Vibrations - what to check?

greenalien said:
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.
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How exactly does that work?
 
greenalien said:
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.
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This isnt a knocking answer, just facts. When I had my first Bene I phoned VP up, Talked to a chap called Lynn (sic) He said that VP used to recommend putting in reverse to lock the shaft whilst sailing and not motoring. He said it could lead in certain circumstances to the box locking in reverse, the answer was to flick the starter which took the load off the box and it would shift to neutral easily. He said they found that peeps wouldnt or couldnt do this so they then recommended leaving in neutral and letting it spin or fitting a brake.
So there you go, there was no mention of the engine turning over whilst in gear.
Stu
 
greenalien said:
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.
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I give answers based on what I have done/seen so that I know that they are correct. Not a difference of opinion just a matter of fact.
Stu
 
I give answers based on what I have done/seen so that I know that they are correct. Not a difference of opinion just a matter of fact.
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Sorry Stu, I don't mean to be deliberately nitpicking, and I'm not disputing the facts of what you have done and experienced - the question in my mind is if the 1mm play that you describe as normal may possibly contribute to more rapid bearing wear, since the generally accepted wisdom - which I base on looking up lots of internet posts on cutless bearings, as well as my own experience - is that there should not be any play at all.
 
skipper_stu said:
So there you go, there was no mention of the engine turning over whilst in gear.
Stu
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Chances of being able to turn a diesel over with a prop are pretty remote, although John Ridgeway claims to have started his old Lister in English Rose while surfing down waves in the southern ocean!
 
greenalien said:
Sorry Stu, I don't mean to be deliberately nitpicking, and I'm not disputing the facts of what you have done and experienced - the question in my mind is if the 1mm play that you describe as normal may possibly contribute to more rapid bearing wear, since the generally accepted wisdom - which I base on looking up lots of internet posts on cutless bearings, as well as my own experience - is that there should not be any play at all.
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From my own personal experience, Benes and their ilk have "sloppy" cutlass bearings, seems to work ok, point I was making in one of my other posts, MABS did things differently, doesnt mean the AWBs are wrong or worse!
Bear in mind that 1mm is 40 thou, so its only 20 thou either side. talking in those terms, not a lot.
Stu
 
greenalien said:
How does that work?
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The shaft freewheels when the gear lever is in 'forward' because the cones in the gear box don't grab when the lever in that direction and the shaft is trying to freewheel. In reverse, the cones grab and the thing stops rotating.

Its a function of how the gearbox works.

I am not getting into the argument about which is best or least drag etc etc except to say that the any arguments about strain on the gear box being put in reverse (and sticking in reverse) is not thinking it through. Compared to the engine driving the thing the strain is zilch. Flicking the starter will 'unstick it' as another peep has said.
 
skipper_stu said:
From my own personal experience, Benes and their ilk have "sloppy" cutlass bearings, seems to work ok, point I was making in one of my other posts, MABS did things differently, doesnt mean the AWBs are wrong or worse!
Bear in mind that 1mm is 40 thou, so its only 20 thou either side. talking in those terms, not a lot.
Stu
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I agree - I don't believe that the play is the issue. My money is on a bent shaft. Just because others have had lines round their props in similar circumstances without bending the shaft doesn't mean it didn't happen to the OP. I certainly don't think that its the propeller - but I could be wrong.
 
greenalien said:
If you leave the engine in gear in forward the prop will continue to freewheel
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How does that work?
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Speaking generally, the gearboxes use cone clutches on spiral splines to give a servo effect which increases the engagement force on the clutch when the gearbox is transmitting torque in the designed direction. Applying a reverse torque to the gearbox (e.g. engaging 'forwards' whilst the engine is stationary & prop being driven in the forward direction by water flow) reverses the servo effect and causes the engagement force on the clutch to reduce as the torque increases. This causes the clutch to partially disengage and slip (not free-wheeling, as such, but 'dragging').

This does the clutch faces no good whatsoever, and is to be avoided.

HTH

Andy

Err.. what John_Morris said !
 
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misterg said:
Speaking generally, the gearboxes use cone clutches on spiral splines to give a servo effect which increases the engagement force on the clutch when the gearbox is transmitting torque in the designed direction. Applying a reverse torque to the gearbox (e.g. engaging 'forwards' whilst the engine is stationary & prop being driven in the forward direction by water flow) reverses the servo effect and causes the engagement force on the clutch to reduce as the torque increases. This causes the clutch to partially disengage and slip (not free-wheeling, as such, but 'dragging').

This does the clutch faces no good whatsoever, and is to be avoided.

HTH

Andy

Err.. what John_Morris said !
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I claim a Lakesailor - or perhaps my typing was quicker?

edit: Ah - you noticed. Great minds think alike; fools seldom differ
 
skipper_stu said:
This isnt a knocking answer, just facts. When I had my first Bene I phoned VP up, Talked to a chap called Lynn (sic) He said that VP used to recommend putting in reverse to lock the shaft whilst sailing and not motoring. He said it could lead in certain circumstances to the box locking in reverse, the answer was to flick the starter which took the load off the box and it would shift to neutral easily. He said they found that peeps wouldnt or couldnt do this so they then recommended leaving in neutral and letting it spin or fitting a brake.
So there you go, there was no mention of the engine turning over whilst in gear.
Stu
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Perhaps the right person to answer this is the gearbox manufacturer. When the shaft is freely rotating, the gearbox though splash lubrication will maintain the required lubrication ( as it does not rely on the engine lube system for this) I understand that it is a matter concering the thrust bearing which is not designed to prolonged astern operation and hence it is recommended to lock the shaft.
With regards to the shaft clearance I tend to agree with you that over 1 mm ( which is nearly 4 times the designed running clearance) is getting towards to what is perceived to be excessive. The trouble with the rubber bearings is that they tend to wear out more at the outboard side whilst the inboard may still be within limits, and unless measured with al ong feeler or clock gauge, one may get the wrong reading.
 
Dear Cmedsailor,

Have you been on another boat to give you a relative comparison?

I also own the best sailing vessel afloat with the same engine and prop as yours. To be honest, I very rarely take the engine to high rpm and load because it does cause a bit of vibration through the hull. I hate to offer the advice "they all do that, sir" but it may be the case?

When I first bought mine I was a little concerned so I checked the shaft with a DTI and it was no more than a thou out (from memory, 6 years ago).

My previous boat had a single cylinder engine and I was convinced that it was going to jump overboard on more than one occassion. A 3 cylinder is a delight by comparison. Can't wait to buy something with a 6 cylinder for its smoothness :D

One day all boats will be fitted with Audi diesels and vibration will be a thing of the past (ducking below the parapet).

Have you noticed any recent changes to the vibration? When running at high speed under load, does the engine and gearbox appear to vibrate when you're looking at it through the steps access point?

My Volvo shaft seal keeps the boat dry and in return I regularly grease it and burp it for any trapped air. I know skipper stu and will willingly and happily take his advice, particularly on issues related to our beloved Beneteaus.

I'm due for haul-out tomorrow so I'll take a look at the seemingly controversial issue of shaft-in-fresh-air play.

Good luck with your investigation.

David
 
Leonidas said:
Perhaps the right person to answer this is the gearbox manufacturer. When the shaft is freely rotating, the gearbox though splash lubrication will maintain the required lubrication ( as it does not rely on the engine lube system for this) I understand that it is a matter concering the thrust bearing which is not designed to prolonged astern operation and hence it is recommended to lock the shaft.
With regards to the shaft clearance I tend to agree with you that over 1 mm ( which is nearly 4 times the designed running clearance) is getting towards to what is perceived to be excessive. The trouble with the rubber bearings is that they tend to wear out more at the outboard side whilst the inboard may still be within limits, and unless measured with al ong feeler or clock gauge, one may get the wrong reading.
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He did answer, his name was Lynn, he was the tech expert that VP finally allow you to speak to if you persevere!
Stu
 
cmedsailor said:
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
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During the (last) weekend I carried out the following two "tests" that would like your opinion whether it proves what some people say about vibrations due to having a 2-blade propeller behind a skeg:
1. Reverse at around 2600rpm (which normally the boat has vibrations), thus eliminating the flow of water FROM the skeg (instead there was flow towards the skeg). Although this test was a bit difficult because it's hard to hold the wheel in the middle for long, the finding was that vibration was much less.
2. Keeping two lines tight at the dock so the boat was not moving and pushing the lever forward at various rpm (2500-3000), therefore the propeller was turning but there was no flow of water from skeg towards the propeller since the boat was not moving. The result was much much less vibration.

So, what do you think? Does it prove anything?
 
cmedsailor said:
Keeping two lines tight at the dock so the boat was not moving and pushing the lever forward at various rpm (2500-3000), therefore the propeller was turning but there was no flow of water from skeg towards the propeller since the boat was not moving.
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I'm not sure that I understand. What you describe is what we call a 'Basin Trial' where the ship is tied to the dock and the engine is put in 'ahead'. Although the vessel cannot move (because it is restrained by the warps) the water is still being pushed by the revolving prop. The water does flow.

Am I missing something?


Edit: Unless you were running the engine while the gearbox was in neutral. In which case the prop wouldn't be turning.
 
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PuffTheMagicDragon said:
I'm not sure that I understand. What you describe is what we call a 'Basin Trial' where the ship is tied to the dock and the engine is put in 'ahead'. Although the vessel cannot move (because it is restrained by the warps) the water is still being pushed by the revolving prop. The water does flow.

Am I missing something?


Edit: Unless you were running the engine while the gearbox was in neutral. In which case the prop wouldn't be turning.
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No, I mean exactly what you described (you can blame my English).
Yes, the water flows but from the propeller backwards. The boat doesn't move so there is no flow from the skeg (which is in front of the prop) towards the prop. So, with the "basin trial" I still have a running engine, a rotating propeller in high rpm that would "normally" cause vibrations but have eliminated the movement of the boat. Result, no vibrations!
 
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