As Jim said 3 blade prop and prop brand will make a big difference...you can also try and run as much pitch as you can... on your 91FS a 13x10 would probably be about as much as it will turn without hurting over all performance. You could also try one of these http://www.davisdieseldevelopment.co...er/4stroke.htm problem here is the size... their not exactly small... they really don't reduce over all DB that much BUT the tone is more pleasing to the ear so it seems quieter... kinda like comparing a Saito 91 to an O.S.91... the Saito is probably just as loud... is just doesn't have the same tone/roar the O.S has given the same prop RPM range.


John