2700 RPM's at WOT

[RC]

1986 Four Winns 190 Horizon OMC Cobra 4.3 liter. 14 1/2 x 19 3 blade stainless steel prop. Manual says WOT is between 4200 and 4600 rpm. Top speed at 2700 RPM's with only me aboard is about 34mph. Boat has always run great. Am I just way under propped? I don't care to go any faster but I also don't want to be lugging down the engine. Thanks

 

[Silvertip]

Re: 2700 RPM's at WOT

Ahhh -- one of three things is wrong here. 1) tach is wrong, 2) prop is wrong, or 3) speedometer is wrong. I suspect the speedometer is wrong. If you are truely turning only 2700 RPM at 34 MPH, the engine is turning at just under half the recommended max. Therefore if it did really run at 4600 RPM your boat would be at something over 60 MPH. You can do the math to get exact numbers. Step 1 is to attach a known accurate tach and verify your dash tach is accurate. Step 2 is to borrow a GPS and get a true speed reading. Pitot tube speedos are notoriously inaccurate. If step 1 and 2 confirm that speed and RPM are accurate (I doubt it) a prop change is necessary to get the engine in the proper RPM band as you are severely lugging it or the throttle is not opening fully. A 19 pitch prop sounds about right however.

 

[Silvertip]

Re: 2700 RPM's at WOT

Ahhh -- one of three things is wrong here. 1) tach is wrong, 2) prop is wrong, or 3) speedometer is wrong. I suspect the speedometer is wrong. If you are truely turning only 2700 RPM at 34 MPH, the engine is turning at just under half the recommended max. Therefore if it did really run at 4600 RPM your boat would be at something over 60 MPH. You can do the math to get exact numbers. Step 1 is to attach a known accurate tach and verify your dash tach is accurate. Step 2 is to borrow a GPS and get a true speed reading. Pitot tube speedos are notoriously inaccurate. If step 1 and 2 confirm that speed and RPM are accurate (I doubt it) a prop change is necessary to get the engine in the proper RPM band as you are severely lugging it or the throttle is not opening fully. A 19 pitch prop sounds about right however.

 

[Silvertip]

Re: 2700 RPM's at WOT

Jeez -- stuttered again!

 

[RC]

Re: 2700 RPM's at WOT

Thanks, how exactly would I accomplish step #1?

 

[Silvertip]

Re: 2700 RPM's at WOT

If you know any mechanic buddies, they may have an inductive timing light with a built in tach. Connect it to a spark plug wire and read the RPM. What does your tach read at idle. If it appears normal 650 - 700 RPM, chances are the tach is reasonably accurate. If its reading way below that or much higher, chances are its not.