Lost on prop selection

[cheaperts]

18' v hull glastron, 470 MC1 combo, 170 HP. Ran her out this weekend 2 people and 3/4-1/2 tank of fuel, after playing with trim had 47 mph indicated (speedo not GPS but I'm stick to it!) @ WOT & 4900 RPM. RPM is too high, I'm looking to get it into 4200-4300 range. I'm assuming I could run bigger pitch, this prop has no visible markings, is an OEM style steel prop. Is there a way to measure pitch? And will the newer style props compare apples to apples, I was told on my last boat that the newer props have more cupping on the blades and take a bigger bite than the same pitch "old style" prop. Probably a dumb question, but I admit it, I'm not a smart man (do you hear Forest Gump when you read that?)

 

[Bt Doctur]

Re: Lost on prop selection

most 3.7`s run the 13 3/4 x 21P

 

[cheaperts]

Re: Lost on prop selection

am I correct in thinking I should up the pitch due to the excess RPM?

 

[steelespike]

Re: Lost on prop selection

Have you looked under the prop nut for numbers?Before investing in a prop I think I would confirm both rpm and speed.
You didn't mention year but assuming a 85 Both tach and speedo could be off.A prop shop can measure the prop.
Beg or borrow a gps for speed. You may be able to borrow a shop tach or automotive tach to check rpm.