ORC
The cat is out of the bag
Multihull sailors in the UK use the Multihull Offshore Cruising and Racing Association (MOCRA) handicap rule – an empirical rule based on boat length, weight and sail area to generate a single-number TCF. It is similar to IRC in its approach, and is used for multihull entrants in the marquee RORC offshore events. MOCRA does a good job, it is well managed and provides handicaps for all boat types, cats and tris, trailer-sailers to MOD 70s. This year MOCRA has issued more than 100 certificates worldwide. Looking outside the UK the number of performance cruising cata-
marans in the 50ft-80ft range is steadily growing. These boats have raced mainly in the Caribbean and before Covid good fleets com- peted under the umbrella of the Offshore Multihull Association (OMA). The racing on coastal courses and traditional buoy racing was keenly contested and soon the shortcomings of the ‘single- number TCF’ scoring became apparent. When some boats could fly a hull and others could not large time
gains could be made. In fresh conditions the experienced crews would push much closer to the edge of capsize. Onboard a monohull a professional helmsman is worth a few lengths per leg, on a multi - hull they’re worth tens of lengths in testing conditions. So the OMA began to look for ways to level the playing field. Larry Rosenfeld, a keen member of the OMA, came up with a plan. He would: l Gather the performance data from the yachts. l Analyse this data with KND to create scoring polars for each boat, in a format similar to that used on the ORC certificate. l Combine polars with wind direction and course geometry to predict an elapsed time sailed for each competitor. l Create a TCF for each boat around that course in the prevailing wind conditions. Using the polars brought into the scoring the different perfor-
mance profiles of the boats and the effects of who was racing them. So far so good. But the process was complicated, harvesting and analysing log
files to generate the polars, post hoc analysis of wind data to deter- mine the average speed and direction. Nevertheless, thanks to Larry’s intellect and perseverance races were run and results pro- duced. However, some features caused problems: inherent com- plexity, the difficulty of adjusting scoring polars if a boat raced with a different sail wardrobe, or displacement, or helm… plus the impression that the boat that Larry raced on won more races! I talked at some length to him about how to resolve some of the
practical issues, apart of course from his high win ratio. The OMA fleet had seen the benefits of Polar Curve Scoring, ie scoring that was sensitive to wind speed and course type, but they wanted a more robust and transparent way of generating the polars, one that could cope with boats changing configuration as they moved from race to race. It became clear that the ORCi system for monohulls had all the
40 SEAHORSE
components, except a VPP to generate the scoring polar curves. There was no question that the existing optimisation routines
and force-model framework could be adapted for multihull use. The question was how to do it quickly and cost-effectively. KND came to the rescue with hydrodynamic data and technical insights; the aerodynamic side of things needed a few tweaks to cope with canting masts and the larger windage footprint of these boats. But we were blessed with the performance database that Larry had compiled to verify our work for catamarans and data from MOCRA and the MOD 70s to tune up the trimaran predictions. First impressions are that a multihull VPP is somehow easier to build than a monohull VPP, but there are a number of features that demand attention. 1. As the boat heels the buoyancy distribution between the two hulls changes, until the leeward hull bears all the weight. Also as buoyancy shifts to leeward the wave-making interference drag between the hulls reduces until it disappears when the windward hull(s) are flying. 2. Although the hulls have lower beam-to-length ratios than mono- hulls the rocker curvature is often quite extreme to control trim and this complicates the resistance calculation.
3.Curved daggerboards and foils on centreboards and rudders are popular, so the influence of vertical forces on drag and righting moment must be included. 4.Daggerboard cases employ a variety of closing devices, the VPP is sensitive to the quality of the seal (when immersed).
5.Cruiser/racer trimarans, where we have introduced an LPP point code to limit heeling so that the crossbeams are not dragged along under the water. Just when we had it up and running Covid took the wind out of
everyone’s sails. By 2022 it felt as if we were all dressed up with nowhere to go, ORC had a fully fruited multihull rule, ORCmh, and about 20 certificate holders, but hardly a sustainable business plan. However, encouragement appeared from several sources. In Europe the Multihull Cup regatta in Palma, organised by the
Allegra team, was raced under ORCmh and the albeit small fleet enjoyed good racing in both 2021 and 2022. Encouraged by the prospect of equitable handicap racing the Highland Fling team committed to a new Gunboat 80 for the 2023 season. At the 2023 Voiles de St Barth the owners of the OMA-style boats
agreed to race as an ORCmh class, with the helpful co-operation of the regatta organisers. Mocra ‘godfather’ Simon Forbes left World Sailing after 37 years
of diligent service, and joined ORC as the Multihull Rating Officer. Simon is now in charge of issuing ORCmh certificates through the ORC infrastructure. He looks after getting the measurement data from the hull scans and liaison with the teams to make sure the freeboard data, inventory and sail dimensions are current. He still
ELOI STICHELBAUT
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