Birdsmouth mast & spars

Hollow-spar construction from our one-of-a-kind Spar Timber Kits

To calculate the stave dimensions for a birdsmouth mast or spar, the first step is to decide whether the diameter of the mast as drawn by the designer is sufficient for hollow construction. If the designer assumed Spruce for a wood species, and the mast was designed for solid wood construction, and the boat was built to the plan, then it may be advisable to increase the dimensions of the mast by 5% to compensate for a very slight loss in stiffness that results when converting a solid mast to birdsmouth. This is not a hard fast rule, but if it is a mast and the mast needs to be very stiff, it may not hurt to bump up the diameter a bit.

To calculate the stave dimensions, use these Birdsmouth Calculators designed by Gaetan Jette which you can fine HERE or click on the image to the left. Calculator #4 is the most useful as all you need to do is input number of staves, N, overall diameter at partner, O.D., and the H-value which should be 0.5. The calculator will then give you the L which is stave width. You can then repeat for each station along the mast or spar, and you will then be able to determine the taper of the stave at each station.

Take a class making spars with Clint Chase in Saco, Maine. Learn more HERE.

We will be building out this page more but feel free to reach out with any questions at boatkits@gmail.com