Vibration in low RPM's

[BiteFactory]

Hit a rock a couple weeks ago, took a good chunk out of my alu prop. I have a 2005 honda 150. Bought a SS prop to replace it. Now when I'm at short range in the low RPM (like 800) I get a lot of vibration throughout the boat that goes away at other settings.

Happens in reverse as well.

Any ideas?

[MacEFL]

Is it a new ss prop? Either it is out of true or you bent your output shaft on that rock. Sorry!

[Starfish]

Actually there might not be anything wrong with the prop or shaft. It's fairly common to have more vibration at low RPMs with a stainless prop than an aluminum one, apparently because of the flywheel effect of the heavier prop and the drive train slack especially on a 4-stroke. You might borrow an aluminum prop to take a test run and see if the vibration goes away, or take the boat to the dealer to verify that your shaft is straight... but I'm betting there's nothing wrong with the boat.

[BiteFactory]

Good thinking on the prop swap test. I was suspecting the prop being off a little as well. It is a new SS prop and looks uniform though.

I was thinking that the SS prop may not flex as much and transmit more vibration, but just a guess.

[Starfish]

It's actually because the stainless prop is heavier and has more momentum. At very low RPMs the crankshaft rotation is not a perfectly constant velocity. The heavier prop acts like a flywheel and maintains a more constant velocity at the prop, and as I understand it the vibration is caused by the slop in the drivetrain between the two. As you speed up the crankshaft velocity smooths out and the vibration goes away. That's my best recollection anyway, based on some posts on walleye central. I don't idle in gear much and I've never noticed much vibration... but mine is a big 2-stroke so I'd probably never notice it anyway :tongue:

[BiteFactory]

Thanks starfish