Purchase ratios
Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=576
Printed Date: 13 Aug 25 at 3:19pm
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com
Topic: Purchase ratios
Posted By: Guest
Subject: Purchase ratios
Date Posted: 24 Mar 05 at 8:57pm
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Have a look at the below picture; The top system is the standard 4:1 purchase on the Musto Skiff, the blocks hook into two loops one each side of the sail (same as a 29er & 49er). What is the purchase ratil of system A & B? In in the case of B is the purchase ratio the same on starboard and port? There must be some mechanics graducates out there ... regards,
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Replies:
Posted By: ioanlavery
Date Posted: 24 Mar 05 at 9:07pm
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can i ask why the interest in getting more purchase? I'm guessing that this controls the cunningham right? ------------- aka YoYo |
Posted By: Guest
Date Posted: 24 Mar 05 at 9:10pm
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For the record I believe A= 8:1 & B=6:1 and that B offers the same advantage both sides. See below; But I am open to correction...
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Posted By: Guest
Date Posted: 24 Mar 05 at 11:05pm
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Yep - cunningham. In a breeze you use loads and an extra purchase makes it easier to uncleat at the windward mark as there is less tension in the rope. Class rules limit this to 6:1 Rick ------------- |
Posted By: Granite
Date Posted: 25 Mar 05 at 1:17pm
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With B I think that you would have different loads going through each of the hooks which may give problems depending on how they attach to the sail. How much space is there between the hooks and the deck and what sort of fittings are there on the deck to attach to? ------------- If it doesn't break it's too heavy; if it does it wasn't built right |
Posted By: JimC
Date Posted: 25 Mar 05 at 1:51pm
| Yeah, not the cleverest of rules that one... Its desirable to have equal loads on both sides of the sail and quite challenging to do that with a 6:1 purchase. All you could do I think would be to have a 3:1 each side of the sail, giving a 12:1, then have a 1:2 maybe on the lead out to each side to bring it down to 6:1 again. If 4:1 is inadequate I think you should start a campaign to change the rule to 8:1 limit: probably be cheaper than 6:1! |
Posted By: Granite
Date Posted: 25 Mar 05 at 2:00pm
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6:1 is tricky if you want to get the tail out to both sides. The only thing that I can think of to give you feed out to both sides is a straight strop from each hook to a moving tripple block in the center then double block at base of mast as a turning for the purchase giving the whole thing as a 6:1 and then two singles to take the line out to each side. ------------- If it doesn't break it's too heavy; if it does it wasn't built right |
Posted By: Granite
Date Posted: 25 Mar 05 at 2:26pm
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Thought I would draw what I was thinking about
It gives the 6:1 in a small space so you should not have any problems with range but is is expensave in blocks ------------- If it doesn't break it's too heavy; if it does it wasn't built right |
Posted By: Stefan Lloyd
Date Posted: 25 Mar 05 at 2:32pm
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I think those ratios are correct. The advantage must be the same on both sides. It is the same bit of string and therefore the tension in it must be the same over its whole length. |
Posted By: ianwat2212
Date Posted: 25 Mar 05 at 5:02pm
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Only if it's inelastic (but thats just being pedantic) ------------- Fireball RSA 14723 Simonis 35 "Scarlet Sun" SA 1500 Royal Cape Yacht Club |
Posted By: Guest
Date Posted: 25 Mar 05 at 5:31pm
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The kicker and boom gets in the way of creating a balanced 6:1 so B seems the best option. Most people are happy with the 4:1 but i'd like a bit more. I think if the rule went to 8:1 people would start destroying the sails!! I was going to test this today but now wind at WHitstable Rick ------------- |
