scratch boat (100) races for 100 minutes, to all be equal the other boats must complete the same course in 57 and 84 minutes.

fast boat does it in 58 minutes, and the slow in 83 mins, then fast boat is third and slow boat 1st (scratch 2nd)

Calcualtion is just a matter of Hcap time = time * rating / 100 to give corrected time.