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Old 28-02-2014, 22:49   #31
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Re: Main Halyard Question

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Originally Posted by Jim Cate View Post
You might give Emmet Ropes in Tingalpa a call for a quote on various bits of cordage. His prices are WAY lower than Whitworths et al, especially on longer lengths, and he can supply most sorts of modern line. We've used this vendor for some time now and had good luck and good prices.

Cheers,

Jim
Thanks, I give them a call on Monday
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Old 01-03-2014, 03:13   #32
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Re: Main Halyard Question

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Originally Posted by transmitterdan View Post
Only if the halyard is led to the cockpit. If cleated to the mast then the compression is the same.
I think not. The luff tension from a 2:1 (pulling the masthead down, directly and indirectly) is the same as for a 1:1 cleated to the mast, but the tension in the halyard fall (also pulling down on the masthead) is only half as much for the 2:1 setup.

That one's easy to get wrong, and at first I thought "OMG he's right" but I'm 90% sure you're not.

Consider a masthead halyard lock: that would mean the only downforce on the masthead would be luff tension.

A 2:1 setup is a BIT like a halyard lock, one with a trigger line which has to be loaded to half the halyard tension, to stop the halyard unlocking.
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Old 01-03-2014, 03:31   #33
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Re: Main Halyard Question

As for whether creep constitutes stretch, it seems to me this risks becoming a pointless fight over the definitions of words. It is conceptually an important distinction, though, methinks.

Perhaps it could be defused by temporarily considering the analogous elongations for an engineering material: if it's a solid rod, the two sorts are labelled "plastic strain" and "elastic strain"

The former is permanent and cumulative, whereas the latter recovers when the load is removed.

For multistrand rigging wire, the terms are 'construction stretch' and 'elastic stretch' - but that's slightly different from solid rod, because construction stretch, in particular, is entirely due to the strands "settling": intertwining more closely, and spiralling slightly less steeply (both irreversibly). There is no "plastic strain" in the individual strands, unless the wire is loaded well past the working limit.

Creep for a braided rope is (I assume, I'm getting outside my field here) largely or wholly a combination of plastic strain of the material within each fibre, and construction stretch as described above.

I'm guessing "elastic stretch" for braided rope is a close analog to "elastic strain" for a solid rod, with an added pinch of (temporary, reversible) settling of the strands into closer, straighter intertwinement.
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Old 01-03-2014, 05:56   #34
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Re: Main Halyard Question

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Originally Posted by Stumble View Post
The 2:1 also reduces mast compression by 25%, which is nice on performance boats but not normally important on cruisers.
Quote:
Originally Posted by transmitterdan View Post
Only if the halyard is led to the cockpit. If cleated to the mast then the compression is the same.
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Originally Posted by Andrew Troup View Post
I think not. The luff tension from a 2:1 (pulling the masthead down, directly and indirectly) is the same as for a 1:1 cleated to the mast, but the tension in the halyard fall (also pulling down on the masthead) is only half as much for the 2:1 setup.

That one's easy to get wrong, and at first I thought "OMG he's right" but I'm 90% sure you're not.

Consider a masthead halyard lock: that would mean the only downforce on the masthead would be luff tension.

A 2:1 setup is a BIT like a halyard lock, one with a trigger line which has to be loaded to half the halyard tension, to stop the halyard unlocking.
Since the original discussion was on mast compression, not halyard/luff tension, transmitterdan is correct. If the halyard is cleated on the mast, the mast compression is the same, purchase or not.
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Old 01-03-2014, 06:31   #35
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Re: Main Halyard Question

^^^
It also depends on your definition of compression. I was thinking about compression at the step. There is another compression on the mast even when halyard is cleated to the mast. The cleat is pulled upward to resisit the halyard tension. That "compression" is 1/2 when using a 2:1 purchase. The mass of the sail and rigging contribute to compression the same regardless of purchase.

PS:The quote attribution isn't quite right.
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Old 01-03-2014, 06:42   #36
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Re: Main Halyard Question

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Originally Posted by transmitterdan View Post
^^^
It also depends on your definition of compression. I was thinking about compression at the step. There is another compression on the mast even when halyard is cleated to the mast. The cleat is pulled upward to resisit the halyard tension. That "compression" is 1/2 when using a 2:1 purchase. The mass of the sail and rigging contribute to compression the same regardless of purchase.

PS:The quote attribution isn't quite right.
The remaining 1/2 of the mast compression comes from the other end of the halyard, usually terminated at the masthead. Hence, the mast compression from a mast-cleated halyard is always same, purchase or not.
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Old 01-03-2014, 07:11   #37
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Re: Main Halyard Question

Forgetting about step forces I think Stumble was right about the 25% reduction. The bolt rope tension is compressing the mast between the goose neck and mast head. This is the same regardless of purchase. But the down lead tension is 1/2 the bolt rope tension (ignoring friction). That additional compression means the total is 75% of a 1:1 setup. For cruisers this is probably negligible but for spaghetti noodle racing masts it may be important. If cleated at the mast none of these loads make it to the step.
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Old 01-03-2014, 07:21   #38
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Re: Main Halyard Question

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Originally Posted by DotDun View Post
The remaining 1/2 of the mast compression comes from the other end of the halyard, usually terminated at the masthead. Hence, the mast compression from a mast-cleated halyard is always same, purchase or not.
Think you might be mistaken there, looking just at the mast..

With single purchase and tension T at the attachment to the head of the sail,
Downward load from masthead to the sail=T
Downward load from the masthead to the cleat at the bottom of the mast=T
Total Downward load =2 x T

With double purchase and the same tension, T:
Downward load from masthead to the sail=T ( 2 x T/2, 1 to the terminated end, 1 to the pulley)
Downward load from the masthead to the cleat at the bottom of the mast=T/2
Total Downward load at the masthead =1.5 x T


I think...

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Old 01-03-2014, 10:28   #39
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Re: Main Halyard Question

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Originally Posted by conachair View Post
Think you might be mistaken there, looking just at the mast..

With single purchase and tension T at the attachment to the head of the sail,
Downward load from masthead to the sail=T
Downward load from the masthead to the cleat at the bottom of the mast=T
Total Downward load =2 x T

With double purchase and the same tension, T:
Downward load from masthead to the sail=T ( 2 x T/2, 1 to the terminated end, 1 to the pulley)
Downward load from the masthead to the cleat at the bottom of the mast=T/2
Total Downward load at the masthead =1.5 x T


I think...

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You're mixing tension on individual purchase segments with total mast compression. When you take a line around a sheave at 180 degrees, the pressure on the sheave equals twice the tension on the line. The pressure on the sheave is transferred to the mast or sail, depending on where the sheave is connected.

The total tension on the luff of the sail is transferred to the mast, creating equal mast compression. There is no free lunch, 500lbs of pull on the sail creates 500lbs of compression on the mast (assuming the halyard is cleated on the mast).

The leverage created by using a purchase only assists in the force required to move the line, the mast doesn't see any of that.
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Old 01-03-2014, 10:58   #40
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Re: Main Halyard Question

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Originally Posted by DotDun View Post
You're mixing tension on individual purchase segments with total mast compression. When you take a line around a sheave at 180 degrees, the pressure on the sheave equals twice the tension on the line. The pressure on the sheave is transferred to the mast or sail, depending on where the sheave is connected.

The total tension on the luff of the sail is transferred to the mast, creating equal mast compression. There is no free lunch, 500lbs of pull on the sail creates 500lbs of compression on the mast (assuming the halyard is cleated on the mast).

The leverage created by using a purchase only assists in the force required to move the line, the mast doesn't see any of that.
Are we talking externally, ie at the foot of the mast? In the case if the halyard is cleated off to the mast then the foot just sees the weight of the sail, everything else is internal to the mast.

And that's contradictory, the first you (correctly) say the force (pressure is something different) is doubled with a line going 180 degree round a block then say 500lbs of force on the sail will result in 500lbs of compression in the mast?

Internally to the mast it does make a difference. In your 500lbs of upward pull on the sail creates 1000 lbs of compression in the mast between the top blocks and the cleat in a 1:1 system. 2:1 will create 750 lbs between the same 2 points.

Unless we're talking about different things here. Been known on forums

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Old 01-03-2014, 11:25   #41
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Re: Main Halyard Question

I misunderstood what transmitterdan meant by compression.

I agree fully with him now, that the mast step sees no difference in downforce from a 2:1 vs a 1:1 provided both are cleated at the mast.

And it seems he agrees with me and Stumble and conachair, now that he understands how we define mast compression.

So now we only have to convince DotDun.

DotDun: do you agree that halyard locks reduce the compressive stresses in a mast tube?
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Old 01-03-2014, 13:19   #42
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Re: Main Halyard Question

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Originally Posted by conachair View Post
Are we talking externally, ie at the foot of the mast? In the case if the halyard is cleated off to the mast then the foot just sees the weight of the sail, everything else is internal to the mast.

And that's contradictory, the first you (correctly) say the force (pressure is something different) is doubled with a line going 180 degree round a block then say 500lbs of force on the sail will result in 500lbs of compression in the mast?

Internally to the mast it does make a difference. In your 500lbs of upward pull on the sail creates 1000 lbs of compression in the mast between the top blocks and the cleat in a 1:1 system. 2:1 will create 750 lbs between the same 2 points.

Unless we're talking about different things here. Been known on forums

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I think we're still talking past each other, but not yet sure we agree...

A purchase is used to amplify force. If you want to put 500lbs of tension on the sail luff and have a single purchase in the halyard (a block on the head of the sail), you put 250lbs of tension on the halyard. As I believe we're talking about a halyard that never leaves the mast, you will have a 500lbs of compression (top to bottom 'squeeze', if you will) on the mast, 250lbs from each side of the purchase. I don't agree that mast compression is amplified by using a purchase.
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Old 01-03-2014, 13:29   #43
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Re: Main Halyard Question

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Originally Posted by DotDun View Post
I think we're still talking past each other, but not yet sure we agree...

A purchase is used to amplify force. If you want to put 500lbs of tension on the sail luff and have a single purchase in the halyard (a block on the head of the sail), you put 250lbs of tension on the halyard. As I believe we're talking about a halyard that never leaves the mast, you will have a 500lbs of compression (top to bottom 'squeeze', if you will) on the mast, 250lbs from each side of the purchase. I don't agree that mast compression is amplified by using a purchase.
What about the 250lb exerted on the masthead by the halyard on the other side of the block at the masthead, the side which is tied off to the cleat?

Mast compression isn't amplified, it's less than without a purchase.


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Old 01-03-2014, 16:21   #44
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Re: Main Halyard Question

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What about the 250lb exerted on the masthead by the halyard on the other side of the block at the masthead, the side which is tied off to the cleat?

Mast compression isn't amplified, it's less than without a purchase.


Don't confuse me with facts! My mine is made up!!

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Old 01-03-2014, 17:50   #45
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Re: Main Halyard Question

If the sail requires 500 lbs force to raise, the compression load from the block at the mast head will be 500#s no matter how many purchases there are in the Halyard. The force required in pulling on the lines will be reduced but the load on the blocks/sail will remain the same.
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