Monday, September 14, 2009

Bird's Mouth Mast

This bird's mouth mast business is cool.  Below is a picture of the trial sample mast section I made while setting up the table saw for cutting staves.



A bird's mouth mast is constructed by gluing together multiple wood strips (usually 8) to form a very strong and light hollow tube.  I chose to use douglas fir because it's readily available at Home Depot and 10 foot 1x2s happen to be slightly bigger than the stave dimensions needed for the GIS mast.  The pieces are scarfed together to yield eight 15.5 foot long staves.  The fir strips range in weight from 1.5lbs to almost 3, yes I weighed a bunch of them while suffering many a strange look from other customers.  If I had picked the heaviest wood the mast would weigh well over 30lb.  I was tempted to use all of the lightest pieces but they were significantly more flexible than the heaviest ones so I split the difference and got half medium heavy and half medium light.
A free standing mast is basically a cantilever beam which means that the stress in it decreases from base to tip.  The mast can therefore be progressively weaker and lighter from base to tip.  This is often done with a taper.  The GIS mast is very slightly tapered along its length but the majority of the taper occurs near the tip.  In order to maximize strength and minimize weight I used the heavy stiff wood on the bottom sections and the lighter on top.  Scarfs are distributed over a 5 foot long section about half way up the mast.  I'm hoping this will provide a smooth transition resulting in a mast which is plenty strong where it needs to be but as light as possible and with a low center of gravity.

Very simple but effective 10:1 taper jig set up on the table saw.  It's made from an appropriately tapered piece of 3/4" MDF with a block of wood glued to one edge.  The wood block allows clamping of each stave to the taper jig then the whole works is fed along the fence.




Gluing up the scarfs.  I let the epoxy absorb into the end grain until it looked like no more was soaking in and then lightly clamped the staves together using a long straight edge for alignment.  The joints fit so tightly that I did not feel the need to thicken the epoxy.




After ripping the staves to the correct dimensions the table saw is setup for cutting the bird's mouth.  For an 8 sided construction the bird's mouth is a pretty straight forward 45 degree cut.  45 x 8 = 360 cool huh?  Each stave is run through once, flipped end for end then run through again.  It is critical the the stave is not allowed to move up or away from the fence.  Feather boards insure this does not happen.  (what's up with all the bird references?) My vertical feather board is far from perfect but it worked well enough.


 
 


Now comes the tricky part.  The mast is tapered at both ends therefore each stave has to be tapered accordingly.  The side opposite the bird's mouth is planed to the profile specified in the plans.  The mast base taper is slight and I just marked a straight line on each stave and planed to shape.  The designer recommends lofting a curve for the taper profile but the difference between a straight line and a curve is minimal in this section so a straight line it is.  The tip taper is much more pronounced and a lot more wood has to be removed.  I made a template for the tip taper by lofting the specified profile on a long piece of 1/4 inch ply.  Then it's just a matter of tracing the same profile onto all 8 staves and cutting to shape.  I used a small band saw for the cuts and followed up with a hand plane.  I found it exceedingly difficult to maintain the planed edge at 90 degrees so a scrap of wood clamped to my plane made a reasonably good guide.



Stave marked and being cut on bandsaw.



All staves tapered.



Mast tip stave shapes.



I have read that bird's mouth mast construction is harder to explain than do and I think it's true.  All the steps went quite well and the staves almost self-assembled into the mast shape, a very rewarding moment.



Mast weight at this point is 19.5lb (8.9kg) including all the line holding it together..  I'm pretty happy with that. Final planing to round will remove a bit more weight but the epoxy will add some so I should come out around 20lbs.  I did a mock stepping and it's very manageable at this weight.

Total time to date: 85hrs

3 comments:

  1. Great idea for the scarf jig! I'm not building a birdsmouth mast, but I have to scarf the chinelogs, gunwale, etc. and I was going to do it by hand, I didn't even think of this idea. Thanks for the tip! B-E-A-U-T-I-F-U-L blades by the way (previous post).

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  2. Thanks for the compliment on the foils. They took longer to make than the mast (so far :-)

    About the only thing to watch out for on the scarfing jig is to make sure the piece you are cutting is clamped square to the jig. I found it easy to clamp it off center vertically and the stave would angle a bit from 90 degrees to the table. I think cutting clearance holes in the MDF for the tips of the clamps will let the clamp grab the thin staves lower down and prevent any tendency to not be square.

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  3. Another way to cut the birdsmouth is to use a 45 degree (90 degrees at the tip) veining cutter in a router, particularly if you can mount the router in a table with a fence. It will probably take 3 to 4 passes across the cutter, increasing the depth each time, so you would do each stave at each depth before readjusting the cutter, but that produces a very accurate, clean groove

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