Engine rattle when engaging forward drive

[Paolo Reale]

When engaging forward drive just before the gear is in I have a 2 second rattle.  Once the gear is in and revs are up, the rattle stops. I don't think it's the drive plate as I've had this before and its a different sound.  However I've watched the propeller shaft while engaging the gear and it seems to flex before the gear is fully engaged ( and while I can hear the rattle) and then runs normally. The odd thing is that it never rattles when engaging reverse.  I'm wondering if there's a bearing issue in the shaft coupling resulting in the flexing shaft and coinciding rattle?   Any thoughts on this would be greatly appreciated. 

[BEngo]

I incline to a an imminent drive plate failure, but, to eliminate other possibilities:

 

Is it all aligned properly?.  Check it is.  Flexible drive couplings generally work better and last longer when the engine and prop shaft are correctly aligned.

 

Check the stern gear seal/stuffing box is cool after an hour or so running in gear.  IA stuffing box  should drip a little, but if it is much more check the packing, and the alignment.

 

 Check the coupling bolts are all tight.

 

Also check the engine mounts are fastened to the bed tightly, with the bottom nut up to the engine foot and the top nut also tight.

 

Then have a look at gear and throttle cables  and water/oil pipes to check that they are not chafing and rattling as the engine goes into gear.

 

 Down the weed hatch, check  the prop is tight upon the shaft, and has its securing split pin.  Check the shaft for up/down, left/right and fore/aft movement.   It should not move unless you have a Kelvin type gearbox when some fore and aft movement is expected.

 

N

 

 

[Paolo Reale]

Thanks for these suggestions.  I'll certainly check as much as I can. Couple of quick questions  - I don't have a stern gland with a stuffing box - it takes silicone grease and is totally try, I forget its name. Anyway thing in particular to look at there? Also do you feel I'm probably wrong about the bearing issue despite seeing the propeller shaft flex while engaging forward? Is this normal?

[Tracy D'arth]

26 minutes ago, Paolo Reale said:

Thanks for these suggestions.  I'll certainly check as much as I can. Couple of quick questions  - I don't have a stern gland with a stuffing box - it takes silicone grease and is totally try, I forget its name. Anyway thing in particular to look at there? Also do you feel I'm probably wrong about the bearing issue despite seeing the propeller shaft flex while engaging forward? Is this normal?

Flex which way? Forward or side to side or up and down?  What type of coupling is it?  A photo of the shaft arrangement may help you get some ideas.

[BEngo]

Only thing you can do to the fancy seals is check they are running cool and in proper alignment.

 

It will be a really odd shaft coupling if it has a bearing- there is often but not always a separate thrust bearing in front of the stern seal but this is not usually part of the coupling.

 

I would expect any  flexible coupling to twist as you put it in gear.  Either several kilos of prop and shaft goes from 0 to about 300 rpm instantaneously (it cannot- ye canna change the laws of physics cap'n) or there is some give somewhere.  A Ffexible coupling is the easiest, and most visible place where you will see give,  otherwise there must be some slip in the gearbox clutch cones/plates, but you will not easily see that.  You should also expect the engine to rock, unless it is rigidly mounted.  This is just the normal reaction to the torque being applied to the prop. The direction of rock will be opposite to the way the prop goes and one way for forward, the other for astern.

 

Photos of the whole set up would help.  General view of the whole engine box and shaft, then closer shots of each individual item, including the mountings,  would be ideal.

 

N

 

[Mad Harold]

The adjustment of the gear cable on some gearboxes is critical. 

After fitting a new gear cable to my boat, it went into fwd ok but trying to engage reverse, graunching and rattling noise but no drive. Adjusted the cable and found the opposite, reverse ok but rattling and graunching but no fwd drive.

The difference between working properly and what I have just described was a couple of turns on the cable adjuster.

Hurth gearbox without stops on the gear selector. 

Lever on the gearbox would go from fwd to neutral, to reverse in one sweep. 

Accurate gear engagement depended on the correct adjustment of the Morse control.

Try removing the cable from the gear lever on the gearbox and engage gears by hand to see if this could be the problem.0

Edited June 16, 2022 by Mad Harold

[MtB]

8 hours ago, Paolo Reale said:

When engaging forward drive just before the gear is in I have a 2 second rattle.  Once the gear is in and revs are up, the rattle stops. I don't think it's the drive plate as I've had this before and its a different sound.  However I've watched the propeller shaft while engaging the gear and it seems to flex before the gear is fully engaged ( and while I can hear the rattle) and then runs normally. The odd thing is that it never rattles when engaging reverse.  I'm wondering if there's a bearing issue in the shaft coupling resulting in the flexing shaft and coinciding rattle?   Any thoughts on this would be greatly appreciated. 

 

 

As always, let's start with what make and model of gearbox, engine do you have?

 

It's almost like asking "my car won't start, what could it be?"