The objective of a pursuit race is that, if boats of different classes are sailed by crews of equal ability, they should all cross the finishing line together, having started at different times related to the Portsmouth Number of their boats.
The race has to be for a fixed duration and this needs to be decided upon early enough to facilitate the calculation of the different starting times for the different boats in the fleet.
Starting times are calculated for a particular race duration (RD). If 1.5 hours (90 minutes) is chosen as the RD related to boat X with a PN of 1200 then the expected RD for boat Y with a PN of 1000 would be 75 minutes and for boat Z with a PN of 1400, 105 minutes. These times are simple ratios i.e. 1200/90 = 1000/75 = 1400/105. Boat Y's start time would be 75-90 = -15 minutes i.e. 15 minutes after boat X. Boat Z's start time would be 105-90 = +15 minutes i.e. 15 minutes before boat X.