More pitch or less?

[TheGaul]

I've got a 1992 90hp Johnson on an 18' boat. Recommended RPM's are 4500-5500. It currently runs just under 4500 at WOT. I'm not sure what size or type prop I currently have. Problem is I have no acceleration. RPM's initially hit 3500 and then slowly creeps up to 4500 as the boat drags and then after about 30 seconds planes and get up to about 35 mph. Is it a pitch problem, and if so would I want more or less pitch for greater acceleration to pull up a skier? 3 or 4 blade?

 

[Dunaruna]

Re: More pitch or less?

[colour=blue]Remove the prop, clean it up - and find some numbers to post.

Need to know what you got.

and welcome to iboats!!

 

[Texasmark]

Re: More pitch or less?

I had a 125 hp Johnson on an 18' heavy tri hull open bow. I had a 17" pitched SS prop on it for water skiing which meant that it had a pretty good hole shot for the weight of the rig.

I could run 39 with just me and regular gear aboard. Didn't have a tach but you could tell by listening to it that it was not lugging.

A good (my opinion) hole shot is 5 seconds from punch to bow down on plane. You can get it with a ported prop (didn't have them back then) and save your pitch for top end.

Running just under 4500 at WOT scares me. If you are getting 35 out of that rig you are overpropped (too much pitch) and lowering pitch could allow the engine to operate in a better region of the torque curve (guessing) which would/could give you both more speed and more rpm's.....but in my opinion, 35 on an 18 footer with 90 horses is humming right along......(but the engine IS lugging and that's not good).

Course not seeing the boat and all makes it hard to just sit here and mouth off. Most of it is just speculation; quasi-educated speculation.

HTH

Mark

 

[TheGaul]

Re: More pitch or less?

So lowering the pitch would give more acceleration? I don't care if I lose some speed.

 

[walleyehed]

Re: More pitch or less?

I believe you need to step down in pitch because your WOT RPM is so far off.
Normally, I would say let's play with the set-up, but we have some un-answered questions...
Do we know we have a healthy engine?
How about tach accuracy...that's a very important gauge that can lead us on a goose chase if it's not correct.
As Dunaruna asked, what is the pitch you have now? what Model?
And, before we discuss 3-4 blade, what type of boat is your 18ft'r?

 

[TheGaul]

Re: More pitch or less?

It's a 13 1/4 X 17. I'm not sure of the model, but there were some numbers on the prop - 391199 and a small D7 and a symbol of an M if that means anything. The boat is a 1992 Four Winns 180 Freedom. Engine is healthy. Tach is accurate.

By myself today it was running at around 4700. Not sure if I throw in more people or a skier. Should I drop to 15 or could I go even more to 13? Does diameter have any effect? Steel or aluminum? Thanks

 

[walleyehed]

Re: More pitch or less?

You're a good 1000RPM below where it should be with just you in the boat....a 15P should be close by application, but your RPM says differently...that's why I asked about the tach....How did you verify the tach?
The prop is aluminum, OMC...
Diameter has some affect, but for different reasons...
SS as a general rule pulls about 100 RPM harder.

 

[TheGaul]

Re: More pitch or less?

I got to a dealer today and they said I was set up for a 13 or 15. I tried out a 13 3/4 x 15 and it was a bit better, but the whole shot was still a little weaker than I wanted. It was running at about 4900 w/ 2 people. The dealer said I could return it so I also tried out a 14 x 13. The hole shot was much better for skiing and I only lost about 2 mph from the original. It's running at about 5300 at WOT. Recommended is 45-5500. I didn't get back to them after the switch. It's w/i the recommended, but is it too much? Anything I should watch out for if it is ok?

 

[walleyehed]

Re: More pitch or less?

I know the book says 4500-5500....
The engine of 1992 was also designed to run on fuel of 1992 quality......14 years ago..
We don't have fuel that is near as good anymore, and we've found that by raising the RPM and using 5500 as the MINIMUM, we have far less coking (ring, piston, cylinder problems), less carbon build-up and a longer engine life...some found out too late.