Prop pitch ?

[tompowers85]

I have a 1975 Mercury 850 that i just rebuilt, and it is on a 15 bass boat. I am currently running the engine at 3700-3800 rpm's at 28 mph with a 21 pitch prop. The WOT is 5500rpms. I would like to get as much speed as possible. I am able to get a sum what decent hole shot with the 21 pitch but i feel the engine should push the boat faster. I have checked the cylinders and they are all at 150 psi and have good spark. Any suggestions would be great thanks.

 

[Silvertip]

Re: Prop pitch ?

Are you saying 3700 - 3800 rpm is the fastest the engine will rev? 5500 is the "recommended" maximum rpm at wide open throttle so what you see on the tach and what the manufacturer recommends are two entirely different things. To get from 3800 to 5500 rpm would mean you need to drop prop pitch from 21P to 13P which is not seem reasonable. I don't know the lower unit gear ratio of that engine but my feelings are that the engine is not performing as it should or that the prop pitch needs to be in the 17P range which would get the engine up on its power curve and then allow it run at 5500.

 

[tompowers85]

Re: Prop pitch ?

it feels as if the prop is slipping in the water. I do not see the rpms jump, but it does not seem as if it is bitting real well.

 

[Silvertip]

Re: Prop pitch ?

If the prop was ventilating or slipping, the engine would over rev. You didn't answer the question about the rpm so I'll ask it in a different way. Is 3800 rpm as high as the tach will go with the throttle wide open? Try pushing the key switch (choke) when the engine reaches 3800 rpm. If it picks up a little and then dies off you have a fuel delivery problem. If not I would be looking at some other engine problem -- like link&sync. It may be that timing is not advancing like it should.

 

[BAYLINER185]

Re: Prop pitch ?

Sounds like the same problem Im having.

I have a brand new BAYLINER 185 open bow ski boat and Im still in break in stage.

Im easily hitting 5k RPM but the max speed is only around 35 mils an hour.

I talked to my dealer today and he says I shouldnt be able to hit 5000RPM and it means I have the wrong pitch prop. They are goign to swap props tomarrow and this weekend Ill try it out.

My prop on the boat stock is a 14 1/2 inch X 23 pitch and I could sware my old boat had a 16" X 14 pitch. I had a 19' webbcraft w/ 4 cylinder engine and it did 45MPH top speed. I have to be able to get a lot more from this brand new bayliner w/ V6 190 Horse power then my old 120 Hp motor.


It does sound like there is too much prop spin / slip in both our cases. My dealer recomends going down 1 or 2 pitch sizes at a time to figure out the best pitch.

 

[tompowers85]

Re: Prop pitch ?

Yes, my max rpm's are 3800. I am only able to hit 3800 if the water is like glass and i am not into a breeze, otherwise my rpm's get to about 3600. I will try hitting the choke next time I am out. I do think I am having a fuel problem. I just cant figure it out. I can have the engine warmed up and hit the choke and the engine does not die. I have replaced everything from the tank up except for the needle valves and check valves, when i checked each valve they seemed to be fine. I am going to try and replace them this weekend. Thanks for the posts.

 

[Silvertip]

Re: Prop pitch ?

Bayliner -- If you are hitting 5000 rpm now with a 23P prop. Going down in pitch will make it rev even higher and make it even slower. Although you are trying to help this poster, your problems are very different than his and therefore you should post your issue in a separate thread.