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Old 21-11-2011, 18:24   #31
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

I don't think any difference in compression from the main halyard going from double to single purchase is significant on the great majority of boats. However, it does seem to me that there is a 25% reduction in mast compression going from one part to two (or 33 1/3% increase going from 2:1 to 1:1).

Attached is a diagram for a 100 unit load. For 2:1 the sum of the parts is 50+50+50=150. For 1:1 sum of the parts is 100+100=200... No?

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Old 21-11-2011, 19:56   #32
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

Quote:
Originally Posted by Ex-Calif View Post
Having shared the math twice I will retire. However I suggest you get with a math and physics guy.

The answers are 200, 800 and 1600 pounds at the head of the sail which is transferred to the tack and depending on the the shape and stifness of the mast will bend it or not.

Conversely to get 400 lbs of luff tension you would pull 200, 50 and 25 lbs respectively. Really, its how this stuff works.
Your 'math' only calculates the luff tension not mast compression. You're missing the forces the halyard applies returning to the base of the mast which as illustrated above, halves with a 2:1.
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Old 21-11-2011, 22:41   #33
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Re: 42' Leopard - do I really need a double halyard?

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Originally Posted by DotDun View Post
I've got the same setup on my 7 year old FP 43'. Never had a problem and using the original block (Lewmar Ocean Series, I think a #2).

I'm wondering if you are driving the block into the top of the mast? I have about 18 inches between the head of the sail and the top of the mast, plenty of room for the block to sit there freely without touching anything.

Or, as others have surmised, the block you are using is just too light, try a heavier duty one.

I grimace at the thought of eliminating the purchase, I like to pull my sail up by hand until it gets too heavy (~ 3/4 of the way) before using a winch. I'm sure I couldn't do that on a single line.
I had a similar thing happen. The halyards went down the side of the blocks and jammed solidly. I assumed it was because there was not enough room at the top of the mast for the block and thimbles etc, and the block would tilt with the inevitable result that the halyard slipped off the pully.

Anyway I abandoned the idea and used a single halyard with a trick loop spliced into it so that as the sail is haued up using a halyard winch, this loop arrives a metre or so above the mast cleat for the halyard. There is a thimble in this loop, and as soon as it is within reach a block and tackle handy billy is hooked into it with a pull release. The other end of the tackle goes into an eye-bolt at the mast base. Once it is fitted, the tackle can take weight and the halyard winch is released from the halyard tail, and the line from the tackle is now passed around the halyard winch, and the final hardening off can be done. The halyard tail is taken around the halyard cleat in the usual way.

I use the same system for the Genoa on the forestay furler. In this case I use just the stainless steel furler on the halyard and a shackle on the furler, and harden them off by using a double purchase with spectra around the arch of the shackle and the yoke of the thimble. It all rolls up very neatly when the sail is furled.
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Old 22-11-2011, 15:06   #34
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Quote:
Originally Posted by tsmwebb
I don't think any difference in compression from the main halyard going from double to single purchase is significant on the great majority of boats. However, it does seem to me that there is a 25% reduction in mast compression going from one part to two (or 33 1/3% increase going from 2:1 to 1:1).

Attached is a diagram for a 100 unit load. For 2:1 the sum of the parts is 50+50+50=150. For 1:1 sum of the parts is 100+100=200... No?

Tom.
This is pretty interesting. Your free ends conveniently tail downwards albeit at an angle. What if the system were upside down? What if the free ends tailed horizontally?

The system is the point between the head plate (where you show a 100 lb weight) and the tack which you dont show. By inverting your image I have made the a class cunningham. If you tail horizontally the pulling load adds zero compression.

If you want to argue that the working end running down the mast adds an additional 100, 50 or at 16:1 an additional 6.25 lbs of mast compression thats fine and it is true. It is true because of the turning block at the base of the mast. The way you draw the free end it could be anywhere from F*1 to F*0 depending on the sheeting angle.
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Old 22-11-2011, 21:04   #35
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

Quote:
Originally Posted by Ex-Calif View Post
This is pretty interesting. Your free ends conveniently tail downwards albeit at an angle. What if the system were upside down? What if the free ends tailed horizontally?

The system is the point between the head plate (where you show a 100 lb weight) and the tack which you dont show. By inverting your image I have made the a class cunningham. If you tail horizontally the pulling load adds zero compression.

If you want to argue that the working end running down the mast adds an additional 100, 50 or at 16:1 an additional 6.25 lbs of mast compression thats fine and it is true. It is true because of the turning block at the base of the mast. The way you draw the free end it could be anywhere from F*1 to F*0 depending on the sheeting angle.
Of course the free end tails downward; it goes from the top of the mast to the bottom. If,as you suggest, the tail goes horizontally from the top of the mast, then you would be right. The number of parts would make no difference. But this is not realistic, is it?

This debate boils down to this: regardless of the number of parts, the compression force on the mast from luff tension is the same. However the additional compression force on the mast from the tail of the halyard --which is real--has been reduced by the purchase when it is a two part halyard.
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Old 22-11-2011, 21:17   #36
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

Quote:
Originally Posted by Ex-Calif View Post
This is pretty interesting. Your free ends conveniently tail downwards albeit at an angle. What if the system were upside down? What if the free ends tailed horizontally?

The system is the point between the head plate (where you show a 100 lb weight) and the tack which you dont show. By inverting your image I have made the a class cunningham. If you tail horizontally the pulling load adds zero compression.

If you want to argue that the working end running down the mast adds an additional 100, 50 or at 16:1 an additional 6.25 lbs of mast compression thats fine and it is true. It is true because of the turning block at the base of the mast. The way you draw the free end it could be anywhere from F*1 to F*0 depending on the sheeting angle.
The tail does, in fact, add to the compression of the panel and it does not depend on a turning block at the base of the mast or even a cleat.

If the system is in equilibrium you will get the compression as drawn.

I'm at dinner at the moment so I will not bother to draw it but imagine that instead of drawing the tails off the picture I had placed weights on them so that they balanced the 100 unit load and imagine that that load is from the force of gravity on a mass (doesn't make any difference but may be easier to imagine). Now, in the 2:1 drawing there would be 150 units of weight suspended from the top of the mast (100 on the left and 50 on the right) and in the 1:1 drawing there would be 200 units of weight suspended (100 on each side). The mast panel would have to support those loads. The load and thus the compression is less for the 2:1 system.

Nothing very fundamental changes when you talk about supporting luff tension instead of a weigh.

The force on the tail of the halyard must be equal to the force on the last part of the tackle for the system to be in equilibrium. It doesn't matter if you hold it or hang from it or put weights on it or cleat it...

I think you may be confusing bending moments and compression. In the great scheme of things l think the rig of a typical cruising boat will likely be sized for much larger loads from the rigging anyway.


Tom.
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Old 23-11-2011, 01:59   #37
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

This one is always a brain teaser. But I've been through it enough times with enough experienced racers and riggers to be sure. Herkam is right. A 1:1 halyard puts more compression on the mast than a 2:1 halyard.

Here's the way I've always thought about it, if it helps. First imagine the situation with mainsail hoisted to a halyard lock. There is no tension on halyard tail. Mainsail luff tension F is a compression force between the masthead halyard lock and the gooseneck (assuming slack cunningham, mainsheet, and vang). Mainsail exerts no compression on any other part of the mast.

Now consider the situtation with the halyard lock disengaged, and 1:1 halyard. The luff tension F exerts exactly the same compression, now between masthead block and goosneck... with addition of a new compression force F caused by the halyard tension between the masthead block and the mast mounted halyard winch. The maximum compression exerted on mast is 2*F.

Same experiment with 2:1 halyard, luff tension F + halyard tension F/2 = 1.5*F.

Just keep engaging and disengaging that hypothetical halyard lock and it will become clear... at least it did for me.

BTW the situation is the same with a halyard turning block instead of a mast mounted winch. If the turning block is on the mast, the halyard compression is between the turning block and masthead block. If the turning block is on deck, the halyard compression force is transmitted by the hull to the mast step. The mast mounted winch is the easiest way to visualize the forces though.

Martin
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Old 23-11-2011, 12:26   #38
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

I like the halyard lock thought experiment. Neat.

As I mentioned above, the balance scenario simplifies the forces to me. I've added the illustrations I didn't do last night. The first shows two penguins supported by a line that is free to move over the top of the mast. One of the penguins can represent the force on the front of the main and the other the opposing force on the halyard. It is left to the reader to decide which is which. In either case the mast supports the load of two penguins.

In the second illustration Tinkerbell has given one of the penguins the ability to fly (or perhaps has just ignited it) and it is just able to stay aloft and support the other penguin at a 90 degree angle. In this case the the mast is supporting one full penguin and about seven tenths of the second penguin. If the burning penguin could achieve vertical flight with a thrust of two penguins the mast would not have any compression loading. If I were to flip the illustration upside down the penguins would auger into the center of the Earth and that would be very sad.

Since the use of halyards is motivated by the desire to hoist sails from deck level the great majority of halyards go up the mast and then back down to somewhere near the deck. In the case where all the forces are in equilibrium (eg. the sail is hoisted) the tail of the halyard will have a force on it equal to the last part of the tackle used to hoist the sail. If the halyard is held fast, belayed or goes through a turning block anywhere near the mast that force will compress the mast.

Tom.
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Old 23-11-2011, 14:14   #39
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

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In the second illustration Tinkerbell has given one of the penguins the ability to fly (or perhaps has just ignited it) and it is just able to stay aloft and support the other penguin at a 90 degree angle. In this case the the mast is supporting one full penguin and about seven tenths of the second penguin...
Ooops. Either 45 degrees or no tenths. Sorry about that.

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Old 23-11-2011, 14:17   #40
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

So will rocket-propelled penguins replace winches?
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Old 23-11-2011, 14:32   #41
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

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So will rocket-propelled penguins replace winches?
Only in the Southern Hemisphere. In the North puffins will be used. I'm told that Harken is working on high end igniter pixies

Actually, reading over my post there are a number of technical errors beyond the horrible one I already pointed out. But the thrust of the thing is correct.

Oy. And now, back to your originally scheduled cruising thread...

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Old 27-11-2011, 20:44   #42
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Originally Posted by Sparohok
This one is always a brain teaser. But I've been through it enough times with enough experienced racers and riggers to be sure. Herkam is right. A 1:1 halyard puts more compression on the mast than a 2:1 halyard.

Here's the way I've always thought about it, if it helps. First imagine the situation with mainsail hoisted to a halyard lock. There is no tension on halyard tail. Mainsail luff tension F is a compression force between the masthead halyard lock and the gooseneck (assuming slack cunningham, mainsheet, and vang). Mainsail exerts no compression on any other part of the mast.
.

Martin
I think we are having a terminology problem to top off everything else. Probably my fault. I'll take the hit. What about these definitions.

Mast Compression Loads - those forces that leave the mast and are transmitted to the boat, classically through the compression post, or keel if keel stepped and the shrouds and stays. These loads are transferred out of the mast and into the boat. Thesse forces include fore, back, and running stays, shrouds and any halyard that "leaves the mast" and is attached to the boat.

Mast Forces(?) - forces that remain in the mast. Imagine the halyard does not run to the cockpit. There is a sheeve at the bottom of the mast and the halyard cleats on the side of the mast - i.e. There is no connection to the deck.

In this case if there were a 16:1 purchase between the tack and the head and you pulled 100 pounds there would be 1600 pounds between the head and the tack tending to load the mast. There would be 100 pounds on the lower sheave and 100 pounds on the cleat on the mast. However the 16:1 block is attached to the masthead there is 1600 pounds pulling on the top of the mast.

The point about the "last sheave" at the top of the mast, the bottom sheeve and the cleat all having an addtional 100 pounds is fine. This of course would not be the case if the block system cleated internally to the block system.

This is how most boom vangs are set up. Mine is 8:1 and cleats at the last block. No additional loads leave the vang system. Same for a lot of mainsheet systems outhaul etc.

Note, of course, that these loads go to the boat in most modern set ups where these lines are run to the cockpit "piano" system.

I think we all agree what is going on but we are talking about different system points. A system, of course, being the two points between which the loads are measured.
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Old 28-11-2011, 20:05   #43
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

Looks like everyone's gone.

I see your distinctions, but I think what you have labeled "mast compression" would really be mast thrust, or force on the step,which is important, but not terribly difficult to deal with, in an engineering sense.

What you have labeled as mast forces(?) is the force trying to compress the mast, which as you earlier noted, is a combination of downward forces from everything-- shrouds, luff, halyard (even if terminated on the mast). This is what can buckle the mast if it is excessive or the rigging to include spreaders, is insufficient to keep it in column. I'd say mast compression is the total force trying to compress the mast section.
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Old 28-11-2011, 20:47   #44
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

I'm here. And I apologize if my last effort was over the top and also apologize because it was sloppy. Certainly my take is that mast compression best describes the compression loads that the mast itself has to take rather than the rigging and mast loads that the hull structure sees. But I can see how the term could be confusing.

Tom.
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Old 02-12-2011, 13:26   #45
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Re: 42' Leopard - Do I Really Need a Double Halyard ?

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Only in the Southern Hemisphere. In the North puffins will be used. I'm told that Harken is working on high end igniter pixies
Time to stock up on penguin fuel. er, food. Do they like chilli?
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