Prop and RPMs

[racemonster]

Ive had my boat for a little over a week now and been learning alot on this site ever since I got it.

I wanted to check my RPMs at WOT.

My tach read 7000 RPMs WOT (1991 Johnson 115HP on a 1990 Smokercraft 17 footer)

That number was a little unbelievable to me.

So the next day I put a tach off of a Racing Kart on the motor,
This tach is made for a single cylinder motor though.
I am assuming that each Cylinder on the Johnson fires once every four Revolutions?
Now because the Racing tach only tells my how often the one spark plug I can connect the tach to fires, I am getting a reading of 4300 RPMs

These are the numbers I found on my current Prop 13-1/4x19
The Speedo in the boat reads 30 MPH and the Speed matches what the fish / depth finder says.

My boat did come with a spare prop it has these numbers on it 14x17

In researching this site. I believe the motor would run best between 5000 and 5500 Rpm?

Am I correct in thinking that the 4300 RPM is more believable than the 7000 RPM? or is it more beliveable that neither is correct?

Any dirrection on the best prop wold be greatly appreciated.

Thanks
Rob

 

[steelespike]

Re: Prop and RPMs

7,000 sounds unbelieveable to me too.My guess is the tach is set wrong.
In my opinion the 19 should put you in your rpm ball park.
On a 2 stroke the plug should fire every revolution.Obviously the crank is staggered so cylinders fire at 90 degrees on a 4 cylinder.

 

[Dhadley]

Re: Prop and RPMs

The correct setting on the tach is position 6. Yours may be set on something else.

 

[Silvertip]

Re: Prop and RPMs

Lets get some numbers straight first. You have a two stroke engine, not a four stroke. A two stroke engine fires every cylinder every revolution of the crank. It doesn't matter how many cylinders you have. So every spark plug fires once every revolution. Next, the tach needs to be set to 6P (pulse) which has nothing to do with number of cylinders since the tach reads stator pulses (6 per revolution).

Now to the readings. Neither is anywhere close to accurate. A 19P prop, 2.0 gears, 30 MPH and using 12% slip yields only 3789 RPM. A 17P prop, 2.0 gears, 12% slip and 30 MPH yields 4235 RPM. That engine, should spin a 19P prop on that rather light boat with no sweat and should put you in the 43 MPH range at 5500 RPM. I would start by making sure the hull is clean, the engine is properly tuned and making full power (as in running on all cylinders) and that you are not lugging around a boat full of water soaked foam.

 

[racemonster]

Re: Prop and RPMs

It does have good compresion, and the Hull is clean.

Any pointers as to how to tell if it is hitting on all four?

How would I tell if it had watersoaked foam?

Thanks Rob