Kiss principle in rating spinnakers and screachers nico boon
Already in 2009 it became clear that crews in Holland did not like the high penalties for using a screacher. They stopped using these sails. In that year the TR system started using a gliding scale for spi as well as scr. The method was partly Influenced by the system used in the OMR, the Australian Offshore Multihull Rule in the same year.
A new idea has been worked out. What will be the result if we calculate the efficiency of a spi and a scr in the same way as other foresails? All of these sails are triangles. Given the calculated area of a free hoisted spi and scr., it is easy to calculate a value for LPG resulting in the same area. This is done by the formula. LPG = area / longest leech (SL1) / 0.5. Using this LPG the new aspect ratio ars = area / LPG ^2. Finally using this ars the efficiency factor
es or escr = 0.72 * ars ^ 0.3. With this eff. factor a new rated area will be calculated for the spinnaker by the formula rsas = es * area – 0.30 *rsag. For the screacher the formula will be rsascr = escr * area – 0.35 *rsag.
By deducting 30 % of rsag and 35 % of rsag the method exactly copies the procedure used in the OMR.
The resulting rated areas prove to be too high to use in rsa.
For the spinnaker a reduction factor of 0.15 is used. For the more efficient screacher this multiplier is 0.25.
To see what the newly calculated rsas and rsascr really mean finally the ratio rsas / msas and rsascr / msascr has been calculated for each individually measured multihull.
It is surprising, that the ratios found in this way, show something, which is remarkable. The average ratio for the spi is around 8 %. With a very limited spread. For the screacher it is around 14 %. With the same results.
This partly can be explained by the clever workings of the designers. The values of rsag and the areas of well-designed spinnakers and screachers are within a narrow range. But this may be not all that can be said about it..
Always practising the KISS principle where this seems to be possible, leads to a really simple solution of the problem how to rate a spi or a scr. This can be done by taking 8 % of msas, the measured area of the spi and 14 % of msascr the measured area of the screacher. That's all.
In the case a spinnaker as well as a screacher is part of the sail wardrobe of a multihull, than in rsa for rsascr will be taken an arbitrary 6 % of msascr only.
In the lists with rating numbers the values of 8 %, 14 % as well as 6 % are included in the Excel formulas in the form of variables. If this is found to be necessary these variables only need to be changed, at the top of the columns with rsas, rsascr or rsa to result in revised TR values,
To make it easy for the reader to judge for him or herself, separately a small Excel file will be included giving all values used for all measured boats in the Dutch 2010 lists. As can be expected this file illustrates that screachers get a higher eff. % than spinnakers The same amazing results were found when the same analysis was made using the list with around 170 boats of the OMR.