birdsmouth spar - should I...?

[dale]

Back to the birdsmouth spar.  I'm thinking about making one for my dinghy.  I need a mast that is 12'6" high (from the step) that can carry a sail of 45 sq.ft. The mast is unstayed.  Lowes carries 1"x4"x14' clear pine that I'm planning on using (I know I should get sitka spruce - but, sheesh, this is just a 7&1/2' dinghy made from luan).  I can make a 6-sided spar by splitting the stock in half and come up with a spar that is about 2&7/8" in diameter; or I can make an 8-sided spar up to about 3&1/2" (or even a little larger if needed).  I would taper the spar down to about 2" at the top.  I would make the bottom 1&1/2' of the spar solid which would make it solid 3/4' above the deck.

Is the 6-sided sufficient?  It would take less material, less labor, and no waste.  Or should I go with the 8-sided?

I have never seen any specs on this.  Just kinda thinking common sense at the moment.  Thanks for any input.

[dale]

BTW - here are a couple pics of the dinghy: one showing a closeup of the mast setup

[Frank Hagan]

I have experience only with making 8 sided birdsmouth stuff; I would think it would be easier to "round off" than a six sided one. 

[PAR]

By my quick estimates, a mast that is 2.25" at it's heel, tapering to 1.25" at it's top will be more then enough to support 45 sq. ft of area on 12' 6" spar. If it's Douglas fir, with a 20% wall in birdsmouth, it will weigh about 5.5 pounds if one of the better spruces, less then 4.4 pounds.

With a 6 stave layout, you'll have 8.7 sq. inches of gluing surface on the staves, per 1" section or 10.8" of gluing surface using an 8 stave layout. Material selection will be less critical with 8 staves then 6. Rounding will make no difference. If you use a symmetrical notch arrangement, rounding will be easier and the spar slightly stronger.

[Guest mikespeare]

Mr. Riccelli,

Can you share with us the math behind calculating what thickness birdsmount mast is needed by sail area. I'm not an engineer but this would be a great formula to have in my back pocket.

[PAR]

Unfortunately it's not a simple formula, but a convoluted set of formulas and "qualifiers" to account for different elements, spar lengths, sail area, rig types, stave thicknesses, wood species, etc.