Thread: The Search for the Perfect Lug Rig Boom for A goat Island Skiff

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.

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.

Originally Posted by BobWes

Hi Matt,

Interesting spreadsheet - even if I don't fully understand the theories and math behind it all - good effort.

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.

Originally Posted by BobWes

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

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.

Originally Posted by BobWes

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.

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

Originally Posted by BobWes

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...

The drawing is not to scale it is there just to illustrate where the dimensions A,B,C,D are.

Originally Posted by BobWes

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.

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.

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.

Originally Posted by surlyone

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.

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%.

Generally you don't want to use ply top and bottom!? You want the strongest and stiffest material on the top and bottom faces. If using ply at all it would be best on the sides, and even then the only reason for using it would be if your side walls were so thin that they risked splitting if they weren't cross-laminated. Apart from that consideration, plain timber will win every time.

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

Originally Posted by Sumbloak

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.


Generally you don't want to use ply top and bottom!? You want the strongest and stiffest material on the top and bottom faces. If using ply at all it would be best on the sides, and even then the only reason for using it would be if your side walls were so thin that they risked splitting if they weren't cross-laminated. Apart from that consideration, plain timber will win every time.

Thanks for the info Sumbloak. If the spreadsheet is a useful tool then I think a "factor" can easily be introduced to adjust for the different MoE of different types of timber/ply.

Regarding the relative stresses on the boom and yard - which of the two has the greater. Which have people found needs the most "strengthening"?

Originally Posted by surlyone

Regarding the relative stresses on the boom and yard - which of the two has the greater. Which have people found needs the most "strengthening"?

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.

Originally Posted by SimonLew

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.

Thanks Simon. So in your opinion the preferred option for a new goat build would be to stick with the solid yard as outlined in the plans and aim for a stiffer boom?

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.

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.

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?

I like that picture.