Should I remove my flexible shaft coupling?

[Hoist]

Bit of background on this.

Our boat suffers from a pronounced noise or drone or vibration when motoring every 3-5 seconds in anything more than tick over

It never changes, it never goes away. You cant rev out of it and anything over tick over its always there

• Moody 336
• original 1" prop shaft
• PSS Dripless seal fitted from before we owned the boat
• Original Perkins Perama 28hp with engine mounts replaced 2020 (in an attempt to fix this issue)
• Gori Race propeller fitted 2016
• Cutlass bearing replaced in 2016
• R&D Flexible coupling

I thought the shaft was possibly bent but a friend did mention this...

1. your engine mounts are rubber
2. your coupling is rubber
3. your cutlass bearing is rubber
4. and the stern gland that used to support the shaft and hold it is gone in favour of the floating PSS seal
5. all of this means your spinning long thin shaft can go where it likes

this issue has been driving me a bit potty. I get used to the drone during the season but every winter I think of a plan to fix it.

the cutlass bearing I fitted in 2016 or 2017 has now worn out again. I think this is too short of a lifespan for it and its the vibration making it worse

Our current plan is to remove the R&D flexible coupling and have the shaft bolted directly to the gearbox. Aside from changing to a saildrive we feel that is our last and only option

has anybody had a similar issue or can share past experience?

 

[MOBY2]

Morning, by no means an expert but something is wrong if your cutlass bearing is wearing out that quick. Perhaps as suggested check shaft isnt bent but first check alignment. Took my Nanni out as stud on mount had snapped... long story short a bed was not perfectly upright and the Nanni had not been installed very well but after sorting it out it was a lot smoother. Vyv Cox on here has a sight that may be worth a look or may reply with better advice than me I suspect ...Good luck hope you get sorted

 

[Arcady]

Morning, by no means an expert but something is wrong if your cutlass bearing is wearing out that quick

Agreed. Vibration in the drive train can destroy the cutless bearing quickly. I once had a badly set- up Autoprop that destroyed the cutless bearing in just one season.

As already suggested, check alignment carefully. If all checks out OK but the problem remains, I would wonder about torsional stiffness and consider removing or changing the flexible coupling. Extending that logic - perhaps examine the transmission drive plate between engine and gearbox. These have been known to wear out. Just a thought!

 

[Poey50]

Bit of background on this.

Our boat suffers from a pronounced noise or drone or vibration when motoring every 3-5 seconds in anything more than tick over

It never changes, it never goes away. You cant rev out of it and anything over tick over its always there

• Moody 336
• original 1" prop shaft
• PSS Dripless seal fitted from before we owned the boat
• Original Perkins Perama 28hp with engine mounts replaced 2020 (in an attempt to fix this issue)
• Gori Race propeller fitted 2016
• Cutlass bearing replaced in 2016
• R&D Flexible coupling

I thought the shaft was possibly bent but a friend did mention this...

1. your engine mounts are rubber
2. your coupling is rubber
3. your cutlass bearing is rubber
4. and the stern gland that used to support the shaft and hold it is gone in favour of the floating PSS seal
5. all of this means your spinning long thin shaft can go where it likes

this issue has been driving me a bit potty. I get used to the drone during the season but every winter I think of a plan to fix it.

the cutlass bearing I fitted in 2016 or 2017 has now worn out again. I think this is too short of a lifespan for it and its the vibration making it worse

Our current plan is to remove the R&D flexible coupling and have the shaft bolted directly to the gearbox. Aside from changing to a saildrive we feel that is our last and only option

has anybody had a similar issue or can share past experience?

I discovered that the excessive wear on my cutlass bearing was caused by a misaligned P-bracket. I think it had previously loosened and a previous owner had done a poor repair.

 

[Moodysailor]

This was well covered n another recent thread, but the combination of flexible mounts, flexible coupling, and no intermediate shaft support should be avoided if the shaft is long. A prop shop could probably adise on the best unsupported length.

In this case, what can happen is "whipping" where the centre of the shaft oscillates whilst rotating. I saw one do it o badly once it wore through the grp shaft tube at the inboard end - had tons of clearance when static, just whipped so much at med/high revs it was touching the tube.

So in your case, replacing the flex coupling wit ha bobbin is a good idea - but also check the alignment is perfect:

• The shaft should be centered in the tube - some shims may be needed to support it at the inboard end
• The shaft should be straight - check the run-out on V-blocks, or have a prop shop do it
• With flex mounts, try t ensure each mounts is "loaded" evenly - that is to say each mount carries an equal amount of engine weight. I've seen some bad vibrations from engines that were loaded like a 3-legged dog. If the bearers are not true this may require shims under the mounts (depending on the securing arrangement)
• If the mounts have studs, try to ensure the engine is as low to the mount as possible. Studs are a bad way of making adjustment in my opinion, but that's a rant and thread on it's own!

 

[jwfrary]

All good advice above.

Only thing to add is 5 years is quite good going for a cutlass on some installs with longer shaft lengths so may be worn and check your props opening correctly!

 

[scottie]

All above is sense but worth getting R&D thoughts first as their couplings are neither rubber nor very flexible perhaps more anti vibration and they are the experts

 

[Tranona]

Couple of observations. The PSS seal gives a similar amount of support to the original stuffing box which would have been mounted on a rubber bellows. Cutless wear needs investigating, if it is wear to one side it suggests an alignment issue either the engine/shaft or the P bracket, or a bent shaft. The R&D coupling is not very flexible, but if you remove it you still might have the alignment problem and you will almost certainly need a new (shorter) shaft or a solid spacer (which you can get from Ambassador Marine) to make up the difference. Was the noise there before you fitted the folding prop? - it could be "prop sing" or an out of balance blade.

I agree with Moodysailer eliminate the easy things by checking the shaft and going through alignment carefully - particularly centring the shaft in the stern tube while you do the alignment and before you push the seal bellows onto the tube.

 

[vyv_cox]

There is a rule of thumb about flexible parts in the drive train. There are three: engine mounts, coupling, stern gland. The rule is that any two of the three can be flexible but not all three. With your PSS on bellows as opposed to directly attached to the stern tube you have all three flexible. Removing the coupling is probably the easiest of the three to accomplish, maybe making up a dummy pair of flanges to avoid having to buy a new shaft and being reversible if it is not effective.

 

[kacecar]

I have long had a similar problem, unwanted shaft vibration when motoring, in my own boat. It is a Sigma, with a very similar set-up to the OP's.

When I first got the boat (many years ago) I was pleased at how smoothly the engine and shaft would run - I remember looking at the shaft and stern gland early in my ownership of the boat and being almost unable to tell that the shaft was actually turning. I can't remember when things changed but it did, and gradually, over years.

I have made multiple attempts at correcting it. At various times (and not in the order presented here) I have: checked the shaft is running true; changed the engine mounts; fitted an R&D flexible coupling; taken it out again (and put it back); switched from a Halyard shaft seal to a PSS; changed the cutless bearing several times; switched propellors (both 2-blade folders); re-aligned the engine to the shaft multiple times, sometimes tweaking it and at other times starting at first principles, and doing it both in and out of the water; taken shaft anodes off, then put them back on again; removed and replaced the p-bracket twice.; endeavoured to ensure the engine is running smoothly (it usually does); renewed the gearbox to shaft connection. I'm afraid to say, Hoist, that nothing appears to have made a significant difference. I'm stumped, almost resigned to it.

I know forumites talk about the wisdom of multiple flexible mounts and the whip inherent in an unsupported length of shaft etc. but, surely, the designers and constructors of Sigma and Moody boats (and many others with similar configurations) wouldn't have ignored the relevant principles - and, presumably, the vibration issues weren't there when the boats were newer.

 

[Graham376]

Does the 336 have a second cutless in the stern tube? My Moody 376 does. If that's the case, then keep the flexible coupling, otherwise don't see why you shouldn't remove it.

 

[penfold]

Removing the flexible coupling doesn't sound like it will reduce vibration; it might remove the annoying vibration you have but introduce another equally annoying vibration, as rigidly mounting it to the gearbox is going to apply an stronger excitation force than exists currently. With a long unsupported shaft a thrust bearing like an Aquadrive may be the best way of stabilising the system, but as noted above check for the simple like a bent shaft and engine alignment first.

 

[dunedin]

I am not an expert in this, but when we got our previous boat (Yanmar 3GM, shaft, P-bracket, two blade Gori) we had vibration above 5 knots, which was pretty slow for a 10m waterline.
Somebody suggested removing the flexi couple, which brought the prop closer to the P-bracket (had to move shaft anode to front of bracket) and transformed the drive. Smooth cruising at 6 knots, and at occasional 7+ knots.

 

[Moodysailor]

I have long had a similar problem, unwanted shaft vibration when motoring, in my own boat. It is a Sigma, with a very similar set-up to the OP's.

When I first got the boat (many years ago) I was pleased at how smoothly the engine and shaft would run - I remember looking at the shaft and stern gland early in my ownership of the boat and being almost unable to tell that the shaft was actually turning. I can't remember when things changed but it did, and gradually, over years.

I have made multiple attempts at correcting it. At various times (and not in the order presented here) I have: checked the shaft is running true; changed the engine mounts; fitted an R&D flexible coupling; taken it out again (and put it back); switched from a Halyard shaft seal to a PSS; changed the cutless bearing several times; switched propellors (both 2-blade folders); re-aligned the engine to the shaft multiple times, sometimes tweaking it and at other times starting at first principles, and doing it both in and out of the water; taken shaft anodes off, then put them back on again; removed and replaced the p-bracket twice.; endeavoured to ensure the engine is running smoothly (it usually does); renewed the gearbox to shaft connection. I'm afraid to say, Hoist, that nothing appears to have made a significant difference. I'm stumped, almost resigned to it.

I know forumites talk about the wisdom of multiple flexible mounts and the whip inherent in an unsupported length of shaft etc. but, surely, the designers and constructors of Sigma and Moody boats (and many others with similar configurations) wouldn't have ignored the relevant principles - and, presumably, the vibration issues weren't there when the boats were newer.

There is the possibility that the vibration has been introduced over time and wasn't there in the boat's earlier years.
GRP boats get heavier over time, and engines get less powerful. In addition, small diesels can develop vibrations doe to cylinder imbalance that cannot be removed by the flywheel - this could be due to a smal change such as one injector down on firing pressure, or a slightly blocked exhaust.
Sometimes the issue isn't actually one problem, but a combination of smaller ones - each one their own not significant enough, but they can all contribute to the issue which manifests itself in the one part which is the most rigid - the shaft.

 

[LadyInBed]

Have you put your finger on the shaft to feel the level of vibration, if not a finger for H&S, then a length of 1x4 wood. You could also apply pressure to the shaft with the wood and listen for any noise frequency change.
If you can feel the vibration is excessive then you know you're on the right track before shelling out money.

 

[jwfrary]

Dial gauge will soon tell you if your shaft is running true

 

[Poey50]

I sometimes see shaft anodes located midway on the unsupported section of shaft. That could cause problems.

 

[Tranona]

A shaft anode should be fitted about 15mm forward of the P bracket. As it is intended to protect the prop it should be as close as possible but the gap is necessary to allow a flow of water into the cutless. If the shaft is made to the correct length with 15-20mm between the prop face and the P bracket there is not space for an anode there. An alternative is to fit an anode nut to the prop as common ly used on many Beneteaus and Jeanneaus.

 

[Topcat47]

It's possible the the shaft is no longer true. As Penfold says, if it is not completely straight from the engine coupling to the screw, it will knock out the cutless bearing first. Another cause of excessive wear of the cutless bearing is an unbalanced prop. This is more noticeable the longer the shaft run from the prop to the bearing. The aqua drive thrust bearing is a good suggestion but you the only boat I have sailed with one had a connection with two universal joints from the gearbox to the flange on the shaft by the aqua drive.

 

[penfold]

The aqua drive thrust bearing is a good suggestion but you the only boat I have sailed with one had a connection with two universal joints from the gearbox to the flange on the shaft by the aqua drive.

That's what an Aquadrive is; a thrust bearing with a short shaft and two CV joints linking to the gearbox. It decouples the shaft from the engine. Greater noise/vibration attenuation can be obtained with softer engine mounts. Prop balance is a good one to check.