An inch in pitch is usually about 200 rpm. So the question is how far from
wot max rpm were you with the previous props. If you were protty close then
dropping an inch may be enough. If you were still in the middle of the
range then dropping 2 inches will not be a big problem.

Right --but with teh 18 pitch I'm just a bit under; as I understand it a 4 blade
is better for towing & heavy loads... wondering if switching to that is better
than going down to the 17...