Prop Choice 17 or 19 pitch

[rainman99]

I have 2 props 13 3/4 x 17 and a 13 3/4 x 19 aluminum never used the 17 pitch. Wondering which one will give me the smoothest ride on choppy water ? I have a 1985 90 Hp. Johnson outboard on a 1973 16.6 Ft. Wells craft Air slot 165 sport boat weight 18000 Lb.s with 2 people WOT 42 MPH at 2800 RPM.using the 19 pitch.

 

[hwsiii]

Re: Prop Choice 17 or 19 pitch

Rain, I show that mtor to have a 2:1 gear ratio, if that is true then your tach is not working right. I show a major negative prop slip.

H

 

[rainman99]

Re: Prop Choice 17 or 19 pitch

Rain, I show that mtor to have a 2:1 gear ratio, if that is true then your tach is not working right. I show a major negative prop slip.

H

Thanks for the help . Now I must ask what is a negative prop slip and what should I do about it if any thing?

 

[rainman99]

Re: Prop Choice 17 or 19 pitch

Think I found the answer to my prop slip question with this old post. I agree my tach must be off. But this still leads me to my first question witch prop should I Use 45Auto;1871522]Slip is pretty much an irrelevent term (but fun to talk about) when it comes to props. You can easily find combos that show "negative slip" (meaning the boat is moving faster than theoretically possible) due to drive combos, prop cup, etc.

Your "20% slip" is due to the angle of attack relative to the water that's required for the prop to drive your boat at those conditions. You'll find that your slip varies from about 70% as your boat comes on plane to around 10% at top speed. 20% slip is about correct for the combination of pitch, RPM, and speed you posted. From the Mercury prop slip calculator:

http://www.mercuryracing.com/propellers/propslipcalculator.php

"Slip is the most misunderstood of all propeller terms, probably because it sounds like something undesirable. Slip is not a measure of propeller efficiency. Rather, slip is the difference between actual and theoretical travel resulting from a necessary propeller blade angle of attack. If the blade had no angle of attack, there would be no slip; but, of course, there would be no positive and negative pressure created on the blades and, therefore, there would be no thrust."

Hopefully the 29 MPH at 3600 RPM is not top speed on your boat. If it is, you have a problem in that your RPM is about 1000 RPM too low. I would expect your boat to run around 40 MPH at WOT (wide open throttle) with your engine RPM between 4400 and 4800 RPM (real similar to what MikDee posted). At that point your slip will probably be 12% to 15%. If 29 MPH is your top speed, when is the last time you had a tune-up and compression check done on the motor? If 29 MPH isn't your top speed, post the conditions at top speed and let's see what your slip is.[/QUOTE]

 

[Randybeall]

Re: Prop Choice 17 or 19 pitch

Hmmmmmm, I hope that you just got the weight wrong, 18000 pounds of boat would be really slow. Humor aside, your question was about what prop would give the best ride on choppy water. I suspect that without changing to a stainless 4 blade prop you will not be able to tell a difference in smooth ride quality.
That said, put the 17 inch blade on it and give it a test. Maybe the engine tune is bad, less load through less pitch will let the rpm come up and give you better overall performance. Looking at the prop calculator I suspect that is close to what you want anyhow.

 

[countvlad]

Re: Prop Choice 17 or 19 pitch

I have 2 props 13 3/4 x 17 and a 13 3/4 x 19 aluminum never used the 17 pitch. Wondering which one will give me the smoothest ride on choppy water ? I have a 1985 90 Hp. Johnson outboard on a 1973 16.6 Ft. Wells craft Air slot 165 sport boat weight 18000 Lb.s with 2 people WOT 42 MPH at 2800 RPM.using the 19 pitch.

if your tach is bad i dont recomend switching the prop... if you're already at you max rpms switching to a lower pitch would definetly hit your redline and posible hurt your motor....

if there s a problem with your tach fix that problem first before you experince with props.....

from what i can tell 42 mph its about right on that boat and motor.... for some reason i think 19 its pretty big, but i might be wrong.....

if i were you i would try this first , make sure tach works correctly.... theres no way we can help you withoout knowing the rpms that the prop is running at....

http://forums.iboats.com/showthread.php?t=190636

good luck

 

[hwsiii]

Re: Prop Choice 17 or 19 pitch

Rainman, those people can say anything they want to about Prop Slip. I will only say this.
At 5,900 RPM one prop has a prop slip of 8% which means he does 49 MPH
At 6,300 RPM another prop has prop slip of 34% which means he does 43 MPH

Both are 19" pitch props but the 34% prop slip is turning 6,300 RPM
The one with the 9% prop slip is turning 5,900 RPM

The motor they are on is rated for 6,000 RPM

Which prop do you want and why if prop slip means nothing

H