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Harbormaster
02-25-2014, 06:15 PM
Is there a rule of thumb about the wall thickness of a glued-up hollow mast?

I've got to make up a 45 foot mast, for a gaff rigged Ketch. It's about 8 3/4" diameter just above the saddle, and if I use spruce 2by stock it looks like the walls will be way too small. But if I use much bigger stock it's hard to buy, and I might as well just go ahead and make a solid stick.

Any good leads on folks in Maine that sell good spruce for masts, or even someone that deals in solid sticks that big? (Even Douglas Fir).

Thanks.

kc8pql
02-25-2014, 06:46 PM
Is there a rule of thumb about the wall thickness of a glued-up hollow mast?
About 20% of the diameter. Also keep in mind that it's usual to include core blocking at the partners and spreaders.

StevenBauer
02-25-2014, 06:52 PM
For spruce, you could check with the folks at Shaw and Tenney. Or give Dennis Day in Scarborough a call. For fir, it would be West coast or one of the guys that reclaim old stuff from old mill buildings being demolished.

phiil
02-25-2014, 07:39 PM
http://www.duckworksmagazine.com/03/r/articles/birdsmouth/mast.htm
or http://www.duckworksmagazine.com/04/s/articles/birdsmouth/

Harbormaster
02-25-2014, 08:19 PM
Thanks - the 20% sounds appropriate, and the charts and the graphs and the calculator are fun, but that really looks like an incredibly thin walled mast for a gaff rigged boat. I look at that and I get scared. I'm pretty sure I would have to go thicker than that calls for.

kc8pql
02-25-2014, 09:47 PM
^^^I don't know if you're thinking about tapered birdsmouth construction or not.
Birdsmouth spars are usually built using the same wall thickness for the entire spar regardless of
the outside taper. This makes the wall thicker than it needs to be
everywhere but the partners. If the staves tapered in thickness, it
would be very difficult to cut the birdsmouth. The interior blocking at the partners
increases the strength there.

PeterSibley
02-25-2014, 09:55 PM
I've always been a bit cautious around the idea of a birdsmouth mast for gaff rig. As you reef the gaff will put point loads on a wide variety of spots , places I'd like more than a 20% wall thickness.

Bill Perkins
03-05-2014, 08:57 PM
Harbormaster ; it sounds like the weight of a solid mast would not be a problem for you , but you’re having trouble finding one locally. Maybe you don’t want the checking of an unseasoned stick either? If these are the reasons you’re thinking of making a birds mouth staved mast you might consider makeing a built up nearly solid mast instead.

I once built up a solid octagonal load bearing column this way. It was composed of four clear Douglas Fir 4by4s with 2 of the octagon's faces milled on each 4by 4 before glue up . I needed a dry ,stable assembly .This looks perfect under stain and varnish ,and supports the roof of a large room .

http://im1.shutterfly.com/media/47a4dd24b3127ccef5dfefb7c88300000030O00BcOGbFq3bsQ e3nwI/cC/f%3D0/ls%3D00107751388020140306014632349.JPG/ps%3D50/r%3D0/rx%3D550/ry%3D400/

You would require eight 4by4s , beveled and ” 16 sided” on the table saw before assembly . I show of such a mast : with two of the milled 4by 4s glued together . Shell thickness 3 -7/16ths in. at the bevel . My scanner is stretching my drawings…

http://im1.shutterfly.com/media/47a4dd24b3127ccef5de9c6aa8b300000030O00BcOGbFq3bsQ e3nwI/cC/f%3D0/ls%3D00107751388020140306014628156.JPG/ps%3D50/r%3D0/rx%3D550/ry%3D400/
The worst defects on the 4by4 can be faced inward ,out of sight and away from the high stress zone on the mast’s perimeter : a big advantage . Also 4by4 can be found with a lower, more uniform moisture content than a huge single stick . You’d scarf them up to length from the available stock .

A thick , flexible epoxy is needed .I used T88 , but Gflex would be good too. It hadn’t been introduced at that time . You want a clear ,gap filling adhesive in case the milling is less than perfect . I didn’t have a table saw that was up to this , or a jointer, and had the 4 pieces of my column ripped and dressed as shown at a good millwork shop . I assembled them in my shop without a problem , with no visible joints .I did just a little trimming of the assembled spar with a hand jointer plane .I was done at that point but you of course would taper your 16 sided spar with a power plane , then round it .

8 piece glueups with a plain bevel have been used for a long time , the point in the past being to produce light , hollow masts for race boats . If acceptable grown spars of your size are no longer available at a reasonable price , could be there is now a case for such construction to get a reasonably dry , high grade, almost solid spar . Sort of a middle path between building up a solid spar from dimension lumber and produceing a light ,thin shell mast. Of course you could taper the inside of the staves before assembly , but maybe it’s better to have a spar that can be strongly screwed to at any point ,and needs no special blocking .You'd have already bought the wood.


If you want to go with Spruce and can’t find 4by 4 you could first glue together 2 piece subassemblies of two by lumber to take their place ; producing a minimum wall thickness of about 2 7/8th in. at the bevels . Here’s 2 such sub assemblies glued together .

http://im1.shutterfly.com/media/47a4dd24b3127ccef5de8c3929d800000030O00BcOGbFq3bsQ e3nwI/cC/f%3D0/ls%3D00107751388020140306014628366.JPG/ps%3D50/r%3D0/rx%3D550/ry%3D400/



A 2by 4 could just make the outside lam with the edges squared up ,and a 2by6 would yield 2 of the inner lams .Again ,the best wood would be placed on the outside ,the lesser wood on the interior . You might even buy a clear grade for the exterior .

I gathered that you were building from a proven solid spar or an already designed one . Is that where the 8 in. diam. came from ? If you decide to build a hollow spar of the same material , strength and stiffness : when you choose a shell thickness you are also choosing a required outside diameter .This last gets significantly larger as the shell gets thinner .