Yamaha 70 on 18' Custom Dory

I am looking for information about the Yamaha props specifically about their 13" x 17" diameter painted stainless. I have a friend who has an 18' Custom Dory with a 7' beam that weighs about 1,800 pounds fully loaded. We have tried Yamaha's Performance series propeller in 18" and 16" pitch.

The 13 1/4" x 18" Performance gets
4,000 RPMs gets 26 MPH and at
WOT 4,700 RPMs gets 32 MPH.

The 13 1/4" x 16" Performance series gets
3500 - 18.8 MPH planing point Performance 16”
4000- 24.1
4500- 28.0
5050- 31.5

When I run the numbers through Crouch and Savitsky I find that the boat ought to do 37 MPH with that Yamaha 70 HP with a 17" prop and a constant of 165 in Crouch.

It has a
Displacement Length Ratio of 164
Beam Length Ratio of .39
Speed Length Ratio of 7.8
and a hull draft of right at 6"

I called Yamaha to find out the specifications on their Performance propeller and john knew absolutely NOTHING except he said it was NOT a progressive pitch. I wanted to find out about the rake, pitch, cup, and DAR. He didn't even know the efficiency of that prop.

Does anybody have the particulars on this prop. I can't make a good decision on which prop to pick without having a starting point and knowing the particular characteristics of this prop. Can anyone please tell me where I might get this information to help choose a prop.


H

 

To continue our quest(LOL), for the correct prop we traded it for one with less cup and rake, to allow the motor to attain more RPMs and mitigate the porpoising. We now have a 13" x 17" and these are the new numbers we are getting from it. Hopefully when the motor is taken off double oil for the break-in period we can get another 150 RPMs out of it. I did forget to mention we also have a TTop on this boat as well.

And this is what we actually got...realize this is an analog tach, so the RPMs could be +- 50, not exact.

1800 - 8.3
2000 - 9.5
2500 - 11,4
3000 - 14.9
3500 - 19.1 Planing point in here
3750 - 21.5
4000 - 24.9
4500 - 27.8
4750 - 28.7
5000 - 30.8
5250 - 32.4
5500 - 33.9
5650 - 35.3 MAX RPM

This prop is far superior to the other 2 Very Happy The engine feels and sounds right now. I can trim it higher without porpoising also, as we suspected. This is right where I was hoping it would be and we can probably expect a little better when I get off this double oil, which is still about 20 gallons away.

After the motor is broken in and we see what the final RPMs are then we can try moving it up one notch and see if it will hold without ventilation becoming a problem.
I would appreciate any thoughts you might have on this.


H

 

as the rev range at WOT is 5-6000 rpm then you are right where you should be ...like you said you are very happy ...so just cruise at 4500 rpm and your fuel consumption will be optimised

You need to lift it up until you have gone too far then go down one hole ....most people have the motor too low ...

 

I do agree with that theory pistnbroke, we would gain more RPMs if we could move it up. What I plan on doing is having Larry try some fairly hard turns at speed, and if the prop doesn't break out and start ventilating bad, then I think it is an excellent candidate for raising it one more hole.


Thank you,