OFFSHORE MULTIHULL RATING (OMR) PREAMBLE – November 2019
Multihull Yacht Club Queensland Inc
The Offshore Multihull Rule by the Multihull Yacht Club Queensland (Inc)
First issued February 1997
Source Filename: OMR PREAMBLE.docx
1. Acknowledgement
1.1 This rule is a development of the Texel Rule, developed in Holland.
The MYCQ gratefully acknowledges the development work of Nico Boon and the KNWV.
(Royal Dutch Watersport Association)
2. Administration
Northern End of Manly Boat Harbour,
22 Trafalgar Street, Manly
P.O. Box 178,
Wynnum Qld. 4178
ABN: 97 324 509 351
Website: www.mycq.org.au
Authority: The authority for the administration and development of this rule is the MYCQ
(Multihull Yacht Club Queensland Inc). Only electronic ratings issued by the MYCQ and appearing on its website register, or ready for uploading by the OMR rating officer, will be accepted in MYCQ events.
Development: This is a development rule. The MYCQ reserves the right to alter or amend the rules, formulae or interpretations as it sees fit, without notice.
Interpretation: The MYCQ Sailing Committee shall appoint an OMR officer, who shall be the impartial arbiter of these rules. The OMR officer shall interpret any part of the text, or sailing innovation in accordance with the spirit of the rule, to ensure fair sailing for all race competitors.
Owner’s responsibilities: It remains the owner’s responsibility:
- To prepare the boat, sails and equipment for measuring
- To ensure that he/she is the holder of a current rating
- To ensure that the vessels measurements agree with the published online rating
- To ensure that, during races the yacht is equipped in accordance with the prescriptions of the rating
A failure to do so shall require the vessel to be disqualified from any race in which the vessel is not properly equipped.
Ratings: A Rating shall be issued by the rating officer and shall remain current for four years from the date of measurement unless a change is made to any parameter applicable to the rating. A full reweighing and measuring is mandatory should any changes be made to the structure of the vessel or its rig.
Re-measurement: If re-measurement is requested at any time, a fee decided by the committee, may be incurred.
Valid Rating: A yacht shall have only one current valid rating.
Notification: A current rating must be available before a yacht’s entry in an OMR event will be accepted. When a regatta includes races of different safety categories, only one rating will be valid for the duration of the regatta. (See also Para 6.9)
MYCQ Measurement Fees: The following fees may be charged by MYCQ. (Any extra costs incurred by the measurer such as traveling, accommodation etc. will be charged additionally.)
1: Weighing Equipment Maintenance Levy: This fee covers cost of weighing equipment and recalibration. For Brisbane-based boats, MYCQ Club members shall have the Weighing Equipment Levy waived in the first instance.
2: Sail Re-measurement: This a processing fee charged for re-measurement of any new sail configuration and subsequent rating recalculation and website listing. The measurer may also charge a further fee.
3: Annual Listing Fee: This annual fee is payable by the owner of each boat, and covers website re-listing of ratings annually.
3. Abbreviations and units of measurement
3.1 Units of Measurement.
All dimensions are given and recorded in meters to two decimal places, square meters and kilograms.
3.2 Abbreviations
The following abbreviations are used.
| AOC | Aft overhang component |
| ARG | Aspect ratio genoa |
| ARM | Aspect ratio mainsail |
| FOC | Forward overhang component |
| LF | Factor applied to Rated Length |
| LOA | Length overall, hull |
| LOAA | Length overall, ama |
| LPG | Perpendicular from clew to luff of Genoa |
| LPM | Perpendicular from tack to longest side of triangle contained by the head, tack and clew of the main |
| MAM | Measured area of rotating mast |
| MSAD | Measured sail area drifter |
| MSAG | Measured sail area genoa |
| MSAM | Measured sail area mainsail |
| MSAS | Measured sail area staysail |
| MSASc | Measured sail area screecher |
| MSASp | Measured sail area spinnaker |
| NC | Number of Crew |
| OMR | Offshore Multihull Rating |
| RL | Rated length |
| RSA | Total rated sail area |
| RSAD | Rated sail area drifter |
| RSAG | Rated sail area genoa |
| RSAM | Rated sail area mainsail |
| RSAMz | Rated sail area mizzen |
| RSAS | Rated sail area staysail |
| RSASc | Rated sail area screecher |
| RSASp | Rated sail area spinnaker |
| RW | Rated Weight |
| VLM | Vertical length overall of the mainsail |
| WC | Total weight of crew in normal fair weather clothing |
| WCD | Declared weight of crew |
| WE | Weight of movable equipment declared |
| WM | Weight of boat and all equipment |
| WS | Weight of boat with racing equipment |
For measurements and abbreviations for sails see relevant paragraphs.
4. Preparation for measurement
4.1. The owner or his representative shall be present when measurements are taken, and shall check and sign off on all forms supplied by the measurer.
4.2. All equipment shall be stowed in the location where it is normally kept while racing.
4.3. An inventory shall be made of all moveable equipment and its location and provided to the measurer at the time of measurement. Inventory forms are available from MYCQ measurers.
4.4. The owner may elect to carry whatever equipment or fittings he desires, (unless otherwise prescribed in the Preamble) so long as all equipment is inventoried and remains on the vessel while racing.
4.5. A copy of the inventory list, signed by the measurer, shall be carried aboard at all times and shall be produced to the race committee on demand.
4.6. The boat shall be dry and bilges free from water. Water tanks and fuel tanks shall be empty or pressed up. If pressed up, the calculated weight of their contents shall be subtracted from WM. Portable toilets, or waste holding tanks where carried, shall be empty.
4.7. Sails shall be laid out flat ready for measurement.
4.8. The measurer shall mark each measured sail near the tack with his name and the date of Measurement.
4.9. Measurement shall be undertaken upon appointment only.
4.10. The vessel must be ready in all respects. If not ready another appointment will be necessary.
4.11. Payment of all fees is the responsibility of the owner.
4.12. Ratings will not be issued until fees are paid. Discounts may apply to MYCQ members.
(See also Appendix 1. Preparing your boat for measurement)
5. Hull measurement
Measurements are taken from the hull or deck surface at the fore and aft extremities whether these fall above or below waterline. Bulb bows shall be included. Extensions or fittings such as prodders, anchor rollers, rub rails, rudders etc. are not included. . All equipment shall be in its normal position as when racing. No persons shall be aboard.
- Measure the length overall of the longest hull. (LOA)
- With the vessel in racing trim in preparation for weighing: if the forward extremity of the LOA overhangs the true waterline mark a position on the stem 2.5% of LOA above the true waterline. (HCE)
- If the stern extremity of LOA overhangs the true waterline mark a position on the stern 2.5% of LOA above the true waterline. (HCE)
- Measure the distance from the forward extremity of LOA to the forward extremity of the hull at or below HCE. (FOC)
- Measure the distance from the aft extremity of LOA to the aft extremity of the hull at or below HCE. (AOC)
- Rated length RL = LOA – (FOC +AOC)
- If the LOA of the Ama of a trimaran is equal to, or greater than the central hull, RL = LOA of the Ama.
6. Weight
Weight of the vessel shall be measured with all fixed equipment and accessories which will be carried during racing as documented in the inventory but without crew, crew personal kit, or consumables including water and fuel. The vessel must be dry and all bilges and lockers open for inspection.
6.1 Weighing equipment must be capable of measuring to an accuracy of + or – 0.5%. {WM}
6.2 Only weighing techniques as approved in the appendix hereto shall be used.
6.3 Equipment which may be added for certain race categories shall not be aboard. These items shall be listed and weighed separately and individually. (WE)
6.4 Crew number (NC) and total weight in normal fair weather sailing clothing shall be declared. (WC)
- A nominal weight of 4kg per crew member is added to the declared weight in the spreadsheet as a crew gear allowance. (NC*4Kg)
- Rated weight RW = WM + WCD + (WE when applicable)
- Where WCD = WC + NC*4
6.6 The WCD shall not fall by more than 2.5% without change to the rating. Extra crew numbers and weight may be carried without change to the rating.
6.7 A Race Committee at its discretion may require the weighing of the crew immediately prior to particular races or regattas for the purpose of rating those specific events only.
6.9 Rated weight RW = WM + WCD + (WE when applicable)
7. Sail Measurement
7.1. The true area of each sail is to be measured as accurately as practicable.
Although parameters and formulas for measurement are given for conventional sails, these procedures shall not restrict the measurer from using alternative means to obtain an accurate area for any sail which is an unusual shape and is deemed to require a different measuring technique. The measurer shall record the method and parameters used together with the actual measurements of such sails. Any doubts shall be referred to the OMR officer for a final decision.
8. Mainsails
8.1. All mainsails are presumed to be full battened mainsails. For sails without full length battens, the MSAM is reduced by 6%. A sail shall not be considered full battened if the Sum of the lengths of the battens does not exceed E and the length of the top batten does not exceed 30% of E.
For vessels with a mizzen or more than one mainsail in fore and aft configuration, the MSAM shall be the area of the largest of the mizzen or mainsail plus 50% of the area of each additional mizzen or mainsail.
For vessels carrying two mains in parallel, the second main shall be rated at 0.8*RSA.
8.2 The measurer shall record,
- B The sum of the lengths of all the battens (if the sail is not full battened)
- Tb The length of the top batten (if the sail is not full battened)
- P The length of the luff from Tack to head
- Pr The depth of the chord, or round of the luff to the vertical P
- E The length of the foot from tack to clew
- Er The depth of the chord or round of the foot to the horizontal E
- ML1 The distance from the head of the mainsail at the luff, to the clew of the mainsail
- ML2 The distance from the outer, or leech end of HB to the clew
- HB The width of the head of the sail at the headboard or square head or parabolic head of the sail measured from ML1 and perpendicular to ML1 to the leech of the sail.
- RDM The roach depth of the main measured perpendicular to ML2
- LPM The perpendicular measured from line ML1 to the tack.
- MC The circumference of a mast in cases where the mast rotates
8.3 Measured sail area Mainsail,
MSAM =0.5(ML1*LPM)+0.5(ML1*HB)+0.66(P*Pr)+0.66(ML2*RDM)+0.66(E*Er)
8.4. Rated sail area mainsail RSAM = MSAM
8.5 Rated sail area mainsail with rotating mast is calculated by rating the main and mast area as a unit.
RSAM = (Windward rating) 0.65 * (MSAM + MAM) + (Offwind rating) 0.35 * (MSAM+MAM)
8.6 ARM = MSAM/E^2 or (MSAM +MAM)/(E+0.375*MC)^2 for rotating rigs.
9. Jibs and Genoas
9.1 The largest sail not otherwise defined set forward of the foremast shall be measured as a genoa
9.2 The measurer shall record,
- LL The luff length of the genoa
- LLrg The luff round (+) or hollow (-) of the genoa measured from LL
- FG The foot of the genoa
- Frg The foot round (+) or hollow (-) of the genoa measured from FG
- LG1 The distance from the head to the clew of the genoa.
- LG2 The distance from the widest point of a quadrilateral genoa other than the clew to the clew.
- HG The perpendicular distance from the widest point of a quadrilateral other than the clew to LG1
- Lrg The leech round (+) or hollow (-) of the genoa measured from LG2
- LPG The luff perpendicular of the genoa measured from LL to the clew.
- (Note: Attention is drawn to Para 7 Sail Measurement)
9.3 Measured sail area genoa
MSAG = 0.5(LL*LPG)+0.5(LG1*HG)+0.66(LL*LLrg)+0.66*(LG2*Lrg)+0.66*(FG*Frg)
9.4 Rated sail area genoa
RSAG = MSAG
9.5 ARG = MSAG/LPG^2
10. Staysails
A staysail is a headsail used in addition to a genoa or other headsail, which is flown between that Genoa or other headsail and the main mast.
A staysail, its area and rated sail area shall be calculated in the same manner as a genoa. Where any sail with a mid-girth of less than 50% of the foot is carried on either a spinnaker pole or a bow pole or any position forward of the forestay, then that sail shall be measured as a genoa, and any additional sail which may be carried between it and the mainsail shall be measured as a staysail.
11. Drifters
A drifter is a lightweight headsail constructed of nylon sailcloth of not more than 1.5 US ounces per square yard (64 gm/m2). It shall be measured (MSAD) in the same manner as a genoa.
Lightweight headsails of other materials shall be measured as genoas.
Rated sail area drifter
RSAD = 0.3((MSAD)-RSAG)
A drifter may not be carried in conjunction with another headsail without rating adjustment
See staysail (10).
12. Spinnakers
12.1 A spinnaker shall be a lightweight sail whose mid-girth equals or exceeds 75% of its foot measured at the mid-point of luff and leech.
12.2 Additionally, the measured area of the spinnaker shall not be less than:
- The measured area of the genoa
- The measured area of the screecher
- 0.36 RSAM
12.3 The measurer shall record,
- SL1 - The longest luff of a spinnaker whether it be symmetrical or not, from tack to head along the edge
- SL2 - The length of the second luff or leech of the spinnaker measured along the edge
- SF - The length of the foot of the spinnaker measured along the edge.
- SMG- The mid girth of the spinnaker measured between the midpoint of SL1 and SL2.
12.4 Measured sail area spinnaker,
MSASp = SF*(SL1+SL2)/4 + (SMG-SF/2)*(SL1+SL2)/3
12.5 Downwind rated sail area with Spinnaker component only = 0.3(MSASp-RSAG)
Downwind rated sail area of Spinnaker component with Screecher = 0.295(MSASp - RSAG)
13. Screechers
13.1 A screecher is a lightweight sail without battens not attached to a stay whose mid girth measured at the mid point of luff and leech exceeds 50% of the foot but is less than 75% of the foot. It shall be measured, MSASc, in the same manner as a spinnaker.
13.3 The measured area of the screecher shall not be less than the measured area of the genoa.
13.4 If the screecher is the largest downwind sail it shall be rated in the same manner as a spinnaker with a 0.35 downwind rated sail area component.
The Downwind rated sail area = 0.35*(“Z” constant * MSASc - RSAG)
And the rating of the screecher shall not be less than 0.36*RSAM
13.5 If otherwise satisfying the conditions of a screecher, but having a smaller measured area than the largest downwind spinnaker, the screecher shall attract the following rating:
Rated sail area Screecher, (RSASc) = 0.055*MSASc – RSAG
14. Centerboards
14.1. All yachts are presumed to have an effective centreboard or fixed keel system.
Yachts deemed not to have such an effective centreboard or keel system by the rating officer shall have their OMR decreased by a factor of 0.98
14.2. Yachts that are fitted with a keel or fin which cannot be retracted when sailing shall have their OMR decreased by a factor of 0.995
14.3. A yacht with a pivoting centerboard where the underwater case opening remains open when the centerboard is deployed and has no means of improving water flow such as flaps shall have their OMR decreased by a factor of 0.99
The measurer shall decide whether such an allowance is applicable and shall record same on the measurement form. Any doubt shall be referred to the OMR officer for a final decision.
15. Propeller corrections.
15.1. All yachts are presumed to have either no propellers, or retractable leg systems. Yachts with propellers remaining in the water while racing shall have their OMR decreased in accordance with the following table.
- One folding or feathering propeller 0.995
- One fixed propeller 0.975
- Two folding or feathering propellers 0.99
- Two fixed propellers 0.9625
The rating officer may apply an appropriate factor for propellers which are considered to be intended to or apparently provide a rating advantage.
15.2. Any yacht having more than one special adjustment, propellers or centerboards shall have the product of each individual adjustment applied to the OMR. For example a trimaran with a fixed propeller and no effective centreboard, the factor would be 0.975 * 0.98.
16. Rating Calculation
Rated Sail Area, RSA = “Configuration RSA” + RSAS + RSAD + RSAMz
The four major sail configurations covering the majority of the current multihull racing fleet are as follows:
16.1. Mainsail and Genoa / Jib only
A vessel carrying no downwind sails will attract a downwind premium on its rated sail area in the RSA algorithm equal to 120% of the rated working sail area. This is a ‘base rating’ which adds the equivalent of 36% of the rated area of the main as a de facto spinnaker, to correct mathematical anomalies in the rating of boats at the lower extreme of sail area relative to their size.
The relevant changes to the RSA formula are as follows:
Rated sail area of Main + Rated sail area of Genoa/Jib + Offwind correction factor
New RSA 6b = (RSAM + RSAG) + 1.2*(Rating of Working Sail Area [= RSAM+RSAG] where RSAM +RSAG is rated in the same manner as a defacto spinnaker)
This equals - (RSAM + RSAG) + 1.2*(0.3* (RSAM+RSAG-RSAG)
This simplifies to: If no valid screecher or valid spinnaker, then
(RSAM + RSAG) + 0.36*RSAM
(The committee will monitor the premium applied to vessels in ‘cruise’ mode without any rated downwind sails and reserves the right to change this correction factor if deemed necessary.)
16.2. Mainsail, Genoa / Jib and Spinnaker only
If no screecher but valid spinnaker, then (RSAM + RSAG) + (0.3*(RSASp – RSAG)
Where RSASp = MASp
16.3. Mainsail, Genoa / Jib and Screecher only
If a valid screecher but not valid spinnaker, then (RSAM + RSAG) + (0.35* (RSASc Screecher – RSAG))
Where RSASc = MASc
16.4. Mainsail, Genoa / Jib, Spinnaker and Screecher
If valid screecher & valid spinnaker, then (RSAM + RSAG) + (0.295*(RSASp – RSAG)) + (0.055*MSASc - RSAG)
(+ RSA Staysail, RSA Drifter, RSA Mizzen, RSA Second Main) if applicable
16.5. The Final OMR Algorithm
OMR = (Drag/Lift Factors) * 0.93 * RL^LF * RSA^0.4 / (RW ^0.325)
Where 0.93 is a constant. This constant has been modified from 1.0 to 0.93 in the 2016 OMR review to correct for the overall shift upwards in ratings with the removal of sail efficiency factors.
17. Data from other clubs
17.1 Each club providing OMR rating measurements or data for on-line listing must ensure clear identification the origin of the data, measurement form and inventory listings, complete with owners and Measurer’s signatures.
17.2 Before accepting a copy of a rating certificate issued by another club, the race committee must validate the copy by contacting the relevant official of the issuing club.
18. Weighing and measuring techniques
Acceptable techniques and paradigms are noted in the appendix. The MYCQ committee shall decide on changes or additions to allowable paradigms and techniques as may be required from time to time.
19. Time Correction Factor
19.1 The OMR Time Correction factor is provided to three decimal places
19.2 Corrected time = Elapsed time * OMR
Appendix (see website menu: OMR | OMR Guidelines)
Preparations for measurement (including weighing procedures)
Sail Measurement (Diagrams)