Higher pitch 2 blade gives more RPM? Cup it?

[55Crestliner]

I have a 1958 Evinrude 35hp Lark pushing a '56 Crestliner 14 foot. Not sure about weight. RPM max for this motor is 4500.

With a 10.5 x 13 3 blade aluminum, I'm getting 4500 RPM (+~30)

I threw on a Bronze 10.5 x 14.5 2 blade, and I'm getting 4770 RPM.

When I had these same 2 props on my smaller boat, with a 25 horse, they did as expected - bronze lowered the RPM.

Why is this increase in pitch giving me an increase in RPM?

Should I cup it?

Am I just seeing an increase in eficiency in the bronze, and the 35 horse is strong enough to use it?

 

[hwsiii]

Re: Higher pitch 2 blade gives more RPM? Cup it?

Crestliner, I looked up your motor in the 1958 Evinrude brohure, so kindly given to me by Steelspike, and I show your WOT RPM are 5,000 RPM and not 4,500 RPM. I also show your gear ratio to be 1.75:1, so atually you are running a little under your WOT RPM. so leave it like it is.

1958 35 Lark

H

 

[55Crestliner]

Re: Higher pitch 2 blade gives more RPM? Cup it?

OH, interesting. My 2 manuals (Seloc and Clymers) don't show the "range", just the "max". That makes me feel better. Thanks.


But why would the Bronze prop with more pitch rev higher on this motor?

 

[sho3boater]

Re: Higher pitch 2 blade gives more RPM? Cup it?

It could be changing the attitude of the boat and going faster, could be cavitating, hard to say you need a tach and gps to get a better idea.

 

[steelespike]

Re: Higher pitch 2 blade gives more RPM? Cup it?

Changing from a 3 to a 2 blade alone could add some rpm.Less resistance.And the bronze is a more sofisticated design.
Just as going from a 3 to a 4 could lower rpm.All else being equal.

 

[Frank Acampora]

Re: Higher pitch 2 blade gives more RPM? Cup it?

The two blade prop has less blade area than the three blade prop. However: The horsepower a prop will absorb is a function of design, blade area, size hull it is on, AND pitch. If you were to change nothing but pitch, you would see that at idle in addition to WOT, a higher pitched prop will lower the RPM, indicating the increased HP requirements the prop needs. This is one of the reasons that bass boats with high pitched props vent the prop hub to give a better idle and hole shot.

On the 25 hp engine, you are not developing enough power to overcome the increased HP requirements of the pitch, even with the reduced area, and the engine drops in RPM. The 35 is obviously about 30% higher in power. THUS, it can swing the higher pitched prop and now the decreased blade area translates to lower resistance at speed so the engine can develop more RPM. SO: With the increase in pitch and RPM on the 35, you should be seeing a higher boat speed (probably 1-2 MPH) unless the prop is ventilating/cavitating.

Unless you actually hear or feel ventilation, do not cup the prop.

You did not say, but if that is the old Michigan bronze prop, It cost 25 bucks NEW back in the 70s. I had mine cupped at a prop shop for 5 bucks. Ahhh! for the good old days.

 

[55Crestliner]

Re: Higher pitch 2 blade gives more RPM? Cup it?

Unless you actually hear or feel ventilation, do not cup the prop.

You did not say, but if that is the old Michigan bronze prop, It cost 25 bucks NEW back in the 70s. I had mine cupped at a prop shop for 5 bucks. Ahhh! for the good old days.

Thanks for all the replies.

Yes, it's a Michigan prop - sure didn't cost ME $25!

I don't hear of feel any ventilation at speed, or just cruising or putting around, but I do feel a very short "slip" when I hit full throttle while sitting still. I can feel the revs go up quick, and then the boat sort of launches. Kinda feels like a car spinning it's tires, then hooking up. It's only for a short moment.