The IRC Yearbook 2003 contained a deceptively simple section on how IRC works on the race course ("So how does it all work?" – page 23). This read as follows:

"It is very simple to work out how much time difference boats give each other. Take the two ratings and deduct one from the other. Each 0.001 difference = approximately 3.6 seconds per hour – eg Boat A rates 1.010, Boat B rates 0.995. The difference is 0.015. 15 x 3.6 = 54 so Boat A gives Boat B about 54 seconds per hour."

Like most pronouncements from the Rating Office, this has been interpreted as if on tablets of stone and applied slavishly in many events throughout the season. The main result of this has been a rash of complaints to race organisers, usually along the lines of "The results are wrong" or "Your computer must have made a mistake".

The truth is less dramatic and infinitely more reassuring to the hard-pressed results personnel. The most important word in the paragraph quoted above (hint – it’s underlined) is "approximately". The rating examples have been carefully chosen because the 3.6 seconds only applies when the ratings of the two boats are at or very close to 1.000. As the ratings increase above 1.000, the time difference decreases which is only reasonable as these boats are meant to take less time to complete the course. Below 1.000, the opposite applies and the time per 0.001 increases.

So how can you tell if you are beating your rivals on the race course? If we look at the same two boats in the above example, if Boat A sails for one hour, her corrected time will be 1.010 hours which is 1:00:36 (1 hour 0 minutes 36 seconds). Boat B will beat Boat A if her corrected time is less than that.

To find out what this means in terms of time, you want to find Boat B’s maximum elapsed time and to do this you divide Boat A’s corrected time by Boat B’s rating - 1:0:36 / 0.995 = 1:00:54. In other words Boat B can sail for up to an extra 54 seconds and still have a better corrected time. As you will see this apparently complicated way of doing things has produced the same answer as above – 54 seconds.

But things change if the boats are bigger or smaller but before we try those calculations it will help that there is an easier way of doing the calculations. We can get the same result if we divide the rating of Boat A by the rating of Boat B – ie 1.010/0.995 = 1.015 and 1.015 hours is 1:00:54.

If Boat C rates 1.110 and Boat D rates 1.095 (the same rating difference of 0.015) the time difference for an hour’s racing is 1.110/1.095 = 1.0137 which is 1:00:49 – equivalent to 3.27 seconds for each 0.001.

Looking instead at smaller boats, Boat E rates 0.910, Boat F rates 0.895 (again 0.015 less). The time difference for an hour’s racing by Boat E is 0.910/0.895 = 1.0168 which is 1:01:00 – equivalent to 4.0 seconds for each 0.001.

Quite reasonably, your navigator tells you he can’t carry all these figures in his head and stop you from running aground so the answer is to find out your rivals’ ratings in advance (remember to check for any last minute rating changes!) and prepare a little table showing all the time differences. Here I am indebted to Colm Barrington of the Corby 36 Gloves Off whose crew at the Bell Lawrie Scottish Series in May 2003 were one of the first to draw this problem (and the note in the 2003 Handbook) to my attention. They had a nifty table of all the boats in their class with the time differences calculated for various lengths of race. The trouble was all the differences had been calculated on exactly 3.6 seconds per hour and as the whole class rated above 1.100 the truth was substantially different.

With Colm’s permission I have reproduced the table here. The first column shows their calculations for one hour’s racing (which we now know are wrong) while the next columns show the correct differences for 1 and 2 hours and 10, 20 and 30 minutes of racing calculated as I have described above. The differences may be small but we all know that races can be won and lost by the smallest margins. You can set up the table quite easily on a spreadsheet but I will leave it to you to work out how!

Ian Macdonald

IRC Council member for Scotland

Ian (as usual!) is quite right. The underlying reason is that to be mathematically correct, the 3.6 seconds per hour relates to corrected time, not elapsed time. On the race course, everybody is of course dealing in elapsed time; hence the errors that creep in as TCCs move away from 1.000.

Colm's table (above) is probably the ideal way to deal with this. For occasions when you do not have that for whatever reason, another simple way of dealing with it is to divide 3.6 by your TCC. eg, 3.6/1.100 = 3.3 seconds or 3.6/0.900 = 4.0 seconds. The answer in each case will then be closer to the correct time allowance per 0.001 rating difference between your boat and a reasonably closely rated competitor. The answer will still be approximate, but rather less so!

Mike Urwin