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Sail Measurement

Posted: 02 May 2009, 06:28
by Graham Herbert
Can anyone tell me the proper way to determine the quarter, half and three quarter height measurement points on the leech of the mainsail and how to take these width measurements. Thanks, Graham

Posted: 02 May 2009, 07:37
by Bruce Andersen
Fold the head point down to the clew point - the crease is the 1/2 leech.
Then take the head point down to the half point - the crease is the 3/4 leach.
Then take the clew point up to the half point - the crease is the 1/4 leach.

The width measurements are from those leech points to the leading edge of the sail.

Posted: 02 May 2009, 21:19
by JThompson
Once you have found the leech points, the width measurements are the SHORTEST distance from the respective leech point (crease) to the luff. You swing an arc and find where the distance to the luff is the shortest.


Posted: 03 May 2009, 18:20
by Don Case
Hi all
A related question. The top width can be 20 mm, when I am folding the sail to determine the leach points do I fold the clew up to the luff end of the top width or the leach end? It won't make a lot of difference but I might as well be right. Could you also point me to the rule that determines this?

Posted: 03 May 2009, 20:07
by Hiljoball
Hi Don,

As the class rule does not specify it exactly, you need to refer to the Equipment Rules of Sailing (The ERS) that may be downloaded from the ISAF web site.

Read section G.

I think the answer is

G.7.2 Leech Length
The distance between the head point and
the clew point.

Also see G7.4

This suggests that the leach be divided based on the head point and that is defined in section G as the point were the luff intersects the head reinforcement.

There is another point called the peak point (G4.5) and that is the projection of the leach and luff and is higher than the head point.

It's a bit confusing.

Posted: 04 May 2009, 00:09
by Jan Dejmo
Hi Don

The introduction of the One Metre class rules state: The rules in Part II are closed class rules. Measurement shall be carried out in accordance with the ERS except where varied in this Part.

A.5.2 Except where used in headings, when a term is printed in “boldâ€

Posted: 04 May 2009, 04:57
by Bruce Andersen
the short answer is the head point is the top of the sail at the luff.

Posted: 04 May 2009, 07:19
by Jan Dejmo
Yes :)


Posted: 04 May 2009, 09:37
by CHATIN Achille
I am feeling better, as the way to measure main sails explained in this topic is the way I was used to do it!

Unfortunately it is clear that everybody doesn't understand it in that way, as it is not obvious at first glance!

In fact, there are 2 things in particular to be carefull at :
- the head point, that must be taken from the corner of the top of the sail at the luff (as well as for the jib)
- the first & third quarter points at the luff : they must be determined by measuring the half way between sheet point/head point and the two quarter LUFF point.

Furthermore, as english is not the native language of many IOM sailors, I would recommend to insert in the IOM Class Rules a sail measurement diagram, (as this would be the clearest way to show everybody the way to measure sails) instead of all the indigest text of measurement of the IOM Class Rules.

Posted: 04 May 2009, 10:20
by Jan Dejmo
Achilles recommends diagrams and I fully agree. Diagrams are often a very good way of illustrating something that can seem very abstract when explained in text and that is why there are so many diagrams in the ERS.

I am sure that Bruce means the right when stating that "the head point is the top of the sail at the luff", but as I do not have English as my mother language I read it as the head point is a point on the actual sail while it in reality could be a point in the air. It depends on how the sailmaker has cut the sail at the head.

I am equally sure that Achille means the right thing when stating that the three-quarter point is a point "half way between" head point and the half leech point. However I read this as the three-quarter point being a point on a straight line between the head point and the three-quarter point while it is the point on the leech where the distances to head point and the half leech points are the same length.

I have probably increased confusion by explaining these two terms in my own words and it is therefore a good principle in rules not to restate other rules, but to refer to them.

Please take a look at the relevant sail diagrams in the ERS and I think you might find that these make the definitions much easier to understand than the texts of some are.

Posted: 04 May 2009, 18:17
by Don Case
Thanks to all of you for the explanations. I was doing it right all along but was't really confident. We all know that a picture is worth a thousand words but in this case a good diagram could double that. It is actually a very straight foward method if you were showing someone but to try and put it into words fails miserably. If I hadn't noticed the orientation of the little dimension lines on the ERS drawing I would still be scratching my head. My vote is for a nice clear diagram to be included in the IOM rules.

Posted: 05 May 2009, 11:43
by Lester

Posted: 06 May 2009, 09:58
by CHATIN Achille
Thanks Jan, very instructive document!

Here is the Naviga 1 Meter Certificate, it may illustrate the way to make a simple document for measuring sails & rigs (and anything else that could be simplified in the IOM Measuring Rules). Although, mainsails measurement way is not clear enough, they do measure it in correct way (I've learn it with them in 2003 as measurement trainee at international event, as all boats of 3 classes were thoroughly checked)

Lester, thanks also. some years ago I've seen that doc. in your "bible" ; very helpfull!