Again I remind everybody that the NMBR system is a construct using the foundamentals of the Texel and ISAF systems. Used these as a starting point to facilitate acceptance. A complete new formula would be better but could probably be too much change in one go. If it isn't then wait and read MBSR thread later.
The issues
-1- Single handed cat rigged boats like the A-cat, M18 are rated to harshly. Some lightweight double handers are so too (M20)
-2- Cat rigged boats with a spinnaker are rated wrongfully against sloop rigged boats with spis.
There is ample anecdotal evidence for both points and even some dependable evidence. However we must all realize how difficult it is to get good dependable data. This is also the achilles heel of the yardstick systems. One really needs to do alot of pre-processing to determine wether a race result is dependable or not.
But what we DO know is this.
Texel and ISAF are relatively accurate when rating middle to heavy weight cat rigged cats without a spi to middle to heavy weight sloop rigged cats without a spi. Afterall this was the data set on which both systems were regressed. Say the span P16, H16, Dart 18, P18's, TheMightyHobie18, nacra 5.2, nacra 5.5, P15's H17 that sort of stuff.
It is a well known fact that sloop rigs are faster around a course when no spinnakers are used even though they difference may be rather small on the upwind leg itself. But only when the right balance is struck between the area's of the main and jib and when the same mast lengths are used. Things maybe different when one of these conditions is not satisfied (A-cats ?).
Both Arvil Gentry and Marchaj show in their respective works that adding a jib has the following effects :
-1- The suction zone on the main near the mast is disturbed leading to less drive produced by the main. This shaves of the low pressure peak. It also prevents the main from stalling easily resulting in a forgiving flow over the main. So it produces less drive BUT is more constant and stable doing so.
-2- The updraft of the main and the low pressure zone above the main result in the jib experiencing extra low suction on its lee sides at reduced apparent angles of attack. At a right balance in area's and slot width the jib will produce twice the force per area as the mainsail.
-3- When going upwind the main must be pulled more inline with the centreline of the boat and the boat must be pointed a little lower.
-4- When going downwind the boat can be pointed lower and the sails stall less easily at these high angles of attack.
If you run the numbers on this you'll quickly end up with a intepretation of the situation where
-1- Where the looses of drive on the main equal about 1 times the area of the jib leaving just 1 amount of jib area to produce extra drive. As a rought but relatively accurate measure.
-2- The factors work against eachother on the upwind as good as cancelling one another out.
-3- The factors work together on the downwinds making the sloop superior here.
Often the sloops carry relatively more area than a comparable cat rigged boat and so the sloops may often be a little faster on the upwinds as well. However we'll get to that later.
Obviously the main benefit of the jibs are found on course that are not upwind beats, although some benefit may still exist there. When a spinnaker is fitted to both boats the spinnaker takes care of most things the jib did. It prevents the main from stalling too easily and provides a large skewing of the apparent wind the main experiences allowing the boat to be driven deep. The jib has almost lost all its benefits except for the small upwind gains.
The problem with Texel and ISAF is that they keep regarding to downwind benefits of the jib as independent of flying a kite. This is arguably very wrong and most sailors know this from personal experience.
I refer to Tornado Alive comments for more data on this :
http://www.catsailor.com/forums/sho...6986&page=&view=&sb=5&o=He gave us some great data on the comparison. I noted it down and research how it compared to what the theory said. It seems the two are pretty close together on this.
Therefor the remedy is quite straight forward.
An example :
Take two identical boats with equal size cat rigs and add a jib to one of them. of about 25 % size than the speed gain is as is currently predicted by Texel. Note how Texel and Yardstick system seem to converge on most boats, suggesting that Texel does do a few things right : I refer to Sam Evans for more info :
http://www.catsailor.com/forums/sho...8033&page=&view=&sb=5&o=So lets keep this feature. Now if we add a spinnaker to both boats than the speed increase due to having a jib as predicted earlier reduces. A good "averaged estimate" is that 2/3 rds of the gains are lost because both boats now use a spinnaker.
We can therefor roughtly compensate for this by reducing the jib area to a fraction of what it was when both were
sailing without a spi. This fraction will only account for the gains found on the upwind legs. Finding this fraction is key but not really that difficult. I have used a constant I have matched with various race result in the example version of the system, it seems to predict performances well.
This correction has the effect that all sloop ratings remain unchanged (bulk of the raced boats) and also leaves the cat-rigged cats without spi rating the same to their non-spi sloop rigs. The only ratings that are impacted are the cat-rigged cats with spis which are definately a minority at this time and are relatively new to the game. Minimal impact for a significant improvement.
In effect we issue a larger speed increase to the cat-rigged setup when they add a spinnaker than we do to sloop rigged boats adding a spinnaker.
The modifications is simple and easy to implement. While still leaving the spinnaker modification independent so we can modify that one to make the comparison between spi and non-spi boats more accurate.
Wouter
