Capt. Neal® wrote:
I would go for a big, honkin', fixed, six-bladed prop in a tunnel,
designed as a system and driven by hydraulics. A streamlined
door on each end of the drive could be closed when under sail
to minimize drag.

For maximum thrust in reverse without a tendency to side slip
one needs tunnel drive. It works for most of the big tugs on
the Western Rivers it should do the job on a relatively lightweight
yacht.

CN

I think you'll find that what most tugs use are Kort Nozzles, rather
than tunnel drive. The idea being to improve bollard pull, rather than
reduce prop walk (prop walk is your friend if you know how to use it).

otn