Our boat exhibits no prop walk that I can detect. It also can't steer
from propwash in forward. It must be moving for any steering to occur.
The prop (18-inch Martec) is 12 feet forward of the spade rudder and
fairly close to the keel, so it has little lever arm to turn the boat.

yeah, 12 feet doesn't leave much "push" still in the prop stream, and too a
prop at about the pivot point can't do much twisting of the boat.