Tach over 2000 rpm off??

[Skyhead22]

23 foot pontoon, mercruiser 3.0. 21 pitch Mirage prop. 35mph wot light load tach reads 2200 rpm. That would give me -67% prop slip.
Realistically I'm turning 4600 rpm or so.
Boat was bought brand new 4 years ago by my parenents. All this time he thought wot was 2200 rpm.
What's the deal?

 

[Scott Danforth]

first, check to see if the tach is set to 8 cylinders vs 4 cylinders

if it is set right, then test it against a diagnostic service tach

if it is really that far off and set right, buy a new tach.

 

[Skyhead22]

first, check to see if the tach is set to 8 cylinders vs 4 cylinders

if it is set right, then test it against a diagnostic service tach

if it is really that far off and set right, buy a new tach.

Would the tach typically have a switch on the back of it?

 

[Scott Danforth]

yes, over 90% of the tachs used have a switch on the back side. that way the boat builder just has a pile of tachs that are the same across many models and doesnt care what motor is connected to it until the tach is installed to the specific boat.

 

[CaptainMak]

Try this: http://www.mercuryracing.com/prop-slip-calculator/ Sometimes I refer to this when I suspect prop acting up.

Has it been doing this for a while (since you bought it), or a recent development ?

 

[thumpar]

I think everything is fine mechanic wise. Check the switch. Even if it is set to 4 switch it back and forth in case the connections are corroded inside. With your numbers and a 12% slip it comes back at about 4000rpm so the prop might be a little steep. You should be around 4600+.

 

[johnkom]

Anything I put into the calculator comes up "NAN." What does that mean??

JK

 

[Skyhead22]

I think everything is fine mechanic wise. Check the switch. Even if it is set to 4 switch it back and forth in case the connections are corroded inside. With your numbers and a 12% slip it comes back at about 4000rpm so the prop might be a little steep. You should be around 4600+.

It is around 4600. The drive ratio is 2.4

 

[thumpar]

OK. That makes more sense. 2.4 is not a common ratio.