Birds mouth mast taper

[StoneCat]

Hmmm, as I sit and contemplate the fabrication of a hollow mast that is straight for 14 ft and tapered for 8, I say, huh ? For the long continuous stave, how are they made so they lay together along the full length while maintaining the same linear central axis (without bending)?
Hmmmm

M.

[StevenBauer]

Are you wondering about how to cut the taper on the staves or how to support the mast during glue up? I'm right in Portland and I could show you how I made my birdsmouth spars.

Steven

[Ian McColgin]

Make the staves suitable for the largest diameter, complete with the bird's mouth routered or cut in first. Easier to handle.

Then make the taper by planing off a bit of the face of each stave opposite the bird's mouth. You may need to taper a bit off the other two sides as well but I'd save that for after dry fit just to see what's really up.

On a proper mast, all of the upper taper is in the sides and front, none along the back edge because you want a nice straight line for the sail's luff. The taper below the partners is symetrical.

You will get this effect by evenly tapering all the staves and then carefully blocking the one that will be along the luff such that that run is kept straight as the mast is assembled with glue.

The niftiest jig I saw for doing that was bits of angle iron temporarily screwed to that stave and secured to a long straight mast bench. Kept the stave straight while allowing room to work around and set up band clamps.

[ 03-08-2005, 07:47 AM: Message edited by: Ian McColgin ]

[ssor]

When I want to plane a long taper on the edge of a piece, I mark intervals along the section that will taper. Then starting from the end that will be narrowest, plane the length from each mark. By adjusting the interval between marks I can vary the rate of taper along the length.

[Billy Bones]

Originally posted by Ian McColgin:
...The niftiest jig I saw for doing that was bits of angle iron temporarily screwed to that stave and secured to a long straight mast bench. Kept the stave straight while allowing room to work around and set up band clamps.Thanks for that!

[StoneCat]

Thanks all, for the posts.
I have solid modeled a couple different spars using two different stave tapering methods. Looks like to make a spar transition from straight to tapered, without bending the staves into contact, is to taper more than one surface of the stave, ick.
Steven, thanks, I will e-mail you to discuss further.

[StevenBauer]

Actually the real beauty of the birdsmouth technique is that you only taper the side opposite the birdsmouth. And the wall thickness of the finished spar stays the same even through the tapered areas. So where the mast is a smaller diameter it is still just as strong.

Steven

[Paul Piercey]

Steven is correct. I've made a few birdsmouth spars and all you have to do is taper the side oppisite the birdsmouth. Very simple. I made a jig with a couple of strips of wood the length of the taper. Lay the strips flat on the floor next to one another. Seperate the strips at one end the amount of the taper. The edges will touch at the other end. Use a couple of pieces of wood to secure the strips in this position. Slide the jig along with the stave through the table saw to cut the taper (the jig is against the fence).

Paul

[kc8pql]

Here's the core for the foot of the mast I'm building. It tapers from the foot to a straight section at the partners and then begins the taper toward the masthead. All the tapering is cut on the edge opposite the birdsmouth as mentioned above.

http://tinypic.com/221ape

[bheys]

I used a table saw and similar techniques described above to achieve tapers on both the mast and yard for a gunter rigged Fulmar. In addition, I found it useful to make simple female "molds" that helped to maintain the uniformity of the taper during the clamping process. The "molds" increase in height as the diameter of the spar decreases.
http://www.imagestation.com/picture/sraid77/p4ad34e016a6e0bd5c47e3d4c9f198ca7/fb3bfd44.jpg

By rotating the mast in the "molds" I could be assured that the mast was shaped according to it's design. The same technique proved handy to get the proper curvature in the yard.

[StoneCat]

That is good news, I guess the flexibility of the individual staves allows them to bend through the transition from straight to tapered.
I will post an update. thx for all the details and help.
Mike

[kc8pql]

Originally posted by StoneCat:
I guess the flexibility of the individual staves allows them to bend through the transition from straight to tapered.
Just ease (round?) the "corner" where the taper transitions from the straight sections a bit and you'll have no problem.

[StevenBauer]

Mike what are you using for stock. Do you know that Rufus-Deering Lumber on Commercial Street still sells spruce "staging planks"? Air dried rough sawn 2x10's. You have to pick through the pile but I've found some beauties. 16 footers cost about $25.
Oh yeah, what are you building? Is the mast for a new boat or old? Got any pictures?

Steven