## The Texel rating and its development since 1984

In the beginning
The Texel rating formula has been used in the Netherlands for the first time for the round the isle of Texel race in 1984. That has been the source of its name too. Before that year yardstick numbers were used to approximate the performance you might expect from catamarans. As these numbers were derived from race statistics, it was not possible to determine handicap numbers for new designs.

As a consequence the Texel Race organization, supported by the national sailing authority in Holland, decided to start using rating numbers. They are numbers calculated from measured data of the designs, by the use of a practical performance formula. This formula is inspired by studies from the British designers John Shuttleworth and Derek Kelsall and supported by Richard Boehmer and others. An article of the American designer Olin Stephens published in 1972 about 20 elements of speed was an extra inspiration. On the places 1, 2 and 3 he mentioned the elements length, sail area, displacement. These three elements were shown to have the greatest influence on the performance potential of a yacht.

A formula has been developed of the general form:

Texel Rating TR = 100 / (c * RL ^ p * RSA ^ q / RW ^ r * various factors.

RL, RSA and RW being the rated areas, calculated using the measured data of the length, the sail area and the weight. The extra factors are multipliers, the rating committees consider to have a substantial influence on the performance.

The unchanged powers for RL and RSA have been 0.3 and 0.4 respectively. The power for RW had been fixed at 0.35 in 1984. In 1993 this value was lowered to 0.325 and in 2002 again, to 0.3. Mainly in order to diminish the disadvantage for very light and fast multihulls, in the group of trampoline cats. The constant c after each update has been fixed at a level that keeps the rating numbers calculated roughly at the same level.

Comparing Handicap systems world wide
For multihulls all over the world there are now three handicap systems using the basic Texel rating formula described above Since 1997 its main formula is used in the Australian OMR rule, for racing with around 170 multihulls, mainly on the East coast of Queensland. In the same year 1997 the French started to use the same TR formula in their Multi 2000 rule. With more than 110 multihulls on the rating list. Technical advisor there is the well-known designer Erik Lerouge. That rule organization now is in the hands of the national sailing Union in France, the FFV.

In both rules, naturally, you find the TR formula used in Holland in 1997 (the updated form since 1993). Both rating committees found not enough arguments to follow our update in 2002, when the power for RW has been lowered to 0.3.

The rating committees wish to harmonize the rules towards one rule for all multihulls, as far as possible. The Dutch rating committee as step 1, decided to return in 2010 to our formula used between 1993 and 2002. At the same time we will start using the same constant of 1 as in the OMR and the Multi 2000 rule. This lowering of the constant factor from 1.15 to 1, only determines the level of the rating numbers, not their relative differences.

Our formula then becomes: TR = 100 / (rl ^0.3 * rsa ^ 0.4 / rw ^ 0.325). The constant 1 could be left out of the formula.

OMR and Multi2000 prefer to use a TCF, a time correction factor, so their basic formula is even simpler: TCF = rl ^0.3 * rsa ^ 0.4 / rw ^ 0.325.

The correction factors for elements considered to influence the performance of the designs in the Netherlands are, the multiplier factor K for catamarans, generally being considered to be a bit slower than comparable trimarans. The factor is based on the values of RSA and RW. (They don't use this factor in Australia, but a comparable one can be found in the Multi2000 rule).

The correction factor P for boats with various kinds of propellers. Finally the correction factor Q for designs with NO efficient dagger boards or centre boards. This factor based on the ratio draft / RL (rated length). For details about the Dutch version of the rating rule, see the formulas in the text of the rule on www.texelrating.org, in the section for cabin multihulls.

The consequence of the announced changes in the basic Dutch formula are that heavy, relatively slow multihulls get a TR number, that is relatively higher than the one used before. The maximum difference between the fastest and the slowest designs will be around 10 % more then with the formula used from 2002 – 2009.

Another consequence of the change in the power for RW and the constant factor is, that the formula predicting Vb (boat speed) from Vt (true wind speed) had to be changed too. This formula now becomes Vb = 164 * Vt ^ 0.75 / TR.

In the Dutch version of the rule, TR numbers as well as TCF numbers have been published for years on the lists with the rating numbers. In these lists all the data of the boats for which they have been calculated can be studied. The lists can be downloaded as Excel files. See the website for them too. There are lists for measured boats and a longer list for unmeasured designs. The data for these boats have been found in designer brochures, on the internet, etc.

To complete the picture of the three rules, it is necessary to explain that in 2009 in the Australian OMR rule as well as in the Dutch TR rule, the method to calculate the efficiency of mainsail and jibs or genoas has been changed. The older method of using the aspect ratio of the main gave to the more efficient flathead mainsail a lower efficiency % than a comparable pinhead mainsail. That had to be corrected. In the Dutch rule the basic ratio used now is the ratio sail area / e ^2 for a mainsail and sail area / lpg ^2 for jibs or genoas. The formula for the foresails is equal in OMR and the Dutch TR rule. The formulas for the main are slightly different but the results are comparable. In the OMR the dimension lpm is used and not e. That dimension is not measured in Holland, because here the Simpson calculation rule is used to determine the area of the main sail. See details in the various rating rules. In Multi2000 a different approach to the aspect ratio calculations has been used in that rule since 1997.

Apart from this return to an older version of the basic TR formula, some changes have been made which have to do with the rating of screachers. These sails can be defined as narrow spinnakers, with ratios of mid girth length (smgscr) divided by the foot length (sfscr) less than 0.75. They tend to be near 0.50 now, or even a bit lower, because of hollow parts in de luff and the leech of these sails.

Spinnakers and Screachers behave like triangles (2010)
In 2010 a new method of calculating rated areas of spinnaker and screacher has been introduced. Based on the idea that spinnaker and screacher are triangle foresails. The efficiency factor can be calculated with the same method as for genoa or jib. Rated areas found in this way are, in the same way as done in the OMR, diminished with 0.30 * rsag for a spi and with 0.35 * rsag for a scr. The final values are multiplied here by 0.15 for a spi and by 0.25 for a screacher. The amazing result is, that using the KISS principle you can take 8 % of the spinnaker area and 14 % of the screacher area.

In case owners use both spinnaker and screacher, rsascr is reduced to the arbitrary 0.06 * msascr. After the 2010 season, the effects of all the changes in 2010 naturally will be evaluated.. The rating committee decided that there is no need in 2011 to change this method.

Edges count too (2011)
In 2011 in the area of a jib is included the area of the rounded leech in the same way as is done in the Multi2000 rule. No deductions for hollows in the sides.

Crew weight more ore less? (2011)
Finally in 2011 a change in the crew weight formula has been introduced to give the smaller and light cabin multihulls a little more advantage. The results are comparable to the ones the OMR started using in 2011. The crew allowance now is: (70 * rl ^ 0.78 -60)

A new view of Spinnakers and Screachers (2015)
In 2015 it is decided to calculate the rating of spinnaker and screacher by using one formula for both sails only depending on the ratio smg/sf for each sail. The formula now is rsas (or rsascr) = 12 / (smg/sf) or (smgscr/sfscr) ^ 1.1 * msas (or msascr) * 0.01 – 0.3 * rsag. As a condition for the spinnaker a minimum value of 0.03 * msas has been added. For the screacher the minimum value will be 0,06 * msascr. If both sails are available together the calculated rsascr will be reduced to 0.8 * rsascr.

The study of the effects of the rating numbers used on the results of the races continually goes on. This is the only way to develop a rating rule step by step.