Thud at 2000 RPM

[Cartouche]

We have a 2002 Mainship 390 with a Yanmar 6LYP 370 Hp I have noticed that when we go out when I bring the boat up to approximately 2000 RPM sometimes there is a thud like the sliding door closing. no vibration and after that all is fine slow down and come back up and never again a thud. any thoughts, is it possible the shaft is re seating into the coupling, Transmission fluid is up,

 

[Comodave]

I would go over the entire drive train and check for anything loose. If you suspect that the shaft is moving in the coupler you can put a mark on the shaft right behind the coupler and check to see if it is moving. You say it only does it on the first time you power up, maybe the shaft is loose and after going in reverse it moves back a bit and when you power up it reseats in the coupler. If you don’t go in reverse again and power up a second time it wouldn’t have to reseat for the second power up. The mark on the shaft would show you if the shaft is moving or not. Check the mark before and after powering up and going in and out of reverse.

 

[sunchaser]

As you go down the correctly suggested drive train checks - add aged, failing or loose motor mounts.

 

[Portage_Bay]

Don't ignore it. I had a similar situation and said "I'll find it fix it when I get a chance... I'm busy!"

Well the shaft backed out of the coupling when leaving the dock, on a windy day, in a crowded marina. It ended well, but luck played a large part.

 

[Ski in NC]

Yep, mark the shaft at coupling edge and check it after rev and fwd. Loose mounts can do it too, look for dust or other signs of motion on mount hardware. Could be backed off prop nuts too. Jump in and see if there is a gap between prop and nut(s). Check coupling bolts/nuts too, seen them loose.

 

[Cartouche]

Many thanks down at the boat this afternoon there was one set screw missing and the other two (had wire through them) were loose, hoping this was the problem replaced missing screw and tightened other two, What if the shaft is not completely seated right now at the dock, and I have tightened everything, also, is there an indent the set screws are supposed to go in, it looked smooth to me where the missing screw went? many thanks for suggesting I look glad I did

 

[RT Firefly]

Greetings,
Mr. C. "... is there an indent the set screws are supposed to go in..." I really don't know BUT I think it would be very well worth checking. It may entail removing the shaft from the coupling but IF there are, in fact, indents and you have simply tightened the screws on the shaft, you may experience another "failure" at the most inopportune time. Also a good excuse to check your engine alignment.

 

[Lepke]

Have someone watch the engine when you rev to 2000. If you have a loose or broken motor mount you'll see the engine lift that corner. Usually at the front, but not always. Thud is when the engine settles back on the mount.

 

[Comodave]

There may or may not be indents where the set screws tighten up on the shaft. First thing I would do is find out if there are indents. If so then line them up so the set screws tighten into the indents. Second thing is to ensure the shaft is all the way into the coupler. Measure how deep the coupler is and mark the shaft that length so you can see that it is in all the way, although if there are indents in the shaft and they are lined up with the set screws then the shaft should be in properly. But most likely there aren’t indents in the shaft so you will need to measure to make sure it is in all the way. Good luck glad you found the problem.

 

[C lectric]

Many thanks down at the boat this afternoon there was one set screw missing and the other two (had wire through them) were loose, hoping this was the problem replaced missing screw and tightened other two, What if the shaft is not completely seated right now at the dock, and I have tightened everything, also, is there an indent the set screws are supposed to go in, it looked smooth to me where the missing screw went? many thanks for suggesting I look glad I did

There should be an indent where each set screw end fits. Pull the screws and look.
I helped a friend years ago who had his shaft pull out of the coupling for similar reasons. His was just farther along.
His shaft was indented with a drill bit. But whoever tapered the screw points did a POOR job with a too long point which was weak. I matched the taper on the screws to that of the indent, resecured it and he never had any more trouble.

We safety wired the heads and also Loctited with #242 Blue which can be undone later but won't let go by itself.

The screw end taper should match the indent angle so the fit is tight at the indent.

 

[Cartouche]

Great advice many thanks we are hauling the boat for the Long Canadian winter next Friday, so I will pull the shaft when on shore. I will have someone check for the engine mount issue, when we head to the haul out dock. although the Mounts appear to be fine. thanks all

 

[Ski in NC]

Since you are already going on the hill, I'd replace the solid coupling with a split type. Get a machine shop to assemble the rig and skim cut the coupling to match.

Those solid couplings suck. For this very reason.