Re: Top end?
That engine on that boat with that prop should turn more than 4200 RPM, ASSUMING the boat is not loaded down with all kinds of weight. In small boats, even multiple persons riding in the boat make a big difference in speed because their weight is a greater percentage of the boat weight.
However, even with the notorious inaccuracy of dash speedometers, with two people in the boat I would expect a little more than 42 MPH. At 4200 RPM with a 17 pitch prop and your gear ratio, your theoretical top speed would be 39 MPH. Figure 10% for prop slip and you are down to 35-- so there are some inaccurate measurements there. But if you bought the tach new, it should be within 2% or 80 RPM at 4000.
A 17 pitch on that size hull is a little fine and I would expect the engine to swing a 19 at about 4500-5000 RPM at about 45-47 MPH. So swinging a 17 pitch at 4200 means you are not getting 125 HP out of the engine. Hell! I'm swinging a 19 cupped with my Chrysler 140 at 4500 RPM on a 21 foot cuddy.
So, Given that, go over to the Force forum and search synchronizing and timing threads. Also do a compression test to be sure all cylinders are in good condition. And reset the idle speed to 700-750 in forward gear, in the water. 600-650 is too low and the engine will regularly stall at idle.
Once you have the engine running correctly, then consider a stainless prop. It is expensive but will usually give 400 RPM and 2-3 MPH more than an equivalent aluminum prop. If you have the black, open hub, semi-cleaver, aluminum piece of crap prop, you will definitely see an increase in speed just by going to an after-market prop.
If you are not having ventilation/cavitation problems, then my favorite prop would be the Chrysler uncupped stainless 13 X 19 pitch prop. You will need to search for one though. They are no longer made and only available rarely on ebay or sometimes through classifieds on The Chrysler Crew. I bought one in good condition for 200 there. Got the prop, thrust washer, prop nut, and aluminum tail-cone adapter. It has a different blade profile and seems to run better on Chrysler and Force engines.
If you are having Cavitation/ventilation problems, then the Michigan 19 pitch stainless (cupped) will be good. While stainless increases RPM, cupped props drop the RPM a bit but "hold" the water better. Cupping does NOT increase speed itself; it allows you to run the prop higher on trim and lift the bow a bit for less wetted area and more speed. You are running your trim out far enough, aren't you? too much trim in (down) lowers the bow and makes the boat plow. Note that to my knowledge, ALL Chrysler and Force props (including the P.O.C.) were made by Michigan--obviously, some were better than others. And mind you, I am stating 19 pitch as an example assuming you get the engine running strong. You may still find it necessary to run a 17 pitch SO TRY BEFORE YOU BUY.
