Spars...

The long snowy Winter has finally ended, Spring has sprung, and boat-building season is in full swing! At the top of the (still) pretty long list of things that have to happen before the boat can launch is making the spars...the mast and boom. I did alot of reading and research over the Fall and Winter and somewhere along the way came across the "birds-mouth" technique for building hollow spars...The basic approach involves cutting eight long strips (aka staves), each having a v-shaped notch cut along the length of one edge (hence the name)...each mouth "eats" the corner of the adjacent stave to create a perfect octagonal wooden "pipe"...certainly more work that making solid wood spars, but the engineer in me found the combination of wood-working and mathematics too irresistible...well, that's my justification and I'm sticking with it!
There were two online articles that I found extremely helpful in the design calculations:
1) Birdsmouth Spars Revisited, by Gaetan Jette (http://www.duckworksmagazine.com/04/s/articles/birdsmouth/index.cfm)
2) Birdsmouth in Practice, by John O'Neill (http://www.duckworksmagazine.com/04/s/articles/birdsmouth2/index.cfm)
To make up for the loss in strength due to the hollow core, a 10% increase in diameter is generally recommended, so the final diameter for the boom was 50 mm, tapering down to about 35 mm during the last 2 feet or so, with a thickness of 10 mm. I was able to get 10' lengths of douglas fir from Vienna Hardwoods, perfect for the boom, which will be approx. 9.5 ft long.
The articles mentioned above give a very good description of the steps involved, so I won't reiterated them here. But there were a couple of "tricks" that have come in handy so far...one is using hose clamps to tighten the staves during the glueup...
I've been puzzling over whether to plug the ends of the spars or not to...one reason I wanted to is because I figured in the event of a capsize, having a plugged hollow mast and boom might provide some additional buoyancy to prevent the boat from going completely turtle...and also, to keep bees and other insects from building nests inside, etc...On the other hand, I've read strong recommendations against sealing the inside, as it will eventually lead to rot due to the trapped moisture. In the end, for the boom I decided to seal both ends with fir plugs. By way of compromise, I made sure every square mm of the inside of the boom was sealed with unthickened epoxy. To accomplish this, it was necessary to do the gluing in two stages. During stage one, four adjacent edges of the staves were taped with packing tape, so after the epoxy cured, the boom could be split in half easily. For the second stage, the split open boom could easily be sealed inside with epoxy, and the two halves then glued together along with the end plugs. Hopefully, any sealed moisture inside will not be able get inside the epoxy encapsulated wood from now 'til eternity...