Wednesday, September 26, 2007

Keeping it Straight

No matter how carefully you select the wood, with eight separate staves more than 26 feet in length, there is bound to be some tendency for some of them to bow or spring out of true, due to natural crown or grain patterns. The only way to insure that you get a straight mast is to laminate the sections with a form that is dead level and absolutely straight. I had already glued the triangular wood fillets to the two mast halves using the straight edge of my workbench. This worked fine, but one of the halves still had a bit of crown in it after removing the screws used to glue in the fillets. It's a simple matter to get it back in line, as long as you have a straight form to clamp it too. I built a lot of straight and curved forms recently while working on the pergola project in Florida, so I figured it would work fine for my mast as well.

The first step was to screw down a platform of 2 x 4s to the work bench, to elevate the mast sections clear of the bench so that bar clamp jaws can fit under them. The bench was already level, but after screwing down the platform boards I checked them all with a spirit level and with a taunt line. The ends were also aligned with the taunt line so that when upright blocks were screwed on at a right angle they were all dead in line. This form allows clamping in two directions, vertically and horizontally, which is necessary when glueing the mast laminates together to be sure all joints are closed up with no voids in the glue line.

In the plans for the Tiki 26 mast the method of assembly is to use rope lashings to pull all the parts together. This is assuming most first-time boatbuilders won't have enough clamps of a sufficient size to assemble the mast. The rope method works, and I used it on my Hitia 17 mast, but with the weather as hot as it is here, limiting the working time of the epoxy, I didn't want the stress of trying to assemble all four parts of a 26-foot mast at one time. In addition, this mast will have wiring conduit inside, as well as aluminum foil for a radar reflector - more stuff to worry about keeping in line while trying to hold and wrap four long, springy pieces of wood. Since the mast diameter is only 5 inches, relatively inexpensive 6-inch bar clamps work just fine for all the assembly steps. I used 40 of them, and this was plenty adequate for close enough spacing to close up the parts. Instead of assembling all four parts at once, I started by glueing one side wall to the aft wall, using the form to keep it all in line. This worked out perfectly, and when the clamps were removed this morning, this half of the mast was rigid and straight with no tendency to bow.

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