Speed difference from Yamaha test

M

MtnExplorer

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Sep 22, 2012
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I have a new to me Duckworth aluminum welded Navigator 172 (17.5 feet in length, 7'9 1/2" width) with a F90 Yamaha motor with a 13 x 19 SS prop. The tested weight is 2,400 lbs. Using a GPS going out and back I recorded a top speed of 32.6 mph at 6,200 RPM. A Yamaha test bulletin (PB_DCK_175NavigatorSport_F90TLR_9-20-06_ALM) with almost the exact setup shows a speed of 38.2 mph at 5,800 rpm. This seems like a significant difference. The only difference in conditions which might affect the speed is that I was at 1,600 above sea level versus 770 in the Yamaha test.

I would like to know if this is an insignificant difference from a careful test with different measuring equipment.

I am not interested in top speed as much as mileage as I travel distances in this boat. Would a higher pitch (say 21) provide better mileage at 25 mph?

I am NOT interested in any hole shot times.
 
pdgs

pdgs

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Sep 22, 2012
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77
Re: Speed difference from Yamaha test

MtnExplorer said:
I have a new to me Duckworth aluminum welded Navigator 172 (17.5 feet in length, 7'9 1/2" width) with a F90 Yamaha motor with a 13 x 19 SS prop. The tested weight is 2,400 lbs. Using a GPS going out and back I recorded a top speed of 32.6 mph at 6,200 RPM. A Yamaha test bulletin (PB_DCK_175NavigatorSport_F90TLR_9-20-06_ALM) with almost the exact setup shows a speed of 38.2 mph at 5,800 rpm. This seems like a significant difference. The only difference in conditions which might affect the speed is that I was at 1,600 above sea level versus 770 in the Yamaha test.

I would like to know if this is an insignificant difference from a careful test with different measuring equipment.

I am not interested in top speed as much as mileage as I travel distances in this boat. Would a higher pitch (say 21) provide better mileage at 25 mph?

I am NOT interested in any hole shot times.
Click to expand...

Yamaha likely prop'd their test boat for the purposes of maximizing performance during the test. Your boat is prop'd just like mine (Jetcraft) so that we can run at a variety of altitudes without having to change anything. I was a little dissapointed in my top end also but after two seasons of runing up and down mountains to fish I'm glad I never changed a thing. My .02 cents is that you should leave it just like it is, enjoy the fine boat that it is and fish like a mad man!
 
Johny25

Johny25

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Feb 8, 2012
Messages
265
Re: Speed difference from Yamaha test

With the info you gave....... Yamy F90 that has a 2.31:1 gear ratio going 32.6mph at 6200rpm with a 19 pitch prop shows that you have 32% slip!!:eek: Something doesn't sound right with these numbers :confused:

The same calculations that you gave for the Yamy test run (assuming a 19 pitch) comes out at 15% slip which is much more reasonable
 
S

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Speed difference from Yamaha test

Agreed your slip is really high You may need to lower the motor one hole.
You will lose about 3% of power at that elevation.
You might want to verify your tach if lowering the motor doesn't help. it seems odd your 300 to 400 rpm higher at 1600 compared to 700 ft.
 
M

MtnExplorer

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Sep 22, 2012
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Re: Speed difference from Yamaha test

Johny25 said:
With the info you gave....... Yamy F90 that has a 2.31:1 gear ratio going 32.6mph at 6200rpm with a 19 pitch prop shows that you have 32% slip!!:eek: Something doesn't sound right with these numbers :confused:

The same calculations that you gave for the Yamy test run (assuming a 19 pitch) comes out at 15% slip which is much more reasonable
Click to expand...

Another thing I have noticed is that there seems like very little variation in speed from 5,500 rpm to 6,200 rpm. Partly I assume is because the higher the speed the more force required to overcome the friction on the boat hull. But the engine just sounds like it is revving higher without much more speed. Could the shape of the hull make a difference at the top end causing dirty water at the prop? The motor was mounted by a well respected dealer several years ago and has not been changed. How is lowering the engine going to make a difference? Is it searching for smoother water under the hull? I have played with the engine trim which seems to make about a 3 mph difference. Will a lower pitch prop possibly reduce the slip?

Thanks for your ideas. I am new to this boat and to speed. But I like both!
 
Johny25

Johny25

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Feb 8, 2012
Messages
265
Re: Speed difference from Yamaha test

I would take steeles advice to start with and see what happens. Because something is not right with these numbers. I suspect your tach could be off. I am finding hard to believe that your turning a 19 on that motor at 6200 with the weight your pushing. Remember your RPM range is 5000-6000. Theoretically the answer would be to go up in pitch to drop your RPM's just under 6k but I do not think that is the answer to this equation given the 32% slip and low MPH for that high of an RPM.
 
jestor68

jestor68

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Joined
Jun 12, 2012
Messages
2,308
Re: Speed difference from Yamaha test

That test used a Yamaha SS 13X19 prop. What is your prop?

That test was conducted with a brand new boat/motor. Your motor is "several" years old. Several years of use, a possible different brand of prop, plus a 3% loss of power(altitude) could easily account for the difference.
 
M

MtnExplorer

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Joined
Sep 22, 2012
Messages
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Re: Speed difference from Yamaha test

Well, thanks for all of your questions. So I had the prop checked that I was told was a 13x19, but it was actually a 13 x 17 ss. I swapped it out for an aluminum 13x19 and guess what, 36mph at 5,800. That is very close to the Yamaha tests. And after several days of running at about 4,500 rpm I am getting 6mpg which I find great.

Next question: how come the fuel foamed out when I filled it? It didn't even shut off the gasoline (ethanol free 92 octane) on the filler hose. How can I tell how much fuel is actually in the tank if it foams up? How accurate are standard tank float meters?

Can anyone recommend an accurate fuel flow meter? I really need to know how much I have in my tank and my current meter does not hardly move.

Thanks for all your ideas.
 
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