Loud clunking sound from boat?

[Fathom]

Just bought a boat and it makes a loud clunk when it planes out. I watched the motor as we took off and it looks like it jumps causing this loud clunk. Any one familiar with this problem and what can be done to fix it?

[Black_Bay]

The first thing that comes to mind is if the prop is tightened properly. Other things are if the motor itself is tight to the transom or if the motor is moving around on the tilt tube.

[RonZych]

Is it a Mercury engine?

[Carpenter]

I had the same problem on our old lund. The motor was jumping out of gear (which made the motor rise since the boat was moving forward and the trim wouldn't keep it down), then kick back into gear causing the motor to slam into the transom. it sounded like i hit something hard. if i drove slower it wouldn't jump out of gear, but if i gave'r a little it would start to jump. it had/has twin 35's so the thump might have been a little more prevalent since the 2nd motor kept the boat moving at about the same speed for the 1/2 second gear jump. there's a place east of hackensack (Tim's marine) that i'm thinking about getting a rebuilt lower unit to swap. just haven't needed to put that boat on the water again.

anyone want an '82 18' lund console with twin 35's??

[jeffreyd]

carpenter will you deliver to phoenix

[marine_man]

Other things are if the motor itself is tight to the transom or if the motor is moving around on the tilt tube.

These would be the first things I would check...

When does it make the noise? Once you're on plane, while you're trying to get on plane, after you back off on the motor throttle, etc?

marine_man

[mamohr686]

Another quick check would be to swap out the props on both motors. If the opposite motor starts to do the same thing, then you have a spun or bad hub on the prop. I saw that happen once with a buddy. The rubber core on the inside of the prop would spin and then catch when it would heat up, causing similar symptoms that you are describing.

[Fathom]

Its a 90hp Johnson

The sound comes just after it has planed out and I've tried accelerating at different speeds but it still makes the clunck, faster speed = louder sound

Seems to be tight to the transom and the prop seems good, not sure how to check the tilt tube. How do you go about to make sure these are all correct?

Thanks for all the help so far.

[marine_man]

With the motor off, I'd tilt up the motor all the way, and grab a hold of the skeg and try and rock the motor to see if you can isolate what's moving.

marine_man

[BoxMN]

I hate being a doomsayer, and not saying I am, cause I am no mechanic.... but with a friends 90hp merc (those old 6 cyl tall skinny ones) he was hearing same type thing. It just would come and go though, not constant. His lower unit ended up blowing and needed rebuilding. I heard it do it before it blew, and it was definitely a loud "clunking" that I heard...

The easiest thing is usually the culprit, so check all bolts, connections, trim pistons (does it do it while trimming? or not?) and hopefully it is simple easy thing. But I would also drain some lower unit lube and check for metal shavings... Hope its not that though... Good luck.

[boatfixer]

You can go ahead and check the items already suggested but it sounds like the clutch dog lugs are wore out and it jumping out of gear.

[Carpenter]

I'd deliver anywhere for a price

[jtroop]

I have an old Johnson 10hp that would "clunk" when under full throttle. Sounded/felt like someone hit the bottom of the boat with a mallet. Turned out to be the clutch dogs, as earlier stated.