source : http://www.roundtexel.com/default2.asp?xlid=en&pid=84&MenuID=6&na=1

"The jubilee edition was sailed in favourable conditions with a south-westerly force three and a calm sea."

Force 3 winds are between 6.6 and 10.5 knots.

South-westerly winds mean that the wind was more or less blowing along the major axis of the island resulting in large legs of upwind and downwind sailing and relatively small reaching legs.

My initial estimate wasn't too far off afterall

Wouter