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16th September 2012, 10:18 AM #121Senior Member
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MIK
I have done some estimating based on the section information in your post. I have assumed that the vertical section would be increased to 50mm and the horizontal section left at 40mm.
Hollow 50mmX40mm Section
Top and Bottom: 28mm x 12mm (timber)
Sides: 6mm x 50mm (plywood)
Compared to a solid 40mmx40mm boom this section would result in a reduction in weight of around 20% (excluding spacers) and increased resistance to bending / deflection of 75% in the vertical plane and about the same as the solid 40mmx40mm section in the horizontal plane. I have also put some of Bob West's hollow section booms in the spreadsheet linked below.
I have based my calculations on second moments of area calculations - that should be checked before anyone relies on them. For the purposes of these calculations I have assumed that the modulus of elasticity of the timber and the plywood is the same. I have enabled public access to this file so feel free to make a copy and/or modify.
https://docs.google.com/spreadsheet/...DRaRjVWYjVvZWc
Cheers
Matt
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16th September 2012 10:18 AM # ADSGoogle Adsense Advertisement
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16th September 2012, 04:35 PM #122
Hi Matt,
Interesting spreadsheet - even if I don't fully understand the theories and math behind it all - good effort.
Some observations:
1. Only my 65mm x 45mm, 16mm boom is ply-sided.
2. The other two were lap-joined, as shown at post #716, https://www.woodworkforums.com/f169/g...0/index48.html
3. How is the Section Area (SA) calculated (and does SA affect the stiffness calculation)? The SA for the 40 x 40 is shown at 1,600 sq. mm - which I understand. The SA for the 50 x 40 is shown as 1,272 sq. mm - which I don't understand. Similar question with the SAs listed for my booms.
4. Minor point - and I'm pretty sure the drawing doesn't affect the calculation - but is the drawing to scale? Looks wider than tall...
5. Point of clarification: My name is Bob Wessel - my forum ID (BobWes) resulted from fat fingers when initially signing up.
I am planning to build another ply-sided boom and another timber boom to take to Sail OK with me. I'll share the specs for those once they are built.
Question: What if the sides were 12mm timber and the top and bottom were 6mm ply? [Assume no taper (much easier to build)]. I may have three new booms to take to Sail OK!
Thanks.Building Gardens of Fenwick, a Welsford Parthfinder
Gardens of Fenwick
Karen Ann, a Storer GIS
Goat Island Skiff - Sacramento
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17th September 2012, 01:32 AM #123Senior Member
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Hi Bob
Firstly let me apologize for getting your name wrong. I searched for an excuse but couldn't find a good one. For me the spreadsheet illustrates how important and how much impact there is in shifting mass away from the center of the section. Timber placed close to the center is nearly worthless in regards to resisting deflection.
I'm not sure I fully understand it all either. It has been 20 years since I did anything like this so it would be great if someone could verify that it is at least headed in the right direction. Then it could be used to get an idea of the relative differences of different designs.
This shouldn't be a problem, my comment that I assume the strength (and weight) of ply to equal timber means that the sides or top/bottom could be either ply or timber. I don't know how valid that assumption is - someone may be able to provide a better estimate of which is stronger and by how much.
The section area is the cross-sectional area of the timber. It can be calculated a number of ways - calculating the cross-sectional area of each piece of timber and then adding these together or calculating the area using the outside dimensions (of the boom) and subtract the area of the "hollow" interior.
Using this second method the section area of the hollow 50x40 section are (50*40)-(26*28)=1272
OR
Using the first method:
top/bottom 28*12=336 there is a top and bottom so multiply by 2, 2*336=672
sides 50*6=300 there are two sides so multiply by 2, 2*300=600
672+600=1272
The drawing is not to scale it is there just to illustrate where the dimensions A,B,C,D are.
I have updated the spreadsheet with your section maintaining the 50mmx40mm outside dimension. This assumes that the top and bottom ply sections are 40mmx6mm and the sides 38mmx12mm. Relative to the solid 40mmX40mm boom weight reduces by ~10% (excluding any spacers), vertical resistance to deflection increases by ~60% and horizontal by ~20%.
The way that the spreadsheet is currently set up means that some thought is required to re-arrange the dimensions needed to represent this section (ply across the top and bottom). Is this a more sensible way to approach building the boom - sides of timber and top/bottom in ply? If it looks like this spreadsheet could be useful than I can add a new sheet that better represents this type of section (ply across the top and bottom, timber in between).
Cheers
Matt
P.S. We have some observed data points so it would useful to check that the theory (spreadsheet) matches these observations at some stage.
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17th September 2012, 08:27 AM #124
Matt,
Thanks for the comments and explanations. They are appreciated.
I have to say... much of this sounds counter-intuitive. Does the orientation of the grain (for timber panels) or construction method have any effect on the stiffness of the boom?
Also, lighter and stiffer yards are very desirable and sought after. I wonder how a taller (hollow rectangle rather than square or round) yard would work. Booms need to be stiff, particularly for loose-footed sails, but it seems weight can be sacrificed a bit for stiffness.
The new yard I've been working on is being revised to simplify construction. I'll report the specs of the new yard and any new booms I make for the Sail OK outing.
Thanks again.Building Gardens of Fenwick, a Welsford Parthfinder
Gardens of Fenwick
Karen Ann, a Storer GIS
Goat Island Skiff - Sacramento
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17th September 2012, 04:04 PM #125Senior Member
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It's not valid. The problem is that ply will have only half its grain running along the boom. This dramatically affects strength, particularly in tension.
Of course, this says nothing about modulus, which is what you need for spars. TBH I don't know if there are any figures available that give modulus across the grain. It's not something that is usually tested AFAIK, because everyone knows timber is crap if you bend it across the grain. Mik might know about the modulus.
I have updated the spreadsheet with your section maintaining the 50mmx40mm outside dimension. This assumes that the top and bottom ply sections are 40mmx6mm and the sides 38mmx12mm. Relative to the solid 40mmX40mm boom weight reduces by ~10% (excluding any spacers), vertical resistance to deflection increases by ~60% and horizontal by ~20%.Last edited by Boatmik; 17th September 2012 at 06:10 PM. Reason: looked a little impolite ... minor edit ... i know it wasn't meant.
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17th September 2012, 06:13 PM #126
Yes!
With a mast you are constrained in cross section ... you want the smallest diameter possible. Or square-ameter.
So ply doesn't make sense because you lose some stiffness and halve the strength.
The boom is not so constrained by cross section because it doesn't really block the wind (using a coarse term)
So you can potentially to out to much bigger cross sections and use ply which won't split.
My expectation is such spars would end up being complicated and expensive though.
So I'm imagining solid timber top and bottom, ply sides and a wider boom than would use solid timber. I don't have the time to sit down and analyse exactly where that would get me!
MIK
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18th September 2012, 09:27 AM #127Senior Member
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18th September 2012, 09:29 AM #128Senior Member
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Regarding the relative stresses on the boom and yard - which of the two has the greater. Which have people found needs the most "strengthening"?
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18th September 2012, 09:49 AM #129Senior Member
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If the boom is loose footed then I would guess that it has the higher loading. However the two spars have different stiffness requirements. The boom should be as stiff as possible without weighing a ton but weight is not as important as it is for the yard. The yard has a stiffness sweet spot so it's not a matter of strengthening but rather finding the right stiffness to match the cut of your sail. It seems like general consensus is better stiffer than too flexible.
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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18th September 2012, 10:00 AM #130Senior Member
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18th September 2012, 11:17 AM #131Senior Member
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Yep, that's a good start. It also depends on how much you want to play with the rig. The stock setup works great so don't feel like you will be missing out on some huge performance boost if your yard is 1mm too thick or thin. If you do want to play you can start with a slightly larger yard and leave it squarish in cross section. Measure deflection and if it's in the ball park of what's in the Wiki, go sailing. If it's much stiffer then knock the corners off some more and re-measure. That was my approach and guess what, it's still the same squarish cross section I started out with.
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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20th September 2012, 12:24 PM #132
Designer's guideline would be now to increase the yard diameter by about 3mm at every point assuming the timber is not too heavy - nice Fir (it varies in weight quite a lot - so some slightly lighter than average density is nice) or Spruce for those who can get it cheaply - If the boards have different weights I would go for a heavier one in spruce.
But Simon's approach of squarish is OK too.
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24th October 2012, 10:54 AM #133Senior Member
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Mid-sheeting boom
I am dropping a good picture of GIR into the boom conversation. GIR has a mid-boom attachment point and in the picture you can easily see the boom and yard flexing. The boom is bending in at least 2 directions up/down and fore/aft, the yard tip seems to be just bending forward while the mast has a very slight bend in it. The sail is at the #1 reef point, boat is heavy due to the amount of gear on board, (Texas 200 event) wind and waves were just plain fun while the mizzen is sheeted just enough so it does not flap!
So with loose footed sail and a mid-boom sheeting system would a box-boom be shaped more like a truss?
JDG
I moved the main sheet to mid-boom so I could get to the mizzen easier and it allows you to sit on the rear tank as you see in the picture. That Mik driving by the way.
Another Question: How would a vang help control the sail in the wind/wave condition shown in the picture?
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25th October 2012, 09:11 AM #134Rusty Member
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I like that picture.
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