Loud noise coming from lower unit/exhaust

[amkilla]

Hey guys, took my 2stroke 25hp 2001 Johnson out saturday, at first I thought I heard a noise kinda loud coming from lower unit/exhaust. Sounded kinda like the exhaust was just really loud. Drove a little bit and it faded. Then it came back but extremely loud. Can't hear it at idle its really only loud once the boat starts to plane off. After it got really loud I got worried so I idled back to ramp. Didn't notice anything outta place from the outside. Sounds kinda like exhaust but might be coming from lower unit. Really can't hear it much until most of bottom of motor starts to get out the water when giving it gas. I changed lower unit oil a couple weeks back but have run it several times since then and seen no sign of issues. Any ideas or places for me to start? thanks for any help

 

[oldboat1]

well now, sure sounds like the prop is out of the water, partially at least. Is the horizontal plate above the prop about even with the bottom of the boat? You could have a motor with the wrong shaft length for your boat (too short), or mounted too high on the transom. Guessing. Hear it when going straight, or just when turning sharply?

 

[F_R]

I also think it is mounted too high. At least it deserves investigation.

 

[Chinewalker]

Is the converging ring on the propeller? It's a plastic ring that goes around the outlet end of the prop on OEM props. Not present on most aftermarket props, so if you have a Michigan, etc., don't worry about it.

 

[amkilla]

Not too high. Been at the same height for months and been driving it without issue many time. Still has plenty of forward thrust when the noise starts. Im not so sure its the exhaust it might have been lower unit somewhere. Just sounded like a real raspy exhaust sound. Appreciate the replies

 

[amkilla]

Im not sure about the ring on the propeller. Its the same one ive had on it for a while and never had any issues with it before. Thanks

 

[oldboat1]

seems you did have issues before -- sounded familiar. You have a 2014 thread on this motor with prop problems -- long shaft, PO had reworked transom to raise motor. Said that was a spun hub. This a different issue?

 

[amkilla]

Yeah different issue. I replaced hub in prop and been working fine since. When noise starts boat still drives like normal, still gets plenty of forward push. Just loud noise.
Thank you

 

[Colthecops]

Hey guys,
I bought a new Yamaha 8hp 4 stroke recently and the noise from the prop exhaust (I think) is really loud when going just above idle.
It’s way louder than my old Yamaha 8hp - does anyone have any ideas of what could be causing it? I put it back into my dealer and they said it was fine.
I don’t know a whole lot about outboards but the sound is best described as that low rumble sound that occurs when the stern comes up in a big wave and the prop is lifted up but it’s like that constantly when I’m trolling just faster than idle.

It’s quite hard to explain - sorry I can’t be more specific!
any ideas?