to brace the outside of the bend with the flashing is worth the potential for asymmetry induced by uneven cooling when wrapping around from one end to the other. I used the end-to-end approach with the aluminum flashing for 3 coaming attempts. But when I later tried to bend the coaming lips, I tried bending them from the front of the coaming symmetric around to the sides.
My second try at a coaming cracked right at the front of the cockpit, where the radius is the tightest due to some grain runout there. By the third try, I finally got a reasonable coaming.
This strip was just under 3/8 inches thick and was steamed for about 30 minutes before bending. I glued with epoxy thickened with wood flour and it looks good enough:
I have since failed on three attempts to bend a coaming lip from strips 3/8 inch by 3/8 inch. Two tries lost to grain runout; the third try today almost worked. The strip came out of the steamer very pliable and went around the form quite nicely. But it insisted on twisting so that the top of the coaming lip wanted to twist to the inside by the time it was wrapped all the way around. I was not able to fix the twisting before the strip cooled off too much and I snapped it while trying to get it to lie flat. Probably it would be better to bend the coaming lip around the bottom of the form where it can be braced against the base. I tried to bend it around near the top of the coaming where it would be in actual use but this made it easier for the strip to develop twist as I was bending it.
I finally was able to contact someone at Garman Brothers lumberyard near Annapolis to inquire about obtaining green, white oak. White oak or ash would be preferable for this purpose, especially if it was still green. I guess I'll try to get out there sometime this week to take a look. This is rough-cut lumber used for exterior fencing and construction so it sounds like a job to pick through and find some suitable boards. I may need to buy a planer, or find someone to surface it for me. Guess I'll take a look first to see if I can even find some suitable boards. This stuff is cheap at least. Exotic Hardwoods in Gaithersburg claims to have quartersawn white oak, but it is all kiln dried and expensive.
I also got in touch with George Dyson to inquire about material for skinning. He sent these PDF files describing the various weights and weaves of nylon and polyester fabrics available. I'm thinking about going with an 8 oz polyester skin and water-based polyurethane coating. I'll order the fabric this week. I also need to order vinyl material to make float bags. Float bags are necessary because this kayak will not have any watertight compartments. Inflatable bags maintain positive buoyancy in the event the frame is flooded.
The frame can be finished and oiled at any time and will then be ready to be skinned. If I can just get a reasonable cockpit coaming, everything will fall into place to finish this off.