correct birdsmouth mast diameter

Discussion in 'Boatbuilding' started by zvi, Sep 14, 2022.

1. Joined: Jul 2005
Posts: 1,418
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Location: 26 36.9 N, 82 07.3 W

LPFlying Boatman

Working with the basic dimensions you posted and these assumption: heel to pardners =1.5 ft, heel to tack = 3 ft, luff = 15 ft and boom = 12 ft, heel to VCE = 8 ft, I estimate a mast diameter of 4". Before saying more though, designing a mast involves more than basic boat parameters. You have to evaluate its intended use and how much safety factor you are willing to live with. The value of wind pressure (taken from Skene I believe) can vary from 1.15 to 1.5 pounds per square foot. Safety factor can vary from 1.5 to 3.5, depending on your needs. A cruising sailing vessel will demand more SF than a daysailer on an inland lake.

The 4" dim. is evaluated at max wind pressure (1.5 ppsf) and a SF (Safety Factor) of 2. Also, per your request, a 3/4" wall thickness is specified. If we drop the wind pressure value to min. (1.15 ppsf) and reduce SF to 1.5 and maintain a wall thinkness (T) of 3/4", the mast diameter can be dropped to 3.25". This a significant drop in size with the associated trade offs. For grins, if you were to maximize wind pressure (1.5 ppsf) and SF (3.5), but keep the 3/4" T, your mast diameter jumps to 5". Ugh. Your designer is most likely going to be conservative his estimations that he forwards to you.

Typically, hollow wooden masts can be designed at 20% wall thickness relative to cross-section dimension. Skene has a formula for solid mast design. At min wind Pressure and min SF (1.15 and 1.5), his solid mast comes in slightly larger than my hallow mast at 3.28" I mention this because you can generally take a solid mast section, add 10% and use the 20% wall thickness for designing a hollow mast. 3.28" x 1.1 = 3.60". 3.6" x 0.2 = .72"

As you can see, mast design can be challenging and involves many tradeoffs. Best of luck with your mast building endeavor.

LP

2. Joined: Nov 2009
Posts: 57
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Location: Macau SAR

voodoochileJunior Member

want to share the building plans?

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