Choosing a new prop...

[jhetrick55]

I have a 1975 Evinrude 70hp with a 13-3/4x15 prop and at WOT I run 3200 rpm, according to my digital tach model: SRT-32R (set to 3p1r). This engine should be in the 5500 RPM range at WOT. I've been looking at a 13-1/4x17 prop, but would like to gain some other perspective. The engine is obviously old, but runs well, and is overall in very good condition.

What size prop should I choose to get those RPM's in the right range?

 

[jimmbo]

Your engine has a WOT range of 4500 - 5500 rpm. That engine does do its best when propped for 5400 - 5500
A 17" prop will put more load on the engine resulting in a drop in rpm
A couple of questions?
How fast is the boat going at WOT?
What kind of boat do you have?
Is the engine running on all cylinders?
You are sure you have the tach set correctly? It reads pulses from the alt, not the ignition system
If your engine is indeed running 3200 rpm at WOT, there isn't a prop on the market that will raise it 2200 rpm, so I suspect you have the tach set wrong

 

[jhetrick55]

You're probably right. The manual with the tach said it needed to be set at 3p1r so that is what I went with. If someone know how many plugs/revolution I should use I can change it easy enough. The only other setting I have tried is 3p2r with then tells me I run about 5700rpm. Maybe 3p2r is right and I should get a small prop to get into the 6000rpm range?

 

[jbuote]

3P1R is 3 pulses for 1 revolution. Setting at 3P2R I believe would be for a 4 stroke..

​If you're on a single plug wire with this inductive tach, it should probably be set to 1P1R.. Slight possibility of 2P1R, but the P part isn't number of cylinders. It's spark pulses per revolution..
Just setting up an inductive tach myself, and that's how I understand it anway... lol

​For what it's worth...

 

[jhetrick55]

jbuote - thanks. You're correct, it is pulses/revolution. I attached a piece from the manual. So, when I have tried to run it at the proper setting it is telling me I am only running 3200rpm at WOT.

 

[interalian]

Post a video on YouTube of the motor at WOT. We'll be able to tell how fast it's running based on the sound.

Like this: https://www.youtube.com/watch?v=Kt4AFg92Cc0

At the 0:45 mark, I'm running around 5500.

 

[jbuote]

Yeah.. I believe the instruction sheet.. I also have a hunch that sheet is assuming you're wrapping the inductive wire around the single wire from a coil that would go to a distributor cap, so in that case 3P1R would be correct.. However, if you're NOT on the coil wire to a distributor, but rather on a single plug wire, then I think It should be 1P1R.. That single cylinder should only have 1 spark pulse per revolution..

​That's my hunch anyway... lol...
Hope it helps..

 

[jhetrick55]

jbuote - thanks I will try that setting - 1p1r. I do have it wrapped around a plug wire now, so I will try from a coil to distributor.

 

[jhetrick55]

interalian - that is about what mine sounds like. I don't trust the gauge being accurate at 3200rpm with how good the engine sounded at WOT.

 

[jhetrick55]

jimbo - my replies are in red

"Your engine has a WOT range of 4500 - 5500 rpm. That engine does do its best when propped for 5400 - 5500
A 17" prop will put more load on the engine resulting in a drop in rpm
A couple of questions?
How fast is the boat going at WOT? - 25mph
What kind of boat do you have? - 16 ft Sierra 168V - fiberglass
Is the engine running on all cylinders? - yes and plugs look good
You are sure you have the tach set correctly? It reads pulses from the alt, not the ignition system
If your engine is indeed running 3200 rpm at WOT, there isn't a prop on the market that will raise it 2200 rpm, so I suspect you have the tach set wrong"

 

[EchoNovember]

I'm thinking a 70HP should push that 16' faster than that, so I am almost wondering if something else is going on to keep your RPMs and speed low. Smarter people will correct me if I am wrong here.

 

[jhetrick55]

EchoNovember - I agree. But everything seems to be running well. If there is something wrong I have yet to figure it out.

 

[flyingscott]

I think you have more of a problem with your tach than the motor. I ran a similiar motor (79 70 hp) on a 16' sylvan which weighed a 100#s less than your boat and never hit more than 30 mph with a 17" pitch prop with T/N/T. Your hull is supposed to weigh 750#s may want to get it weighed also not having T/N/T will hold back your speed. What hole is the trim pin in? I will bet the 15" pitch will be a good prop for your motor.

 

[jhetrick55]

flyingscott: Trim pin is in the 2nd hole closest to the transom. And I agree, I need to get that tach figured out before I can tell anything for sure. I've got some ideas on that now.

 

[jimmbo]

My calculations say 3200rpm with a 15" prop and 10% slippage places the speed at 17 mph. 25mph based on the same parameters comes in around 4750 rpm,

I would expect a 70 to push a 16ft in to the 30mph range and usually a 17 is the prop of choice.
So, do you have the compression numbers? Is the spark advancing and set correctly? Are the carbs opening fully?

 

[oldboat1]

Might explore prop venting -- probably increase rpm, and provide a better hole shot. (But think it wouldn't increase speed.)

 

[jhetrick55]

jimmbo - carbs are opening fully, and I am going to rebuild them soon. The compression is all between 120-125lbs. I haven't changed anything on the spark advancing at all, but I do know all plugs/cylinders are getting spark.

 

[jhetrick55]

This is my boat after a different prop, 13 1/4 X 17 three blade prop. What do you think of the WOT rpm running?https://m.youtube.com/watch?v=U4uV106FkYo

I can now get about 27 - 28 mph.

 

[jimmbo]

about 5100 - 5200 rpm is my guestimate

 

[randyray41]

From what I saw in the video it is running well. I have a 78 70 hp evinrude and that sounds like its running in the range of WOT.