I have a Prostar 197 with the 310 HP Predator motor in. I am boating at 1400 above sea level and is finding it difficult to pull out heavy skiers. I have been told to change my prop to a 3 blade 13. 12. from a 4 blade 13. 14. will this work for me. has anyone had similar experiences?
Daniel, the change from a 14" pitch to a 12" pitch will give you 17% more torque coming out of the hole. With that prop you would not have trouble getting a skier up. It will also raise your RPM by about 17% at 40 mph so pay a bit of attention when your are running flat out to see you don't over rev the engine.
Your top speed is not likely to change much. You've still got extra RPM above your calculated level. The torque to drive the boat is lower at 4800 than at 4000 but you've picked up that 17% because of the effective gear change with the prop. You might find your top speed goes up.
Luciano, the 17% is going from the 14" per revolution that the boat is driven forward to the new prop that only has to move it 12" forward. That's less work that needs to be done. The difference is 14/12=1.17 or 17%. In essence the engine will go through 17% more revolutions for the boat to travel as far. That is where the torque increase comes from. It's like using a longer arm on a wrench. If it's 17% longer the end of it where you apply force moves farther in distance than the shorter one. There are lots of small issues that arise from prop slip and hydrodynamic forces but they apply to both props so it's not a big deal.