I have a problem with rpm on boat?

[drrm123]

I finally took out boat I bought about 1 month ago. And engine runs fine and it get on plane ok. Once there The max rpm is about 2100-2200 at about 35 mph. Manual says should be 4000-4400 rpm. Throttle should be wide open. I've rev'd boat to about 3500-4000 with muffs on. Could prop be slipping or could I have a different problem. I tried trim up and down no difference. 1981 beachcraft about 18ft. 3.8 omc engine v6 and omc sterndrive prop 15X17 if any other info needed let me know.

 

[Chris1956]

Re: I have a problem with rpm on boat?

It is doubtful your engine is planing that boat at 21-2700 RPM. I suspect your tach is wrong.
BTW - don't rev up the motor in neutral, or it will blow up!

 

[Haut Medoc]

Re: I have a problem with rpm on boat?

I finally took out boat I bought about 1 month ago. And engine runs fine and it get on plane ok. Once there The max rpm is about 2100-2200 at about 35 mph. Manual says should be 4000-4400 rpm. Throttle should be wide open. I've rev'd boat to about 3500-4000 with muffs on. Could prop be slipping or could I have a different problem. I tried trim up and down no difference. 1981 beachcraft about 18ft. 3.8 omc engine v6 and omc sterndrive prop 15X17 if any other info needed let me know.

You need to verify the tach with a test tach & you need to verify your speed with a GPS for starters.......
One of them is wrong....
You are not getting 35 mph out of 2200 rpm....

 

[drrm123]

Re: I have a problem with rpm on boat?

thanks I'll check tach. What you said about being on plane at 2100 rpm makes sense. I'll try tach gauge and next time I take boat out I'll watch tach to see if it changes with engine sound also.

 

[BAYLINER185]

Re: I have a problem with rpm on boat?

Sounds like a tach issue to me. My brand new 185 did the reverse. My tach would hit 5000 and the boat would do 35. Motor sounded like it was reving high to me but it really wasnt. Tack was sent wrong and once fixed foud out I was only at 3000 rpm and doign 35 then was able to push the throttle to 5000 to do over 50mph

Ill bet your tack is set up for the wrong motor. Your prop may be a little smal in pitch and you may find you easily over rev the motor easily do to being under propped.

For referance my 18 foot bayliner has a 23 pitch SS prop and the WOT is 4200 rpm. the 23 AL prop WOT was about 4800 RPM

 

[drrm123]

Re: I have a problem with rpm on boat?

I checked the tach today and there are four marks on the back 1 2 3 4. It was set to 1 I have a v-6 and changed it to 3, I was thinking the one is for a single cylinder or 2 stroke the 2 is for a 4 cylinder the 3 for a 6 cylinder and the 4 for a v-8. I dont know what brand tach this is no name on it just some design on the face of tach on 1981 beachcraft with 3.8 if any of this sounds correct let me know?

 

[Don S]

Re: I have a problem with rpm on boat?

It could be any make, model, and year tach, no way in the world of knowing.
Get a known accurate shop tack, hook it up, then find the proper number on your boats tach that give the proper readings.
You HAVE to do this, no one here can guess what you have or what the settings mean, and be 100% accruate.

 

[drrm123]

Re: I have a problem with rpm on boat?

The setting on the tach was wrong. I got it as close as i could by ear with engine running wide open got it around 4000 rpm. The engine is supposed to red line about 4200-4400rpm. Will get tach gauge to get it correct. has 15X17 prop alum v-6 engine 3.8 does 37 mph around 400rpm. Which if fastest enough for me. the tach has positions between each number on the back so when I dig out a tach gauge or timing light with rpm's on I get it set accurate. Thanks for the help again.