Flexible coupling on 47
I am experiencing strong vibrations of above 2000rpm and I am hauling the boat for checking the alignment, together with calibration of the shaft and propeller.
I will put new bearings and new antivibration mountings, too.

On my boat there is a flexible coupling from Vetus.
The engineer of the yard has told me that he does not think that this coupling is necessary in case there is a proper alignment of the engine to the shaft. To his opinion the shaft is not that long to require this coupling (I will have in case to install a new coupling because I broke it last May in Greece and refixed it on board with screws of bigger thread).

I wonder if this is correct and if the other swan 47 owners ( or similar boats) have a flexible coupling between the gearbox and the shaft.

The engine is a 62hp Yanmar of 1994.
Thank you, and the opinion of the professor would be much appreciated.
Matteo

Dear Matteo
With the original 1.5" diameter and 5' long shaft there is no need for a flexible coupling unless a second bearing has been installed on the shaft. Adding a flexible coupling actually makes the shaft more prone to whirling, but for this shaft the whirling rpm is close to 2000. Assume there is a reduction gear and that the shaft rpm never goes that high. Without a flexible coupling the shaft whirling rpm is about 2600.
It is likely that misalignment or unbalance is behind the problems. Engine power, rpm, reduction ratio, propeller size and type, number of blades, maximum achievable speed and rpm may give some hints.
Which hull number is it?
Kind regards
Lars

Dear Lars,
I am confused. I have to admit that I never looked closely at the shaft coupling of my boat but I always assumed that it was somehow flexible, at least in the capability of absorbing rotational shocks. I will certainly look at it next time I go to the boat but: assuming that it is still the originally mounted one, am I not going to find any presence of rubber between the two flanges of the coupling?

Daniel, 411/004

Dear Lars,
thank you for your prompt reply which is,as always, very exhaustive. My boat, Grampus II, is hull #16. It might be possible that the coupling was installed at the moment of fixing the new engine Yanmar 62 hp in substitution of a Volvo.Probably this was a way of adjusting a misalignment of the new engine that had a different geometry. Anyway the problem increased this summer and was probably due to a bad hauling of the boat ( I was not present attending that time) and something went wrong.The propeller is a three-blade max-prop, I presume 20 ". Speed with 2900 rpm and clean hull is 7.0 knots, at 2500-2600 6.5 knots. At 2200-2300 as it was this summer it was 6.0.
I presume that the gearbox has a reduction of 2:1
Should the gearbox be checked, too?
Thank you very much,
Matteo

Dear Daniele, I believe that all reduction gear boxes have internal flexible couplings to reduce shocks. In my case the flexible joint that I am mentioning is external and very bulky, about 18 cm diameter and should provide mainly to tolerate small angles between the gearbox and the shaft.
Thank you,
Matteo

An interesting discussion.

Lars, can you expand on what you mean by “whirling rpm”?

We put a flexible coupling in a few years ago.  The logic being that the engine/gearbox are on flexible mounts and move rotationally and fwd/aft under load; whilst the other end of the power train, i.e. the shaft is relatively fixed in the P bracket and shaft gland/seal.  From a purely engineering perspective this arrangement does not make much sense.  My understanding is that the intention of the flexible mount is to allow the prop shaft to remain “fixed”, and allow the engine/gearbox to move on their mounts.  Or are the movements small enough that it doesn’t really make any difference?

I would have though that you should be able to get a fairly decent alignment on the shaft line.  If you have along shaft line and it was originally set up on land it is worth checking everything again in water, as the boat will probably sag slightly.

Be interested to hear if you resolve it as we are suffering something similar, but it is annoyingly intermittent.

All the best
Adrian

Dear Matteo
Thank you for the information
If the vibration increased after hauling it needs to be checked whether the shaft and/or the propeller have been damaged for example by lifting strops causing them to bend.
I would suggest that the flexible coupling can be removed and proper alignment of shaft and engine needs to be done. Before doing the alignment check that the shaft is centered in the stern tube - when the shaft flange at the forward end is disconnected from the reduction gear, lift the shaft up and down so the upper and lower clearance limits in the stern tube are found, and leave the shaft in the intermediate position. It would be sufficient to check the condition of the reduction gear flange unless there is trouble with the gear shift.
It appears your reported speeds and used engine powers are rather low. At 7 knots about 20 hp is required, at 6.5 knots 15 hp, and at 6 knots 13 hp. When the installation is in order it is suggested you report the full throttle speed and rpm.

Daniele, if you have an engine installation drawing in your manual it is suggested you check whether a flexible coupling is mentioned in the parts list. I would agree with Matteo that the reduction gear has a flexible element included.

Adrian, suggest you google whirling of shafts, this is something that definitely needs to be avoided. Fortunately it is rather easy to calculate the critical length of a thin rotating shaft, it could be added that it is not advisable to go very close to this, as shafts usually are not perfectly straight, and this may cause vibration already when approaching the critical length. With one bearing aft the forward end of the shaft is free to move with the engine due to the flexible shaft seal. You are right that shaft alignment may change slightly after launching.
Best regards
Lars

thank you very much, your suggestions are very clear. Meantime, I have found topics regarding the flexible coupling in this blog : http://www.samlmorse.com/forum/read.php?5,9449 .
I do not know if the opinions are correct, but there is good common sense and some hints.
It looks like the Yanmar mounts should be soft ( softer than the Volvo ) and should be checked frequently.
Then comes the issue: do we need a flexible coupling?
Probably yes, it helps if there are not many maintenance controls in the engine system and it is so a kind of protection.
Probably not, it is an extra piece and what you can do without won’t break. I broke my flexible coupling last May, without it I would felt the vibrations and checked something else, the mounts of the engine, the propeller and anything else.
It interesting to read that some people think that a flexible coupling helps when motoring with heavy seas.
Dear Professor, what do you think? Most probably you might assess that, not belonging to the original building project, we can do without it. Or no?
Thank you very much,
Matteo

Dear Matteo
I would suggest that a flexible coupling is not really needed in your case. There is, however, considerable pressure from people with commercial interests towards installing these, and the sales talk claims many reasons.
Best regards
Lars

Dear Lars,
your suggestion is very reasonable.
I will contact you as soon as I will check what has probably happened. In case, better a new shaft than a new flexible coupling!
Thank you as usual for your help, I appreciate very much.
Matteo