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Old 31-01-2008, 22:34   #1
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New Gearbox problems

Can anyone help?
I have just bought a new Beneteau Oceanis 43 (6 months ago) with the standard 55hp Yanmar diesel motor.
I have had a number extras added, including a 3 bladed Gori Prop.
After 4 months of owning the boat, with the motor having done less than 40hrs, the gearbox had a catastrophic failure. (Luckily I had just moored the bow on to the dock). On examining it, a large hole was evident in the front of the gearbox. It appeared as if the drive shaft had been forced through it. Yanmar were very good at responding and had an engineer on site quickly. They replaced the gearbox but could not identify the problem, putting it down to a manufacturing fault in the gearbox itself. They described the damage as if the boat had been reversed at speed and hit something solid forcing the driveshaft through the gearbox, but as there was no damage to the rudder this was impossible and I can assure you it didn’t happen.
After the motor had registered another 20 more hours, yet again and in exactly the same manner, the replacement gearbox failed. Yanmar have also replaced this one, but they still can not give any explanation for the fault. Their investigation seems to be leading towards the prop, but as their Reseller supplied both, they are insisting the specs are correct and have explained that there are many boats with this engine/prop combination.
Has any one got any ideas?
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Old 31-01-2008, 23:33   #2
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...are you getting shaft whip?
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Old 01-02-2008, 00:08   #3
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Army, marching in step...

Is it possible that you have the "Army marching in step over the bridge syndrome"?

That is, the propeller/shaft is vibrating in synch with the engine/gearbox/mounts.

Is it an aluminium gearbox?

Are there any shiny areas (as opposed to crystaline) where the metal has broken?

Can you feel any strange vibration during normal operation?

Are there any large weights bolted to the gearbox?
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Old 01-02-2008, 01:39   #4
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Thanks chaps
There are no unusual vibrations etc. The engineer who has fitted both gearboxes has tested it for shaft realignment and suggests this is defiantly not the problem.
Prior to both gearboxes failing there was no indication that there was a problem i.e. no unusual vibrations, no unusual or concerning noises. In fact I only noted the second gearbox failure after seeing oil in the collection tray beneath the engine.

One issue I have noted though is that I can only get the engine up to 2800 revs at full throttle giving a SOG of approx. 6 knots. The supplier suggests I should be able to get it to 3300 revs at 8 knots. I will be diving on the boat tomorrow to see if the prop is fouled or stuck in overdrive. Yanmar suggest while this may be a problem it should not cause the type of problem I am having.
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Old 01-02-2008, 01:46   #5
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A good friend had a new 53hp yanmar installed by "trained" yanmar people & had 2 gearbox failures before it was found the installation was at fault. The shaft rotation was wrong ie the boat was being driven f'ward when the box was in reverse!! It lasted about the same time as you are indicating. The mechanical box is not designed to be able to work in this way. Just a thought.

Regards Bill Goodward
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Old 01-02-2008, 02:11   #6
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Thanks Bill
Despite my inexperience at these things your explanation seems logical.

Did your friend’s gearbox suffer the same type of failure i.e. catastrophic (hole in the side?)
When you say the shaft rotation installation was at fault. I assume you mean the gearbox is fitted correctly but the "cog" on the shaft is the wrong way round?
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Old 01-02-2008, 02:27   #7
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Quote:
Originally Posted by bill good View Post
The shaft rotation was wrong ie the boat was being driven f'ward when the box was in reverse!!
What is the standard, Clockwise or anticlockwise? Or is it a case of each to his own?
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Old 01-02-2008, 02:36   #8
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No none of the above issues are likely the problem. Certainly running in reverse will not cause that sort of failure. It may cause issues, but not that kind of failure. Although several types of boxes can happily run in either direction.
However the details we need to give you a more accurate idea of the problem are missing.
The first thing that would help would be the model of the box.

The drive shaft does not actually go all the way through the box. So it won't be the drive shaft. Do you know what part actually went through the box?

I also assume you still have fwd and revs gears working when the hole is in the box, seeing as you state that you only noticed an issue when you had noticed the oil. Is that correct?

The actual driving load is seen on the "final drive" end of the box. There is a large Bearing that takes the fwd and revs thrust imposed on the box by the shaft. A hole at the front of the box is in the area's of the gears. They will have also have thrust bearings, but this is to take the thrust of the helical cut gears as the load tries to force them along the line of the helical cut. Normally these bearings are held in place by circlips. Unless you have a compleatly wrong box as far as Hp rating to the engine Hp rating, I doubt that the load the over pitched prop is placing on the box will be causing any issue's like you are experiancing. But then again, you can never say never in mechanics.

Yes the prop is overpitching. But just how much so is determind by the max RPM unloaded minus the max RPM loaded. The difference should be no more than 10%. 200RPM ruffly is a good mark. You may not operate the engine at full RPM all the time, but it has to be able to reach full RPM if you open it up fully.

If Pat(never monday) reads this, he may know of an advisory from Yanmar perhaps. But I doubt this has been a common issue. I have never heard of it before myself.
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Old 01-02-2008, 03:49   #9
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Thanks Alan
I may have misled you somewhat, in that I was only repeating the Yanmar engineer’s explanation of the damage. The whole in both gear boxes as you suggested was at the rear.
They could not explain what caused it, only that it looked like the drive shaft had pushed through it, but there was no indication that this had actually happened. (end result is they do not know what caused the hole)
Their explanation re the prop was the same as yours, in that I may have problems with the prop but it is unlikely to cause the problem we are experiencing. They have also searched their “worldwide” data base and cannot find any similar problems.

The make of the engine is a Yanmar 4JH4AE – 54 hp
However I have not got the gear box details on hand. I can only assume that given this is a common motor and Yanmar themselves are working on it, that they would have ensured the correct gearbox to engine/prop was installed, now 3 times
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Old 01-02-2008, 03:50   #10
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yanmar g/box

I was ref to the prop shaft. The failures were internal & I was told due to the way the oil was dispersed. The mech gear box cannot be ran for long periods in reverse but the hydro can. Check the specs for your installation.
The initial prob was the prop rotation was in the reverse sense & the prop pitch was adjusted to suit the mistake!!
Regards Bill
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Old 01-02-2008, 04:07   #11
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Thanks Bill and further to Alan’s comments:

The first failure occurred as I was coming in to dock. In this case there was a loud noise as I put it into reverse gear. I then lost all power. The engine collection tray was full of oil. It could have however lost the oil along time earlier as I had not looked in the engine compartment (a few weeks) and did not notice any problems with the gears prior to the failure.
On the second occurrence I noticed no performance issues or vibration, noises etc and only realised there was a problem when I was safely on the mooring and looked in the engine compartment for an unrelated matter.
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Old 01-02-2008, 04:18   #12
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It is starting to sound like history repeating I will send an email

Bill
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Old 01-02-2008, 04:51   #13
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Please post the model and ratio of gear box. I'd assume KM4P, but want to be certain before commenting.
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Old 01-02-2008, 05:23   #14
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Thanks Never Monday,
not sure what model gearbox. Will check tomorrow and let you know.
I can assume though, it is the standard that comes with this motor and that they have replaced the damaged gearboxes with the same model.
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Old 01-02-2008, 12:46   #15
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Quote:
It is starting to sound like history repeating I will send an email

Bill
Ummmm??? not sure what you mean?!?
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