Vibrations - what to check?

[cmedsailor]

I have always believed that the vibrations of my boat are caused due to having a 2-blade propeller behind a skeg as shown in the photo. This winter most probably I will replace it with a 3-blader though I am still a bit reluctant adding this additional drag because during the last 4 years my propeller has proved really efficient and perfectly sized.
Anyway, until then (beginning of next year) I still have time to check a few things and see if anything can be improved. So I am listening for suggestions on what I could do.
Just a few things to add:
- The engine (Volvo Penta MD2030D) despite some vibrations at idle (probably normal for a 3-cylinder) does not vibrate at any other rpm. It’s the boat that vibrates! And you feel the vibration more on the cockpit slightly starboard size above where the engine is.
- Vibrations up to 2000rpm are minimal (in fact I better say non existing), up to 2500 “acceptable” but from 2500rpm and above start to get annoying.
- 2 years ago I replaced all the engine mounts with new ones and the engine was aligned (never aligned again seen then). The main difference I remember to have seen was the less vibration of the engine (not the boat).
- 3 years ago I replaced the cutlass bearing. The “lips” (or “hat” whatever you want to call it) of the bearing coming out of the shaft ware “wounded” after catching a thin line. I don’t remember to have noticed any change since then. But the cutlass bearing is around 15cm so it’s hard to believe that damaging 1cm of that thing could cause any changes.
- 3 years ago I replaced the Volvo seal. It’s still completely dry (hopefully that means that the shaft is straight!).
- If I catch the propeller and shake the shaft there’s a slight movement (1-2mm probably) in any direction. This can cause some water escape from the Volvo seal inside the boat.
- When the engine is off and I am sailing, the rotation of the shaft causes again some vibration. The engine, or the gear box doesn’t seem to vibrate (though by touching the gear box you can feel the movement inside but cannot understand if it is vibration or normal movement of parts inside)
- The propeller has some corrosion spots on it (some very very small holes). Apart from this it looks to be OK.

Any suggestions on what I could check?

Thanks

 

[Gordonmc]

The “lips” (or “hat” whatever you want to call it) of the bearing coming out of the shaft ware “wounded” after catching a thin line.

- 3 years ago I replaced the Volvo seal. It’s still completely dry (hopefully that means that the shaft is straight!).

Any suggestions on what I could check?

Thanks

I would not discount the possibility your propshaft has been slightly bent out of true, particularly as the line-catching incident was traumatic enough to damage the bearing housing.
From your pic drawing the shaft would appear to be uncomplicated. At the least, drawing the shaft and getting it tested on a lathe might eliminate it as the culprit.

 

[greenalien]

If I catch the propeller and shake the shaft there’s a slight movement (1-2mm probably) in any direction.

This has to be the root cause - there should be no detectable movement. Could be wrong size cutless bearing - 1 Inch bearing on 25mm shaft, for example. Cutless bearing should be a tight fit when dry.

 

[Heckler]

This has to be the root cause - there should be no detectable movement. Could be wrong size cutless bearing - 1 Inch bearing on 25mm shaft, for example. Cutless bearing should be a tight fit when dry.

On those rubber top hat cutlass bearings there is a mm or so of clearance from new. The line around the shaft is a red herring, I had a 12mm one around mine, it forced the bearing up the housing and didnt do anything to the shaft
It looks like a bene?
Stu

 

[greenalien]

On those rubber top hat cutlass bearings there is a mm or so of clearance from new.

A cutlass bearing with rubber inside shouldn’t have any play when installed. It's a bearing, should be a close sliding fit on the shaft, when wet. 1mm is way too much play.

 

[cmedsailor]

On those rubber top hat cutlass bearings there is a mm or so of clearance from new. The line around the shaft is a red herring, I had a 12mm one around mine, it forced the bearing up the housing and didnt do anything to the shaft
It looks like a bene?
Stu

Yes it is a Beneteau Oceanis 361.
Sorry (for my English) but what does it mean red herring? Should I replace the cutlass bearing? It was a fishing line that was caught by the propeller. The hat was damaged (not completely cut) but appart from this the bearing did not move.
In fact I tried to replace it the last time the boat was out of the water by removing the two screws and then trying to pull it with two screwdriver but no way.

 

[cmedsailor]

From your pic drawing the shaft would appear to be uncomplicated.

I wish!! You need to drop the ludder first.

 

[Ammonite]

You mention some small holes on the prop - which side are they? If they are the back of the prop (closest to the deadwood) it could be cavitation. I had this and ended up dropping the engine back an inch (an alternative would be to have got a longer prop shaft) and the problem disappeared completely. The following diagram is a useful guide:-

http://www.yanmarhelp.com/i_propclr.htm

In your case "C" looks like the dimension to check. It looks pretty tight at the top but it is difficult to judge from the photograph angle

 

[cmedsailor]

Just to make clear (in case I didn't describe it correct) the slight gap/movement it's between the shaft and the cutlass bearing. The cutlass bearing in the shaft tube is tight.

 

[PuffTheMagicDragon]

It seems to me that one of the mountings is twisted. This usually happens when not enough care is taken (during the final tightening after alignment) that the stud does not turn with the nut.

 

[Heckler]

A cutlass bearing with rubber inside shouldn’t have any play when installed. It's a bearing, should be a close sliding fit on the shaft, when wet. 1mm is way too much play.

Ok, why do peeps have to argue? on beneteaus there is a top hat bearing made of rubber. Nothing like P bracket bearings. It is held in place by two plastic screws. It has slots in it to allow water to circulate. WHEN NEW IT HAS A CLEARANCE OF ABOUT A MM ON THE SHAFT!!
I know because I have changed them 4 times on my Benes. Thats how it is, I dont care if your tufnol rubber lined p bracket things have no clearance, thats how it is on a bene.
In the olden days, on MABs they used to have stuffing boxes, that used to drip water and fling grease everywhere, on new nice boats they just have a nice DRY Volvo seal, that acts as a bearing and a seal. New nice boats have dry, dusty bilges, the VP seals dont leak. Point im making, things move on, the cutlass bearings on benes, even when new are slightly sloppy! They are a softish rubber, no tufnol, no bronze!
Stu

 

[Heckler]

Just to make clear (in case I didn't describe it correct) the slight gap/movement it's between the shaft and the cutlass bearing. The cutlass bearing in the shaft tube is tight.

We have had this conversation before, see here for instructions, http://www.ybw.com/forums/showthread.php?t=270735
I would get the old bearing out as detailed, fit a new one from Lake in Poole. Then think about the prop, I suspect an imbalance in the prop.
Stu

 

[cmedsailor]

Stu,
THANK YOU VERY MUCH.
Yes, indeed we had this conversation before and your answer was:

"Easy peasy, I did it on my 351, (a rope had gone around the prop and then the shaft, pushing the lot up the tube) Just get two big self tapper screws, drill a pilot hole in the rubber bit, screw the screws in then use mole grips and a big screwdriver as a lever and it comes out relatively easy.
Stu"

I wasn't brave enough to use the two screws method because I though that if I managed to destroy the stupid thing inside the shaft tube then removing it would have been a great trouble ...drop the rudder, remove the shaft...I don't even want to think about it.
But next time I will try it.

 

[Heckler]

Stu,
THANK YOU VERY MUCH.
Yes, indeed we had this conversation before and your answer was:

"Easy peasy, I did it on my 351, (a rope had gone around the prop and then the shaft, pushing the lot up the tube) Just get two big self tapper screws, drill a pilot hole in the rubber bit, screw the screws in then use mole grips and a big screwdriver as a lever and it comes out relatively easy.
Stu"

I wasn't brave enough to use the two screws method because I though that if I managed to destroy the stupid thing inside the shaft tube then removing it would have been a great trouble ...drop the rudder, remove the shaft...I don't even want to think about it.
But next time I will try it.

Trust me, they are pretty easy to get out, even worst case scenario, two drill holes along the length will split the damn thing, its only 100mm long.
Stu

 

[greenalien]

I know because I have changed them 4 times on my Benes.

things move on, the cutlass bearings on benes, even when new are slightly sloppy! They are a softish rubber, no tufnol, no bronze!

So that's how things are with these particular bearings, doesn't mean they are OK, the fact that you have to change them so often surely indicates that there's some sort of design flaw - the cutless bearing on my propshaft is still going strong, with no play, after over 20 years (including 4 trips through the French canals). If there's any play, it's bound to give some vibration, which causes wear, which in turn causes more play.

 

[Heckler]

So that's how things are with these particular bearings, doesn't mean they are OK, the fact that you have to change them so often surely indicates that there's some sort of design flaw - the cutless bearing on my propshaft is still going strong, with no play, after over 20 years (including 4 trips through the French canals). If there's any play, it's bound to give some vibration, which causes wear, which in turn causes more play.

Stop digging, if you had read the posts you will see that I had one damaged, I change them as a matter of course when I get the boat. I expect this one to last for years.
Stu

 

[greenalien]

Stop digging

Oh dear me, how dare someone hold an opinion different to you, please forgive me for daring to contradict you.

BTW - no boat is perfect, surely the point of this forum is that we can all learn from each other's problems - the OP described 2mm play, which is way too much on any type of stern bearing. You seem happy to accept 1mm play, well it's your boat and good luck to you.

 

[Leonidas]

I have always believed that the vibrations of my boat are caused due to having a 2-blade propeller behind a skeg as shown in the photo. This winter most probably I will replace it with a 3-blader though I am still a bit reluctant adding this additional drag because during the last 4 years my propeller has proved really efficient and perfectly sized.
Anyway, until then (beginning of next year) I still have time to check a few things and see if anything can be improved. So I am listening for suggestions on what I could do.
Just a few things to add:
- The engine (Volvo Penta MD2030D) despite some vibrations at idle (probably normal for a 3-cylinder) does not vibrate at any other rpm. It’s the boat that vibrates! And you feel the vibration more on the cockpit slightly starboard size above where the engine is.
- Vibrations up to 2000rpm are minimal (in fact I better say non existing), up to 2500 “acceptable” but from 2500rpm and above start to get annoying.
- 2 years ago I replaced all the engine mounts with new ones and the engine was aligned (never aligned again seen then). The main difference I remember to have seen was the less vibration of the engine (not the boat).
- 3 years ago I replaced the cutlass bearing. The “lips” (or “hat” whatever you want to call it) of the bearing coming out of the shaft ware “wounded” after catching a thin line. I don’t remember to have noticed any change since then. But the cutlass bearing is around 15cm so it’s hard to believe that damaging 1cm of that thing could cause any changes.
- 3 years ago I replaced the Volvo seal. It’s still completely dry (hopefully that means that the shaft is straight!).
- If I catch the propeller and shake the shaft there’s a slight movement (1-2mm probably) in any direction. This can cause some water escape from the Volvo seal inside the boat.
- When the engine is off and I am sailing, the rotation of the shaft causes again some vibration. The engine, or the gear box doesn’t seem to vibrate (though by touching the gear box you can feel the movement inside but cannot understand if it is vibration or normal movement of parts inside)
- The propeller has some corrosion spots on it (some very very small holes). Apart from this it looks to be OK.

Any suggestions on what I could check?

Thanks

Some questions that may help:

Q1: Is there a flexible joint between shaft and gearbox?? If yes does it work properly ?
Q2: What is the prop shaft/ cutlass bearing clearance ?
Q3: Have ever checked with a clock dial or other means that the shaft runs true?
Q3: Is the propeller fitted properly on the cone or is it loose?
Q4: Have ever checked the balance of the propeller?
Q5: if you disconnect the shaft at the flange and rotate it by hand do holes align again without too much effort??
Q6: Did this vibration develop suddenly or it has always been there??
Q7: Does the vibration change when you turn the rudder at same rpm??
Q8: Are you sure that all engine cylinders are firing properly ??
Q9: Have you ever made any structural modification , added / removed weights in way of the area where vibration is more pronounced ?

By the way, ( and this may be the subject of yet another blog) a school of thought says that you should lock the shaft in the astern position when you are sailing. This is in order a: to reduce drag and b : to prevent the shaft from rotating and possibly damaging the gearbox.

 

[Neil_Y]

As a supplier of shaft bearings on everything from very high speed (12,000rpm shaft) to submarine and workboats (Plymouth pilot boat)

Things have moved on from rubber as a bearing surface for water (or oil) lubricated shaft bearings, the material of choice for a number of Navies, workboats, pilot boats, race boats and submarines is a composite. Not lined with another material or surface layer but a homogeneous material. Clearances are known and specified for the full range of shaft and carrier sizes. It is a hard material which resists shaft flex or movement that can allow vibration to develop. For some of our customers they want no flex or noise from the shaft line. You can see our standard clearances here if you put in your shaft size and carrier size.
http://www.h4marine.com/Downloads/Maritex AQUARIUS General Calculator REV 20100519.xls

 

[greenalien]

a school of thought says that you should lock the shaft in the astern position when you are sailing.

The problem with doing this is that it will continually apply a pressure that's trying to turn the engine backwards, so encouraging it to suck in water from the exhaust system. Doesn't seem like a good idea to me, although putting the engine in forward gear is OK.