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3rd August 2012, 02:06 AM #31Senior Member
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engblom, The rope wrap is real simple to do but hard to explain lets see if this makes any sense. You need to use a stretchy line like nylon not a high tech one. The stretch is what make this work well. Start by laying a length of line parallel to the mast (call this the first end) and then wrap the line around the mast back over the first end that is parallel to the mast. Continue wrapping until you have the desired number of wraps. Put a lot of tension on the line while wrapping. Once you are done wrapping tuck the second end of the line under the coils beside the first end of the line that is already under the coils. Since you used stretchy line you can lift up individual colis and keep working the second end of the line under them. Do this until the line is under several coils or if you have the patience work it under all of the coils until it comes out the other end. Use a razor blade to trim the two ends of the line flush with the coils and you are done.
Your idea for a boom should work. I've seen wood epoxy masts made this way to look like solid wood. Like you said it's less structurally efficient than putting the carbon on the outside but you get the appearance of an all wood boom.Simon
My building and messing about blog:
http://planingaround.blogspot.com/
The folks I sail with:
West Coast Trailer Sailing Squadron
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3rd August 2012, 02:31 AM #32
An H profile is an I-beam. I-beams have terrible resistance to torsion. I can post pics of 12" steel I-beams which failed catastrophically when torsed.
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3rd August 2012, 03:08 AM #33Senior Member
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Tru dat. The torsional stiffness of the boom would drop but it may still be enough to work. I'm not rushing out to change the boom. If I get the Goat and my skills so fine tuned that I can percieve the differance of dropping half a kilo of weight from the boom, I'll sign up for the olympic trials
Simon
My building and messing about blog:
http://planingaround.blogspot.com/
The folks I sail with:
West Coast Trailer Sailing Squadron
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3rd August 2012, 04:59 AM #34
Check the WIKI to see data on Bruce experiment trying to make sure really bendy boom stiffer with carbon.
To cover with carbon makes negligible difference. You need thickness.
Also remember that a larger cross section is much more powerful at making the boom stiff than the material.
Finally, construction method you are suggesting.. The most efficient way is to have the carbon fibres in the longitudinal direction. But if all that way the boom will split. Same with a thin wall timber boom. But if you have some of the fibres perpendicular to the wood grin direction it will prevent either carbon or timber from splitting.
Michael
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3rd August 2012, 05:19 AM #35Senior Member
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MIK you may have missed my previous post in this forum on my adventures with carbon. I totally agree that increasing depth is the most efficient way to increase stiffness but I added carbon to an existing boom and got a 65% increase in stiffness for a 12% increase in weight. I'm thinking that's a success. Not the most efficient structure but it's now a rock solid boom and it retains the nice look of a slender spar.
Simon
My building and messing about blog:
http://planingaround.blogspot.com/
The folks I sail with:
West Coast Trailer Sailing Squadron
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3rd August 2012, 06:06 AM #36
Similiar to this:
Common Whipping | How to make a Common Whipping | Scouting KnotsDave
StorerBoat Builder, Sailor, Enthusiast
Dave's GIS Chronicles | Dave's Lugs'l Chronicles | Dave's StorerBoat Forum Thread
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3rd August 2012, 08:41 AM #37Rusty Member
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I pull the rudder blade if I'm going to row for a significant distance. (and the board too obviously). I find that the foils create a noticeable amount of drag. Without the rudder blade the tiller can be bungied to the gunwale out of the way. The other day though, I was sail rowing, and kept the foils working. Then I just bungied the tiller to the traveller like a wrapped python. It was perfect. If I wanted to tweak the helm, a little nudge with my foot set it right.
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3rd August 2012, 08:55 AM #38
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3rd August 2012, 11:33 PM #39
Simon, I need some help with the numbers as I don't understand the calculation for the 65% increase in stiffness. I see the 12% difference in weight but can't figure out the improvement in stiffness. I come up with about 38% - which is impressive enough! - and am curious about the calculation.
Great looking boom, by the way.Building Gardens of Fenwick, a Welsford Parthfinder
Gardens of Fenwick
Karen Ann, a Storer GIS
Goat Island Skiff - Sacramento
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4th August 2012, 03:04 AM #40Senior Member
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Bob, Stiffness (if I'm remembering correctly) is the slope of the load-deflection line so I calculated the slope of the two lines using only the end points no interpolation. Slope for the wood boom is 2.20 and 1.33 for the wood carbon boom. 2.20 is 65% greater than 1.33 ...... OH CRAP that's backwards isn't it. The lower slope is the stiffer one so we need 1.33 is 40% less than 2.20 which is where I assume you got 38% eyeballing the chart. Is that right?
Simon
My building and messing about blog:
http://planingaround.blogspot.com/
The folks I sail with:
West Coast Trailer Sailing Squadron
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4th August 2012, 03:56 AM #41
I did miss that Simon.
I have a list of goat booms on the WIKI do you have the dimensions of yours and then the amount of carbon you used? The existing data is halfway down this page .. a table for booms.
WIKI for setting up and tuning Lug and Sprit Rigs | Michael Storer Wooden Boat Plans
I was referring to Bruce ... who covered a very bendy timber boom with carbon but not enough so it made negligible difference. That data is in the WIKI already.
MIK
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4th August 2012, 04:56 AM #42
Thanks, Simon,
I did eye-ball it using the data points (roughly 21mm and 13mm) for the 10Kg load, did the math ([a-b]/a) to get the 38%. I'm not savy enough to know stiffness is represented by the slope of the line. I figured there must be a function of the graph representing stiffness but had no idea what it might be.Building Gardens of Fenwick, a Welsford Parthfinder
Gardens of Fenwick
Karen Ann, a Storer GIS
Goat Island Skiff - Sacramento
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4th August 2012, 11:06 AM #43
Howdy.
Gosh BobWes, you just made me think of my undergraduate engineering.
I really only do simple comparisons for my design work.
Usually because the forces are too uncertain or even indeterminate for the starting inputs for calculation. Plus it gets doubly complicated when the spar itself is tapered.
But yes. If you start with the forces and locations you can draw a bending moment diagram. Then use calculus in steps to get slope then finally deflection.
Gosh I'm so rusty. I can barely describe it without feeling uncertain.
MIK.
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4th August 2012, 01:04 PM #44Senior Member
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MIK my boom data in a nutshell, blog post has pics and details. This boom is too heavy but I think the same approach can be applied to a lighter wood or ply section with similar stiffness improvement and a lot less weight.
Douglas fir constant width box section, 37mm wide by 52mm deep tapering to 45mm at the front and 43mm at the back
Boom is hollow with 12mm side wall thickness and 7mm top and bottom wall thickness
The carbon was laid into routed channels so the boom has the same exact geometry before and after carbon.
Carbon tow-epoxy is 4mm thick from tack 1.5m back then 2mm all the way to clew, top and bottom of boom.
Weights:
Hollow wood boom: 2815g
With carbon added: 3150g (wood 2460g, carbon-epoxy 690g)
Deflections:
weight | wood | wood-carbon
10kg | 20.5mm | 13.1mm
12kg | 24.8mm | 15.5mm
14kg | 29.0mm | 17.5mmSimon
My building and messing about blog:
http://planingaround.blogspot.com/
The folks I sail with:
West Coast Trailer Sailing Squadron
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4th August 2012, 01:10 PM #45Senior Member
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Simon
My building and messing about blog:
http://planingaround.blogspot.com/
The folks I sail with:
West Coast Trailer Sailing Squadron
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