Scott Vernon wrote:
Any idea how much speed is lost from one, if any?

None at all, if you get the really cool expensive retracable kind.

There are a couple of things to reduce drag from the tunnel. One (simple
& cheap) is to work up a sort of fairing at the leading edge of the
tunnel so that the flow is diverted when the boat is moving forward...
helps but not perfect, obviously. Another is to make the tunnel as small
as possible, except that this reduces the amount of horsepower you can
get into play; which can be helped by putting it deeper & further forward.

I agree with Oz1, there are times & places where a bow thruster is
really handy. It greatly extends the range of maneuvering possibilities
in tight places and crosswinds.

Fresh Breezes- Doug King