Keelboat news and development: square top jibs

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square top jibs

Printed From: Yachts and Yachting Online
Category: Keelboat classes
Forum Name: Keelboat news and development
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URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=9009
Printed Date: 04 Aug 25 at 3:05pm
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Topic: square top jibs

Posted By: antc
Subject: square top jibs
Date Posted: 19 Feb 12 at 10:42am

I’ve seen on the Volvo boats and some if the 18ft skiffs they are using square top jibs,

I am wondering, why do they work? is it because they are just bigger, because if so why don’t they just make them taller (move the hounds up). or is it because of the aerodynamic of having a fatter head that gives you more efficient power.

I’m sure there are some sail makers out there somewhere who know.

Replies:

Posted By: Lukepiewalker
Date Posted: 19 Feb 12 at 11:53am

Without looking into it such things are normally an exploitation of the rules. If you are limited in luff length or hounds height then you can sometimes get extra area by squaretops and related shenanigans, subject to rules on reinforcement and the like.

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Ex-Finn GBR533 "Pie Hard"
Ex-National 12 3253 "Seawitch"
Ex-National 12 2961 "Curved Air"
Ex-Mirror 59096 "Voodoo Chile"

Posted By: laser193713
Date Posted: 19 Feb 12 at 2:32pm

An extract from Quantum Sail Design Group talking about IRC optimisation. This could be where the trend has stemmed from.

Our research over the last few years has shown that, for upwind headsails, IRC favors a relatively short luff length relative to the rated forestay length. It also disproportionally penalizes LP (luff perpendicular, measured from the clew to the closest point on the luff). This prompts most teams to go with the smallest No. 1 jib—IRC measures the largest jib in the inventory—they can use without significantly impairing their light-air performance. IRC allows a fairly generous head width on headsails, up to .8 percent of the luff length. On a typical IRC 52, the jib head can be as much as 6 inches wide. A moderate reduction in hoist can lower your rating, while a maximum jib head width will help make up for any lost sail area.

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Posted By: Whirlwind
Date Posted: 20 Feb 12 at 12:01pm

This started off in the old IACC boats as away of trying to get extra area within the rule. They had issues with supporting the big heads they were using (inflatable battens were used) and sometimes extra drag due to the interaction on the vortex leaving the jib head outweighed the extra lift.

The 18's could be using them to try and emulate the square top mains de powering characteristics, I don't think they do it for rule based reasons.

Posted By: fudheid
Date Posted: 20 Feb 12 at 2:20pm

the square top main is mostly to do with drag....... there thats a can of worms opened. If you rread cj marchanj seminal work on how aerodynamics he pointed to this along time ago......

In fact Lionheart the uk Ac boat in the 80's developed the bendy rig to reduce the drag and the eighteens (or maybe the otherway round?) ran with it by taking the head width up past perpendicular there is research to show you cut down drag and hence are faster.

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Cheers you

only me from over the sea......

Posted By: antc
Date Posted: 20 Feb 12 at 3:15pm

is there any litrature out there with sail design and this sort of thing in that anybody can recomend?

Posted By: fudheid
Date Posted: 21 Feb 12 at 11:05am

if you can find it CJ Marchanj - Aero-hydrodynamics of sailing or Sail performance. It is not a glossy book contains large amount of thinking with only simple diagrams - not cheap either.....

But the one who influenced most sail designers

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Cheers you

only me from over the sea......