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Thread: Birdsmouth for GIS Main Mast
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18th October 2009, 08:35 PM #46
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18th October 2009 08:35 PM # ADSGoogle Adsense Advertisement
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18th October 2009, 11:30 PM #47
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19th October 2009, 07:01 AM #48Senior Member
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19th October 2009, 08:08 AM #49
I want to ask a serious question ... but I'm not game enough too
Okay, I've put the siamese on Beaver Alert and given the dog a shotgun to go with his normal defensive armoury (which doesn't really extend past rolling over and looking cute )
Why is that that PAR? If you've taken a round spar and just made it wider in one direction, wouldn't it be strong enough? Or are you referring to spars that have been narrowed?
Richard
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19th October 2009, 12:05 PM #50
Mostly Richard, the beaver's take exception to the wider staves in oval section spars.
Seriously, (this is difficult at times) the oval section will not offer a uniform radius of gyration (or modulus) that a round section stick does, in a free standing application. This is also true of a square section mast, though to a lesser degree, but the corners are still farther away from the center then the sides. When you have this variance in stiffness in a free standing column, you have differing bending moments at different angles of attack. This means the mast bend will go from stiffer to less stiff, then back to stiffer as the boat and wind direction change. This cycling through less stiff and stiffer sections of the mast, means the stick will break much sooner then one of uniform section.
In a stayed rig this is also true, but the stays hold the stick in column for the most part and it's not an issue. In fact, using non-round sections can let the designer use thinner, lighter wall thicknesses, because of localized stiffness in the direction the most movement is expected. This is why you see oval sections on larger stayed masts and tear drop in the smaller ones.
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19th October 2009, 03:05 PM #51Senior Member
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Paul,
I guess I'd never thought of it, but it makes sense from an engineering point of view. I wonder, though, if there's a point where a stick is short enough, carrying a small enough sail, that it makes no significant difference - functionally?
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19th October 2009, 04:04 PM #52
It's not the height of the stick that's most important, but the section diameter for the sail area it must endure. In other words, a slightly smaller diameter section, can have a huge impact on the ability of a mast to stand, but a slightly shorter (or longer) stick isn't affected as adversely. In regard to the GIS, I'd say the stick is overly strong. This is usually the case with small craft, except for full up racers, who might shave a stick until it breaks, just to have an advantage in light airs on the race course.
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19th October 2009, 04:27 PM #53Senior Member
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Paul,
I'd say the GIS mast is strong. Ours is the original solid, round version. Our PDR mast , otoh, is the hollow, square version for the 85 sq. ft. balanced lug rig.
I've thought of replacing the GIS mast with a round birdsmouth type (same diameter). I love the lightness of the PDR stick, but it's experiencing some abnormal wear at the squarish corner where the yard rides on it. Might could do a round birdsmouth for that one too... but that's further down the wish list.
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20th October 2009, 08:57 PM #54
Steal your wife's plastic cutting board (HDPE) and rip a 1" strip off the edge and put it back before she catches you. Screw this strip to the corner of the mast where it's getting eaten up by the yard. Round over the crisp edges and you'll not have this problem again. Don't tell your wife where you got the idea, I got my own troubles.
85 square feet on a low aspect rig like that, you can use a 2.5" round birdsmouth mast, tapering to 1.75" at the top, with a .75" thick stave at the bottom tapering to .5" at the top.
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24th October 2009, 12:37 AM #55
Hey PAR,
A biiiig thankyou for filling the technical breach while I am travelling - it has been a huge help!
MIK
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24th October 2009, 09:12 AM #56
What technical help. I've been making things up as I go along . . .
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24th October 2009, 11:41 PM #57
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27th October 2009, 05:18 AM #58
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5th January 2010, 05:04 AM #59SENIOR MEMBER
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Thought I'd bump this because I'm playing Birdsmouthology in my shop this month and ....
Does anyone know the weight of your rig w/o sail? The hollow box + spars would be most accurate.
Thanks,
Clint
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5th January 2010, 06:21 AM #60SENIOR MEMBER
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Clint,
9250 grams for my completed mast with all fittings and the halyard rigged.
This is the box mast as drawn by MIK for the GIS according to the exact specs provided in the plans:
- mast staves - 12 mm spar quality oregon pine
- core filler for the bottom part - softwood
- 2 layers of light (160 grams) glass on the bottom 3 ft of the mast, mast fully coated in 3 layers of epoxy
- mastfittings: 1 saddle in the top of the mast attached with 2 bolts and 1 small block, 1 small saddle with screws, 1 cleat with screws
- halyard of 12 meters 6 mm dyneema
- 6 layers of varnish
My (standard) completed GIS boom weighs 2400 grams (oregon pine, 3 layers of epoxy, 6 layers of varnish, leather at mast area). I guess the yard will be about the same.
Hope this helps.
Best regards, Joost
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