What's my Purchase? - SailNet Community
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What's my Purchase?

I've got the stock mainsheet rigging on my Javelin, but it's in this funky configuration, and I just can't wrap my head around the blocks to the point that I can calculate my purchase. Can one of you rigging gurus make sense of this:

(I whipped it up in photoshop )

I'm betting it gets me some mechanical advantage 2:1? 3:1? But I'd love to be able to cite a number when chatting with my sailing buddies, you know?

Thanks!
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Old 09-08-2010
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2:1
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Thanks for the quick answer, davidpm. Now, can someone lay a bit of theory on me so I can explain this if someone asks?
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Old 09-08-2010
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You've actually got closer to a 3:1 purchase, rather than just a 2:1. If the lines came straight down to a block with cam cleat at the transom rather than further forward, it'd clearly be a 3:1 purchase, but since it comes forward a bit, you're losing some leverage, as the lever arm is shorter compared to the other lines.

Basically, to figure out the purchase, you need to look at how many lines are attached to the moving part of the tackle, which is the boom in this case. There are three lines... so the purchase is almost 3:1.

See the diagram here:

Of course, the diagram in question is actually upside down for our purposes... since the boom is pulling away from the cockpit. Of course, another way to measure it is to see how much the boom moves when you haul in 3' of line. If it moves about a foot, it is 3:1, if it moves about 18", it is a 2:1.

Sailingdog

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Telstar 28
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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sailingdog is offline
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That's awesome, sailingdog. Thanks for the rule of thumb, and the practical test. Next time I get her rigged, I'll take a photo, and update y'all on the results.
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Old 09-08-2010
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I'm going with 2:1 as drawn... the two blocks on the transom, and the two blocks on the boom add nothing to the purchase as far as I can see....

(I know.... defy the Dog at your own peril.... )

Edit... never mind... (sigh)...

Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)

Last edited by Faster; 09-08-2010 at 01:37 PM.
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Old 09-08-2010
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Faster—

As I said, if all three lines lead to a single double block or fiddle block at the end of the boom, it would be a 3:1 pretty clearly. The line going forward to the block mid-boom does add some purchase—since the two blocks effectively act as a single large diameter block, but probably not as much as if it was at the boom's end.... mid-boom sheeting gives less leverage than end-boom sheeting. That is why I say it is closer to a 3:1, but not quite 3:1.

Sailingdog

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Telstar 28
New England

You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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