That Rudder – Positive Caster or Canted Axis?

Saturday, 10 July 2010 21:08 | Written by 
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The CAR rudder on a new EOS 660 ski The CAR rudder on a new EOS 660 ski Credits: www.surfski.info

[Editor: Phillip Nye writes to say that Johan van Blerk’s rudder may well work – but it’s not a “Positive Caster” rudder]

“Positive Caster” is a Misnomer

Firstly - and this is nothing to do with the pros or cons of Johan's design - the name "Positive Caster" is a complete misnomer based on a misunderstanding of mechanics.

The caster effect is the tendency of steering to "self centre" and return to its straight ahead position when moving forward. In a steerable wheel this comes from its point of contact with the ground being behind the point where the steering pivot axis intersects the ground - angling the pivot axis is one way to achieve this but does not define positive caster, offsetting the wheel axis from the steering axis is another way - the casters on the bottom of your furniture have vertical pivots but still exhibit the effect - in fact they give their name to it. Bicycle steering has a tilted pivot, but the wheel axis is offset in the forward direction (by curving the forks) to reduce the caster effect as any good frame builder understands - indeed some tandems offset the axis sufficiently to reduce the caster effect to near zero and achieve "neutral steering", but they still have the tilted headset angle.

Translating the caster effect to a rudder means that the centre of drag of the rudder comes behind its pivot axis. This is the case for any practical rudder and every ski I've ever seen has a "positive caster" rudder - you only have to try and steer your ski with its rudder whilst paddling backwards at any speed to feel what negative caster is.

So let’s call it a "backward canted rudder axis" or something of the sort.

[Editor: Let's call it the “Canted Axis Rudder” or CAR]

The Benefits of CAR

Now what are the benefits and where do they come from? In a straight line and for very small rudder deflections there is no difference between the backward canted axis and vertical axis because the rudder is in the same position in the water. When running straight down a swell with the rudder more or less straight ahead, no amount of axis cant is going to change the performance and it certainly will not stop the bows dipping (or rising).

[Editor: Dale Lippstreu's new EOS ski arrived - equipped with a Canted Axis rudder...  The first trials were held on flat water and were inconclusive.  A review of the ski's performance will be written as soon as we have some downwind conditions to try it out in!]

conventional surfski ruddercanted axis surfski rudder

Conventional v Canted Axis rudder - note how the CAR tilts forward as it turns

As the rudder is turned further, the effect of the canted axis becomes greater and the time it really comes into its own is when a ski is wanting to broach and the paddler is typically putting the rudder hard over to fight this.

When a boat begins to broach, the wave tends to sweep the stern sideways across the water whilst the bow continues to track forward. In these circumstances the operation of the rudder is pretty complex as the stern is not moving forward but diagonally, and over steering will easily make the rudder stall. In a stall, rather than flowing evenly around the rudder from front to back, the water flows from the upstream side round both front and back edges and into a chaotic region of turbulence covering the whole downstream side of the rudder. In these conditions the rudder exerts virtually no turning force (sometimes even a slight "wrong way" force) but simply slows you down and exacerbates the broach.

Steer Small and Paddle Fast

With a conventional vertical axis rudder, gentle steering and faster forward movement is the best answer though this is not always easy! And as Dale Lippstreu's excellent article on rudders points out, a rudder with a well rounded leading edge and fatter cross section can resist stalling much better than a thin section.

...but the Canted Axis Rudder may help

With a backward canted axis, there are two effects at play - the first is a genuine feature of the axis angle and way the rudder tilts outward as well as sideways as it is turned: The angled rudder surface pulls the stern down and makes it much harder to push the hull sideways through the water. This keeps the boat tracking forward and reduces the stall tendency. The second effect is coincidental but probably significant: the canted axis design limits the rudder angle achievable and makes over-steering and consequent stalling much less likely - this could be achieved with a vertical axis rudder by simply adding stops to limit its travel.

The real benefits obviously depend on conditions but also on paddler technique, weight and distribution, hull design and rudder geometry. For paddlers who experience broaching or rudder stall problems often, a canted axis should be a real win and apparently can make a huge difference. For paddlers/boats which rarely experience broaching or loss of steerage a canted axis may not be worth much - these are likely to be experienced paddlers who sense the broach early and are confident to power forward and steer gently - but the canted axis may still allow them to push their boats and performance that bit closer to the edge.

[Editor: Phillip Nye is an engineer based in the UK.  Although his degree included mechanical engineering and fluid dynamics he’s now an electronics and software consultant working in stage lighting.  He’s been paddling since age 6 although he’s a relative newbie in surfskis.]


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