Evinrude prop pitch question

[Shadow662]

I saw a new prop on craigs list for my 76 70HP size is 13 1/4 x 17. My question is the prop that originally came on the motor is a 13 1/4 x 19. I used prop wizard and it said for best speed to use a 13 1/4 x 17. Won't if I reduce the pitch from 19 to 17 reduce my top speed?

 

[Dhadley]

Re: Evinrude prop pitch question

The prop wizard doesn't take into account set up, hull design and / or prop design. If your set up is allowing for a good rpm and slip % then in theory - and all things beigng equal, yes you will slow down. Hard to say without knowng more about your current set up.

 

[Silvertip]

Re: Evinrude prop pitch question

Look at the sticky at the very top of this forum that is titled "READ THIS FIRST"

 

[Shadow662]

Re: Evinrude prop pitch question

Ok answers for the read this first section.

1) Performance issue you are trying to correct.
2) Current prop manufacturer, model, aluminum or stainless and as a minimum.
3) Current prop diameter and pitch (required).
4) Wide open throttle RPM and speed with an average load (very helpful)
5) Engine/drive make, model, year, and HP
6) Boat make model, year, length and weight

Answers
1) None, Like to have more speed if possible
2) Stock, Model ?, aluminum
3) 13 1/4 x 19
4) 35 MPH at 6.5K RPM's
5) 1976 Evinrude 75 HP
6) 1964 Starcraft Holiday 18' 875lbs

My questions again.
After inputting all the info into Prop Wizard Inluding make, model, weight (inlcuding fuel, gear, persons), engine, hull material, etc. Prop wizard says to use a 13 1/4 x 17 prop for max speed.
The prop I have is 13 1/4 x 19. Wouldn't droping the 3 in. in pitch reduce the speed of my boat?

 

[Dhadley]

Re: Evinrude prop pitch question

If you reduce the pitch you'll gain more rpm. If you are indeed turning 6500 you don't need more. I'd suggest going to at least a good ss 21. That should indeed get you more speed and bring down the rpm some. That's assuming the 19 you have is in good shape. If it's worn and your rpm is accurate then you're simply gaining slip.

Since you're at 30% slip I suspect it's badly worn or the tach isn't accurate.

 

[Shadow662]

Re: Evinrude prop pitch question

The prop that is on it looks to be in good conditon to me. No dents on the blades and no vibration suggesting that it's bent. The tach is new and the engine idles around 800 in the water and around 1000 out of the water. The prop may not be a 13 1/4 x 19. Thats what Evinrude parts manual says came standard on it and since I got the motor for free I don't know if the previous owner ever changed it. They had it on a 16ft Fiberglass tri hull. Is there a way I can measure the pitch? or is there a part number stamped on the prop itself?

 

[Dhadley]

Re: Evinrude prop pitch question

The numbers are stamped on the prop. Near the nut if it's an OMC prop.

 

[Shadow662]

Re: Evinrude prop pitch question

Ok, just checked and it is a Michigan prop. but can't see the numbers under the washer so after the holiday I will take the prop off and look to see what I've got. Thanks

 

[hwsiii]

Re: Evinrude prop pitch question

Shadow, I found your boat to weigh 750 pounds dry. With the motor fuel and 3 people I came up with a total weight of 1,800 pounds. The motor I found to have a WOT RPM of 6,000 RPM witha gear ratio of 2.42. If the gear ratio is wrong my numbers will be wrong.
My program says you need an 18" Pitch prop and you should get about 37 or 38 MPH top end speed at maximum RPM, which is good for the motor.

Boat and Motor

Prop Picker

Prop Change

My opinion is your tachometer is probably wrong, and if this is so then you are probably turning about 5,700 or 5,800 RPM and showing a prop slip of about 17% if you are indeed running a 19" prop. If your tach is right then you are most probably running a theoretical 16" pitch prop with a prop slip of 14%.

You might be able to raise your motor up a litlle higher and run a 19" prop but it would be better to have a stainless if you are moving the motor up to keep your prop slip percentage down.

H