Even More Birdsmouth Joints

[Howard]

Due to the recent comments on various parts of these forums, I decided to try my hand at a birdsmouth mast for my Spindrift.

Lacking a table saw, I went looking for alternatives and found these:

http://www.leevalley.com/wood/page.aspx?c=2&p=45160&cat=1,46168

I gave them a try and below is the results (not glued up yet).

I ran across a couple articles in an issue of BoatBuilder, and in that they indicated that all things being equal, the more staves you had, the stronger the mast. I think that is to do with how little you have to plane off to get them round. Also, at the Lee Valley site, you can download the instructions that come with the bit, and it mentions that if you off center the tip of the cutters, you can create ribs or mostly rounded. Closer to the center and it gets mostly round, with little to take off in the process.

As far as I can tell, it works as advertised. Using the bits would be about 100 times easier than setting up a table saw to cut perfect 30 and 60 degree cuts at the right height. But 12 staves are a handful to get put together. Like herding cats with a broom.

My staves were 3/4", which is supposed to result in a 2 3/4" mast and it came out to that almost exactly.

Bring on that Doug Fir!

[Stump]

neat. looks like a cool tool. 8)

[Greg Luckett]

Howard,

Which of the 3 bits did you use? Are all 3 needed?

This is exciting to me and I thank you for this posting!

Greg.

[Guest Oyster]

WOW!! :shock: This old guy may even try one, since some of your guys are so convincing with your live action shots. Thanks,

[Charlie Jones]

Greg- you don't need all three. Each of those bits cuts a different angle, so you can use a differing number of staves. The Birdsmouths I cut on the table saw is a 90 degree angle and requires 8 staves.

His bit (that he used) cuts the proper angle for using 12 staves. Others would use less, or (perhaps more) depending on the angle cut- you'd have to choose which you wanted FOR THAT MAST.

Actually they look pretty good- I"m with Oyster- may look into one myself, although since I have a shaper I don't have a router table. But I've been looking Shaper bits are getting harder to find, and more expensive, while router bits are proliferating rapidly

[Greg Luckett]

Charlie,

Thank you for pointing that out....I figured it out from reading some more too. I guess what I am not sure about is what number of staves would be needed to build a mast or spar and that this probably varies somewhat. The bit choice is determined by the number of staves to be used. Howard must have used the one for 6 or 12. You would use the one for doing 8. I am thinking that those two would be the bits to have. I do not see the need to do one with 16 sides...do you? I printed out the Lee Valley information and am going to sit down and read it this afternoon.

Thanks again

[Howard]

Had I known I might need it someday, I might have paid more attention in Geometry class. :oops:

All of the cutters leave a 90 degree cut, but the ones for 6 sides or twelve sides cut at 30 and 60 degrees. (360 / 60 = 6 and 360 / 30 = 12). The same cutter can be used for both, depending on which angle you put to the center. The cutter for an 8 sided mast cuts both at 45 degrees (360 / 45 = 8 )

I got the 6/12 and 8 sided bits. They make one for 16 sides, but I'm not going there. The 45 degree cuts for an 8 sided mast would be easy to do on a table saw, especially if the cuts are centered. But those 30 and 60 degree cuts would be a nightmare.

I went ahead and glued those staves up and hit it with a plane and a bit of sandpaper. Not much work at all, and with those 12 sides, very little material had to come off. If these bits will cut to 7/8", they could be used on a mast up to about 4 1/4" to 5 3/4" diameter, if stave thickness is 15% to 20% of diameter.

I think that bits like this would be ideal for a shaper, especially if building the larger masts. I think the larger size would really put a strain on the router.

[Howard]

Charlie:

when you do these masts, do you plug the ends solid, or do you leave holes to drain moisture and/or let the inside "breathe"? I guess if they are plugged solid and water tight, in theory, no moisture should ever get in.

I would think you would have to be very carefull around any bolt holes for sprits, booms, blocks, etc.

[Dave R1]

Howard, your test joinery looks very good but I would suggest some caution and perhaps a bit more math before you build an entire mast or other spar using the bit you used for your test. The resulting wall thickness in your example isn't very much. If you stick close to the scantlings given for a solid mast, you may find you don't have the needed strength or stiffness.

I have a section in my Excel-based birdsmouth mast calculator that gives a rough comparison of strength between a solid spar and a hollow one. Unfortunately I think the calculator is at work so I can run any numbers right now. In any case, please do some investigation before you go to the trouble of making a twelve-sided mast.

Good luck with it, though.

[Charlie Jones]

Howard- on small spars I usually plug 'em solid. On larger ones- say over 2 inches, I leave a small hole for venting. I've started epoxy coating the entire inside of the spar before it's final glued so that may not be really needed, but I feel better having the opening.

[Greg Luckett]

I ordered the two bits, one for 6 or 12, and the other for the 8. I ordered a couple of other things too....plans for that work bench .

Then, a little bit ago, while going through some old Wood mags I saw an ad from 2002 for these bits from Lee Valley.

[Howard]

The resulting wall thickness in your example isn't very much.

Dave: This was just a test, and the thickness used was 3/8" x 3/4". It gave me a mast diamter of 2.75 inches. At that thickness, it comes out to 13.6% of mast diameter, which is too light. From what I've read, it should be 15 to 20%.

If I stick with the 3/8 inch thickness and build a 2.25" mast it will be 16.7% at the bottom, and tapered to 1 1/2 inches, 25% at the top. I'm hoping to get three staves per each cut from inch and a half stock. It could be made thicker than 3/8" if my saw cuts don't take out too much. With these cutters, it makes no difference how thick it really is. You can easily adjust them to the stock. But with those scantlings, if anything, it may turn out to be too stiff.

The top and bottom will be plugged solid.

Charlie.....is the vent hole in the side above the plug or in the bottom? If I plug it solid for 3 or 4 feet, I'm not sure how I would vent the bottom unless I put a couple saw cuts along one side of the plug and left the groove up while the epoxy cured. That might leave a vent hole. I could leave a piece of heavy string in the groove and pull it out about about the time the epoxy kicks to the gell stage. I would think it would stay open after that.

[Howard]

BTW....I understand there is a plenty of scantling information published on mast diameters, etc. No reason to re-invent the wheel. But I don't have it or know where to get it. I'm going from "rule of thumb" information I've picked up here and there.

Does anyone have those tables and can tell me what it should be?

[capt jake]

Dave has an excell calculator he has designed that works real well. I am sure he would send it to you. I have a copy of it, but it is also on the other computer, and it is in the shop right now.

One of these days I need to spend HOURS transfering all of my information from one computer to the other. :roll:

[Charlie Jones]

Either rout a small groove the length of the lower plug, or run it through a table saw and rip the groove. Doesn't need to be much- just enough for condensation to escape and air to equalize.

[Greg Luckett]

Dave has an excell calculator he has designed that works real well. I am sure he would send it to you. I have a copy of it' date=' but it is also on the other computer, and it is in the shop right now.

One of these days I need to spend HOURS transfering all of my information from one computer to the other. :roll:[/quote']

Dave, I would like to have that calculator too.

Thank you.

[Dave R1]

Greg, I just sent it.