Sorry about raising the dead but it seemed pointless to start a new one when correcting an issue presented as fact by a World renowned designer as the OP.
Originally Posted by Richard Woods
Or put it another way
It is extremely rare for a cruising catamaran to capsize in waves with no sails set (trimarans are a different matter) because despite what the media say, waves over 20ft high (the average beam of most ocean going multihulls these days) only occur in F10 conditions or more.
This is incorrect as the following nomogram shows: Wave height nomogram
The fetch and wind speed given the appropriate duration determine the wave height. So for a short duration squall, yes, you are correct. For a long constant blow, waves can exceed 20 feet in surprisingly low wind speed, 30 knots for about 26 hours will do it. So an F7 for just over 24 hours gets those wave heights.
Likewise, 4 hours in 80 knots will get you 20 footers.
Some of the more interesting aspects to this are in the trade winds. If they are blowing a bit higher than normal, say 26 knots then it won't take long to get 20 footers if the wind picks up to say 40 knots , it's about 12 hours.