Is tiller vibration with semi balanced rudder normal?

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maxwell edison

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I recently sailed on a new boat and was very surprised at the extent to which the tiller vibrated under power. To be fair it was a long tiller but even so it vibrated constantly by about an inch which I thought was a lot. Changing throttle setting made no difference. I have been told that this is common with a semi balanced rudder. I would be interested to hear of the experiences of others. Just to complete the picture, the boat had a saildrive and kiwi prop but I have been told that neither of these factors would be an issue. I should add that she handled perfectly undersail with a very steady tiller to which she was very responsive.
 
In my experience yes, if (as they normally are) in the turbulence of the prop - I think because the turbulence acts on the rudder both forward of and behind the centre of rotation of the rudder so the changing pressure differences across the rudder faces forward and aft of the shaft cause the wobble (in my experience more of a wobble than a vibration).

Not so noticable with a wheel but we still get it ourselves even so, that with a high geared (1 turn from stop to stop) wheel which is probably the worst wheel case.

As you say, not an issue when sailing when their efficiency really comes into its own.

John
 
Unfortunately only too common. What is happening is that the rudder is probably very close to the prop and picking up pulses from the prop blades as they pass. Often confused with cavitation, this is a very difficult thing to cure I am afraid and more a design fault than anything. If the rudder performs well under sail any mod to it which might cure the problem will make the steering under sail heavy.
 
If it was cavitation it would also happen under sail. As already said, cused by turbulence from the prop
 
Interesting replies so far and thanks for them. To pick up just one point I am not sure of the distance between prop and rudder but they are certainly not close to each other. I guess the prop is approx midway betweek aft edge of keel and the rudder and she is a 9m boat

Just to pose another thought. Would autohelm
a) work on a vibrating tiller such as I have described
b) eliminate the vibration as it holds the tiller with rigid steel as opposed to a flexible arm

I ask this as, to me, it seems one thing to manually steer short distances but a prolonged session would be a touch wearing
 
I cannot comment directly on tiller mounted autopilots but never heard of a problem - certainly linear drives mounted under the deck (and connected to the rudder shaft by a short tiller so much the same) have no problem and lock solid so no wobble observable.

However, any slackness in joints are inclined to rattle when under power, so, for example, on an underdeck drive unit if there is side to side slack on the pins it swivels on then perhaps need to be taken up with washers or similar. I suspect the same could be found to apply to similar joints on a tiller unit.

As an aside, I think we are all assuming there is no slackness in the rudder shaft bearings which is showing up under power, but would expect that to have been noticed.

John
 
Jimi
You beat me to it. This sounds like a case of loose rudder stock bearings, unfortunately boat builders aren't engineers so even if they are new I'd bet the bearings only fit where they touch, so under sail the forces are all in one direction - no vibration, but under power in the prop wash you get vibration.
The next time the boat is pulled grab the bottom of the rudder and give it a shake - I bet it rattles

Peter.
 
Not necessarily the bearings - ours are purrfect but we still get the "wobbles" feed back from the semi balanced spade rudder (and thats on a 12 tonne boat). But certainly worth discounting as a possibility.

John
 
John
The original poster said that he had a sail drive, this tells me that the prop should be well clear of the rudder - so his problem is probably down to bad bearings but if you have a conventional shaft drive it may be a design problem which is a bigger problem to overcome.

Peter.
 
Hi Peter

You may very well be correct - I had been assuming that the reference to a semi balanced rudder (and a new? boat) inferred a spade rudder in which case its draft would likely far exceed that of the saildrive.

For example, in our own case the spade rudder foil is far deeper than any saildrive prop ever would be (in fact is not a geat deal less in draft than the keel)(we are shaft drive with a sail optimised fixed 3 blade prop) and I suspect most are the same. I have not got a measurement as to the distance between our prop and the rudder (I am on the boat and the drawings are at home) but it is a long way as the rudder is as far back in the boat as it practically could be and would be more than most.

As you say though, is maybe so especially if not a spade.

John
 
Thanks guys for the continuing help with this. I have re read all the posts this morning and yes, as was suggested early in the thread, I guess the problem is more a wobble than a vibration. I do not know whether she has a spade rudder, in fact I don't know what a spade rudder is(!) but the drawing I have of the boat shows a rudder going down to the same depth as the lower edge of the keel, the sail drive is shown as being slightly more than half the depth of the rudder. The boat is a modern design and the one I sailed on was only four months old. If anyone has any more thoughts I would be glad to receive them. Thanks
 
We had this problem (and we have a sail drive where the trailing edge of the prop is at least 800mm from the rudder) until we changed the prop so it was definitely the guilty party. There was no movement in the rudder stock bearings in our case but quite unpleasent "tiller wobble" under power above 2000rpm. Even at its worse (around 2200 rpm) it never affected the autohelm.
Hope this helps.
 
Max
Basically a spade rudder is one that is not supported on the bottom or another way of putting it is "once it exits the hull there is no more support.

Peter.
 
I said... often confused with cavitation. This is not cavitation. Propeller cavitation is a completely seperate subject. If experienced it will only be experienced under power. You cant get prop cavitation when sailing Jimi!
 
As you said it was a new boat Max I assumed, as Cat did, that the rudder bearings were not worn. In fact my own experience is that rudder shakes due to worn bearings will often (but not always) happen under sail and power. I think that the prop being "too near" the rudder is relative. It will obviously be worse the nearer it is. One other thing that has not been mentioned is the type of prop. If this is a saildrive (especially Volvo) it is probably fitted as standard with a horribly inefficient 2 blade folding prop. Replacing this with a 3 blader will very likely help a lot. A fixed 3 blader will of course knock speed off the boat but something like a Brunton Autoprop 3 blader would probably help as the hydrodynamic efficiency is far superior.
 
Has boat been stationery in water for some time.

I get vibration on the tiller about halfway through summer - due to barnacles and mussels on and around the prop (despite be used at least 2/3 times per month.

The livestock cause a lot of turbulence which is transmitted to the rudder - it can also be seen in the wake.

A good scrape on a warm day sorts the prob.

Magic
 
Re: Has boat been stationery in water for some time.

The prop is a kiwi which I believe is a three bladed feathering prop. I doubt that the boat has been stationary in the water for an abnormal time but I think that on some tides she may occasionally settle in the mud.
 
Re: Has boat been stationery in water for some time.

I have found with my own boat that vibration occurred when I changed from a 3 blade prop to a two blade and disappeared when I abandoned the two blade experiment and went back to three. If you already have a three balde prop I suspect propellor damage or (if feathering or automatic) poor operation.
 
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