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Granite View Drop Down
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    Posted: 14 Jan 19 at 9:24pm
I work it out as 5:1 also. However if you turn the green line the other way up i.e through the upper ring first 2:1 and then do the black line from the floating ring down through the bottom up to the floating and to your hand that gives 3:1. So a total of 6:1 with the same range and number of blocks as before.
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Posts: 106 		Post Options Post Options   Quote pondlife1736 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 9:46pm
Originally posted by mozzy

Originally posted by pondlife1736

Don't think it matters which end is fixed.

I think the reason it matters is because if the top was fixed, then the 10kg you pull on the fall line would not count in the system.  


It certainly would, assuming frictionless pulleys. It just depends on your frame of reference. Remember Newton - every action has an equal and opposite reaction, and therefore the shroud and base forces must be equal and opposite otherwise the system will not be in equilibrium
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Posts: 106 		Post Options Post Options   Quote pondlife1736 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 9:50pm
Originally posted by 423zero

I was always told number of strings not counting one in your hand and cascade doubles, so 4to1 under that system.


That's only valid if all strings have the same tension. In the example given, the green carries 2x the black tension.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 10:16pm
Originally posted by pondlife1736

Originally posted by mozzy

Originally posted by pondlife1736

Don't think it matters which end is fixed.

I think the reason it matters is because if the top was fixed, then the 10kg you pull on the fall line would not count in the system.  


It certainly would, assuming frictionless pulleys. It just depends on your frame of reference. Remember Newton - every action has an equal and opposite reaction, and therefore the shroud and base forces must be equal and opposite otherwise the system will not be in equilibrium

Well, no, really no. Yes, I get that the holding tension in the line stops the whole thing from unravelling. But whether that line comes directly from the object you are working (moving in relation to yourself), or from a fixed pulley (not moving in relation to you) does change whether that counts as a purchase or not. 

The reference frame is the mast foot. You could say the caps are fixed and that's the reference. But then you could have account for the mast, boat and person pulling the line all moving... but the result would still be 5:1.

Imagine a simple block on the ceiling. Your lifting a weight, with a line tied to it. The rope goes up to pulley on the ceiling and down to you. That is a 1:1 purchase. There is no mechanical advantage (see A below). 

Now, dead end the rope on the ceiling, take it around a pulley attached to the weight, and lift up with rope. You now have 2:1 (b). 

So, it really does matter whether that line you're pulling is coming from the working end (moving in relation to oneself) or a fixed point (relative to oneself, like the ceiling).




Edited by mozzy - 14 Jan 19 at 10:18pm
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Posts: 106 		Post Options Post Options   Quote pondlife1736 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 10:32pm
Originally posted by mozzy

Originally posted by pondlife1736

Originally posted by mozzy

Originally posted by pondlife1736

Don't think it matters which end is fixed.

I think the reason it matters is because if the top was fixed, then the 10kg you pull on the fall line would not count in the system.  


It certainly would, assuming frictionless pulleys. It just depends on your frame of reference. Remember Newton - every action has an equal and opposite reaction, and therefore the shroud and base forces must be equal and opposite otherwise the system will not be in equilibrium

Well, no, really no. Yes, I get that the holding tension in the line stops the whole thing from unravelling. But whether that line comes directly from the object you are working (moving in relation to yourself), or from a fixed pulley (not moving in relation to you) does change whether that counts as a purchase or not. 

The reference frame is the mast foot. You could say the caps are fixed and that's the reference. But then you could have account for the mast, boat and person pulling the line all moving... but the result would still be 5:1.

Imagine a simple block on the ceiling. Your lifting a weight, with a line tied to it. The rope goes up to pulley on the ceiling and down to you. That is a 1:1 purchase. There is no mechanical advantage (see A below). 

Now, dead end the rope on the ceiling, take it around a pulley attached to the weight, and lift up with rope. You now have 2:1 (b). 

So, it really does matter whether that line you're pulling is coming from the working end (moving in relation to oneself) or a fixed point (relative to oneself, like the ceiling).



OK I see where you're coming from, I'd assumed the line was cleated, so all forces reacted between shroud and deck. If you hold the fall then the 10kg or whatever is reacted through your weight as you become part of the system, and there is consequently 10kg less at the deck. 
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Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 10:36pm
Originally posted by Do Different

After all that I'm inclined to think the system purchase is 4:1, that is to say the tension being applied to the shrouds is four times the effort applied to the black line.

Surely: to be in balance forces are opposite and equal. Therefore your later diagrams showing 50kg of tension above the system and 40kg below the system cannot exist.

I couldn't work out what was happening and made the same assumption so I built a test rig and measured it. And, indeed the advantage was 5:1. Pics on page 2 in my earlier post.


Edited by Sam.Spoons - 14 Jan 19 at 10:37pm
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 10:40pm
That's what makes this system difficult to understand. The cleat is in the middle of the 'system'. Usually our control lines in the boat are lead to a fixed block / cleat in the boat where we adjust the system from. Yet, in this case, the cleat is on the moving part. Therefore when you are working the line, the line in your hand becomes part of the purchase system. 

That and the cascade dead ends are also on the moving part. 

Makes it all a bit weird to look at! I know. I've been looking at it all season thinking it can't be 4:1! But I only recently came to thinking about altering it (not because of purchase, but because it rubs on the mast)... but that got me looking at the class rules and thinking it through, then of course posting hear to have my thinking challenged!
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Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 11:01pm
Originally posted by mozzy

That's what makes this system difficult to understand. The cleat is in the middle of the 'system'. 

I should have got this at the beginning now I think about it as I used a system with a moving cleat on my Raceboard luff downhaul setup (effectively a cunningham) which was a 16:1 dual cascade setup. Though it was two separate 4:1s and didn't have the two tails attached to the moving block which is, I suspect, what threw me....... . Embarrassed
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Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 11:04pm
Originally posted by mozzy



This explains the above scenario extremely well  Wink http://www.youtube.com/watch?v=zZUJLO6lMhI
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Posts: 822 		Post Options Post Options   Quote PeterG Quote  Post ReplyReply Direct Link To This Post Posted: 15 Jan 19 at 6:01pm
Originally posted by mozzy

Originally posted by pondlife1736

Don't think it matters which end is fixed.

I think the reason it matters is because if the top was fixed, then the 10kg you pull on the fall line would not count in the system.  

It depends on your definition of "fixed". Quite clearly neither the top or bottom are "fixed" in this example, the aim of the purchase is to bring them closer together, as far as the purchase is concerned is doesn't matter which "moves" - though of course they both will, even if one moves less than the other.

The reason it's 5:1 not 6:1 has nothing to do with that, but that the last pull is effectively pulling on the top block and not acting via the cascade - as the diagrams show.
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