mast blank glue-up

[Dennis Rioux]

I have managed to shake off my wintertime inertia (haven’t done a thing boatbuilding-wise since the end of October -- d’oh!) and am getting ready to make a mast for a Penobscot 14. All the boatbuilding books I have looked at typically show two equal thickness halves (or three equal thickness thirds) being glued up for a solid mast if one is not lucky enough to have something big enough right from the start. The mast is supposed to be 2.5” in diameter at its thickest point. I have a straight-grained doug fir board that will probably be 1.75+” thick when I get done planing it. It seems a shame to rip the thing in half width-wise, glue it up to 3.5” thickness, and then promptly rip that back down to 2.5”. I have thought about just adding enough to my 1.75” piece to make up the blank, but then I got to thinking about stresses and thicknesses and got myself confused. I will admit to not being thrilled with the idea of resawing a 12’ long board too. Is there anything wrong with doing unequal thicknesses? Or should I just let it go and try to get over wasting some good wood? Thanks for your help and advice. (And, yes, when I get done inventing my patented Way-Back time machine, I will go back and buy 6/4 S2S instead of 8/4. Near as I can figure, I must have been so excited to find a source of decent doug fir that I divided 3 by 2 and got 2 instead of the slightly more precise 1.5.)

Dennis

[Don Maurer]

Why not make a hollow bird's mouth mast? For a 2.5" diameter mast you need 8 staves 1" x .5". If you are resawing on a table saw, you should be able to get 3 out of 1.75" stock. That will give you 9 staves out of 3.5", 8 for the mast and 1 to practice with.

[Bruce Hooke]

If you don't like the birdsmouth idea I don't see anything wrong with gluing together two pieces of different thicknesses. The glue lines will be a bit more obvious because the surface of the mast will cut across them at an angle, but I can't see that it would be any weaker, or at least not enough to matter. I might be a bit more concerned if this was going to be a 10" diameter mast!

[Bob Smalser]

I like to laminate or lay up in increments of three because I was taught to do it that way where possible.

Messers Cleek and Fleming explained why not too long ago...something I'd never thought much about:

A lamination of two boards will break its glue joint before a lamination of three boards of the same thickness because the seasonal wood movement of the center board tends to cancel out the movement of the outer boards in stress on the glue joints. Thicker wood moves with more power than thinner wood.

Alternate the growth ring cups when laminating or laying up to reduce that stess even further.

When exposed to the sun or some other factor that causes moisture change and the resulting seasonal movement, growth rings try to straighten themselves out as the wood shrinks or swells, so the stress on the glue joint is in two dimensions, not just across the grain. Accordingly, the board with the most cup goes in the center of a 3-board lamination.

[ 02-01-2005, 04:08 PM: Message edited by: Bob Smalser ]

[Jim Budde]

Don, you suggested a bird's mouth alternative of same dia as solid. I am faced with the same delima as Dennis and had considered your suggestion, but wondered if a 2 1/2" solid dia converted to a 2 1/2" bird's mouth, or does one need to add to dia a wee bit for strength?

[Don Maurer]

A hollow mast using these scantlings is supposed to give you about 90% of the strength of a solid mast of the same diameter. If the plans call for a spruce mast and you use douglas fir, you should be able to get by without increasing the diameter. For the same species, you should increase the diameter a little (maybe 10%) to achieve the same strength.

[Chris Ostlind]

Use this URL to download a set of simple-to-use shareware programs to determine hollow and solid wood mast size.

http://www.carlsondesign.com/#Fun_Shareware

You can select from different species as well as intended use applications such as offshore, cruising, etc.

Fun and simple to use.

Chris

[Dennis Rioux]

Thanks very much for the feedback. I am leaning toward the three board solution Bob talks about since I certainly have enough material to give that a go. I will go off and do the other narrow diameter spars first, though, and look into the bird's mouth technique since it appears I have enough material to give that about two goes. Seems quite advanced to me at my stage of learning and I have all sorts of questions about it. I know I have seen threads on it here, so off I go to the archives. Thanks again.

Dennis