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Sam.Spoons View Drop Down
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    Posted: 15 Jan 19 at 6:05pm
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Posts: 382 		Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 15 Jan 19 at 6:29pm
I think that the difficult way to look at this is as a mechanism that is moving trying to work out what happens as one piece is moved, relative to the others. it is all a bit confusing. If you consider it as a static structure with the load between the two ends being shared by the various elements in the structure then it is easy to work out what the load is in the line that is being pulled and how this relates to the load applied by the system.
PS The only reason I got this quickly is I have just had a similar "brain teaser" at work and have struggled with it until I got my brain around it. When you do, its obvious and you kick yourself for having taken so long.
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Posts: 3400 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 15 Jan 19 at 6:52pm
I took it that one end was fixed, if that was the top the fall was irrelevant (as I said in my post), what I didn't get was the effect of the top moving (with the bottom fixed) but the black rope also acting on the cascade. Think I have it now (and simply mocking it up and measuring got to the right answer in a few minutes).  Thumbs Up
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 15 Jan 19 at 6:53pm
Originally posted by PeterG

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.
Well, i don't agree. 

Clearly the person hauling on the caps, , the hull and the mast foot don't move in relation to one another when the system is pulled on. When I'm stood on the deck pulling it on, the mast doesn't slide toward me. The only thing which moves in relation to hull, mast foot and myself is the caps. Therefore, it helps to define a system where those are considered fixed and the caps are moving. 

You could say the caps are fixed and calculate around there, but then you'd have to integrate the input force moving toward the fixed object at the same time the secondary part is moving too. It would complicate things further, but you'd still get 5:1. 

Or you could say everything is moving, and use some universal positioning system and account for orbit of earth etc... but when all integrated, you'd still get 5:1. 

And, if the whole thing was the other way up, it would be 4:1, not 6:1. And this has everything to do with the take off coming from the moving end and therefore contributing to force applied to the moving end.   


Edited by mozzy - 15 Jan 19 at 6:57pm
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PeterG View Drop Down
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Posts: 822 		Post Options Post Options   Quote PeterG Quote  Post ReplyReply Direct Link To This Post Posted: 16 Jan 19 at 9:47am
Clearly the person hauling on the caps, , the hull and the mast foot don't move in relation to one another when the system is pulled on. When I'm stood on the deck pulling it on, the mast doesn't slide toward me. The only thing which moves in relation to hull, mast foot and myself is the caps. Therefore, it helps to define a system where those are considered fixed and the caps are moving. 

I hate to admit this, but I think you are right. One end is "fixed" in relation to the applied force, and that's the key to the effect of turning it upside down.
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Posts: 131 		Post Options Post Options   Quote PeterV Quote  Post ReplyReply Direct Link To This Post Posted: 16 Jan 19 at 10:30am
Goodness me, on the 5th page and a lot of people failing on basic mechanics.  As stated by several a fixed point acts only as a turning point and adds nothing to the purchase, in seamanship terminology this is a tackle 'rigged to disadvantage' so it's a 3:1.  A bit of simple rearranging could easily make it a 6:1.
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Posts: 3400 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 16 Jan 19 at 10:33am
Originally posted by PeterV

Goodness me, on the 5th page and a lot of people failing on basic mechanics.  As stated by several a fixed point acts only as a turning point and adds nothing to the purchase, in seamanship terminology this is a tackle 'rigged to disadvantage' so it's a 3:1.  A bit of simple rearranging could easily make it a 6:1.


But we have already established that is it neither 3:1 or 6:1? The tackle drawn gives a 5:1 advantage both in theory and by experiment.




Edited by Sam.Spoons - 16 Jan 19 at 10:33am
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Posts: 476 		Post Options Post Options   Quote Granite Quote  Post ReplyReply Direct Link To This Post Posted: 16 Jan 19 at 12:44pm
Originally posted by Sam.Spoons

Originally posted by PeterV

Goodness me, on the 5th page and a lot of people failing on basic mechanics.  As stated by several a fixed point acts only as a turning point and adds nothing to the purchase, in seamanship terminology this is a tackle 'rigged to disadvantage' so it's a 3:1.  A bit of simple rearranging could easily make it a 6:1.


But we have already established that is it neither 3:1 or 6:1? The tackle drawn gives a 5:1 advantage both in theory and by experiment.



The system as shown is 5:1 but by using the same blocks in a different arangement you could get 6:1 without any change in range. So unless there is a rules mandating a max of 5:1 I don't know why anyone would rig it this way.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 16 Jan 19 at 1:31pm
The rules mandate it is 4:1... yet this is how it is supplied by RS / Selden and how nearly everyone in the class has it rigged. 

So somewhere between the supplier and the person who wrote the class rules there has been a mistake. 

Perhaps the class felt the need to specify a purchase to stop people over tensioning the uppers and damaging their masts? I've not been in the boat long enough to know the history. I don't think running more tension on the uppers would be an advantage, so I don't really see the point in the rule.   

But, the current system rubs on the mast. There is a steel protective plate but this comes off all the time. So I was thinking if something kinder on the mast could be rigged up, and that when I noticed the rule.  
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Posts: 219 		Post Options Post Options   Quote ohFFsake Quote  Post ReplyReply Direct Link To This Post Posted: 16 Jan 19 at 2:38pm
3:1

Really it's a 2:1 purchase but with the tail led to advantage so the overall purchase is 3:1

Turn it upside down and it would be 4:!




Edited by ohFFsake - 16 Jan 19 at 2:40pm
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