Re: Questions on props
Yes, you do have a Mercury designed lower unit and yes, SOME mercury props will fit it. It is the small gearcase so props from smaller Mercs like a 90 are the ones to fit it. You need a prop that will fit a 4 inch gearcase. You can use Force props, BUT they must be the open hub, "Thru-hub exhaust" props like the Merc Because your engine does not have an exhaust snout.Here are a couple of examples of props that you can use. Note that they are used on Chrysler and early Force engines but they are all 13 inch diameter with 15 splines and open hubs. First is a Michigan stainless, next is a Ballistic, next is a black aluminum Vortex, after that a blue Merc test prop, and finally another shot of a Ballistic.
Hydro-foils--"fins" have supporters and detractors. Some swear by them and some swear at them. It is common for them to reduce top speed as in your case but not an absolute certainty. Again, in your case, it sounds like the hull does better without it.
Right now you are running at the top Of the RPM range of that engine. IF you are using an aluminum 13 X 19 and IF you go to a stainless 13 X 20 RPM will stay about the same and top speed MAY improve by about 1-3 MPH. If you go to a 13 X 21 stainless RPM will drop to about 5300 and again, top speed may improve a bit. The higher pitch will make the hole shot a little worse, but not much.
If you are currently using s stainless prop, stepping up from 19 to 21 pitch will drop RPM by about 400 and speed may stay the same or may increase a little.
Your hull and engine combination is doing well and you can not expect major improvements in speed with a prop change.
