Frank Acampora
Supreme Mariner
- Joined
- Jan 19, 2007
- Messages
- 12,004
Re: how fast would a bigger motor make it go!
You can not use the cube formula because we are dealing with a planing hull. The four times formula is only an estimation to show that, as most people would assume, twice the horsepower would yield twice the speed is not valid.
The variables will certainly be weight and load in addition to wetted area of the hull. Over about 40 MPH, air resistance will also come into play. Given the same load or total weight--plus or minus some for the weight of the larger engine-- The higher horsepower engine will lift the hull higher for less wetted area, thus drag will not increase as much as expected. Disclaimer: Every hull is different and will respond differently to increased horsepower however, it is a given that significantly more horsepower will increase speed within certain parameters.
NOW: This hull was at 55 MPH with a 90 and when repowered with a 125, speed jumped to 60--that's 5 MPH for a 35 Horsepower increase. HOWEVER: Since the hull was already riding very high (about the last 18 inches) the larger engine did not lift it higher so there was an increase in drag (both water and air) due to speed. And, the faster I get this hull to go, the more horsepower it will take per MPH increase. Thus another 5 MPH may require 100 horsepower increase.
You can not use the cube formula because we are dealing with a planing hull. The four times formula is only an estimation to show that, as most people would assume, twice the horsepower would yield twice the speed is not valid.
The variables will certainly be weight and load in addition to wetted area of the hull. Over about 40 MPH, air resistance will also come into play. Given the same load or total weight--plus or minus some for the weight of the larger engine-- The higher horsepower engine will lift the hull higher for less wetted area, thus drag will not increase as much as expected. Disclaimer: Every hull is different and will respond differently to increased horsepower however, it is a given that significantly more horsepower will increase speed within certain parameters.
NOW: This hull was at 55 MPH with a 90 and when repowered with a 125, speed jumped to 60--that's 5 MPH for a 35 Horsepower increase. HOWEVER: Since the hull was already riding very high (about the last 18 inches) the larger engine did not lift it higher so there was an increase in drag (both water and air) due to speed. And, the faster I get this hull to go, the more horsepower it will take per MPH increase. Thus another 5 MPH may require 100 horsepower increase.