three blade prop
From my limited experience with a two blade prop they have less ability to provide thrust in both forward as well as reverse thus less manuvering ability. I would imagine that to fully realize the desired effects of 2 vs. 3 blade, one would have to actually experience the use of both props on one specific boat as boats all handle somewhat differently. A 2 blade on a light displacement boat may be sufficient for that boat and provide desired manuverability. The 3 blade will cause some drag, however unless you have a race boat I believe the drag created would not be noteworthy. With sailing, almost everything is a compromise.
As far as the HP rated per ton let me say this... If you are in a headwind, a heavy sea, trying to buck the tide the more HP available the better off you''ll be. On my 17000 displacement 38'' cruising boat with a 50hp engine and 3 blade prop I can make 6.5 knots under power in a flat sea. In a headwind and heavy chop I do well to make 3 knots. If I had a 27hp engine (for example) I doubt if I would make much headway at all. For the formula you mention of 1hp per ton of boat my boat then would have an 8.5 hp engine which may get me underway from the slip with no wind at all, but that''s about it. Then on the other hand, there are many sailboats that have circumnavigated with no engine at all. So, I guess it boils down to what one expects from their boat. For me, I want the power and I would personally would not want any less HP than I currently have.
In addition, in a crusing boat one is likely to have a high output alternator of 125 amps or more maintain charged batteries and operate power hungry "electronics of the day" and refrigeration. A larger engine (than 8.5 hp)is required to provide the hp necessary to turn the alternator (I''m likely to get hits on this one). But then again, the purists with no engines typically don''t or can''t have refrig., radar, cold beer and color TV. :^)