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Old 12-04-2015, 06:17   #1
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Double Purchase Through Single Low-Friction Ring

Does anyone see a problem running more than one line through a single low-friction ring, in order to increase purchase?

I suppose I could rig a second ring, but I hardly see the point -- the lines will run only when adjustments are made. There will be some chafe of one loop against the other.

This is for my new twing. It will look somewhat like this:

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In the photo, the twing makes a single loop through, giving 2:1 purchase. I would like to double that to give 3:1, but that would involve two loops of the twing line butting against each other in the ring.

I will be using not a simple stainless ring like in the photo, but something like this:

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Old 12-04-2015, 06:39   #2
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Re: Double Purchase Through Single Low-Friction Ring

Every time I get fancy and try two purchases through a ring one part of the line overrides the other and destroys by friction any hope I had of increasing my purchase.


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Old 12-04-2015, 06:57   #3
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Re: Double Purchase Through Single Low-Friction Ring

What is the block fixed to the track? A low friction ring as well?
I guess one of these at the track end and a standard ring at the other end with the line terminating on the upper ring would give 3:1
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Old 12-04-2015, 07:07   #4
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Re: Double Purchase Through Single Low-Friction Ring

Quote:
Originally Posted by monte View Post
What is the block fixed to the track? A low friction ring as well?
I guess one of these at the track end and a standard ring at the other end with the line terminating on the upper ring would give 3:1
Attachment 100276
Yes, also a low friction ring, attached to a tang on the plunger car.

The end of the twing will be spliced around the outside of the upper ring. It will go down and loop through the lower ring, then back up through the upper ring. Then back down through the lower ring once more.

I think that gives 3:1. But there will be two loops through the lower ring with the loops rubbing against each other. My question is whether that is a problem or not -- I'm thinking not, but interested in views.



The Figaro race boat which I saw which had a similar arrangement had a huge multiple purchase -- a cascade of these rings along the doghouse. I don't really want to do it that way, because I want the purchase (or at least the main part of it) near the sheet where everything is stronger. I will lead the twing line to the cockpit by leading it along the rail using stanchion lead blocks (more low friction rings). The turning block (yet another low friction ring) will be on a padeye screwed into my teak cap rail. This will not have nearly the strength of the car, so have to arrange a bunch of purchase before getting there.
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Old 12-04-2015, 07:47   #5
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Double Purchase Through Single Low-Friction Ring

So something like the second drawing? The first would give 3:1 and the second 4:1. I don't know about the friction with 2 lines, it would certainly add some. You could also double the purchase along the toerail run somewhere easily enough by terminating the pulling line with another ring and running a new line to the next staunchion aft, through the ring and back past the staunchion. Maybe this is the setup you mentioned on the race boat?
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Old 12-04-2015, 07:48   #6
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Double Purchase Through Single Low-Friction Ring

Forgot the pic. Click image for larger version

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Nb. I don't think there would be a chaffing problem on the first lower ring because of the different direction of pull for the two lines. Maybe on the second drawing upper ring though...
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Old 12-04-2015, 07:59   #7
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Re: Double Purchase Through Single Low-Friction Ring

Concerning the loads:

The sheet load of the blade at 40 knots is 1,700kgf. At a 60 degree deflection angle (the most we can imagine here), that makes 850kgf 1,700kgf of load on the twing. [edit: I was confusing towing loads with load on the block]

So the load on the car will be 1,700kgf plus 567kgf = 2266kgf, well within the 3.5 ton SWL rating. But the 567kgf load will be exerted from a different direction, so the actual force will be less.

Triple purchase will turn the force on the twing, as it leaves the car, into 567kgf.

Double purchase at the turning block (by anchoring the line on another rail-mounted pad eye) will turn that into 283kgf.

The load on the pad eyes will be 142kgf each plus the the force resulting from turning, which seems to be ok.

The twing purchase will be 6:1 altogether, which will require 6 meters of travel at the end for 1 meter of adjustment range.

The Spinlock XAS clutch is rated at about 500kgf holding power for a 10mm double braid line, which seems to be ok for handling 283kgf of force.

10mm double braid polyester rope is rated at 2700kgf breaking load, which seems ok.

Seems like all the loads are within conservative parameters, doesn't it?
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Old 12-04-2015, 08:01   #8
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Re: Double Purchase Through Single Low-Friction Ring

Quote:
Originally Posted by monte View Post
Forgot the pic. Attachment 100278
Nb. I don't think there would be a chaffing problem on the first lower ring because of the different direction of pull for the two lines. Maybe on the second drawing upper ring though...

The pic is good! Yes, it would be like the left-hand one, so no chafing in the upper ring.

In the lower ring, as one can see from your drawing, the second loop of the twing doesn't loop all the way around.

It seems ok to me.
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Old 12-04-2015, 08:02   #9
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Re: Double Purchase Through Single Low-Friction Ring

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Originally Posted by Sailmonkey View Post
Every time I get fancy and try two purchases through a ring one part of the line overrides the other and destroys by friction any hope I had of increasing my purchase.
Yikes -- that would be a bad scenario.

I guess I could use two rings below, if this happens.
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Old 12-04-2015, 11:47   #10
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Re: Double Purchase Through Single Low-Friction Ring

Would it make sense to use dynema for the forward section to minimise bulk and friction and change to 10mm double braid at the aft ring where line handling becomes an issue?
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Old 12-04-2015, 11:55   #11
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Re: Double Purchase Through Single Low-Friction Ring

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Originally Posted by monte View Post
Would it make sense to use dynema for the forward section to minimise bulk and friction and change to 10mm double braid at the aft ring where line handling becomes an issue?
It's a good question.

I definitely don't need Dyneema for strength -- the loads will not be great with all this purchase.

So cost would tend to indicate polyester double braid. On the other hand, it's such a small amount of cordage that the cost doesn't really make much difference. Maybe if Dyneema is easier to splice, that would be a good reason, plus the slipperiness.

I could go down a size for the forward section to 8mm polyester -- still plenty strong enough.

I'm going to have to do a ton of splicing, so I'm now thinking about that issue, too. I have good instructions from SWL on how to splice single-braid Dyneema (like Amsteel). Is it greatly easier to splice than polyester double braid?
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Old 12-04-2015, 12:06   #12
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Re: Double Purchase Through Single Low-Friction Ring

I'd guess amsteel is a lot easier to splice looking at the finished splices. It's not so much about strength as being able to reduce the line diameter. Probably from 10mm to 6mm. Probably less stretch as well. It may not matter depending on the inner hole size of the low friction ring, remembering you need to fit the sheet as well.
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Old 12-04-2015, 12:20   #13
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Re: Double Purchase Through Single Low-Friction Ring

Quote:
Originally Posted by monte View Post
I'd guess amsteel is a lot easier to splice looking at the finished splices. It's not so much about strength as being able to reduce the line diameter. Probably from 10mm to 6mm. Probably less stretch as well. It may not matter depending on the inner hole size of the low friction ring, remembering you need to fit the sheet as well.
Another issue is that a smaller diameter line will turn better over a given radius ring.

But I'm not sure any of this matters -- 8mm polyester will also be fine.

I think bottom line is probably how hard it is to splice. If Amsteel is a whole lot easier to splice, then that sounds like a winner.
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Old 12-04-2015, 12:29   #14
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Re: Double Purchase Through Single Low-Friction Ring

We are adding a similar purchase system on our 50' to move the lead forward when reefing the 110% genoa. With a sheet load of ~ 5,000 lbs, I am putting a strong bail ahead of the track with a large backing plate to spread the load because the upward load component will be ~ 4,000 lbs. The bail will be f&aft and will have a spacer to keep the lines apart. We will use Dyneema line for its slipperiness (even though the adjustments will be made when the sheet is pretty slack) and a Ronstan Constrictor jammer mounted on the side of the cabin. Our sailmaker is making the ss rings at no cost!
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Old 12-04-2015, 12:37   #15
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Re: Double Purchase Through Single Low-Friction Ring

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Originally Posted by Utahsailor View Post
We are adding a similar purchase system on our 50' to move the lead forward when reefing the 110% genoa. With a sheet load of ~ 5,000 lbs, I am putting a strong bail ahead of the track with a large backing plate to spread the load because the upward load component will be ~ 4,000 lbs. The bail will be f&aft and will have a spacer to keep the lines apart. We will use Dyneema line for its slipperiness (even though the adjustments will be made when the sheet is pretty slack) and a Ronstan Constrictor jammer mounted on the side of the cabin. Our sailmaker is making the ss rings at no cost!
Cool!

Do you have a drawing of your system?

Will you be able to adjust the height of the ring from the cockpit, or not?
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