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10 reasons why catamaran sailors prefer the Texel Rating system.
Nico Boon, Groningen, 5 may 1996
1. If you have a choice between a number of rating systems all giving about the same results, then you will
choose the most simple one. In the IOMR procedure for small cats only, after having determined rated
length, sailarea and weight, you need 7 formulas to calculate a rating. The TR system needs only 1 formula
for small and all catamarans. That one formula is even more simple than 4 out of the 7. The complete IOMR
system for all multihulls needs at least 9 formulas and a maximum of 27 formulas.
2. If sailors in many countries have the opinion that the more simple system gives even better results than
the complicated one, it is evident why they prefer the more simple system.
3. The American IOMR rating for multihulls started in 1968. It has been criticized all the time, especially in
England. Why ? The IOMR was based on tanktests, but of a monohull and not even a sailing boat. After
modifications the set of formulas gave a curve which could be produced by a simple formula too. This
simple english formula was also based on tanktests, but of a 24 m. long multihull (trimaran) by two
english designers, Derek Kelsall and John Shuttleworth. Extensive correspondence in the english monthly
Multihull International. A.o. in nr. 117 of october 1977.
4. As there are cheap computers since 1980 there are no practical objections against an extensive
complicated calculation system as such. But already since 1983 users in the USA (California) had
complaints about the results of the IOMR (PMA) rating system. Because of this and the criticism from the
English side, the wish to look for a more simple system grew. Such a system had to be metrical, to be easy
to change and easy to be supplemented with modules for other elements of speed than the three
elements used, if the sailors ask for it.
5. In 1985 during a symposium in Annapolis a number of experts spoke about a simple system. A.o. the late
Hugo Myers, one of the advisors for the original IOMR system, was an advocate for a formula which had
the same general form as the final Texel Rating formula.
6. In the period from 1982 to 1984 there was in Holland a change over from a pure statistical system like the
PY handicap system, to a pure measurement system. In the beginning, naturally, the IOMR system has
been used, being the oldest one, adopted by the IYRU since 1974. But because of the theoretical
objections mentioned in point 3. and the practical complaints in point 4. a period of experimenting started
with the english formula of Kelsall and Shuttleworth and other formulas. In 1985, after consulting a.o. the
american scientist Richard Boehmer, the Texel Rating formula finally came into being. In 1993 the power
for the weight factor in the formula has been changed and as consequence the constant too.
7. When the IOMR and the simple formula are both supposed to be good, then it must be possible to express
the set of formulas of the IOMR in the form of a simple power formula. Using a multiple regression analysis
method on various collections of boats it has been a surprise that it was possible indeed. With such a high
correlation between the results, that the simple power formula found, can replace the set of IOMR
formulas.
8. What is said in point 7 means that a formula of a type like the TR formula is a universal formula for
multihulls. The formula is easy to adapt for circumstances when there are very light winds. In strong winds
the influence of factors like bigger waves and stability are so high, that there is no need to change the
formula.
9. As the IOMR, the english (MOCRA) formula as well as the TR formula approximate the performance of
optimal boats, additional corrections are possible and sensible for designs which are not optimal. That has
been done in the TR system in the same way as has been done in the original IOMR (PMA) system in
California. For open cats without efficient centerboards or keels, the handicapnumber is raised by 3 %. It
is not correct that in the new IYRU system for small catamarans the centerboard correction has been
based on the aspect ratio only and not on its area in connection with the dimensions of the boats and the
sails. (So a certain centerboard on a design long 3 m. gets the same correction % as the same
centerboard on a design long 6 m. This apart from the fact that is it better to correct for NOT having a
centerboard.)
10. The TR formula is metrical : 100 / (0.99 * RL0.3 * RSA0.4 / RW0.3). To serve boatowners in the USA
where crews use imperial measurements (feet, sq. ft. and lbs) you only need to change the constant
factor from 0.99 to 0.3397. Simple and practical. To use the new IYRU system for small cats in the USA,
you have to convert all input dimensions of the boats from imperial to metrical measurements.
With the IOMR written as a power formula :
100 / (1.00 * RL 0.311 * RSA 0.42 / RW 0.318)
you only need to change the constant factor from 1.00 to 0.3275. for use in any area where people work
with imperial measurements and weights.
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