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Strawberry View Drop Down
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Posts: 1337 		Post Options Post Options   Quote Strawberry Quote  Post ReplyReply Direct Link To This Post Topic: Sail Measurement
    Posted: 25 Jul 06 at 1:55pm

I'm looking at doing some sail measurement, and I have the ISAF sail
measurement spreadsheet to do it. But there's a couple of measurements it's
asking for that I don't really understand.

"luff round offset: This is the offset between AB (as laid out, so B is
probably not on the sail) and the luff at the greatest point."

"luff round length": This is the length from the peak to the point where AC no
longer touches the sail."

"foot round offset: "This is the offset between BC (as laid out, so B is
probably not on the sail) and the luff at the greatest point."

"foot round length": This is the length from the peak to the point where AC no
longer touches the sail."

Does anybody else have any ideas?

Thanks for your help
Cherub 2649 "Dangerous Strawberry
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Bruce Starbuck View Drop Down
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Posts: 124 		Post Options Post Options   Quote Bruce Starbuck Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jul 06 at 3:10pm

That all sounds very strange.

The current ISAF Equipment Rules of Sailing (ERS) is what you need.  It's a really good document which amongst other things, clearly and simply defines all the sail measurement points.

It makes no mention of words like "peak" and "offset". What exactly is asking for these measurements? The class rules or the ISAF document?

If it's the class rules, then you should remember that class rules are a law unto themselves. Some are very good and adhere to the definitions in the ERS (470, 420) and some invent their own definitions, veer wildly off on a tangent and turn something which should be black and white into a million shades of grey. (most of the older UK classes!).

Sounds like this could be what's going on here. In that case, you should put down the ISAF guide and try to interpret what the class rules mean.

 
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JimC View Drop Down
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jul 06 at 6:28pm
Unfortunately ERS isn't anty help for this exercise. This is the ISAF sail measurement rules, which are quite a different animal. They are used for some development classes, Cherub for one, and are a reasonably precise measure of real sail area as opposed to the somewhat nominal measures used by some classes.

At a guess you've got the spreadsheet interpretation of the ISAF rules which I knocked up a few years ago. You'll have trouble unless you read it with teh actual rules, as here, and most especially the digtrams, linked, which should make the arcane numbers a bit clearer.

http://www.sailingsource.com/cherub/sailrule.htm

The key thing here is that you need the sail pegged out on the ground,together with a length of light line. AB is the distance between the mast black bands, and BC the foot one. This diagram should help loads if you don't have it...



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Strawberry View Drop Down
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Posts: 1337 		Post Options Post Options   Quote Strawberry Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 06 at 1:19pm

I think this probably is the spreadsheet you made. It's titled "UK Cherub Sail Measurement Sheet" and already has the data on it for 2641, 2685, and 2637. I got it from Will.

I do not understand AB and BC on a Cherub which has no black bands.
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JimC View Drop Down
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 06 at 1:34pm
Originally posted by Strawberry

I do not understand AB and BC on a Cherub which has no black bands.


Well put them on then. They are required by sail measurement rule 3.2.

If you haven't got any you have to assume the end of the boom, the mast tip and probably the gooseneck (unless you've got a 49er style cuffed sail in ahich case the floor) and your sail will measure bigger than it really is. If a sail sockets onto the mast tip then use the mast tip as the top black band location of course.

Jim C

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Bruce Starbuck View Drop Down
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Posts: 124 		Post Options Post Options   Quote Bruce Starbuck Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 06 at 2:31pm

As I thought, this is a case of class rules creating a complicated job out of what could be a very simple one.

I can now measure a Fireball main in about 20 seconds, because they have converted their old rules to comply with the current terminology of the ERS. Well done the Fireball class!

I suppose a class which specifies a max sail area (like the Cherub) as opposed to one which specifies max individual edge dimensions (like the Fireball) will always need slightly more complicated rules, as they need to include a method of calculating the area.

Have just found the relevant document on the ISAF site, so I can see where this came from. The document is from 1985, and although "official ISAF" seems a bit messy. Surely there must be a way of determining the area of a sail which is more up to date in terms of the nice new ERS terminology.



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I luv Wight View Drop Down
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Posts: 628 		Post Options Post Options   Quote I luv Wight Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 06 at 3:02pm


The cherubs no longer need black bands.

BUT you can't now use this method for cherubs, unless the sail is triangular ( with only a bit of leech round ) which cherubs sails are obviously not.

Currently the approved cherub method is by triangulation -ie succesive triangles to the batten pockets.


The moths used to use this (above )ISAF system, but after similar meas problems, now measure the sail area more 'accurately' using this spreadsheet :
http://www.moth-sailing.org/download/sail-area-form-ver2.xls

This would be also OK for cherub. - be interesting to see the differences.
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Posts: 2415 		Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 06 at 3:40pm

Originally posted by I luv Wight



The cherubs no longer need black bands.

BUT you can't now use this method for cherubs, unless the sail is triangular ( with only a bit of leech round ) which cherubs sails are obviously not.

Currently the approved cherub method is by triangulation -ie succesive triangles to the batten pockets.


The moths used to use this (above )ISAF system, but after similar meas problems, now measure the sail area more 'accurately' using this spreadsheet :
http://www.moth-sailing.org/download/sail-area-form-ver2.xls

This would be also OK for cherub. - be interesting to see the differences.

 

The Moths use something similar to what we use for Cats:

http://www.schrs.com/schrsmeasurement.asp?id=main

 
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JimC View Drop Down
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jul 06 at 3:54pm
Originally posted by Bruce Starbuck

Surely there must be a way of determining the area of a sail which is more up to date in terms of the nice new ERS terminology.


No, there isn't. I looked into it.
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