boat wants to turn left with new prop

[TexasIvey]

I had to replace my prop. I had an aluminum black max prop (PN 48-78118-17) to an aluminum turning point 14.25 inch 17p prop. I crossed the number to the turning point prop (size & pitch are the same but the rake I can see is different). I went for a test drive last night (1990 18.5' starcraft, heritage 1901, mercruiser 3.0L alpha 1 gen 1) and when I was running near or @ WOT the boat wants to turn left & was very hard to turn the wheel to the right. if I let go of the wheel the boat would have snapped to the left. At low and mid range speeds boat tracked fairly straight. Any thoughts. RPMs about 4600 @ WOT & speed was 30mph (fully loaded 6 people).

 

[Bondo]

Re: boat wants to turn left with new prop

I had to replace my prop. I had an aluminum black max prop (PN 48-78118-17) to an aluminum turning point 14.25 inch 17p prop. I crossed the number to the turning point prop (size & pitch are the same but the rake I can see is different). I went for a test drive last night (1990 18.5' starcraft, heritage 1901, mercruiser 3.0L alpha 1 gen 1) and when I was running near or @ WOT the boat wants to turn left & was very hard to turn the wheel to the right. if I let go of the wheel the boat would have snapped to the left. At low and mid range speeds boat tracked fairly straight. Any thoughts. RPMs about 4600 @ WOT & speed was 30mph (fully loaded 6 people).

Ayuh,... Welcome Aboard,... Adjust the torque tab above the prop,...

If that don't work, sell the prop, 'n try another,.... yer hull don't like that 1....

 

[Bamaman1]

Re: boat wants to turn left with new prop

+1 on above. If it's hard steering to the left, move the tab just a little to the left and tighten it back down. It's trial and error.

 

[TexasIvey]

Re: boat wants to turn left with new prop

Thanks for the help. I will try adjusting the tab...I did have to go to a narrower version of tab for prop clearance. If I have to find a prop that likes my hull that sucks! I did the slip calc for this prop and came up with 18.9% this seems high. (rpm 4600, gear ratio 1.98, pitch 17, wot speed 30mph) I need to verify speed with GPS next time I go out. What reduces slip? cupping? Current prop is cupped.

 

[steelespike]

Re: boat wants to turn left with new prop

Is your speed by gps?I feel the typical prop would be a 19 or 21 with a speed at least in the upper 30s
Some aluminum boats are harder to push with all those rivets,seams and convolutions so may produce slightly higher slip numbers. Your speed is in the ball park for 4600 but a 3.0 would normally over rev with a 17" pitch. The turning point is usually called aggressive so it might be holding down the rpm.
How did the old prop do?

 

[TexasIvey]

Re: boat wants to turn left with new prop

Old prop was in the same neighborhood of 4600rpm & 30 mph. You say 19 or 21 pitch should yield upper 30s...max speed calculations yields 37mph so would I be able to get 33mph (~10% slip)? If I have to change the prop to correct my steering issue should I change manufacturer's or models or will just changing pitch help? Has anyone tried the new 4 blade Turning Point with 2 different blade shapes? One provides hole shot and other provides top end (this is the company line)?

 

[steelespike]

Re: boat wants to turn left with new prop

What I'm saying is that typically a 3.0 in a 18 or 19' boat would run a 19 or 21" prop at
about the 4600 and upper 30s Usually a 17 is suggested for water sports with a load.
with a caution to watch rpm.
Something is up with your setup. Is the speed by gps? Your rpm in relation to the speed seems ok but the 17" should rev much higher.
Could be as simple as the throttle not opening all the way, perhaps the timing is set wrong or not advancing all the way.
Is the bottom clean and smooth? does the boat respond to up trim?
Wouldn't hurt to do a compression check and note the condition of each plug. As far as a prop change I would wait till we know the motor is right.

 

[TexasIvey]

Re: boat wants to turn left with new prop

Sorry, it took awhile to sink in but I got it ...a 2" increase in pitch trumps the loss of ~200rpm. I noted in my previous post that I need to verify speed with gps. Not sure how to verify RPM though.

 

[steelespike]

Re: boat wants to turn left with new prop

No.I guess I didn't make my point very well.
First increasing pitch lowers rpm and decreases power like overdrive on a car.
Decreasing pitch increases rpm and power. like a lower gear on a car.
Rule of thumb a 1" pitch change nets "about" a 200 rpm change.
Any change in speed with both would depend on the rpm and speed with the original prop.
You have an issue with your motor not your prop. A healthy 3.0 should over rev with a 17" prop.
You need to find out why the motor can't turn a 17"prop over the rated wot max rpm.
Answer the questions in my other post , may help figure out what is holding back the performance.

 

[TexasIvey]

Re: boat wants to turn left with new prop

I adjusted the torque tab and the boat handles much better. I think with 1 more tweek I will have the turing left problem solved. I did check on rpms @ WOT and I was wrong it appears I am running more like 5200 rpm but my guage is flakey above 5000. Could the gauge be bad? it bounces between 5500 and 5000 with no increase in speed. @ 5200 rpm i was @ 32mph (still need to verify with gps...i will do this tomorrow). I am now inclined to go with a pitch of 21.