Wayne.B wrote:
It's difficult if not impossible to quantify the drag but it will
definitely be significant. Racing boats go to great lengths to either
plug up, or put mylar flaps over, through hull fittings that are far
smaller than a bow thruster aperature. The bow thruster aperature is
also in just about the worst possible place as it will disrupt
attached flow over the entire length of the hull. If you are serious
about sailing performance, I would not install a thruster.

I understand what you write but I happened to encounter quite a few
sailboats with bow thrusters; usually 40" or longer.
I remember all of them very well: entering with nonchalance her narrow
berth while I am struggling with her backing that makes her wander to
all places except the right one!
I wonder if it is possible to put in front of the tunnel openings
something like a flap valve or a venetian blind device. This would
reduce the impact on the flow pattern.

Daniel