curious rudder balance problem

I have a Van de Stadt 34 with a spade rudder. The rudder was nicely under-balanced when sailing ( i.e. just a bit of feel), but overbalanced under power (i.e. tiller would go to its stops if let go). I modified the rudder by adding chord length aft (retaining its NACA section, just going from about 0012 to about 0010). This reduced, but didn't eliminate, the overbalance under power, but has made the helm rather heavy when under sail. I do not understand why the rudder is overbalanced under power but not sail. The stern gear is conventional p-bracket, with the prop about 0.5m forward of the rudder leading edge. The overbalance problem has not changed since swapping from 2-blade folding prop to 3-blade folding prop.
It happens when motoring at any speed and also whilst tied up alongside.
If the tiller is let go, it ends up maybe 60% of the time going to stbd, 40% to port. That small bias is most probably due to the rotational flow component in the prop wash.

Any suggestions as to what is going on, and how to get the balance acceptable under both power and sail?

 

Kim,

So the only difference between "nicely-balanced" and "over-balanced" is related to the speed at which the flow of water is passing across the rudder surface. Is that correct?

 

It's a very common behaviour (or a problem). Very few boats keep their course while motoring. Mine does it surprisingly well. It has happened a few times that I thought I had autopilot on, but did not press the button hard enough or pressed the wrong button. The boat would go straight for even a minute or two and then starts to turn at a rapidly increasing rate.

In your case the propeller is so close to the rudder that the rudder will be turn the propeller wash very effectively. This makes the problem worse and also makes the rudder oscillate. In my boat the distance is about 2 m.

Usually rudders are designed to give good feel on a beat and reasonable force on a reach. In those conditions there is quite a lot of side force carried by the rudder and making the feel. But feel doesn't have much to do with the rudder staying in the middle. If you let go the tiller while you have feel, the tiller will turn and the boat as well.

When you are motoring (or on a run) there is no side force on the rudder.

I wouldn't try to solve the "problem" you had. Just keep you hand or autopilot on the tiller while motoring. Or is it so bad that it needs to be solved?

 

It might be, but I have no proof. The only definite thing is that it is overbalanced under power and underbalanced under sail, regardless of boat speed. Being under power changes more things than just flow speed over the rudder (e.g. no leeway, though even downwind the rudder is underbalanced under sail))
I first tried extending the chord over just the spanwise region lying in the prop wash, the idea being to push the centre of pressure aft under power but not (very much) under sail. Didn't have the desired effect - slight reduction in overbalance under power and underbalance under sail. I could have done a much bigger chord extension in that region, but it would just look ridiculous, and doesn't get to the root of the hydrodynamic problem - why should such a radical shape change be necessary?

 

It is so bad it needs to be solved; in my view an overbalanced rudder on a hand-steered boat is dangerous. I have sailed quite a lot of boats and this is the only one that exhibits this strange characteristic. do you have the names of the designs that you have found also have this problem? I could then look up their plans and see if I can find the common design characteristic. I think you might be right, that it is because the prop is close to the rudder (though 0.5m is not very close).

 

Did you notice any difference between the rudder behaviour right after the seasonal hull cleaning, and after few months in water?

 

Position of Rudderstock

It would be helpful to know the position of the turning point (axis of rotation) relative to the chord length (e.g. 20% c). Is this position (in %) the same at the tip and at the root of the rudder?
The prop creates a different angle of attack at the tip compared to the root of the rudder. The hydrodynamic center is usually at 25% chord. The lever-arm from this center to the turning point is crucial.
Uli

 

No difference. Good thought though.

 

OK, so it's not normal. How deep is the rudder and which part of the rudder does the propeller wash hit. Could it be that the balance is very different at the propeller wash region of the rudder? The velocity is much higher in the propeller wash and forces go with V^2. Thus while motoring the wash part of the rudder is the one that determines the balance and while sailing it is the full rudder.

What happens during acceleration or when you put the engine on neutral at cruising speed? In the former that propeller wash is even faster and in the latter it doesn't exist.

 

Is your boat built like this?

 

Is it possible for you to post a pic or a drawing of the rudder and of the rudder stock?
The above picture posted by Joakim doesn't show much, besides having a too small resolution.

 

So, if speed is not an input, then what about the attitude of the vessel.
Under power is the boat no heel/list and little trim?...whereas under sail...heeling, and trim and if so different from when under power?

 

Excellent point, AH.
Generally, sailboats have some positive trim under sail and neutral or negative when motoring. So the problem can be related not only to the hydrodynamics but also to the CoG of the rudder, relative to the stock.
That's why it is necessary to have a detailed drawing and/or pictures of the rudder, in order to pinpoint the problem.

 

My boat is as per the profile drawing posted by Joakim (the solid line profile, not the dotted one). picture attached. it is just a rectangle with NACA Section (approx 0012 reducing to 0010 at tip), with stock at around 10% aft of leading edge i.e. what would usually be considered too far forward. It is heavy (maybe 50-70kg) so not buoyant. Stock is vertical to within a couple of degrees.

Thanks to all for suggestions, need to follow them up with more sea trials. I am off-line for about a week now.

 

Seeing that rudder setup and weight, the following thing comes to my mind:
- if your boat trims by bow (positive or bow-down trim) when motoring, then the turning moment caused by the weight of the rudder could be a cause of such behaviour. In other words, you might have the rudder CoG too far aft (relative to the stock), and the weight-induced moment cannot be overcome by the hydrodynamic moment. If this is true, then adding more chord can help alleviate the issue (by adding a hydrodynamic counter-moment) only if it doesn't shift the CoG further aft. Otherwise it may do nothing or may worsen the situation.

In order to test this theory, you should put a weight at the stern to decrease (or invert) the bow trim and see if the situation changes.

If it turns to be the cause, then you should find a way to shift the CoG of the rudder towards the stock, either by making the structure of the aft part of the blade lighter or by adding weight to the fwd part. The latter solutions will require a check of the rudder stock scantlings though.

Cheers