RE: Board/sail combo for gusty flat water sailing
Not to really counter the many useful comments offered above, I think that it's important to recognize different personalities and preferences when making decisions of this nature. Yet, without a doubt, I have to agree wholeheartedly with geo regarding the need for balance and tune between the board, fin and sail. Still, with that said, I think that what might ideal for one person wouldn't necessarily be the same for another, even if they might be in the same general size range.
I know from experience that I like somewhat bigger fins, especially if I'm working with very light winds. If stronger currents come into play, the path kind of goes in two directions. If the wind and current are opposite of each other, my call is the smaller fin size. Conversely, if the wind and current are moving in the same direction, my call is the larger fin size.
One thing I think is important to recognize is that the formula for absolute top speed can sometimes have different attributes, especially if control factors become an issue. In a fair amount of wind, the length of the fetch can have a massive affect on the water state. In my local conditions, the solid 20-25 knots winds that Maximus refers to, I wouldn't go any bigger than a 5.0 sail, and I wouldn't even consider a 111 liter board because the sea state would make things too crazy. Yet, on a lake or protected venue with a relatively short fetch, it's a whole different ballgame.
When considering Foulweather Jack's predicament, what kind and size of board would be best? Given fairly flat conditions, I think a slalom board would an excellent choice due to the straighter/quicker rockerline. The real question would be how much volume would be appropriate. Although it would be quite possible to go up to 122 liters, I would tend to recommend something in the 101-111 liter range. With gusty conditions, I think the scale tips towards the 111. Still plenty of float for a 75kg sailor and a suitable platform for sails in the 5.8-9.0 range.