Prop, which way to go

[dreadinger]

I have a new boat to me, it is a Tracker 19' magna cc. I took it out for the first time yesterday and it only ran at 4700rpm's and 30mph after trimmed out. It has a 1975 johnson with a 13 1/4 x 17 prop. I need some advice on how to get the rpm's into the optimum range.

 

[hwsiii]

Re: Prop, which way to go

Dread, I need to know what horse power engine, gear ratio of lower unit and manufacturer and model of propeller. As well as minimum and maximum RPM at Wide Open Throttle.

H

 

[dreadinger]

Re: Prop, which way to go

Sorry I left it out,115hp 4500-5500rpm, I dont know what mfg the prop is.

 

[dreadinger]

Re: Prop, which way to go

book says propeller gear ratio is 13:26

 

[dreadinger]

Re: Prop, which way to go

One of the options in my service manual is a 14x13, any thoughts on that to increase my rpm's?

 

[Silvertip]

Re: Prop, which way to go

Your engine has 2:1 gear ratio. Running the numbers through the calculator shows that the 13P prop would give you just a slight (1 mph) speed increase and raise the rpm to 5500 rpm. Slip comes out to about 10% vs over 20% with the other prop (based on the numbers you provided).

 

[dreadinger]

Re: Prop, which way to go

That will work for me, my main goal was to increase the rpm's. The top end speed is fine. Thanks!

 

[junior1113]

Re: Prop, which way to go

first make sure setup is maxed out it engine up high enough is cav plate above or below the bottom of transom seems that motor should do a lot more than it is on this application also think it would be worth it to chech out tach

 

[hwsiii]

Re: Prop, which way to go

Dread, my calculations show a drop of RPM to under 5,200 RPM and a slight loss in speed, if you stay with the same model of prop you have now. If you change to a different model prop it is impossible to compare the geometry differences in the props without knowledge of each prop and come up with a real assessment of the number differences between them in my opinion, unless they are both just standard aluminum props.

H

 

[steelespike]

Re: Prop, which way to go

first make sure setup is maxed out it engine up high enough is cav plate above or below the bottom of transom seems that motor should do a lot more than it is on this application also think it would be worth it to chech out tach

As suggested raising the motor could improve your performance at no cost.
an inch higher could net a couple of mph and some rpm.Then careful prop selection cold get even better performance.A slight improvement in mileage should also result.

 

[hwsiii]

Re: Prop, which way to go

Steele, I agree with your statement wholeheartedly, and it only takes time and NO money dread.

H