Vang attachment point

[RichH]

I agree with your static analysis .... but my concerns have always been the presence of enhanced buckling failure mode due to the induced compressional-axial loads from the vang.

Standard 'thin beam' formulas can be applied or derived from the 'generalized Euler Buckling Load formulas or the 'tangent modulus' formulas .... and this certainly isnt the place to discuss them.
Nevertheless as the vang attachment is moved 'aft' and towards 'midspan' of the boom, the 'length to radius ratio" in all such formulas becomes larger and you get greater 'deflection' due to applied load coupled with the 'horizontal' induced compressional forces from the vang system .... the boom becomes more vulnerable to buckling failure ... and more so with a loose footed boom than a boom that has 'slug' attachments (distributive counter-load applied), etc. All depends on I^E3 or 'boom stiffness'.

Ive also broken a more than few (thin sectioned) booms over the years, especially when they become 'dipped' into the water at 'speed'; hence, my interest and curiosity.

 

[Joe]

Jesus!

 

[Bill Roosa]

So Quoddy
why do you want a more powerful vang?
I'd love to get involved in the "structures class" rich and Jack are putting on but have learned over the years to ask why first.
Sometimes the customer wants to do xxx and makes the determination that yyy is how to do it. Then he asks the engineer to give him yyy and guess what, it is not the best way to provide xxx and the customer returns unhappy.
SOOOOOOO
Why do you want a more powerful" vang.

 

[Bill Roosa]

For the record Rich, the length to radius ratio is not used in that fashion in this case.
I believe you are confusing lateral torsional buckling with normal bending in the plane of the force.

 

[Rick Webb]

I am Humbled by This Discusion

I am now however puzzeled by my original suggestion. The boat (the 260 in question and my 23.50) came with a small simple three block purchase (maybe four). Adding a block to the mast step then running a line from the original bail thru the block to the bottom of the original vang attached to an additional new bail worked wonderfully on my boat. The added bonus was it was cheap a block a peice of line, a bail, and a couple of rivets no noticeble additional bending in the mast and it is easier to reach now from the cockpit. Thes boats by the way do not have a loose foot and are mid-boom sheeted.

Here is what I am wondering adding the extra block the way I did does it:

A. give me a 4 to 1 purchase with the extra block
B. double the 3 to one I already have
C. not add any mechanical advantage just change the direction

 

[Bill Roosa]

Your purchase is easy to determine. Imagine a plane passing between the two sets of blocks perpendicular to the lines. The number of lines cut by the plane is your purchase. So if 4 lines are cut then 4:1.

 

[Joe]

I am now however puzzeled by my original suggestion. The boat (the 260 in question and my 23.50) came with a small simple three block purchase (maybe four). Adding a block to the mast step then running a line from the original bail thru the block to the bottom of the original vang attached to an additional new bail worked wonderfully on my boat. The added bonus was it was cheap a block a peice of line, a bail, and a couple of rivets no noticeble additional bending in the mast and it is easier to reach now from the cockpit. Thes boats by the way do not have a loose foot and are mid-boom sheeted.

Here is what I am wondering adding the extra block the way I did does it:

A. give me a 4 to 1 purchase with the extra block
B. double the 3 to one I already have
C. not add any mechanical advantage just change the direction

Your original vang is 3:1....... you count the number of load bearing lines to figure that. By adding the additional line you were attempting to double the purchase by rigging a "cascade". Note in the picture that all moving blocks of the vang and the cascade are attached to the boom.... My understanding is that yours is reversed with the cascade's block on the mast instead. I'm not sure if that still constitutes a cascade..... You might try flipping it around to see if it makes any difference... if not.. then you are cool... and now have a six to one vang purchase.

 

[RichH]

For the record Rich, the length to radius ratio is not used in that fashion in this case.
I believe you are confusing lateral torsional buckling with normal bending in the plane of the force.

Simple "L over d" ratio .... . TMI for a 'sailing' forum.

 

[kloudie1]

For Quoddy, we can all agree then that the vang is more effective at 2:1 than it is at 1:1.. and we can all agree that it moves the load point to a place that puts the boom at more risk for collapse.
Rick, can ya give us a quick and dirty sketch of your vang? I am not able to visualize what ya have ..

 

[Jackdaw]

I'd love to get involved in the "structures class" rich and Jack are putting on but have learned over the years to ask why first.

Not me, I'm just an enthusiastic interloper. Kloudie1 is the other real brain here.

 

[kloudie1]

Jackdaw, My fiancee,Susan, would not agree ! She said today.. "If you think you are an engineer, why don't you make a trap door and lift, so that we don't have to disassemble the tree?.. just open the ceiling and push a button and whoosh it is up in the attic !"

Until I put pencil to paper, I'd have lost money on a bet that the vang becomes more effective at providing downforce as the boom attach point moves aft. It is not intuitive that it behaves that way.

 

[Rick Webb]

My Vang

Mine looks exactly like the picture Joe posted just turn it on its side so the boom becomes the mast and visa versa.

Wouldn't a simple vector diagram illustrate the "down" force? Straight up and down it is all down out to the end of the boom none of it is. The issue for a simple guy like me seems to be how far out you pull on the boom to keep it down.

 

[Rick Webb]

My Vang

Mine looks exactly like the picture Joe posted just turn it on its side so the boom becomes the mast and visa versa.

Wouldn't a simple vector diagram illustrate the "down" force? Straight up and down it is all down out to the end of the boom none of it is. The issue for a simple guy like me seems to be how far out you pull on the boom to keep it down.

 

[kloudie1]

Rick, as Joe said, because you are pulling on a 2;1 with a 3;1, you are getting a 6:1 advantage. Both the attachment ends (one end has two points with half each) have the same force on 'em.. Ya have a vang that has a lot of poop !