I am building a Skerry and will have a lug sail for a 12 foot mast. I hope to build a round, hollow mast using the birdsmouth technique. I am wondering what would be a reasonable minimum diameter that would provide a rigid mast without excessive weight. Anyone have experience with this method and suggestions on an appropriate diameter and number of sticks to use in the process (my preference is to use only 45 degree cut angles)?
If the originally specified mast is square, use the length of a side as the outer diameter of the birdsmouth mast. If it's rectangular, or you expect it to be a highly stressed mast, use the diagonal. If it's circular, use the same diameter.
The last drawing shows the neutral components of a square mast, the material that makes little or no contribution to its resistance of the wind load. By removing that material you get the classic birdsmouth shape.
Laszlo
http://www.duckworksmagazine.com/04/s/articles/birdsmouth/
Everything you ever wanted to know about birdsmouth masts.
George
The first two links below will take you to a couple of birdsmouth mast calculators that may help you with the number of staves etc. The third limk will take you to pictures of my birdsmouth mast and spars.
I made a birdsmouth mast and spars for my Skerry. It was a great project, you will enjoy building your spars with this method.
https://www.woodenboat.com/calculator
I built an 8 piece mast for my NE Dory. After cutting the parts with a table saw, I planed A taper on the square side on the top 6 feet. I coated the insides with epoxy. Then after that cured, I laid them all out on pieces of duct tape, stickyside
Up. I then buttered up all the joints with thickened epoxy and rolled it up and put several hose clamps on.
Be sure the mast is straight when you tighten the hose clamps and keep it straight as it cures. My mast was 2.5 OD and 1.5 ID.
I also made the boom and top yard hollow too.
All from aircraft grade Sitka spruce.
Christine Demechant and the birdmouth mast for her Skerry:
https://www.christinedemerchant.com/mast.html
https://www.christinedemerchant.com/mastupdate.html
Ruud