...on an otherwise still day, would it move ❓
Note: requires a really long extension lead.
[s]If the fan was substantially big enough yes, however it would be massively in-efficent, you would be much better to invert said fan and hang it off the back of the boat creating a propeller.[/s]
Aye it would like totally
Yep but works much better if you mount the fan under water:
[img] 
[/img]
Ooooh, a Bonwitco! We had one of those as a tender for some years. Lovely planing hull, especially on flat water!
But if you mounted the [s]big fan[/s] motor underwater, surely the sails would fill as you went forwards and stop you.
But if you mounted the big fan motor underwater, surely the sails would fill as you went forwards and stop you.
Slow you down, but not stop.
Besides, the OP never said the sails were up.
It would depend on a few factors but, generally speaking, If it's standing at the north pole, the speed would be almost zero but at the equator, where the circumference of the earth is greatest, it's speed would be about 1,038 miles per hour (1,670 kph). The mid-latitudes of the U.S. and Europe it would speed along at 700 to 900 mph (1125 to 1450 kph).
HTH
just turn the fan round
[img] 
[/img]
just turn the fan round
Yeah that'd work too. You might need a bear though.