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PostPosted: Sun Nov 03, 2013 3:45 am 
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AntonLargiader wrote:

I think your position on this is entrenched and this is pretty typical of internet tech spats. Those who get the physics already know who is right and wrong, and you don't need to be an engineer to get the physics of it. But those who don't know might get something from what I've written. Or, read Matt's post at the top of page 2. He sums it up very cleanly.


You are the one turning this into a spat. :-)

If anyone would like to continue the discussion based on science and objective measurements we can all learn more about the systems. It does not change the science, but I am a professional rigger. Some of the statements made in this thread seem to indicate that the posters do not understand sailing rigs at a technical engineering level.

Cheers,

Randy

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PostPosted: Sun Nov 03, 2013 3:51 am 
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aschaffter wrote:
If you studied statics and dymamics you would understand.


I did (but waving my credentials isn't my style), so let me see how close we are to being on the same page. For these purposes, A refers to the area of the mast at the tang, and B refers to the area where the various jib halyards attach, around and below the boom.

1) if you were to install a cleat at A and cleat the halyard there, the halyard would not add to mast compression between A and B.
2) the original jib halyard adds the jib luff tension to the mast compression between A and B
3) the Aussie halyard adds 1/3 of the jib luff tension to the mast compression between A and B

If you disagree, can you clarify exactly what you disagree with?

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PostPosted: Sun Nov 03, 2013 9:29 am 
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AntonLargiader wrote:
aschaffter wrote:

1) if you were to install a cleat at A and cleat the halyard there, the halyard would not add to mast compression between A and B.

If you disagree, can you clarify exactly what you disagree with?


I disagree. If you apply a downward vector to anywhere on the mast for instance increasing the tension on the forestay/jib halyard (and obviously the weight of the jib and wind forces) between the bridle and the tang (A) regardless of what happens on the mast between the tang (A) (or a cleat at (A)) and cleat at (B), that force must be counteracted by an equal and opposite upward vector at the bottom of the mast! You are making the same mistake many people do. An external force vector applied to the mast must be countered by an equal and opposite external force or the mast will (and must) move down through the boat! Regardless of what you do on the mast, the vector applied by the forestay/halyard eventually is transmitted to the base of the mast. It just doesn't get absorbed by the mast or disappear! Draw the vector diagram and it will be (or should be) obvious! In a static situation all vector forces MUST be balanced! The only way this would not be true is that once the forestay/jib halyard was tensioned it was cleated at the tang (no blocks) AND most importantly the forestay/halyard tension was totally released (you'll still have compressive forces caused by the weight of the jib and wind forces, however.) Also, contrary to popular myth, forestay/jib halyard mast compression doesn't just occur between the tang (A) and cleat (B), it occurs between the tang (A) and the foot of the mast. I won't get into the compressive force applied to the mast by the mainsail halyard- same deal though.

(Though it plays no part in this discussion, obviously a vector force applied by a tensioned jib halyard has a horizontal component along the hulls to frame/mast step (in addition to the vertical component down the mast). Just like the compressive force on the mast the horizontal component is a compressive force on the hulls/frame between the bridle anchors and mast foot/step.)

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PostPosted: Sun Nov 03, 2013 12:49 pm 
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I agree with everything you say, but you're not answering the question. I asked about the forces specifically between A and B caused by the jib halyard tension. Not the mast step reaction, not the mast support loads. Let me reword them so as to acknowledge the external forces:

1) Cleated at A: mast compression equals mast step reaction force
2) Aussie system: mast compression equals mast step reaction force plus 1/3 jib luff tension
3) Older system: mast compression equals mast step reaction force plus all of jib luff tension

Sound right? Or not?

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PostPosted: Sun Nov 03, 2013 4:04 pm 
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The one purpose I had for installing the Aussie was to remove the bend from the mast in an effort to create a smoother tacking process. All I can say is it worked (especially since I sailed solo most of the time). Hadn't missed a tack since installing the Aussie and all of the penalties I have to pay (longer lines, a place to store it, the twisting of the system, taking longer to raise the jib) for having one are worth it. There ARE counter actions for the penalties, you know. All I can say is "I love it !!!" :D

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PostPosted: Sun Nov 03, 2013 8:17 pm 
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AntonLargiader wrote:
I agree with everything you say, but you're not answering the question. I asked about the forces specifically between A and B caused by the jib halyard tension. Not the mast step reaction, not the mast support loads. Let me reword them so as to acknowledge the external forces:

1) Cleated at A: mast compression equals mast step reaction force
2) Aussie system: mast compression equals mast step reaction force plus 1/3 jib luff tension
3) Older system: mast compression equals mast step reaction force plus all of jib luff tension

Sound right? Or not?


Lets look at this another way. In a no wind situation (no reactive forces), without the main rigged, and assuming the main shrouds are not slack (rarely the case but not important for this discussion), you have the vertical component of four external force vectors acting on the mast. Three act downward- the forestay/halyard, both standard and Aussie, (includes the weight of the jib if rigged), and the two shrouds. You also have the weight of the mast but we don't need to include that for this discussion.

Those vectors are summed and applied to the mast at the tang (A) regardless of whether the halyard is cleated at the tang, whether it is run through a block at the tang and cleated lower on the mast (B), or it is run through an Aussie halyard (multi-sheave block attached to the tang by a pigtail and a second multi-sheave block mounted lower on the mast) cleated at (B). The fourth vector supplied by the boat's structure acts upward through the mast step and is and MUST be equal to the sum of the other three. The amount of downward force (mast compressive force) can be adjusted by tensioning the forestay/jib halyard. As you shorten the forestay/halyard the line (steel or fiber line) you will be attempting the stretch the halyard and bend the hull bows upward (through the bridle). The elastic modulus of the line and hulls will cause the tension to increase. It just doesn't matter where you terminate the forestay/jib halyard, at the tang (A) with a block, cleat, or Aussie pigtail, or at a cleat elsewhere else on the mast. The resultant total compressive force is transferred to the mast at the tang and acts on it between the tang and the foot/step. It doesn't matter how many sheaves you have, where on the forestay/jib halyard they are located, or where the end of line is cleated. Compression on the mast is the same in 1), 2), and 3).

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PostPosted: Mon Nov 04, 2013 10:27 am 
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My take on this:

Let's assume A: AntonLargiader defines mast compression as: The mast being compressed between the tang & the jib halyard cleat as does Hobie.
And we will assume B: Aschaffter defines mast compression as: the mast being compressed between the tang & the mast step attached to the forward crossbar.

Assuming I am understanding both of you correctly, this thread will never end.....

Steve.

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PostPosted: Mon Nov 04, 2013 12:13 pm 
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SnSBuck wrote:
My take on this:

Let's assume A: AntonLargiader defines mast compression as: The mast being compressed between the tang & the jib halyard cleat as does Hobie.
And we will assume B: Aschaffter defines mast compression as: the mast being compressed between the tang & the mast step attached to the forward crossbar.

Assuming I am understanding both of you correctly, this thread will never end.....

Steve.


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PostPosted: Mon Nov 04, 2013 12:41 pm 
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Especially when there are engineers involved!

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PostPosted: Mon Nov 04, 2013 12:56 pm 
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SnSBuck wrote:
My take on this:

Let's assume A: AntonLargiader defines mast compression as: The mast being compressed between the tang & the jib halyard cleat as does Hobie.
And we will assume B: Aschaffter defines mast compression as: the mast being compressed between the tang & the mast step attached to the forward crossbar.

Assuming I am understanding both of you correctly, this thread will never end.....

Steve.


Not quite right- I'm using Anton's reference points, but the fact is the compressive force generated by the forestay/jib halyard is the same everywhere on the mast below the tang- it is the same between the tang and the foot/step as it is between the the tang and the jib halyard cleat, or for that matter between the step/foot and the jib halyard cleat. It may be a hard concept to wrap your brain around but it is an engineering fact. If it wasn't true and if is wasn't for the weight of the mast, you could almost remove the section of mast between the halyard cleat and foot/step- a floating mast- a bit ridiculous, right?- it can't be done because the compressive force created by a tensioned jib halyard is eventually applied to the foot/step (and the boat).

If anyone is still in doubt- build a simple mockup and test it for yourself- use a piece of plywood for the boat, a dowel for a mast, some wire or string for rigging, a compression (bathroom) scale and some fish scales to take measurements. Surprise, surprise, the mast compression is as I say.

Here is an extreme analogy- by adding more sheaves to the Aussie rig you can reduce the force required to raise the jib and tension halyard, and also reduce the tension on the halyard cleat, correct? Theoretically, if you add tons of sheaves you could reduce that force to next to nothing, a small fraction of the original amount. But the tension between the bridle and tang and resulting compressive force on the mast isn't affected. All you are doing is changing the mechanical advantage required to tension the halyard (you also end up with a lot more excess halyard to stow.)

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PostPosted: Mon Nov 04, 2013 1:00 pm 
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Well, I was rebuked for suggesting there was a terminology discrepancy between us, so that can't be true... :D

Alan, there's more to the jib halyard than the luff line. I keep talking about the cleat... move your thinking a bit.

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PostPosted: Mon Nov 04, 2013 1:08 pm 
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AntonLargiader wrote:
Well, I was rebuked for suggesting there was a terminology discrepancy between us, so that can't be true... :D

Alan, there's more to the jib halyard than the luff line. I keep talking about the cleat... move your thinking a bit.


It doesn't change the fact that the Aussie rig does nothing to reduce mast compression .

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PostPosted: Mon Nov 04, 2013 2:13 pm 
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Edited.....I decided not to enter this fray....not just yet at least.

Yup......I'm another engineer.

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PostPosted: Mon Nov 04, 2013 2:46 pm 
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We agree the force at the mast step doesn't change, and we agree that we are both talking about the mast compression between A and B.

Looking at the vertical forces on a section of the mast around B, I don't see how, for a non-zero halyard tension Fh and constant mast base reaction Fbase, that Fab would not be different when you have three Fh (stock), one Fh (Aussie) or zero Fh (cleat at A) forces.

Image

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PostPosted: Mon Nov 04, 2013 5:22 pm 
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AntonLargiader wrote:
We agree the force at the mast step doesn't change, and we agree that we are both talking about the mast compression between A and B.

Looking at the vertical forces on a section of the mast around B, I don't see how, for a non-zero halyard tension Fh and constant mast base reaction Fbase, that Fab would not be different when you have three Fh (stock), one Fh (Aussie) or zero Fh (cleat at A) forces.

Image


In the image you provided, there cannot be a non-zero tension Fh. For there to be a non-zero tension at Fh you have to include the rest of the system.

You cannot evaluate the load and it's potential effects until your graphic representation of the system has a total moment of 0 around the centre of mass.

Any 2D diagram can only represent the loads in one plane. To evaluate the JibLuff/Halyard - Mast loads you must diagram a 0 net moment in that plane.

The real simple proof is that without a 0 net moment load diagram the mast falls down. So when you try to figure out the effect off the tensions in just the jib luff and halyard you have not considered the other forces that allow the jib luff to develop tension.

The system is much more complicated than the simple arguments put forward here.

Cheers

Randy
the rigger :-)

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