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Thread: Birdsmouth for GIS Main Mast
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12th June 2009, 01:24 PM #31New Member
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Birdsmouth mast for GIS - draw a curve for stave taper?
I'll be building my GIS this fall, but am planning to get a jump on the build by constructing the spars and possibly the boards this summer, on the cooler days.
I've bought the lumber for the birdsmouth mast build, and am looking over the plans preparatory to cutting and tapering the staves.
Michael's birdsmouth addendum specifies the various stave widths as you progress up the mast. My question is, do you just draw straight lines between each of these measurement points, and cut the taper, or should you draw a nice curve through the points (ie, with a batten), and cut to that?
Most of the taper is pretty gentle, so I suspect for that part it wouldn't really matter, but the taper increases significantly at the top of the mast...
Thanks,
Neil
Carrboro, NC, USA
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12th June 2009 01:24 PM # ADSGoogle Adsense Advertisement
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12th June 2009, 02:03 PM #32
Hi Neil,
It will be good to get another birdsmouth mast under way. Clint made one for Dana's boat.
The staves are marked with a curve through all the points using a batten. You are correct.
Michael
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13th June 2009, 04:16 AM #33New Member
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Taper
Thanks, Michael!
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12th October 2009, 04:58 AM #34Senior Member
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birdsmouth or square?
Hi all,
So I just bought my 1/4" Okumee ply for my GIS build, and at the same time I bought my mast and spar timber. I really didn't want to have to do a bunch of scarfing, so i picked up two 16' long 7" wide by 4/4" sitka spruce boards for the mast. Not cheap, but considering how light and flexible they are, i think it was worth it. My dilema, is that to make the box mast i'm going to have to turn 50% of these planks into sawdust. I dont have any problem with the square mast, it is nice enough on my pdracer, ant that one is made up of an ancient doug fir scaffolding plank with a million nail holes. At the same time i'd kind of like to give the Birdsmouth mast a try, after all, i have all winter, and i'd love to take a little extra time and learn a new process. I dont want to bug MIK while he's on Vacation, the colorado river pics are really something else! But i would like to know if I can accomplish the birdsmouth mast with the material I already have. If it looks like i can do it with existing materials I might just bug him for the supplement when he gets back down under. In the meantime I think i'll start gluing up my foils and drawing lines on plywood. This is going to be one cool boat!
Thanks in advance,
Al
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12th October 2009, 07:47 AM #35Senior Member
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Hi Al,
I just finished my square boxmast for GIS, made of 18mm light fir with slighty rounder corners than specified in the plans. Finished weight incl epoxy and some glass at 9.0 kg. I just put a second coat of varnish today. Attachment 118813
Just to get you an idea of a light-wood boxmast. I have used 18mm european fir and planed that down to 15mm towards the top. Outside shape is (about) according to plan. I made a square foot where it is stepped, because I like that. It is a very strong and stiff mast now, I probably could have planed it down even more, but I want to be on the safe side. With spruce you must be able to cut weight even more.
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12th October 2009, 12:59 PM #36
The same mast, with good white or sitka spruce it would be in the 6.5 kg range completed, 6.7 - 6.8 with some of the other spruces.
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12th October 2009, 01:30 PM #37
Just to show the difference between an asymmetric and an symmetric stave arrangement. Note you can do oval too. The asymmetric type is the the one I prefer. It's easier to round and make octagonal columns too. This is handy for mast lowers and bow sprits.
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12th October 2009, 04:41 PM #38Senior Member
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Watermaat,
you are really tearing through your build, and i am very impressed with the quality of the work you are doing. It is quite inspiring! I can't wait to get properly started myself. The last two days here were nice blustery days albiet a little cool, and i could neither sail my PDR or work on my GIS cause I had a rush job i had to complete.
So i dont want to hem and haw about it too much, because as far as i see it the birdsmouth and the square mast are essentially the same from a performance standpoint, the difference is more an aesthetic choice, round or square. Personally I normally lean toward the simple, more rugged choice, and if you had a look at my PDRacer up close, most of you would be appauled by the slopped on glue and really, really, really cheap materials. But if i am going to build a Goat I want something I can at least be proud of. (not a museum piece though as launching it from where i am means lowering it 8 feet down a rotten wooden bulkhead onto the barnacles) Also, I am a pretty accomplished woodworker (at least in my own head) and would like to try hollow round mast if it will hold up as well long term as the square one (asymmetric method PAR, i've been over your earlier posts on the subject, and really appreciate the depth of information) Most important in the end though is rugged and light, I am still not a very good sailor, and this boat will be launched and pulled out every time it is sailed, and there aren't many ramps where i live.
Thanks for the feedback,
Al
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17th October 2009, 10:12 AM #39
Hi Al,
That's one of the things I like about this boat gig ... every boat has a different approach - even from the same person!
Infinitely variable!
MIK
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17th October 2009, 02:56 PM #40SENIOR MEMBER
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Birdsmouth
So true! I made a B'mouth yard for my 11' sailing dinghy using the symmetrical approach PAR mentions and using two staves a little wider to get an oval shape. It was more for the exercise of it than gaining a real performance edge.
I made a square-hollow mast for Dana, actually. Currently, I have staves cut for a GIS mast. I have been playing with techniques and trying to nail down a process for doing them on a more production basis as part of my business. I have been mostly tuning the process for the table saw and have yet to use a router table with a B'mouth bit. My router table is sub-PAR (no pun intended). Do you use a table saw, PAR, or a router?
Keys to doing quality staves on a TS:
-a sharp blade! I still have not concluded whether a regular 24T or a 40T does better
-a 0-clearance table insert that is stiff nice and flush with table (make your own)
-featherboards to hold staves down and in tight to fence (you can make your own too)
-make extra staves to use as test pieces for setting blade angle, height and fence position. Set fence position first, cut a scrap piece of wood until you get a 45 degree cut, perfect. The raise blade height until it meets the exact middle of the stave.
-mill and rip staves dead square and use one square edge to test fit when cutting test staves. Again, cut a scrap of wood to make sure your saw is ripping a square edge
-don't be too tempted to cut the birdsmouth/dado such that the cut produces a feather edge on stave...leave a millimeter or of wood
-cut all staves in one operation and don't change any settings midstream, thus the importance of the test pieces.
Have fun!
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17th October 2009, 06:08 PM #41Senior Member
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Production Tapers
Can you do a batch of staves however works best for you, tablesaw, router, etc. and then trim the taper to size with a flush trim router bit and a hard plastic template that you screw down to the part? you could probably even use aluminum strip for the template if you had a number of parts to make. more difficult to fabricate the first time around, but very durable and available in long lengths.
just a thought,
al
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18th October 2009, 03:06 PM #42SENIOR MEMBER
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The simplest way to cut tapers on a non=production basis is to clamp 4 together and plane to the taper per the plans. Then do the other four.
Al's idea is a good one but wouldn't really work. You could do it if the 'part' was bigger.
Clint
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18th October 2009, 03:16 PM #43
I use a set of highly trained beavers. They were orphaned when a boat prop took out their mother, so I brought them in. They eat a lot, so I decided they could earn their keep and in a fit of brilliance one evening, I knocked the bigger of the two out with a few raps of a wooden soup spoon. While he slept, I used a rotary file and ground a perfect 45 degree straight cut into his front teeth. While I was "in the mood" I took advantage of his sibling and ground a cove a bead set into his front teeth, which makes strip plank edging a lot easier now. They truly love their work, literally eating it up and though it has taken some effort teaching them to chew in a straight line, a few simple jigs has them well employed.
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18th October 2009, 03:23 PM #44SENIOR MEMBER
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Now that joke is truly sub-PAR!
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18th October 2009, 04:31 PM #45
I should add that oval masts need to be stayed and shouldn't be used as a free standing stick.
I've found most doing birdsmouth spars, make more work for themselves then they need. Custom ground router bits, tapering jigs, etc. most of this isn't necessary.
I taper all the staves at once on small spars, what ever I feel comfortable with really. I set up an inclined ramp on each side of the staves, usually a couple of 2x4's wedged to position. Then the staves are solidly wedged in position, between the two ramps. One of the ramps always has the in feed shoe of the power plane or hand plane riding on it. This keeps the taper angle correct and the plane square with the work. A few passes with a power plane, followed with several swipes by hand, I don't spend a lot of time on them. Good setup always make your job easy and short.
Cutting the notch is easiest on the table saw and I use a datto on larger stock, but make two passes on skinny stuff, unless of course you've got some trained beavers. You don't even want to know how I get steam bent wood . . .
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