do I need a flexible coupling?

[contessaman]

Hi. Re engining my boat. Old motor was md21 on flexible engine mounts connected to shaft with a rigid coupling. Shaft is quite short, with a proprietary dripless seal and a rubber cutlass bearing at the prop end. I want to fit the new engine with an r&d flexible coupling. Will this be okay or does my shaft need to be fixed at the engine end since it effectively floats with the rubber shaft seal? It will be a big help if I can use a r&d coupling - the new engine will be shorter so I could use the spacing effect of the coupling...

 

[scottie]

Rule of thumb only 2 of 3 should be flexible but R&D may be stiff enough

Phone Rodger at R&D and ask

 

[KAM]

I fitted an R & D coupling to my installation which has short shaft, flex mounts and Volvo rubber seal and rubber cutlass bearing. I think the Volvo seal instructions said not to use a flexible coupling but its run 700 hours without any problems.

 

[Topcat47]

I replaced an MD1 with a Yanmar 1GM10. The old volvo was about four times the weight of the Yanmar which has much softer mountings. It jumps around a lot more than the old volvo did. The flexible coupling is vital.

I have a friend whose 1GM10 is rigidly mounted to the engine bearers on his boat. It's a lot noisier than mine when it's working, but he manages without a flexible coupling.

 

[contessaman]

Sounds like giving r&d a call is a good idea then. Part of me thinks that it just seems wrong to have a prop and shaft supported in a rubber cutlass bearing, a flexible shaft seal and a flexible coupling... As mentioned here with rule of thumb, it needs to be fixed somewhere....

 

[Old Troll]

Sounds like giving r&d a call is a good idea then. Part of me thinks that it just seems wrong to have a prop and shaft supported in a rubber cutlass bearing, a flexible shaft seal and a flexible coupling... As mentioned here with rule of thumb, it needs to be fixed somewhere....

What would the problem be with all three parts being flexible? Excessive wear of the cutless bearing?

 

[scottie]

What would the problem be with all three parts being flexible? Excessive wear of the cutless bearing?

It needs to be fixed somewhere!!!!!!!!

 

[ghostlymoron]

Don't forget that the thrust from the prop is what drives the boat forward ( or astern). This is transmitted through the shaft to the engine mounts unless you've got a separate thrust bearing so needs some sideways restraint.

 

[rogerthebodger]

Hi. Re engining my boat. Old motor was md21 on flexible engine mounts connected to shaft with a rigid coupling. Shaft is quite short, with a proprietary dripless seal and a rubber cutlass bearing at the prop end. I want to fit the new engine with an r&d flexible coupling. Will this be okay or does my shaft need to be fixed at the engine end since it effectively floats with the rubber shaft seal? It will be a big help if I can use a r&d coupling - the new engine will be shorter so I could use the spacing effect of the coupling...

In a nut shell if you have a rigid seal or a cutlass at the inner end of your prop shaft you need a flexible coupling.

If there is no rigid seal or cutlass bearing at the inner of the prop shaft you MUST have a rigid coupling at the gearbox to shaft coupling and it needs to be aligned correctly.

If you fit a flexible coupling with a flexible shaft your shaft will move too much and possible start to hit the shaft log and damage your stern seal.

 

[billcole]

The installation instructions for the Vetus Bullflex say that if the propshaft has two rigid bearings you should remove a centering ring from inside the coupling, but if the only rigid bearing is the cutlass then the centering ring should be left in. Not sure how that relates to R+D couplings, but it does at least imply that some degree of coupling flexibilty is permissible with inboard end of the shaft not running in a rigid bearing.

If it's just spacing you need then ASAP supplies sell rigid bobbin spacers (also R + D ) to go bewteen the gearbox and the coupling

 

[Old Troll]

It needs to be fixed somewhere!!!!!!!!

My question remains unanswered about problems which would arise. Your quote is the two footed tackle with the studs showing.

 

[scottie]

Sorry if my studs were showing but the answer is relatively simple in that the entire mass of engine and shaft needs to be solidly mounted at least one point to stop it jumping about . Likely problems include damage due to unrestrained rotating shafts excessive vibration and torsional loads and in extreme cases shafts damaging stern tubes causing water ingress boat sinking and other such havoc. But at least your shins may survive.

 

[contessaman]

The installation instructions for the Vetus Bullflex say that if the propshaft has two rigid bearings you should remove a centering ring from inside the coupling, but if the only rigid bearing is the cutlass then the centering ring should be left in. Not sure how that relates to R+D couplings, but it does at least imply that some degree of coupling flexibilty is permissible with inboard end of the shaft not running in a rigid bearing.

If it's just spacing you need then ASAP supplies sell rigid bobbin spacers (also R + D ) to go bewteen the gearbox and the coupling

Now that sounds perfect! I have to fit a new four inch flange to the shaft. My next move is to buy the new gearbox (prm 120) and bell housing and take this to the boat. I can see what spacing if any is necessary and as the bell housing has the rear engine mounts, get this welded so that the engine mounts line up with the old bolt holes in the bearers. After this, fit gearbox and bellhousing to engine, crane into boat and then make up front mounts to suit... If only it was as simple as it sounds!!

 

[dougo]

hi all if your shaft is rigid inside the hull and rigid on the stern bearing and you flexible mounts you will snap your shaft in time i have repaired a large catamaran with 3inch shaft snapped in half had inner bearing in hull cutless on rear and solid mounted engine the engine needs a flexible mount suited to that engine and cutlessat rear end engine still has to be lined up correctly pss seals allow for some flex hope this helps

 

[IanCC]

In a nut shell if you have a rigid seal or a cutlass at the inner end of your prop shaft you need a flexible coupling.

If there is no rigid seal or cutlass bearing at the inner of the prop shaft you MUST have a rigid coupling at the gearbox to shaft coupling and it needs to be aligned correctly.

If you fit a flexible coupling with a flexible shaft your shaft will move too much and possible start to hit the shaft log and damage your stern seal.

Hmmm

R&d flexible couplings are not, like, flexible flexible. The engine needs to be perfectly aligned. They just absorb a bit of vibration. Unless the gap between the shaft exterior and the interior of the log is only a mm or 2, i am struggling to see how they could touch as a result of the flexible coupling. Perhaps i lack imagination ?

 

[vyv_cox]

hi all if your shaft is rigid inside the hull and rigid on the stern bearing and you flexible mounts you will snap your shaft in time i have repaired a large catamaran with 3inch shaft snapped in half had inner bearing in hull cutless on rear and solid mounted engine the engine needs a flexible mount suited to that engine and cutlessat rear end engine still has to be lined up correctly pss seals allow for some flex hope this helps

Nice example of the way that fatigue fractures occur. Long term cycling at stresses far below the UTS of the component. Do you have any photos for use on my website?