Windvane Choice

Servo or Auxiliary Rudder?

There are several basic concepts of a windvane steering system. This brief overview looks at the differences between them.

A servo pendulum model steers using the ship’s rudder via control lines led to the ship’s steering system, whether wheel or tiller. An auxiliary rudder model is generally based on the same concept, however the pendulum is actually directly physically connected to an additional aft-hung rudder, which can steer the ship independently.

Wind – True or Apparent

True wind is what one feels on their face while standing still. Apparent wind is the amount of wind that is measured while travelling at a given velocity. For example if you are sailing into a 10 knot headwind at 4 knots, the apparent wind is 14 knots. Conversely, if you sail downwind in the same conditions at 4 knots, the apparent wind will only be 6.

It is clear the air foil sees less apparent wind and therefore, less wind pressure sailing across and especially down wind on any given day.

Explaining the servo mechanism as applied to these systems

Have you ever rowed a dinghy? The oar is simply a lever with your effort applied at one end and the load at the other. The oar, as a lever provides a mechanical advantage that will move the dinghy along quite nicely with little effort from you. At a very low speed, if you take the oar and put the blade into the water parallel to the flow of the water (feathered), the flow passing over the blade creates very little drag and the dinghy continues to move ahead.

However, when the oar is rotated, even slightly across the flow of the water, the dinghy will turn around the oar. A result of the hydrodynamic energy created by water flow.

Servo Pendulum Model

The initial course is set with the air foil upright or a little bias to hold against normal weather helm. When the vessel deviates off this set course the apparent wind direction changes and the increased pressure acting on one face of the airfoil pushes it left or right.

This movement is transferred down to the rudder via a pushrod, turning it left or right using a gear. This turns the rudder against the flow of the vessel over the water, which creates an immense amount of water pressure on the servo rudder.

Since the servo rudder is allowed to pivot, literally like a pendulum, the water pressure will force it to tilt, which pulls the control lines attached to the opposite side of the rudder’s pivot axis. These lines in turn pull at the vessel’s steering system, turning the main rudder until the vessel’s heading returns to allow the wind to flow neutrally over the airfoil.

This concept is the most common in the world today steering a range of vessels from 20′ to 65′.

Modern designs are lightweight, simple, user friendly and less expensive than the complicated auxiliary designs. They are not suitable for hydraulic steered vessels unless the hydraulics can be by-passed when in use.

Advantages

  1. Servo designs are built small, strong and compact suiting most vessels from 10 to 65ft (5 to 25mtrs).

  2. They can be designed to allow quick, easy complete “vane only” removal from the vessel.

  3. The servo rudder can be shipped out of the water and protected by a replaceable sheer pin or swivel joint when in use.

  4. Units incorporating gears in the air/water drive train provide positive feedback to apparent wind change preventing over steer.

  5. Will commence steering at a low hull speed of 2 knots and provide tremendous steering power in higher winds.

  6. Can be controlled by a linear drive autopilot for light air use.

  7. Simple to operate and repair.

  8. Less expensive than auxiliaries to build.

Disadvantages

  1. Dependant on the vessels main rudder and steering system. Any deficiencies in either will affect performance. Tight rudder bearings, sloppy or worn pintails will effect the units capability, especially in light airs.

  2. Control lines which must be just right, not “too slack” not “too tight” routed through low friction sheaves.

  3. The control lines in most cases are run on deck so careful routing is required.

  4. Centre cockpit vessels require longer control lines and extra turning blocks, adding to friction and magnifying line slack/stretch problems.

  5. A servo rudders lateral swing and developed hydrodynamic energy needs hull speed to work. An initial hull speed of around 2 knots is required to overcome rudder mass, inertia and control line inefficiencies. This is the steering start point. Smaller vessels reach this hull speed with less wind.

Servo Auxiliary Model

A servo rudder is fitted mechanically to the auxiliary rudder, moving it directly, rather than the vessels main rudder which would require control lines. It functions and performs within itself as a servo direct system, but providing the auxiliary rudder with constant apparent feed back through a mechanical connection. The main rudder is locked off slightly bias to apparent wind allowing the auxiliary rudder to make any small corrections. This design provides tremendous power to the auxiliary rudder.

Advantages

  1. The slack, wear, stiffness, slop and other inefficiencies in the main steering system have no detrimental effect on the auxiliary rudder.

  2. It is independent of the main rudder, as opposed to a servo which is not and this is greatly affected by the problems above.

  3. With no control lines required to the wheel or tiller, line stretch and time lag is eliminated completely. Centre cockpits which require longer lines magnify the problem. Accompanying pulleys and turning blocks can develop friction and inefficiencies and these are eliminated. Considering the obstacles, it is easy to see a servo has a lot of catch up before it even starts to reach the auxiliary rudder efficiency, which is already steering the vessel before it gets a handle on the situation.

  4. The main rudder still does the lion’s share of the work (as it was designed for) so only small auxiliary rudder adjustments are required to steer the vessel in the lightest of airs. The servo rudder always requires more starting wind to generate enough hydrodynamic energy from hull speed to overcome rudder inefficiency, weight and turning inertia, as well as “control line baggage”.

  5. Hung well aft, the semi-balanced Auxiliary Rudder, with a long lateral turning movement, produces great power in a compact rudder profile. An area as small as 20 percent on the main power will turn the vessel when set up correctly. Downwind, with the main rudder amidships at the slowest of hull speeds and the lightest of winds, the vessel will steer through the auxiliary rudder because it does not turn the main rudder, which always requires more hull speed relative to more wind.

  6. The most powerful auxiliary concept available uses 800/1 water/air density advantage.

Disadvantages

  1. The unit is heavier, more complicated and expensive than a servo device.

  2. The Auxiliary rudder since January 2009 has now incorporated the new lift out feature eliminating the previous disadvantages of permanent rudder in the water both in wear and manouverability.

Quick Comparison

SERVO AUXILIARY/ SERVO
Steering Responsiveness Vessel Dependant Always Optimal
Downwind Light Air Standard Optimal
Deck Control Lines Yes No
Main Rudder Locked No Yes
Emergency Rudder No Yes
Hydraulic Steering No Yes
Cost & Weight Less More

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