Under Propped??

Joined
Jun 6, 2010
Messages
27
Hope this is the right section. (mods please move otherwise)

But im having some problems with my tach on my boat. Im reading 6000RPMs and doing anywhere form 25-30mph. which is very high, but the boat sounds nice and runs smooth.

The consensus in the other topic is that i might be under propped, bad tach, or stripped splines/bumper on the prop.

***The shaft/splines/rubber bumper on the shaft is in perfect condition and not spinning.

***Im not too sure about the tach being in full functioning order and yet to have it look at.

But for the prop. Right now im running a (J 15 x 17 P) on a 17ft 1974 Glastron with a 140 hp I\O merc with the R series out-drive. usually carrying >400lbs

Whats it mean to be "under propped". and Any ideas or suggestions of that being a good/bad prop or right or wrong prop for this particular boat. Thanks in advance guys...
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Under Propped??

Under propped means too low a pitch allowing the motor to rev too much.
I'm pretty sure you have a tach problem 17" at 6,000 would probably be about 40+ mph.Your likely also under propped a little.Be sure tach connections are clean and tight.also there may be a selector switch on the back note the setting
and operate the switch a few times to clean the contacts.Of course we are assuming it is set right in the first place.I feel that a 17 should top out over 5,000 rpm.Once have an accurate rpm and speed we can get an idea of what prop you need.
 

Bifflefan

Commander
Joined
May 27, 2009
Messages
2,933
Re: Under Propped??

Under proped is not having enough pitch in the prop to keep you at the proper WOT.
17p, 19p and 21p all should be fine on your boat.
As for the speed, on my boat at 3000 RPM is about 33 MPH with a 19p.
It should be about the same for you.
 
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