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Old 11-04-2015, 15:09   #1
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Reeving a Tackle to Advantage -- What am I Missing?

I've been reading this:

Purchase & Tackles | 4th Gillingham Sea Scouts

Which has left me thoroughly confused .

How the hell does this work:

Click image for larger version

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Why would it make any difference, from what direction you shorten the distance between the anchor points? uzzled: It makes no sense whatsover to me -- can anyone illuminate the question?
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Old 11-04-2015, 15:49   #2
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Re: Reeving a Tackle to Advantage -- What am I Missing?

It all depends on which end you want to move

Consider the force applied on the two anchor points (trolley and fixed). It is proportional to the number of falls at that end of the system since the tension is all falls is the same.

In the top figure, there are two falls at the trolley and three at the fixed point so the force on the trolley is less than the force on the fixed point. You are putting more effort into fulling the fixed point out of the ground than you are into moving the trolley.

In the second, there are more falls on the trolley end so the load on that end is greater, you are putting more of your effort into moving the trolley and less into pulling the fixed point out of the ground.

It will require less effort in the second situation to move the trolley.

Edit: Think about rigging the same thing between two fixed points and then pulling the rope. The attachment point with the most falls will always fail first because it has more force on it.
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Old 11-04-2015, 15:58   #3
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Re: Reeving a Tackle to Advantage -- What am I Missing?

Consider the block that is connected to the boat in your illustration at the top, rove to disadvantage. Assume the load of the boat is P. The block to the left has two lines pulling on it, so it can move the boat with a line tension of P/2. This is a mechanical advantage of 2. That is, the line would be pulled forward 2 feet for every foot that the boat was pulled forward.

Now look at the lower illustration, rove to advantage. The block to the left has three lines pulling on it, so the block can move the boat with a line tension of P/3. The mechanical advantage is 3 and the line would have to be pulled 3 feet for every foot that the boat was pulled forward.

A way to think about it is that rove to advantage means that you're pulling on both the load and the block that is connected to the load. When rove to advantage, the required line tension is reduced, but the line has to move further to move the same load.
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Old 11-04-2015, 16:14   #4
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Re: Reeving a Tackle to Advantage -- What am I Missing?

Would it make sense if the text was in German or Russian? Perhaps the perspective would be different?
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Old 11-04-2015, 16:14   #5
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Re: Reeving a Tackle to Advantage -- What am I Missing?

Harken website has a set of diagrams and they are almost correct.

Sorry I cant find my link on their website as I have downloaded images of the ones I need (tripplels with a becket)
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Old 11-04-2015, 16:21   #6
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Re: Reeving a Tackle to Advantage -- What am I Missing?

Here is the Harken link


Harken



At least most of the old codgers that could complain about you reeving the "wrong way" are now dead.


Ooops! I didn't even read the main question! Sorry! The reason is that a pully can becaome a turning block. Turning blocks have no mechanical advantage. So if your pull is only 90 degrees from the rope before the sheave you get no mechanical advantage, but it its 180 degrees you do.
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Old 11-04-2015, 16:34   #7
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Re: Reeving a Tackle to Advantage -- What am I Missing?

I'll be damned! Re-rigged the blocks on my mainsheet a while ago and have had difficulty sheeting in to gybe - never occurred to me that I'd reeved my sheet to disadvantage (wouldnt have even known what that meant). Still learning, maybe I'll know it all just before I die...
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Old 11-04-2015, 16:40   #8
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Re: Reeving a Tackle to Advantage -- What am I Missing?

Quote:
Originally Posted by charliehows View Post
I'll be damned! Re-rigged the blocks on my mainsheet a while ago and have had difficulty sheeting in to gybe - never occurred to me that I'd reeved my sheet to disadvantage (wouldnt have even known what that meant). Still learning, maybe I'll know it all just before I die...
It's quite common to reeve main sheets to disadvantage deliberately, especially with high VRs. It's a trade off between slightly easier sheeting in versus more convenient location of the running end/jammer down by the traveller. (
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Old 11-04-2015, 17:07   #9
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Re: Reeving a Tackle to Advantage -- What am I Missing?

"In the top figure, there are two falls at the trolley and three at the fixed point so the force on the trolley is less than the force on the fixed point. You are putting more effort into fulling the fixed point out of the ground than you are into moving the trolley."

This is backaswwards, the force on both objects remains the same and equal. The drawing isn't very good at explaining it either. The disadvantaged shouldn't show the end if the line leading toward the fixed block as at the same angle they would provide the same purchase.
If it was rove to disadvantage the purchase would be 2:1 if the block acted as a turning block. As it's drawn the purchase in both systems would be 3:1 when the direction of pull is along the line of the two blocks and 2:1 when it's at right angles and somewhere close to 2.5:1 if the line exits at 45 degrees.
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Old 11-04-2015, 17:18   #10
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Re: Reeving a Tackle to Advantage -- What am I Missing?

Quote:
Originally Posted by monte View Post
"In the top figure, there are two falls at the trolley and three at the fixed point so the force on the trolley is less than the force on the fixed point. You are putting more effort into fulling the fixed point out of the ground than you are into moving the trolley."

This is backaswwards, the force on both objects remains the same and equal. The drawing isn't very good at explaining it either. The disadvantaged shouldn't show the end if the line leading toward the fixed block as at the same angle they would provide the same purchase.
If it was rove to disadvantage the purchase would be 2:1 if the block acted as a turning block. As it's drawn the purchase in both systems would be 3:1 when the direction of pull is along the line of the two blocks and 2:1 when it's at right angles and somewhere close to 2.5:1 if the line exits at 45 degrees.
Finally something which makes sense. Thanks.

So the way it's drawn, there's no difference.
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Old 11-04-2015, 17:19   #11
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Re: Reeving a Tackle to Advantage -- What am I Missing?

Quote:
Originally Posted by StuM View Post
It's quite common to reeve main sheets to disadvantage deliberately, especially with high VRs. It's a trade off between slightly easier sheeting in versus more convenient location of the running end/jammer down by the traveller. (
exactly why i did it, just seemed tidier - just never occurred to me that there was a leverage involved - I'll have to go back to the old, untidy setup.
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Old 11-04-2015, 17:27   #12
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Re: Reeving a Tackle to Advantage -- What am I Missing?

Quote:
Originally Posted by Dockhead View Post
So the way it's drawn, there's no difference.
Actually, it is the way it's drawn, but it does make a difference.

Very simply, the mechanical advantage can be determined by counting the number of lines to and from the movable block. In this example, when reeved to advantage, it's three total lines, hence 3:1. It has nothing to do with any "angles" of pull. All this assumes, from high school physics, a massless rope and frictionless pulleys.....

Dave
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Old 11-04-2015, 17:33   #13
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Re: Reeving a Tackle to Advantage -- What am I Missing?

Quote:
Originally Posted by monte View Post
The drawing isn't very good at explaining it either. The disadvantaged shouldn't show the end if the line leading toward the fixed block as at the same angle they would provide the same purchase.
If it was rove to disadvantage the purchase would be 2:1 if the block acted as a turning block. As it's drawn the purchase in both systems would be 3:1 when the direction of pull is along the line of the two blocks and 2:1 when it's at right angles and somewhere close to 2.5:1 if the line exits at 45 degrees.
Sorry Monte. That is totally incorrect. You are confusing lots of readers.

Several members have explained it correctly, but a picture tells a thousand words. I will draw a diagram in a sec that should make it clear .
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Old 11-04-2015, 17:41   #14
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Re: Reeving a Tackle to Advantage -- What am I Missing?

https://en.wikipedia.org/wiki/Block_and_tackle
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Old 11-04-2015, 17:53   #15
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Re: Reeving a Tackle to Advantage -- What am I Missing?

Quote:
Originally Posted by Seaworthy Lass View Post
Several members have explained it correctly, but a picture tells a thousand words. I will draw a diagram in a sec that should make it clear .
Same diagrams as in Dockhead's first post have been drawn below, showing the purchase more clearly:
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