in a handicap race, the ior length was used to compute a "time allowance". in europe this was calculated on the duration of the race, in seconds per hour, known as time on time, whereas in the usa they preferred to base it on the length of the race, as seconds per mile, known as time on distance. time on distance is easier to calculate at any point in the race, but can cause significant anomalies in tidal waters as the distance sailed through the water can differ significantly from the distance over the ground, due to the effect of the tide.