Prop Selection

[jhetrick55]

I have a 1988 Sierra 168V (16 ft fiberglass) with a 1975 Evinrude 70hp engine. I have a 3 blade propeller, not sure of pitch (I will try to find out and post it later - it was on the boat when I bought it). I can hit about 25mph the way it sits. Since the is my first boat I am learning about prop selection but am wondering about advice from people with more experience. Since I don't use that motor to troll I thought maybe I would increase the top end speed a little and go with perhaps a 4-blade 21 pitch prop.

Mostly I am looking for suggestions and insight. Thanks in advance for your help!

 

[racerone]

Fact----The motor does not have enuff power to spin a 4 blade 21" pitch on a 16' boat !

 

[jhetrick55]

Thats good to know - as I said, I am new at this. Any recommendations??

 

[ob]

Select a prop that will allow your engine to run in the factory recommended wide open throttle operating range. On your engine that would be between 5500 and 6000. Near the upper end of that with a light load would be optimal. It may already be propped to attain that range. I'd speculate you'll end up with around a 13 1/2 X 15 three blade to attain this on your rig. Check to see which prop you're currently running and post back with what your WOT rpm is, as propped.

 

[racerone]

Step # 1 in this process is to determine that the engine is in good condition and producing the rated power.

 

[flyingscott]

Your motor is rated at 5500 rpm not 6000. So 5500 rpm is where you want to be when propped with a light load.

 

[jimmbo]

I thought the 1975 70hp was rated at 5000 rpm with a Full Throttle range of 4500 - 5500 rpm

 

[flyingscott]

I thought the 1975 70hp was rated at 5000 rpm with a Full Throttle range of 4500 - 5500 rpm

You are correct I usually just tell people to prop those motors right to 5500 rpm. That is because if you prop it for 5000 rpm and load the boat up the rpms fall off very quickly. So if you prop for 5500 rpm on those when you run with a load you can generally run above 5000 rpm which is kinda their sweet spot for running.

 

[jhetrick55]

So I may have the right prop if I am sitting at about 5500 rpm at WOT? I'll check it next time I am out. Thanks!

 

[jimmbo]

So I may have the right prop if I am sitting at about 5500 rpm at WOT? I'll check it next time I am out. Thanks!

Yes, you would have a prop that lets the engine run at the top of the rpm range.

 

[jhetrick55]

The prop i have is 13 3/4 x 15. Yesterday I did a little over 5500rpm WOT with this prop. My guess is it is propped correctly.

 

[jimmbo]

How much over 5500?

 

[jhetrick55]

5600-5700 tops.

 

[jhetrick55]

So, my digital tach is telling me I am only running at 3300-3400rpm at WOT. (I had to make sure my tach was programmed correctly) However, this means that my boat probably isn't propped right if I am only running that many RPM at WOT. Or, there is another problem I am unaware of.

Ideas?