Yamaha 70 on 18' Custom Dory

[hwsiii]

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

 

[Dhadley]

Re: Yamaha 70 on 18' Custom Dory

If those numbers are accurate it's doing quite well. Less than 10% slip and just about 5% slip. If I were working on it I'd work with the 18 and try to get the rpm up. Have you palyed with the X dimension (motor height)?

 

[hwsiii]

Re: Yamaha 70 on 18' Custom Dory

Dhadley, WOT on this motor is supposed to be 6,000 RPMs, we are only getting 5,050 RPMs. this in turn is putting way too much stress on the motor lugging it. And it may could be moved up one notch, but that isn;t enogh to move the RPMs high enough to help much.


H

 

[Dhadley]

Re: Yamaha 70 on 18' Custom Dory

We wouldn't be looking for a 800 or 900 jump, but rather a couple hundered. And see if the prop still holds. Dropping pitch to gain rpm without maxing the set up usually results in less efficiency. Moving the motor costs almost 0 but another prop requires a few $$.

Question - with the motors as it is now, how much of the trim range can you use at wide open?

 

[hwsiii]

Re: Yamaha 70 on 18' Custom Dory

We can use a decent amount of trim at this time. But trying to go all the way with the trim causes a porpoise effect. That is why we want to change props, and get one without so much rake or cup in the blade tips. I guess I approach it from the opposite direction you do. LOL When I know I have the wrong prop I do not try to raise the motor, until I finalize the prop. If i have to use a prop with very little rake and cup then I will most likely have to lower the motor because of excess ventilation. But i do thank you for your thoughts on my post. If you think of anything else please let me know.


H

 

[steelespike]

Re: Yamaha 70 on 18' Custom Dory

It does seem the 16 is closer to what you need.
I don't know how you can select a $prop$ when your not running your present best setup.The wrong setup could mean a 2 mph difference in speed or more and of course rpm as well.If your expierencing porpoising before venting the motor is probably too low. Once setup a prop change will be apparent and may allow a further X demension change.Raising the motor also changes its affect on the boat.Imagine the motor as a lever if you raise the motor you are shortening the lever and dragging less through the water.
I know nothing about the ratios but when I think "DORY" I don't think speed. 37 seems like a stretch on a 1800lb 18 ft with a 70 hp.
A 18ft 120 hp I/O propped for top speed is lucky to see 40+.

 

[hwsiii]

Re: Yamaha 70 on 18' Custom Dory

Gentlemen, I appreciate your thoughts thus far, and 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

 

[steelespike]

Re: Yamaha 70 on 18' Custom Dory

I guess I under estimated the potential of your setup.I have a feeling the "Dory" is more speed friendly than a typical dory design.While Your prop change was surely effective it seems to me that most likely your approach assures the need of an additional prop or prop modification.
I would think your present prop would be more likely to vent in a higher setup.And without raising the motor it would seem your dragging to much through the water.

 

[hwsiii]

Re: Yamaha 70 on 18' Custom Dory

Steele, I don't quite understand why you say that the prop is too deep in the water. I don't wholeheartedly disagree with your statement, there is a good chance that we can raise the motor another notch up and decrease drag. I first plan on having Larry try some fairly hard turns at higher speeds to see if we get much ventilation, and if we don't I am going to try and get him to move it up another notch. If we can I expect to gain another 150 to 200 RPMs if this is possible.
But making the statement that the motor is too deep in the water, I don't understand how you can make that correlation, with what I have said. I do understand the theory that the present prop will ventilate quicker than the previous ones, as it does have less rake and less cup and that is why it attains the higher RPMs, that makes sense. And I don't know why you say that my approach assures an additional prop or prop modification. I am fairly well versed in the theory of propellers and all of the different elements in them and what each change will theoretically do to the behavior of that propeller.
I would appreciate your comments.


H

 

[steelespike]

Re: Yamaha 70 on 18' Custom Dory

Has nothing to do with how deep the prop is.The prop needs to be at a height and design to produce minimum venting and max speed.
To me it is pretty simple if your dragging more lower unit through the water than asolutely necessary its more difficult to achieve max speed.
As little as an inch can result in a 2mph increase at the speeds you are dealing with.And thats without any other changes .I think you can imagine the affect an inch might have at 50 or 60 or more.
Many folks will tollerate occasional venting to get that extra height and the right prop will minimize the problem.
If you want sport cornering and no venting then you may be at your max height.Just as a prop suitable for skiing isn't likely to work for speed and vice versa.A boat setup for maximum speed isn't likely to be as corner and swell friendly without some venting.Depends on what you want to accomplish.A boat that isn't setup for the best speed with minimul venting isn't likely to be as efficient.Though the difference might be difficult to measure.

 

[hwsiii]

Re: Yamaha 70 on 18' Custom Dory

The depth of the prop is directly proportionate to the height of the motor on the transom and usually has a correlation to the potential venting problems. He also intends on using the boat in the ocean as well, I should have mentioned that, because it will make a difference on how high we can raise the motor without causing ventilation from the extra fore and aft movement of the boat as he negotiates any formed seas.
I do agree with your statement on the more of the lower unit in the water the more drag that will be produced and hence the lower speed. Thank you very much for your posting, as it always helps to have other peoples thoughts and ideas, in case I have forgotten something or am on the wrong track with my thinking, as I sometimes do.



H

 

[steelespike]

Re: Yamaha 70 on 18' Custom Dory

It appears with the ocean use youre probably about where you need to be.
Wouldn't hurt to try a little height if you think there might be some room.

 

[hwsiii]

Re: Yamaha 70 on 18' Custom Dory

Thank you very much Steele, as I am hoping we can move it up one more notch as well. That would make it even better if we don't get ventilation.


H