Steering & handling

hazza said:

On the long keel boat, the head of rudder stock is below the prop shaft. Both are on centreline. I was thinking I could add a small spade rudder further aft at the end of the waterline, behind the prop
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On the shorter keeled boat, I am planning to reduce the amount of balance on the rudder as suggested, as it seems this may be what may be causing the helm to lock over into a turn and not self-centre when released.

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If you are going to add/change the rudder of either vessel, you want it to be a horn style balanced rudder, with the balanced area beneath the thrust of the prop. This will improve both tracking and maneuverability without adding too much additional load on the tiller/wheel.

A horn balance is difficult to put under the keel line as it is impractical for slipping and can cause damage to the steering if I run aground.
That is why I am thinking of a separate rudder further aft.

I am also interested to understand why load must be continually applied to the keel hung rudder in order to turn the boat, but then returns to centre and tracks straight when released.
Is this due to the CLP being well aft of the CG, making the boat directionally stable or perhaps does the angled rudder change the camber of the keel, moving the centre of pressure of the keel aft. , or is it just style the rudder itself?

hazza said:

A horn balance is difficult to put under the keel line as it is impractical for slipping and can cause damage to the steering if I run aground.
<snip>
I am also interested to understand why load must be continually applied to the keel hung rudder in order to turn the boat, <snip>

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You need to take a dedicated Naval Architecture course. You have many wrong assumptions and lack of knowledge of how the maneuvering coefficients work for/against each other.

I was hoping that in such a forum on 'boat design', some of these issues could be discussed or even helpfully explained.
I can find no mention in texts such as Larson nor others on the subject of directional stability of yachts other than to say the subject is complex, nor are the differences in steering I am experiencing explained.
Perhaps you can provide some clues where this font of knowledge resides.

As a rudder is rotated away from center

The amount of drag increases

On the keel hung rudder - all of the drag is aft of the pivot. Therefore all of the drag is trying to return the rudder to center. The position of least drag.


On your spade rudder -
The drag on portion of the rudder forward of the pivot is trying to spin the rudder around to face aft.

On a well balanced rudder the forward portion has just enough drag to counter act some of the drag from the aft portion.
BUT not enough to overwhelm it like yours seems to.

I occasionally sail on a 28' fin keel yacht with stern hung rudder which is a completely unbalanced spade type.
You can never leave the helm unattended because the boat immediately starts spinning in a circle.
Once it starts to turn, the rudder locks itself into the turn and the only way to stop it is to pull hard to bring it back on centre.
This is very similar behaviour to my own boat, but it does not seem to have anything to do with the balance of the rudder.
This is why I am thinking there is more at play here.

hazza said:

I was hoping that in such a forum on 'boat design', some of these issues could be discussed or even helpfully explained.

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Realistically, there is not enough bandwidth. I am not being facetious. It is(!) very complicated.

hazza said:

I can find no mention in texts such as Larson nor others on the subject of directional stability of yachts other than to say the subject is complex, nor are the differences in steering I am experiencing explained. Perhaps you can provide some clues where this font of knowledge resides.

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So by "Larson" you mean Lars Larsson's "Principles of Yacht Design"? Ok, it has to do with where the forces are being applied and how the vessel resists the turn. You need to gain an understanding of the 2nd moment of lateral area of the hull. Try SNAME PNA or Rawston and Tupper's Basic Ship Theory for the derivatives of stability in yaw. I am not trying to say this is part of the "Illuminati", but there is an expected knowledge of how a vessel turns. In both underbodies there are issues that are obvious to those who are looking for what makes a "good" underbody.

To understand what's going on, do a simple experiment. Fill the sink with water, take a knife, tape it to a bottle and put into the water. Now try to spin it with two fingers. Take another bottle and tape a big cutting board to it and try to spin it. If you use the bathtub you can also give them a push and observe how they track.
This is the primary behavior of the two types of underwater shape, and it can't be changed. The long keel boat is hard to spin, it always wants to go straight. The finn keeled one is easy to spin and will rotate with minimal force.
Turning force comes from the rudder and depends on rudder area, water speed over that area and distance from the pivot point. The farther away the rudder is from the middle of the boat (asuming the pivot point is there, wich of course it isn't) the more authority it has because the lever arm is longer. This means you can use a smaller area to achieve the same turning force. Same thing with speed, the faster the water speeds over the rudder, the less area you need. This you know because as speed increases you need to make smaller rudder movements to achieve the same results.

Your long keeled boat has a small rudder at the end of a longish keel, but not at the extreme end of the boat. This means low rudder authority, the boat wants to go straight and anytime you let go of the rudder the water flowing along the keel pushes it in the lowest drag position wich is inline with the keel. Simply put (it's more complicated actually) the boat overrides the rudder.
The short keel boat has better rudder authority (bigger area and longer lever), the rudder turns the boat and stays into a turn because this is now the lowest drag position. This time the rudder overrides the boat.

Waterspeed over the rudder can be created two ways, you move the boat trough the water, or you accelerate water over the standing rudder. The second case happens when the propeller sends water over the rudder, and your two boats exemplify this perfectly, one has no propwash at all and the rudder "won't bite" until you have a certain speed, while the other responds to the tiller sooner.

In sailing circles a "horn style" is known as a semi-skeg rudder, and is seldom found on smaller boats. That's because if the "horn" is to actually support the rudder it creates complications in building the structure. Cantilevered rudders are much simpler to implement and can easily be made to whitstand whatever force is desired. Of course the "horn" doesn't have to actually be structural in relation to the rudder, and in reality many skegs are only play pretend in this department, more hydrodynamic aid (or hindrance) then actual support.

I don't think the amount of "balance" is the issue. Since the rudder is free to swing, it's almost like it isn't there as soon as you let it go. I suspect that some shock cords that are strong enough to gently return it to center would make a big difference. If there is more balance area in front of the rudder's axis, and the bearings don't have much friction, the shock cords don't have to be as strong. OTOH, if it's overbalanced, you'll have problems. If you don't like the shock cords, and you don't mind complication, look up anti-servo tabs. An anti-servo tab will require a little more force on the tiller, though, while making it a bit more powerful.

I wonder what the cross section of the rudder is like. It looks pretty crude. A crude shape may stall prematurely and cause more drag. A shape like, say, a NACA 0012 would have more lift before it stalls, and less drag.

P.S. I was talking about the spade rudder. I the situation is analogous to stick free vs stick fixed longitudinal stability in an airplane.

I think what you have here, with the spade rudder boat, is a prop wash issue.

What I think happens is that the blast of the prop hits the forward part of the rudder (the area ahead of the rudder shaft) harder than it does the rest of the rudder. This prop blast makes the forward part much more effective, making it over balance the rudder.

I noticed that this forward part has a dead vertical leading edge, almost to the top of the rudder. So the top part of this leading edge is right behind the prop.

A possible cure for this would be to start the aft slope of the leading edge about 1/3rd up from the bottom. This would reduce the forward area significantly, maybe by a third or more.

But then there would be considerably less forward area right behind the prop, and the rudder would be far less inclined to over balance.

If this rudder has an airfoil cross section, changing its profile shape may be more difficult, as the newly cut aft sloping bit will have to have a fuller nose on its foil section.