My Gulfstar 43 has a bow thruster that the previous owner had installed. I wondered before buying this boat if this would be a useful thing or not? I also wondered if having a 10 inch tube with a propeller inside the middle and protruding through the bow from one side to the other, that while under sail might have some sort of aqua dynamic impediment as a result?
Since my purchase of this boat in early July, EAGLE, I have sailed her several times and I am impressed and pleased with her sailing performance so I believe I can rule out her bow thruster for changing the dynamics of hull performance, however, I can never really know for sure since I've never sailed this boat without the thruster but her performance is very good for what she is.
As far as usefulness, backing into, or forward into, a slip, on a windy day, and making a perfect landing on the dock is amazing, and for getting the bow to point on the mooring ball on a windy day, also amazing!
While the above maneuvers are quite well done by skilled captains without the aid of a bow thruster, I do believe in their usefulness for the many who's skills at docking, or pointing on a mooring ball, less than perfect (the weekend sailor

) those who only use their sailboat 10 or 15 times a year, the bow thruster may help them from careening down the side of your boat!