Posted by tedious on 06/21/16 - 4:47 AM
#21
jgortva wrote:
Will the lower pitch improve top end? It seems like the higher pitch would have a higher top end but a longer planning time while the lower pitch would have a shorter planning time but a slower top end speed?
Jim G.
Jim, it is counterintuitive, but yes, a lower pitch is likely to improve your top end. If you were still going to max out at 5000 RPM, a lower pitch prop would indeed result in a lower top speed. But the lower pitch will let your motor spin up higher, and the net is likely to be more speed.
The F70 is rated 70 horsepower at 5800 RPM. At the 5000 RPM you are currently hitting, I'm guessing the motor is producing between 50 and 55 horsepower (just a guess) so you are leaving a lot on the table. The rule of thumb is 1 inch pitch = 200 RPM, but like all rules of thumb, sometimes it works, sometimes it doesn't. You have a lot of RPM to play with, all the way to 6300, and in your situation it would be best to put your WOT RPM right up there - it will improve your performance in the rougher water when you can't go that fast anyway.
Some math:
5000 RPM, 16 pitch, 30 MPH
5000 RPM, 13 pitch, 24.3 MPH
6300 RPM, 13 pitch, 30.7 MPH
Of course, this assumes things are linear, which they aren't, but it gives you some idea why the 13-pitch won't necessarily slow you down.
Tough call between the 14 and the 13 - I guess, having thought about it a bit more, you might want to try the 14 first. With the usage model you describe, I would certainly keep trying props until you at least hit that 5800 RPM where your motor makes 70 horses.
Another possibility is that the SCD3 is just the wrong prop for your boat - I hope not, but it's not an exact science.
Tim