Mast scantlings, hollow vs. solid?

[Norm Bernstein]

Hi,

I'm doing a bit of planning-ahead on my Ellipticat project, and am debating some issues about the mast with myself.

The original plans of the boat that the Ellipticat is loosely based on call for a hollow mast, 3 3/4" in diameter, unstayed. For a number of reasons, I'm contemplating building the mast as solid, rather than hollow, 3 1/4" diameter, and with a forestay and sidestays, connected a bit above the top of the luff of the mainsail...

Any reactions? Condemnations? Endorsements?

(project log and photos at www.marisystems.com/ellipticat (http://www.marisystems.com/ellipticat) )

[Tom Robb]

Sorry to rain on yer parade, but isn't that oak venerred ply you mention on your website IN-door cabnet stuff. I've built cabnets w/ it & the veneer is paper thin & the glue probably isn't waterproof.

[Norm Bernstein]

Actually, Tom, I didn't end up using it for the centerboard case... I used solid white oak, instead, splined for width. I DID use the big-box-store plywood for the centerboard itself, but that was heavily glassed and filled.

[mmd]

Calculate the section modulus of the spec'd hollow mast, then extrapolate a diameter for a similar-modulus solid spar. Schaum's "Properties of Materials" is a good inexpensive references and available at any college-town bookstore.

Another method would be to use the mast formulae in Skene's book and calculate the diameter and stress in the extreme fiber of the stick. Stress figures less than 5500 lb./sq. in. will be fine for 'most any wood species you will use for the sticks.

A third method is to gather data fron specs for other boats of similar size and displacement, plot the data on a parametric graph and do regressional analysis to arrive at the scantlings suitable for your application.

[Tom Robb]

Building with what's available is usually reasonable. smile.gif Since the design is a guestimated experiment, I wouldn't throw too much money at it.
That solid spar will probably be heavier for a given stiffness and the standing rigging will add to your costs, if that's a consideration.
The people who've built birdsmouth spars seem to have found the experience enjoyable.

[Venchka]

Originally posted by mmd:
...

A third method is to gather data fron specs for other boats of similar size and displacement, plot the data on a parametric graph and do regressional analysis to arrive at the scantlings suitable for your application.

:eek: :rolleyes: :confused: I always thought that my friends in Nova Scotia spoke English. :cool:

For what it's worth, my unstayed, solid spruce mast supports a 131 sq. ft. balanced lug sail. It is 3 1/8" sq. at the partners, turns round just above the partners with a diameter of 3 1/8" for about 30" and then tapers to 2 3/16" at the head. It is 16'-9" long. That's one data point for Michael's linear regression. Or, it's your mast design. :D

Wayne
In the Swamp. :D

[Nicholas Carey]

Using Carlson Design's Handy-Dandy Spar Calculator (http://www.carlsondesign.com/#Utilities),

for a 15-foot spar, supporting a 139 sq. foot sail, with a head taper that starts 3 feet below the masthead and a foot taper that starts at 1-1/2 feet, it suggests the following (using sitka spruce):

solid unstayed spar. diameter: 3.81 ins, tapering to 2.36 ins at the head and 3.24 ins at the foot. (Skeene's Rule).

solid stayed spar. diameter: 2.61 ins, tapering to 1.62 ins at the head and 2.22 ins at the foot. (Baader's Heuristic).

hollow stayed spar. diameter: 2.72 ins, tapering to 1.68 ins at the head and 2.31 ins at the foot. (Baader's Heuristic).

The software is free.

[kc8pql]

Originally posted by Nicholas Carey:
Using Carlson Design's Handy-Dandy Spar Calculator (http://www.carlsondesign.com/#Utilities)These programs seem to be quite accurate. I've checked the results against known successful spar dimensions and they are very close. I don't know if the programs on Carlson's site are the latest versions or not but Robert Tyrrell will send you versions that will calculate spars up to 75' long and 24" in diameter. Neat programs, particillary for those of us who are mathematically challenged.

[ 12-03-2004, 09:59 PM: Message edited by: kc8pql ]

[Norm Bernstein]

Thanks to all who posted the links to the 'Handy Dandy Spar Calculator' website. From the programs I found there, it appears that my substitution of a 3 1/4" solid stayed mast, in place of the unstayed 3 3/4" one specified in the plans, appears to be fairly reasonable: it will be somewhat heavier, but will be more than adequately strong.