Hello to Pitchpole Guy...
It's relatively simple, you call the WSSRC ( World Sailing Speed Record Committee ) of ISAF, England...ask for Sir Reginald Bennett and he will explain you the best way.
Probably, the best and cheaper way could be 2 WEBCAMS ( WC )linked to a single PC's screen, located 100 m away from each other on strong pedestals...a simple soft could start the clock as soon as a blast-cat cross the 1st WC field ( joke... ) and shut it down when she reaches the 2nd...
Friendly Yours,
Frenchie
