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14 Forums • Jib Clews - how to measure them consistently
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Jib Clews - how to measure them consistently

Posted: Thu Sep 26, 2013 8:22 pm
by Haywire
Hi All,

Here's a topic that should generate some healthy discussion...

Several but not all of the national class measurers were in attendance at the recent worlds in Toronto. After the opening ceremonies, we met in the RCYC bar and discussed the challenge of consistently measuring the sail area of headsails given the creative jib clews that have appeared in the last few years.

Here's the problem:
The preamble to our class rules says "...ISAF Sail Measurement instructions shall not apply. The ISAF Equipment Rules of Sailing definitions shall apply to words in bold text. They shall be used in the sail measurement processes. Where a definition is amended or modified by these rules, the modified definition shall be used." at the same time, Class rule 14 (b) (ii) states that the jib clew "...is defined as the intersection of the foot and the leech, extended as necessary". Since we can' t use the ISAF Equipment Rules, we do not have clear instructions in our rules to clarify how to extend the foot and leach and specifically when it is necessary to extend them. This leaves it up to each measurer to interpret when and how to define the clew - and this can result in different sail area measurements from different measurers.
Normal corner.jpg
For a simple clew that ends in a distinct corner like the one in this photo above, all measurers agree that the corner should be the point where the Lp, Leach and Roach are measured.
Big Clewboard.jpg
Similarly, for a clew which has been cut at about 45 degrees and has a clewboard less than 229mm (like the one above), all measurers agree that the foot and leach should be extended with a straight edge along the foot and leach to define the point for Lp, Leach and Roach measurements.
High Batten.jpg
But what happens if the leach cuts in below the lowest batten to a corner that is closer to the tack (like the photo above) - should the leach be extended from the point above the batten, or should the measurement point be the back grommet-corner with no extension required? When the lower batten is far above the corner (say around 1m above) all measurers agree that there is no need to extend the leach from above the batten and they would use the corner with the grommet as the point for Lp, Leach and Roach measurements.
mid batten.jpg
So what if the batten is closer to the corner - should we extend the leach down from above that lower batten? As this batten gets closer to the corner grommet, more of the measurers agreed to extend the leach from above the batten to define the measurement point. Unfortunately, there is no specific distance at which all measurers extend the leach - it is an undefined interpretation. So it is possible that some measurers might measure to the grommet-corner and some might extend the leach - and so we could end up with the same sail measuring differently depending on the measurer.
concave leach.jpg
And how would you do it for the sail above? Use the lower grommet, use the upper grommet, use the corner below the leachline cleat or extend the leach down? And what happens if the foot round curved more smoothly so that it eventually intersects with the leach without any distinct corner on the foot from which to extend? What if a clewboard were placed 200mm forward of the leach along the curved foot - should the foot-round be extended and from what point? There was a jib like this at the Toronto worlds.

To solve these ambiguous situations, I think we need to add a bit more language to our class rules (through a formal rule change) so that all measurers will be able to calculate the same sail area for any headsail they measure. One suggestion discussed is to add the following sentence to 14 (b) (ii): "The leach and foot must be extended if there is any curve (concave or convex) in the last 500mm from the bottom aft corner of the sail." Perhaps that would solve the problem? If you can think of better language - please let me know!

Haywire
CAN 601
Canadian Measurer

Re: Jib Clews - how to measure them consistently

Posted: Thu Sep 26, 2013 9:37 pm
by rand
This should be fun :D

What if, instead of adding more verbiage, we went the other way, like how we now "measure" the kite?

With the jib rigged and the boat on its side, measure the foot from the base of the mast at the hull to where the forestay intersects the top of the hull, the luff up to where the halyard attaches, and the leach from there to the front of the mast were we measured the foot.

That is your measured area, have fun!

My measured area would drop, as I've still got one of those ancient pre-self tacking setups, others could see measured area rise.

But you now have freedom to build and deploy the jib that suits you best, and it is easy to measure.

Could we end up with boats with crazy roaches? huge genoas? Maybe, do we care?

However maybe we have more fun arguing over how we measure out boats, instead of sailing them?

:D

What if we did the same kind of thing with the main?...

Re: Jib Clews - how to measure them consistently

Posted: Fri Sep 27, 2013 9:28 pm
by Colin Smith
Peter

Thanks again for getting this conversation started. A couple of comments:

1. Not relevant to the questions of whether and how to project from straight edged sails, but as regards sails with curved edges, I was interested to see there is an RYA submission to this year's ISAF conference proposing a change to ERS to deal with this issue - basically they propose extending the curve as needed using a batten. See
http://www.sailing.org/tools/documents/ ... 616%5D.pdf

2. One solution proposed in Toronto was to define the clew as the point on the lower part of the sail furthest from the luff. I played with some simple numbers, this approach does indeed have a good degree of self-adjustment in it, as if the clew is defined further forward, the area of the body of the sail falls but the roach increases - and vice versa. Where I think it has a potential problem, though, is if there's a batten holding the leech out some way, such that the sail edge below the batten end is nearer horizontal than the luff. Then the furthest point from the luff is at the batten end and the area below the line from batten end to tack is unmeasured. This could need an additional measurement to pick up this "missing" area, and would bring foot round into the measured area, which could mean the area limit gets changed. Starts getting complicated...

3. One final thought - do we need to clarify the head point also? As we start to see more square heads on the headsails, do we project the highest point of the sail onto the extension of the luff at 90 degrees to the luff as per ERS, or just take the "top edge" of the sail as in the class rule? There seem to be some different approaches being taken here also. I suspect this might be a good time to adopt the ERS definition for "head point" - move to the standard terminology while the square heads are still quite modest

Assuming we continue to measure headsails at all of course!

Cheers
Colin

Re: Jib Clews - how to measure them consistently

Posted: Sat Sep 28, 2013 6:09 pm
by Shu
Rand's idea of simply measuring the effective foretriangle is certainly the simplest and most foolproof. However, in a development class, it has one drawback: it makes it difficult to experiment with changing the sail area between main and jib. I suppose one option would be to change the location of the jib halyard block; that would be an easy change. Another potential drawback is that it would encourage genoas, and I certainly do not want to go back to those.

Re: Jib Clews - how to measure them consistently

Posted: Sun Oct 06, 2013 2:35 am
by rand
Steve I'm not clear on how this makes it hard to change the balance of the sail area between the main and jib. Since you are just measuring the basic triangle it seems you have even more freedom to do what you might want. Huge, roach, huge foot, square top.

The bigger concern might be much larger unmeasured area, but then if that is fast, might that not be a good idea?

If unmeasured area in a genoa would be an issue we could just measure the triangle between the three attachment points, still unmeasured area, but if you have a clew aft of the front of the mast at least you would be capturing some of that area.

Re: Jib Clews - how to measure them consistently

Posted: Tue Oct 08, 2013 12:30 am
by Shu
Rand,
I just typed up a marvelous and eloquent reply. Apparently, I timed out and lost it all. I'll try again tomorrow.
-Steve

Re: Jib Clews - how to measure them consistently

Posted: Tue Oct 08, 2013 3:31 am
by rand
It's always easier the second time right?

Re: Jib Clews - how to measure them consistently

Posted: Tue Oct 08, 2013 4:35 pm
by Shu
Rand,
My concern with the jib triangle measurement system you propose is that it gives free sail area to roachy and overlapping sails. This essentially encourages roachy and overlapping jibs, which I think make sailing 14s more difficult. The roachy, stiff-battened jibs don't tack well in light air, and don't gybe well in light to medium air. As far as overlapping jibs go, I really don't want to think about the jibsheets in a gybe. My current self-tacking rig has a very high "I" measurement, which allows me to have a good-sized jib with almost no roach. I really appreciate how I don't have to think about it in tacks and gybes.

The system we have now, though flawed, attempts to measure all the jib sail area. This current system encourages people to incorporate roach or overlap (or not) because they are intrinsically better for their particular boat, not because they give free sail area.

My previous, now missing, write-up had an eloquent discourse on the history of the class. The Cliff's Notes version is: The I14 class has a history of encouraging higher performance and easier handling. I don't want us to devise a rule which tends to make handling our little over-powered boats more difficult. My understanding of the fundamental driving philosophy behind our class rules is that they encourage real improvements to performance and handling, improvements that have over the history of our class trickled down or up to other classes.

Now, if we modify your proposed rule so we measure the triangle between the tack, jib halyard block, and jibsheet lead block, then measure any sail area that falls outside that triangle, I can get behind that.
-Steve

Re: Jib Clews - how to measure them consistently

Posted: Tue Oct 08, 2013 4:45 pm
by Shu
I think a fairly easy solution is to measure jibs like we measure mains, substituting the luff measurement for the "A" measurement. We should also limit the size of the "square" heads on jibs, or measure the area they provide.

Re: Jib Clews - how to measure them consistently

Posted: Thu Oct 10, 2013 3:39 am
by joe.bersch
Steve:

Your solution as it relates to the head sounds simple and consistent with how we treat the main- at least as i understand it. But since the jib clew can also be tortured similar to the head, it is a bit more complex than the main alone. Would you propose similar treatment? I appreciate Haywire's dilemma but wonder if this is really an area of significant concern. I myself am more concerned about the absence of a uniform all up "sailing" one design weight than i am this issue.

It seems to me that in recognition of some of the older boats and in order to keep them competitive we should look at this issue. I supported reducing hull weight but wonder about the virtue of allowing the arms race to allow the development of 4 pound rudders and 5 pound centerboards which advantage the best sailors further but are not really durable or reliable for the majority of the class. Does anybody have any l idea of what the real sailing weight of the top boats versus the middle at the Worlds?

Anybody else worry about this issue? Aren't we an anomaly with no all up weight especially in high performance boats?

Sorry for the hijack.

Joe

Re: Jib Clews - how to measure them consistently

Posted: Thu Oct 10, 2013 9:39 am
by Rich
Basically the old rule is based on the premise that the jib is a triangle which it no longer is. Would it make more sense to start from scratch? For example a jib is basically a trapezium and a triangle.
img1.png
img1.png (2.43 KiB) Viewed 6449 times
just measure and add the 2 shapes together and you should get an area with out too much free area.

Trapezium is foot, luff head, rear head, start of clew

triangle is foot , clew, bottom corner.

this should in theory work for all possible sail shapes as it doesnt matter if the triangle = 0

Re: Jib Clews - how to measure them consistently

Posted: Thu Oct 10, 2013 4:10 pm
by rand
Good idea Rich. My previous ideas were focused around making the measuring process easier. Along the lines of Rich's idea, we could measure the three triangles (head, tack, clue), luff and foot. All measured the same way .5 * base * height and added together.

Re: Jib Clews - how to measure them consistently

Posted: Fri Oct 11, 2013 8:32 am
by Rich
rand wrote:Good idea Rich. My previous ideas were focused around making the measuring process easier. Along the lines of Rich's idea, we could measure the three triangles (head, tack, clue), luff and foot. All measured the same way .5 * base * height and added together.
the point is the red shape is not a triangle. No Jib is. Their is always some cloth at the head either 50mm for a traditional head or 300mm for a square top (for example). someone clever than me can work out what the impact is and if it creates a massive loophole....

Re: Jib Clews - how to measure them consistently

Posted: Fri Oct 11, 2013 4:37 pm
by rand
I was thinking that triangles are super easy to measure, so go with the triangle. To capture all the sail area, we can just add more and more triangles. I believe that the old Australian rule for main measurement did something like this.

We could simplify jib measurement by measuring the main triangle and then adding more triangles as need for extended roaches and foots. Maybe if the height of the triangle exceeds a fixed size or a percentage of that side of the jib then we measure, otherwise we don't?
Jibmeasurement.png
I'd rather err on the side of simplicity than complexity. I also don't think we need to worry about genoas, I don't see us going back to them, even if they were fast sailing they make the boat harder to tack and I don't anybody wants to go in that direction, but if it is better for the boat then why not?