The basis of Bosham SC Cruiser ratings and how they are produced
The ratings we use currently are TCF (Time Correction Factor) ratings derived from the Island SC system (ISC). To achieve a corrected time it is only necessary to multiply the elapsed time by the TCF.
In the past we have used Portsmouth Yardstick numbers as do the dinghy fleet. However, PY numbers, while satisfactory for dinghy handicap racing, do not work well for cruiser racing. PY numbers are based on observed times and results gathered from large numbers of boats and races, so do not readily handle small numbers of boats and races. Very few cruising boats have published PY numbers and there is no reasonable way of estimating one. For these reasons we decided some five years ago to move to a more useful regime.
There are two principal choices in the Solent. Both are widely used by cruisers and racing yachts and both are calculated from formulae which attempt to assess a boat's performance from a set of parameters (eg length, beam, weight, sail area and a number of others). In both cases the formula is kept confidential to hinder strange designs to cheat the rule. The more serious racing yachts use the IRC rating (which was previously called CHS), which is provided by the ORC Rating Office belonging to RORC. This rating requires a degree of actual measurement of the boat, costs an annual fee and is renewable every year.
A more sensible choice for casual and club racing such as ours is the ISC rating which is calculated by the Island SC using their own formula, and requires little or no measurement and no renewal fees.
Regrettably there is no direct correlation between the IRC and ISC ratings (generally ISC ratings for a particular design appear a little higher than IRC). However as long as we all race and calculate times based on the same regime this is of no relevance.
Because of the reduced effort and costs we decided to move to ISC ratings. At that time the ISC would provide a rating for each of our boats for a princely sum of £3 (once off) when we sent them a single page document with some basic data on it. Most of our fleet active at that time signed up and got official ISC numbers. Unfortunately a year or so later the ORC complained to ISC that they were spoiling their business (in selling IRC ratings) and ISC agreed to stop providing their club rating to members of other Clubs.
ISC still use their own rating for their own races, such as the Round the Island race and some other similar events, so if one of our boats enters one of their races we can get an ISC rating for the boat. Equally ISC publishes each year the TCFs of entrants for the RTI so it is possible to obtain generic ISC ratings for a large number of designs. So at present in BSC races we have some boats which have got official ratings direct from ISC, some whose design type has an ISC-published rating, whilst for a few we have to try to do an educated guess at a speed comparison between the candidate boat and our other fleet members with an ISC rating. In that case we have to try to assess from experience whether a modification is required.
Of course all rating systems are generally imprecise and there is always scope for discussion about how unfair your rating is, but hopefully by using external objective rating formulae we shall get it reasonably right most of the time.