Mid range random vibration?

[pyro225]

Hi guys I changed my prop a couple of days ago from a 10 to 15 pitch a couple of days ago. Yesterday took it out for the first time and it was fine. However around 3000rpm there was like a judder. You could back off and it seemed to go away or rev past it? I do have a slight idle misfire... Any ideas? Only noticed since changing prop but may have been less load on old prop so no problem? Cheers it's the mid 80s merc 80 btw

 

[MH Hawker]

I would change back to the old prop and see if it goes away. Then go from there.

 

[Texasmark]

Where'd you get the new prop and what's it's history? No reason for what you say other than a bad prop. Changing pitch alone will not do what you said.

Mark

 

[pyro225]

Hi Mate, its one off my other boat i changed when i got a bigger pitched one. i dont remember it being bad the only thing is it had about an inch gouge out of one of the blades so i filled it with chemical metal... if it was out of balance would the above happen?

 

[Texasmark]

Hi Mate, its one off my other boat i changed when i got a bigger pitched one. i dont remember it being bad the only thing is it had about an inch gouge out of one of the blades so i filled it with chemical metal... if it was out of balance would the above happen?

Probably just what you are experiencing. It's hitting an unbalance resonance and like anything else as you move into the resonant freq, the vibrations start and get worse till you continue on past the peak and they subside just like the started. As far as how wide is the "spectrum" depends on the subject matter and excitation.

Mark

 

[pyro225]

hi bringing this back up again, so i took the 15 off a while ago and went back to 10 and the judder isn't really noticeable and the light misfire is barely noticeable.

Today a forked out for a reconditioned 19 pitch prop so good conition no issues at all... i put it on and i experienced the same problems as the 15 but amplified by about 2 or 3 times... from just going past idle the whole boat was shaking and it continued shaking quite badly up until about 3k when it disappeared and was nice and smooth all the way to wot... wtf is going on? i made some videos not sure if they can help you may be able to hear though
https://youtu.be/2iPgx4QoDhs
https://youtu.be/NKtOnCHS5p4
https://youtu.be/SXvggqPKRnw

cheers

 

[pyro225]

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