Re: Not full RPM?
It is a 10 3/8 diameter, 12 1/2 inch pitch prop. The Chrysler number 293 defines it and only a 10 inch nominal diameter prop will fit on the shaft.
Just because your engine says it produces 55 HP at 5250 RPM does not mean it will be able to reach that RPM. A lot depends upon engine condition and load.
SO: since you have set points, now check compression ratio. Factory new was around 145 PSI and anything over 120 would be considered very good.
If compression is OK, then you need to try a lower pitched prop assuming your current prop is in reasonable condition--11 1/2 or even 10 1/2 pitch
However, you really also need to check the way the engine is set on the transom and the trim setting. If the engine is set tucked too far in, the boat will "plow" and speed and RPM will be reduced. Generally, you should se the tilt lock bar in the second hole in from the end. This will set the engine cavitation plate about parallel with the hull bottom.
Now, you appear to have the engine on a V143 Glastron which was rated for around 60 HP. I can tell you that set correctly, my 1967 Chrysler 55 on an V153 Glastron (15 feet long) Turned 5500 RPM and 27 MPH with a 10 3/8 X 13 Bronze columbian prop. This was with me alone. When I had 2 or more passengers I dropped the prop down to an 11 1/2 pitch spare. and speed dropped to 24-25. Small boats are very sensitive to load.
The stock prop is way regressive with blades well over 1/8 inch thick and lots of drag. Search the auctions for a used bronze prop or even a used Michigan. They run way better than stock. You can use a prop for a 35 -50 Force--same splines and pin drive. You can also use a Chrysler 45-55 prop. Chrysler made two versions of the 35 and some 35 props may not fit.
Here is a Bronze prop that I needed to "pin" because hubs are no longer available and the original one "spun"
Here is also a photo of my 1972 50 that is on a 10 foot tunnel hull. It swings a 10 1/2 X 15 cupped bronze prop at 5000 and 45 MPH. I don't know if you can see the prop in the last photo. It is just behind the girl in pink holding my "Chick Magnet."
