Curious rudder balance problem

[Kim Klaka]

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, and the overbalance problem has not changed since swapping from 2-blade folding prop to 3-blade folding prop.
Any suggestions as to what is going on, and how to get the balance acceptable under both power and sail?

[Butterman]

A few questions:

1) Does this happen when motoring at any speed through water, or only when the boat is still relatively stationary? (still accelerating)

2) From a neutral position, does the tiller seem to favor one stop over the other?

3) Is it on backwards? (Joking)

[Kim Klaka]

Originally Posted by Butterman

A few questions:

1) Does this happen when motoring at any speed through water, or only when the boat is still relatively stationary? (still accelerating)


2) From a neutral position, does the tiller seem to favor one stop over the other?


3) Is it on backwards? (Joking)

1. It happens when motoring at any speed and also whilst tied up alongside.

2. Not really. 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.

3. Nice thought :-) Incidentally, when going backwards under power, it is heavily overbalanced as expected. Haven't managed to sail backwards yet ;-)

[Stumble]

You may want to ask on boatdesign.net and get some designers input, but if I had to guess (and I haven't seen this before) I would bet that the prop shaft is installed just a bit off center say an inch or so, but angled back towards centerline. So the thrust is always hitting one side asymmetrically.

The other things I can think of that would effect helm balance, like an off center rudder or bent shaft, would effect it equally under sail or power.

one thing I can think of to eliminate it, instead of changing the section profile, would be to cut away the leading edge atthe top of the rudder. So the flow from the prop is working on an unbalanced rudder. But this will have serious drag penalties under sail.

[Butterman]

Originally Posted by Kim Klaka

1. It happens when motoring at any speed and also whilst tied up alongside.

2. Not really. 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.

3. Nice thought :-) Incidentally, when going backwards under power, it is heavily overbalanced as expected. Haven't managed to sail backwards yet ;-)

Yes, I was hoping it was simply the rotational effect of prop wash.

I am a bit surprised that boat speed doesn't alleviate the issue, at least to some degree.

The only thing I can think of, is the rudder is simply too close to the prop. As the prop wash disperses, it is putting more effort on the leading edge of the rudder, and less on the trailing edge. You can try balancing it even farther aft, but you will be taking from your left hand to give to your right; making steering under sail even more under-powered.

[Butterman]

Originally Posted by Stumble

You may want to ask on boatdesign.net and get some designers input, but if I had to guess (and I haven't seen this before) I would bet that the prop shaft is installed just a bit off center say an inch or so, but angled back towards centerline. So the thrust is always hitting one side asymmetrically.

The other things I can think of that would effect helm balance, like an off center rudder or bent shaft, would effect it equally under sail or power.

one thing I can think of to eliminate it, instead of changing the section profile, would be to cut away the leading edge atthe top of the rudder. So the flow from the prop is working on an unbalanced rudder. But this will have serious drag penalties under sail.

Yes, definitely agree with getting an engineer's opinion on this.

[Kim Klaka]

Originally Posted by Butterman

Yes, definitely agree with getting an engineer's opinion on this.

I'm a bit embarrassed to admit to this, but I am a naval architect specialising in yacht design and hydrodynamics research (>40 years of it!). I was hoping the combined practical experience of the forum would throw up something obvious that I have missed.

[Snowpetrel]

I think Stumble is on the right track with the causes and a posible fix.

It's a pretty common issue when the prop is close to the rudder. The best cure I have seen was to remove some of the balance area near the prop.

It used to be common on some early IOR/RORC designs with partly balanced partial skeg rudders. The usual cure was to remove some of the balance in way of the propwash.

Another random thought would be to keep the aft rudder extension in way of the propwash but remove it elsewhere. Prehaps you could add some balance away from the prop area to restore balance under sail?

Any pics?



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[Butterman]

Originally Posted by Snowpetrel

I think Stumble is on the right track with the causes and a posible fix.

It's a pretty common issue when the prop is close to the rudder. The best cure I have seen was to remove some of the balance area near the prop.

It used to be common on some early IOR/RORC designs with partly balanced partial skeg rudders. The usual cure was to remove some of the balance in way of the propwash.

Another random thought would be to keep the aft rudder extension in way of the propwash but remove it elsewhere. Prehaps you could add some balance away from the prop area to restore balance under sail?

Any pics?

Yes, my thoughts exactly

Not sure about pics, but here is the schematic from Van De Stadt:
http://www.stadtdesign.com/designs/s...n_de_stadt_34/

[Kim Klaka]

Originally Posted by Butterman

Yes, my thoughts exactly

Not sure about pics, but here is the schematic from Van De Stadt:
Van de Stadt Design - Van de Stadt 34

Yes, that's the boat. In fact my boat is the one in photos 3 and 4 of that website.

Originally Posted by Snowpetrel

It used to be common on some early IOR/RORC designs with partly balanced partial skeg rudders. The usual cure was to remove some of the balance in way of the propwash.

Another random thought would be to keep the aft rudder extension in way of the propwash but remove it elsewhere. Prehaps you could add some balance away from the prop area to restore balance under sail?

Any pics?

Pic of the rudder attached (just after haulout). It's just a rectangle with what would normally be considered not enough balance - stock is at around 10% chord aft of leading edge. I have tried just changing the balance over that part of the span over which the prop wash goes, but it didn't work. It slightly decreased the overbalance under power and slightly increased the underbalance under sail, as expected. This led me to think that it is not in fact a prop wash effect. Can't think what else it could be though.

[Wotname]

If you are motoring at hull speed in calm water with no sails hoisted and go into neutral, is it still overbalanced as soon as the prop stops?

[IdoraKeeper]

Suggestion- Optimize the balance for sailing. Add an anti servo tab that is engaged only when motoring.

[Kim Klaka]

Originally Posted by Wotname

If you are motoring at hull speed in calm water with no sails hoisted and go into neutral, is it still overbalanced as soon as the prop stops?

That's a very good question; I don't know, will try it next time I go out.

[jimbunyard]

I'd guess it was normal; the rudder is slightly underbalanced underway with sail, but when underway with power, the balance of the rudder is the same until the force of the prop wash is added... As for fixes, going way out on a limb, maybe trying something a bit more radical, like a variable chord, shifting in depth and placement over the length of the rudder, and/or a more efficient shape (maybe a little taper, for instance) might help. Of course, now we're walking new rudder territory...

[a64pilot]

Island Packet is a full keel boat with a Spade rudder, and exhibits exactly what you describe. In the owners manual it suggests tightening down on the friction as a way of dealing with it. I have always assumed the rudder was overbalanced a little so that they could use the little wheel they use and not have excessive steering forces, or have to overgear the wheel to lighten up steering forces, but end up with six turns lock to lock or something. A compromise
It is prop wash induced obviously, and I'm sure when you remove prop thrust, it will go away just like it does when sailing.
Scalloping out the rudder in front of the prop, removing some of the balanced portion should reduce it, or just tighten down the friction and deal with it