Transom hung rudder vibration under power

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Moonbeam

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Hi All

We have just completed our first season with a 'new to us' 1984 Westerly Konsort. Very pleased with every aspect of the boat except an annoying shake/wobble/vibration through the tiller from the transom hung rudder when motoring. The vibration increases with the revs. It's silky smooth while sailing. Our two previous boats (Trident 24 & and Centaur) had spade rudders with 3 blade fixed props and did not have this problem. Our new Konsort has a fixed two blade prop which appears well matched to the Bukh DV24. The P bracket and cutlass bearing are both sound.

My questions are...

1: Is this vibration under power common/inherent to transom hung rudders?

2: Or is the problem the 2 blade prop and might we be able to reduce the vibration by switching to a 3 blade prop?

This opens another can of worms :D We are reluctant to sacrifice sailing speed with the fixed 3 blade, so we might consider a 3 blade feathering like Darglows featherstream or 3 blade folding like the flexofold. But would these cause more vibration?!
 
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I have a Stag 28 with a transom hung rudder and a three blade fixed prop and get the same symptoms, I believe most of this is due to the fact it's a balanced rudder. It makes it nice and light when sailing but is a bit oversensitive in the propwash. It's a feature and normal on these boats.


Edit: just looked at a Konsort and the rudder is not heavily balanced so will think further...
 
Vibrations like that are quite common. Partly it is your 2 bladed prop and partly (possibly) wear in rudder hangings - not enough to notice when sailing, but enough to get excited by the turbulent prop wash.

A Featherstream will be a big improvement under both sail and motor in terms of speed performance and may well reduce the vibration.
 
Used to get this with my Impala, the rudder also clonked a bit at anchor.
I 99% cured it by reaming out the fittings and getting rid of the slack. Changing from a pin that moved in both the rudder and the hull gudgeons to bolts that only moved relative to the rudder also removed one source of slack.
 
Pintles and gudgeons are not a good fit. Under sail a fairly constant pressure on one side.
Under power the aerated water passing over the rudder creates cyclical movement at the fulcrum.
 
I had someone, who shall remain nameless, work on my auxillary. For some unknown reason (well maybe not - he was refurbishing his own boat at his clients cost) he replaced the two front engine mounts. When I got the boat back the transom hung rudder had this very noticeable shake/vibration. Of course he denied causing the problem and in the end, mainly to save my blood pressure from going through the roof, I got a "professional" in who replaced all the engine mounts with the correct type and realigned the shaft. Never had a problem since.
 
Thanks for all the advice so far, very helpful, I shall check all the rudder fittings first and try to iron out any loose fittings before moving onto engine feet!

Tranona... I'm interested that you sounded positive about the featherstream out of my two choices on possible prop replacement, but from reading up on all the folding/feathering debate on this forum, I gather you have a two blade flexofold which you are happy with? Any particular reason you did not mention the flexofold? It looks from the French Voiles and YM tests that I'm likely to get more power/speed from the flexofold design compared to my fixed egg whisk. The flexofold (2 blade) is by far the cheapest, but the featherstream seems to have it's advantages too! Probably less vibration? Seem to be happy owners of both these brands out there.
 
I also have a boat with a Featherstream. The 3 blade Flexofold would be a good alternative if you have room. My Featherstream is on an old fashioned boat with a keel hung rudder and it is the only type that will fit. The Flexofold is on a saildrive and is fine, but if a Featherstream was available for a saildrive I would prefer one of those. The other 3 blade feathering props (except the Kiwi) are very expensive for a saildrive.
 
You are into a complex area here Stuey. Your tiller and rudder set up is a very sensitive one - its one of the joys of a tiller set up that you can feel what is happening so much better than I ever will with a wheel. But it also means you will feel turbulence much more than I will.

As any turbulent water flows over the rudder, it will cause the rudder to flip back and fore through any slop there is in the mountings. So the first step is to tighten up the rudder mounts.

The problem is invariably worse under power and I reckon this comes about in part because of prop walk which means the prop wash is never flowing along the line of the rudder but tends to flow at an angle to it. Its also because the prop forces a much faster flow of water over the rudder than you get under sail. Changing the prop might help but it might also make it worse since its not a matter of good prop / bad prop so much as does the prop concerned work better with your boat.

Try and compare notes with someone else who sails your model of boat.
 
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