The Windmill (left) is used for grinding grain. It turns slowly, starts at low wind speeds and has multiple blades.The Wind turbine (right) is used for generating power. It turns fast, works only at high windspeeds, and generally has few blades.
So, we should really be using the word wind turbine in connection with driving boats, since we are certainly not grinding grain, but it does not really matter.
The autogiro (left) uses the thrust force on the blades directly, in the same way that a sail force is used.The wind turbine (right) converts the wind force and transmits it through gears to drive a propeller under water. The wind turbine boat can go directly upwind, but the autogiro boat has roughly the same sort of limitations as a sail boat.
This confuses a lot of people, because these were not very well-named in the early days, and the original names have stuck. It doesn't actually depend on which way the axis is pointing, but on the orientation of the blades to the axis.
A turbine is called "horizontal axis" if the blades are at 90 degrees to the axis - no matter in which direction the axis is actually pointing - and "vertical axis" if the blades are in the SAME general direction as the axis - whether the blades are straight or curved. Therefore, believe it or not, a helicopter counts as "horizontal axis", even though its axis is actually vertical.
This terminology gets really screwed up if the blades are at a different angle to the axis. What should we call the last two above - there is not actually much difference between them.
Incidentally, the proper technical term for the study of windmill-type devices is MOLINOLOGY. Whether that is the proper name for the study of autogiro boats is another matter.
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