Speed vs Tach Question

[Ky_Captain]

Still working out the kinks in my 1991 Sylvan 20 foot pontoon boat with a 40 HP Mercury outboard. The owner?s manual for the motor says RPM should be between 5,000 and 5,500. At WOT my tach reads 4,300. GPS speed was 18 MPH with only me on board.

Never had a tach or speedometer on a boat before and wonder if 18 MPH is about right for this boat/motor combination. Backing off to 4,000 RPM produced 13 MPH.

Is my tach slow or do I need to go change the prop? Don?t know what prop is on it now.

All comments are appreciated and thanks in advance.

 

[BLC]

Re: Speed vs Tach Question

I just went through this last year with my pontoon boat. You will need to change the prop to a less pitch so the rpm is within the factory specs WOT range or you will be lugging the engine and you will not get the best performance out of it. It is also possible to damage the engine if it luggs too much too long. I went from a 13 pitch to an 11 pitch.

 

[QC]

Re: Speed vs Tach Question

That ^^^^^ assumes the tach is right. Verify that first. Take a read in the prop forum to better understand this. ALL recreational marine engines are propped in this manner. No stock prop, no perfect answer. Requires testing.

 

[MH Hawker]

Re: Speed vs Tach Question

18 MPH or so is about right for a 20 foot with a 40 HP, I would start out buy double checking the tach not saying you cant get a bit more out of it but you have to know the motor RPMs or you risk blowing the motor. Their are a couple of cheep temporary tech you can use to test yours for around 30.,00

 

[Ky_Captain]

Re: Speed vs Tach Question

Thanks to all who responded. I talked to my mechanic yesterday and he’s going to check the tach and prop next week. . I’ll look around for a test tach.

Thanks Hawker for the speed being about right. I thought that was pretty good. I didn't buy it to tear up the lake so 18 MPH is fine with me. It uses less fuel and rides smoother at about half throttle.

 

[MH Hawker]

Re: Speed vs Tach Question

It may very well also be that the tach is set to the wrong pole / pulse setting to.