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TR RATING NUMBERS (version 2002).
1. How to calculate .
2. How to use.
3. How to determine length.
4. How to determine weight.
5. How to measure the sailarea.
6. How to handle spinnakers.
7. How to handle centerboards/keels.
1. How to calculate.
The TR number is an integer of two or three digits. It is the result of a formula using normal rounding rules.
TR number = 100 / (0.99 * RL^0.3 * RSA^0.4 / RW^0.3),
RL, RSA and RW are the abbreviations for length, sail area and weight, after prac-ticing corrections to bring them at the level of their effective (rated) values.
2. How to use
The corrected time is calculated from the sailed time by dividing that time by the TR number and multiplying by 100. For that purpose the sailed time (hours, minutes and sec-onds) must be expressed as a decimal number.
(example : 2 hours, 12 min., 45 sec. = 2.2125 hrs). It is quite usual to express the sailed time in seconds. (example : 2.2125 * 3600 = 7965 sec.). To get the right sequence of the results of a race on handicap this is not necessary, neither is the multiplication by 100.
3. How to determine length.
Start determining the total length (LOA). Then measure the overhangs at the stem and the stern (called FOC, AOC respectively), from a basic line parallel to the waterline at a height of 2 % of LOA above the waterline. (for a boat long 6 m. this means a line 12 cm. above the waterline). In practice the after overhang component AOC generally will be set at 0. A formula has been developed to determine RL, using LOA, freeboard and the top angle of the stem only. This requires no difficult measuring of a waterline.
For small open trampoline catamarans the overhangs are measured out of the water, from a basic line at a height of 2/3 of the hull at the front-beam location or at the side-stay attachment location (whatever gives the highest height).
RL = LOA - AOC - FOC.
4. How to determine weight.
The weight of the boat is determined by weighing the boat including standardequip-ment. For the crew, an additional weight is added to find the displacement (RW) the boat has when sailing.
For designs with a LOA <= 4.00 m. for each crewmember add 65 kg.
<= 4.80 m. 70 kg.
> 4.80 m. 75 kg.
If total sail area >= 11 m2 than for crew 1 man boat always take 75 kg
For cabin multihulls the crew allowance is: 40 * RL - 70 (kg.)
5. How to measure the sail area
The real sail area is determined by one of the usual methods.
This area is converted to a rated sail area (RSA) by using an efficiency %.
That requires the calculation of the aspect ratio (A) of each sail.
The aspect ratio is the vertical length (VLM for the main, VLG for the jib), squared, di-vided by the sail area of each sail.
A = VLM * VLM / area
When a swiveling mast is used of which the section is not a pure circle, the area of the mast is added to the area of the mainsail, before A is fixed.
area mast = 0.5 * maximum circumference mast * VLM
eff. % = 40.1 + 18.31 * A - 2.016 * A^2 + 0.07472 * A^3
RSA = real sail area * eff. % * 0.01.
6. How to handle spinnakers.
The real area (MSAS) is determined by using the next formula :
MSAS = SF * (SL1 +SL2) /4 + (SMG - SF / 2) * (SL1 + SL2) / 3
SF is the length of the foot, SL1 and SL2 the lengths of the sides, SMG the distance be-tween the middle of the sides measured across and along the sail.
if the condition SMG >= 75 % of SF is fulfilled, then RSAS is added to RSA, to be calcu-lated as follows :
RSAS = 0.11 * MSAS (for open multihulls only)
RSAS = 0.07 * MSAS (for cabin multihulls only)
A screecher is a sail like a spi with an SMG between 50 % and 75 % of SF
RSAScr. = 0.1 * MSAScr (for cabin multihulls only)
The maximum length of a spinnaker-boom (for open cats) is fixed on the distance be-tween the place of the foot of the boom on the mast or the mast-beam towards the uttermost point on the stem of one of the hulls + 80 cm.
For designs with a LOA <= 22 feet a special rule is in force :
Max. area spi for boats with LOA <=16 ft. 17 m2
do. > 16 <= 19 ft. 21 m2
do. > 19 <= 22 ft. 25 m2
For these types the TR number (without spi) is lowered with 4 points.
7. How to handle centerboards/keels
For beach catamarans without centerboards the calculated TR number is multiplied by 1.03. For cabin multihulls without boards the TR number will be multiplied by a factor Q cal-culated as follows:
Q = 0.94 / (draft / RL) ^ 0.037
If you have any questions about the rating, please contact:
Nico Boon or Geert Ruesink
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