In the case of a 32ft WLL, 26,000 lbs. sailboat the short answer is 36HP for a nice cruise on Biscayne Bay and 100HP when trying to claw off a lee shore at 3am after you have screwed up your navigation.
Good old Nigel Calder has an excellent table on page 226 of his Marine Diesel Engines (3rd ed). He uses the example of a 32ft WLL, 26,000lb auxiliary sailboat, which is close enough to a fully loaded Whitby 42 to make no difference. Please refer to Calders book for details, but the basic figures are;
Hull speed = 7.58 Kts.
Shaft HP to reach hull speed allowing for 40% prop efficiency = 36.25HP.
Add 33% for power train loss = this gives a BHP of 48HP.
This ties in with all other tables I have seen. Obviously add a few HP for alternators and other bolt on's.