Thanks for the advise. My 188 hurricane deck boat is turning 5400 r p m. I usually run lightly loaded. I currently run a yamaha s. steel prop 14.5 X17. Max r p m is 6000.
As I said, the data you have provided yields a -3.5% prop slip. You can’t have negative prop slip. Let me reiterate, it is impossible to have negative prop slip.
Yamaha has a performance bulletin on the 188SS w/115 yamaha w/17P aluminum prop. The 115 uses a 2.15 gear ratio.
Under a light load, That boat topped out at 37.7mph at 6000rpm. They used a 17 pitch aluminum prop. They had 16% prop slip.
You could have something slowing you down, or you could be receiving inaccurate information from your guages. If your boat, with a smaller, less powerful engine can turn a 17P prop all the way to 6K rpm, then I assure you, the problem is not your prop.
I would verify your speed with GPS. The test boat had the motor mounted in the #2 position and the anti ventilation plate was 1/4” above the boat bottom. I would look at yours and see where it is. It doesn’t cost anything to look at the back of your boat. It shouldn’t cost much to get an accurate speed.
If you’re actually doing 45mph, then your tach is inaccurate. If your GPS speed is 38-40, then your tach is accurate. Then it brings us to motor mounting height.
Verify speed with gps.
Check motor mounting height.
Check the height of the anti vent plate in relation to the bottom of the hull.
do these things before spending money on a prop.