SO 34.2 Rudder Bearing

[MickeyB]

Hey all,

Start with a question.  Steering feels good, positive and firm.  But, when rocking from port/starboard and vv I hear a thud.  Thud is putting it very strongly, maybe a mild deep tap but you get the jist.  It is as if the rudder is wobbling.  I dived on it and shook it as much as possible and it is not moving, at all!

However, from the top of the shaft, if I grab and shake then there is maybe 1mm movement which makes the noise.  The 'wobble' occurs across the boat, not length ways.

So the question.  I cannot remember this from before, so is it normal?  I assume not, bearings should be tight I think.

Which now brings me to question 2.  The bottom of the rudder is good, can I replace just the top bearing or should I do both? (Do I have two??)  I wasn't planning on pulling her till March/April, so can I do this in the water?

Does anyone have a schematic of the rudder system on the SO34.2?  I tried searching but couldn't find anywhere - I know I have seen one somewhere - but cannot refind it.

I contacted Jefa but they don't know my model, so any help on where to go would be great.

Mike.

[rene460]

Hi Mike,

I would suggest the wear is normal, so the question is only whether the amount is acceptable.

Lets start at the beginning, the force on a rudder is always at right angles to the direction you are sailing.  If I remember correctly the centre of effort is slightly in front of the centre of lateral resistance of the hull and keel, and the rudder provides a sideways balancing force in the opposite direction to the keel.  It is obvious that the rudder is streamlined, so very low resistance in the direction of travel, so a much smaller force in that direction.  So the wear you have described is as expected.

If you are stationary, and rock the boat, the rudder rocks within the slack the bearings allow, but when sailing, the force only changes sides as you go through a tack or gibe, so you probably don’t notice it then.  

So so how much is too much? As an engineer, I always find that the hardest question.  But then I always dealt with new equipment.

One mm in a rotating shaft bearing such as the cutlass bearing, I think would be too much, it will allow the shaft to whip around a bit, and the vibration would be quite noticeable.  But the rudder really only has minimal movement.  When the boat is well balanced, it might make the steering a bit imprecise if you are sensitive to that, but otherwise I suspect it is not a problem.  

I suggest that if it is not causing troubles with cable tracking, and there is plenty of bearing material remaining to ensure the shaft does not rub on the bearing housing, a bit of lubrication and don’t worry too much.  You need a lubricant suitable for the plastic bearing, I suspect some silicone grease, or oil, but not hydrocarbon, something suitable for plastics.  The idea of the lubricant is to reduce the rate of further wear.

Always difficult to know how much wear is too much, so I am interested to hear what others think.  We all need a process to work it out.

rene460

PS - I realise there is another thread reopened by docobdo, and there is some good information in that.  However, I think they were generally talking about more serious problems.  Still, well worth reading.