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    Posted: 14 Jan 19 at 11:15am
Picture below shows purchase system for applying tension to cap shrouds. Purple is attached to shroud ends, orange is fixed to mast foot. Red circles are sheaves. Green is one line, and black the other, both dead ended at the caps (basically a cascade). The black line cleats at the top after rounding the top sheave. 

What purchase is this system? 
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Posts: 382 		Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 11:27am
5:1 I think
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I think that's 4:1. Green and black both give 2:1. Multiply them together = 4:1.
Nick
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I'm having trouble believing my calcs, but it seems like it might be 2:1 -> 3:1 depending on the angle pulling down on the tail. The other way up it would be 4:1. The "cascade" doesn't seem to be doing anything if I read it right. But I'm not at all sure I am correct.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 11:50am
If it were the other way up, then it would be a 4:1 cascade. 2:1x2:1. That much I'm sure of.

But... the line then rounds the moving part. In my mind this adds another purchase. The cleat is on this moving part, but, the sailor is moving that part relative to the mast foot / hull. So, the cleat is at 4:1, but the sailor when using it realises a 5:1...

If you measure the velocity ratio relative the mast foot (orange), the black lines move 5cm for every 1cm that the caps move (purple).

I remember in Topper (just), that the kicker was 3:1. But, if you mounted it upside down, so the cleat was on the boom, then it added a purchase. It feels like the same is happening here. 
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 12:13pm
Originally posted by mozzy

If you measure the velocity ratio relative the mast foot (orange), the black lines move 5cm for every 1cm that the caps move (purple)


That's not what I'm getting, so maybe I've finally lost it!

Lets say that the distance between sheaves is 10cm as drawn. If you extend the black line to level with the lower sheave then the total length of the black line is 30cm.
OK, now lets pull it down to 5cm between sheaves. The mobile sheave is now level with the top, so there is ~0cm between sheaves. So the end of the black line is now, according to my sums, 30cm below where the bottom sheave was. Or have I lost it and need to construct a model?
I had lost it. The bottom sheave doesn't move. So 20cm of line moves through the top sheave,and the top sheave moves down 5cm, so the end of the black line is 25cm below where it was. 5cm of movement of the system as a whole, 25cm on the bitter end - 5:1 purchase.

Edited by JimC - 16 Jan 19 at 5:29pm
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 12:29pm
Well, you'd never be able to get the top and bottom sheaves block to block, because it's a cascade. 

You could get the cascading block to block out at the top sheave though. 

So, say the distance between the top and bottom sheave is 10cm when fully extended (cascade block at bottom). You would have 10cm of green lines, and 20cm of black line in the system (but 30cm of black line measured back to fixed lower block). The top and bottom sheaves can only get 5cm apart before the cascade blocks out. At which point, 20cm of black line will have passed round the top sheave and through the cleat. That gives you the 4:1 of the cascade. 20cm of line for 5cm of movement...  

But, that's measuring the amount of black line that moves through the top sheave. The top sheave is moving closer to the fixed bottom block as well! 

So... measuring relative to fixed bottom block, 20cm will pass it, as 20cm comes round the top block. But that top block is now also 5cm closer to bottom block itself. So that's a further 5cm of black line that moves down past the fixed bottom block. Add that on = 25cm of black line moves past the fixed bottom block when the top block is moved 5cm closer.  25:5 = 5:1.


Edited by mozzy - 14 Jan 19 at 12:34pm
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Posts: 3048 		Post Options Post Options   Quote jeffers Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 12:41pm
Pretty sure that is 5:1, I have a very similar setup on my cunningham.
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Posts: 382 		Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 1:27pm
The way that I got 5:1 was looking at the number of black ropes that are applying a downward force (3) and the number of green ropes applying a downward force (1) . the load in the green rope is double that of the black rope so we now have a downward total force of 3+(2x1)= 5 times the load in the black rope
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 1:35pm
Yup, that makes sense too. 
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