the best prop?

[Mercs and such]

On my Mercury 75hp inline, I currently have a SS prop with the numbers: 48 72500a4 21P. I know its a 21 pitch and at WOT, I can go about 34 mph maximum. The hull is only a 15' fish n' ski and i know i can do 40+ does anyone know how i can or what prop??? also there are 3 small exhaust holes behind the prop blades that could slow it down. any help would be great!

Mercs and Such

 

[trendsetter240]

Re: the best prop?

On my Mercury 75hp inline, I currently have a SS prop with the numbers: 48 72500a4 21P. I know its a 21 pitch and at WOT, I can go about 34 mph maximum. The hull is only a 15' fish n' ski and i know i can do 40+ does anyone know how i can or what prop??? also there are 3 small exhaust holes behind the prop blades that could slow it down. any help would be great!

Mercs and Such

What is your RPM at WOT?

The 3 small exhaust holes are vents and they are there to improve hole shot. I don't see how the vents would take away much from the top end.

Are you saying that you had this boat running 40+ before? It could be that you have water logged bilge/foam that is slowing you down. Also any growth or slime on the bottom of your boat will increase drag. Check motor height; anti-ventilation plate should be even or higher than the bottom of the boat.

 

[Mercs and such]

Re: the best prop?

anti vent is good. the boat has been overhauled so no slime or soggy bilge foam no excess water. very light hull probably about 500 pounds. The boat has never gone 40 but i have heard of people changing props and they go from 25 to 35-40 mph. so i figure i can get the boat somewhere between 40-45 MPH. How do i do it and what prop???

boat was running about 5250 RPM @ WOT

the 3 vent holes i wont worry about then.

what about hydrofins? the things you put on the vent plate (look like wings) ??

Mercs and Such

 

[junior1113]

Re: the best prop?

sounds like a lot of slip. but 5250 is low. 19 pitch would be better. fins help underpowered boats get on plane and put more stress on transom and motor mounts.