Windvanes 101Crash Course in Selfsteering Systems
Classification of Vane Gears by Course Correcting SystemStraight Vane to Main Rudder
In this type of self-steering device the air vane is connected to the main rudder directly on the rudder or the rudder shaft or through lines and blocks to the wheel or tiller.
It is fair to say that these arrangements are inadequate and do not work. Keep in mind the very slight force generated by the vane and the often large forces needed to overcome the friction in the main rudder suspension, the resistance from the surface area of the main rudder, and the friction and resistance in the vane-to-rudder linkage.
When the wind is light the air vane is almost powerless and cannot overcome the friction in the system. When the wind picks up, the resistance from the rudder also increases. Although the air vane now has more power, this is still inadequate to overcome the helm of the boat. The system fails because the air vane does not have enough power to control the course correcting device, in this case the boat's own rudder.
Straight Vane to Auxilary Rudder
By coupling the air vane directly to a smaller auxiliary rudder you gain several advantages over the previous solution. Friction and rudder torque can be reduced considerably by making the auxiliary rudder small and balanced. The main rudder is not occupied by the self-steering and can be used instead for trimming the yacht.
The problem with this principle is that you are still asking an air vane, with its very limited power, to directly turn a rudder big enough to correct the course of a large and heavy boat. Even though the auxiliary rudder can be made comparatively small, it will still have to be big enough to turn the yacht and sturdy enough to withstand the forces acting on it. Consequently, there is still considerable resistance to the weak signals from the air vane.
The vane-to-auxiliary-rudder is an inferior solution. Downwind, the vane itself does not have enough power to control the auxiliary rudder efficiently. Reaching, the auxiliary rudder often is too small to handle the yacht in gusty conditions, even though the vane may now have enough power to control the rudder. The system fails mostly because too much is being asked of the air vane. If the self-steering rudder is made small enough for the air vane to control, the rudder is not large enough to control the boat.
Correcting Devices Using Servo-Power
A solution to the problem of the air vane's limited power is to amplify it through an intermediate mechanism, designed to increase the force of the signal from the vane. This extra power is derived from the flow of water past the hull in motion.
The devices used to create such "servo-power" are trim-tabs and servo-pendulum oars (servo-oars). Servo-oars generate a much greater amplification of the force of the vane's signals than do trim tabs because they use leverage through the oar shaft.
Vane gears using servo devices can be classified according to the type of servo and the type of course correcting device that the servo operates:
Trim Tab on Main Rudder
This arrangement is generally individually tailored when the boat has an outboard rudder, which facilitates the air vane-to-trim-tab linkage and makes installation of other devices more difficult. Since the main rudder is usually quite large and difficult to turn, it takes a comparatively large trim-tab to do the job. As more is asked of both the air vane and the tab in this system compared to other servo-systems, it will not be able to self-steer a yacht in lighter winds or when there is a lot of helm and friction fighting the self-steering. Nevertheless, the introduction of the trim-tab makes it possible for the air vane to turn the main-rudder. The systems works although it is not very forgiving of difficult conditions and operator mistakes. It is very difficult to use on a boat with a lot of weather helm.
Trim Tab on Auxiliary Rudder
In this design a relative smaller tab is used to turn an Auxiliary-Rudder, considerably smaller than the main rudder. Since less is asked of the air vane in terms of the area it has to control, the trim-tab on Auxiliary-Rudder is a more sensitive system than when the tab is driving the main-rudder. Also, the main-rudder can now be used for trimming the boat and the better balance simplifies the job for the self-steering gear.
In light air this system is extremely sensitive since it does not turn the boat's own rudder, which is much larger. The system works best on boats less than 45-48 feet LOA since the Auxiliary-Rudder gradually becomes too small in rougher conditions on larger boats. Auxiliary-rudder systems often require strengthening of the boat's transom. They cannot be removed at sea when not in use. They are often preferred on boats with a poor main steering system (too many turns lock to lock, excessive play or friction and hydraulic steering). There are no lines to the cockpit and they can be used as an emergency rudder. The auxiliary rudder will also improve the directional stability since it is located far aft.
Servo Oar on Auxiliary Rudder
Instead of a trimtab controlling the auxiliary rudder this rudder can be steered by a servo paddle behind the auxiliary rudder. This method provides plenty of power to turn the auxiliary rudder. The servo oar is more vulnerable than the trim tab since it travels from side to side while the trim tab is "hiding" behind the protection of the auxiliary rudder.
The question is still if the size of the auxiliary rudder is large enough to control the boat in rough conditions.
Servo Oar to Main Rudder
The most efficient way to steer a boat is to use the boat's own rudder, which was designed to steer the boat in the first place.
The development of the servo-pendulum oar represented a break-through in self-steering by providing a means of drastically increasing the force of the signals from the vane.
In this design the vane rotates an oar on the transom or stern. The oar gets forced by the slip stream of the water to swing to one or the other side of the hull. Through the pendulum oar a very powerful leverage is created which is used to turn the main rudder through an on-deck linkage with lines and blocks.
Because of the tremendous increase in power created by the long lever arm of the servo-pendulum oar, the blade itself does not need a very large surface area, consequently it does not take much force in the signal from the vane to produce the rotation of the oar and bring about the course correction
The Servo-Oar-to-Main-Rudder principle is the most popular principle today. It works well on surprisingly large yachts since it is using the boats rudder to control the boat. The more it blows the faster the boat will move through the water. With higher boat speed the power of the servo oar increases and this gives ample power to turn the boat's large rudder.
It is important that the boat's own steering system is as friction free as possible and also has minimal play. Wheel steering systems should not have more than four turns lock to lock. Strong wind is normally not a problem and with a good gear, properly used, you should expect good performance even in slow speed-down to approximately two knots of boat speed.
One unique property of the servo-pendulum type of self-steering is that it acts to prevent the boat from yawing downwind. When the stern moves to the side this produces a swing in the opposite direction by the servo-pendulum. The swinging oar will turn the main rudder to counteract the yawing as it prevents a broach. This positive yaw damping is an extremely valuable feature, which is only present in servo-oar devices. Trim-tab devices actually work exactly opposite to increase yawing.
Clearly, the less resistance there is to the servo-oar in the boat's own steering, the better the servo-oar-to-main-rudder gears will work, especially in light airs.
Hydraulic steering systems, which tend to slip, and have too many turns on the wheel, do not lend themselves to this type of self-steering device, but many times the hydraulics can be bypassed and the pendulum lines rigged to steer directly through an emergency tiller the main rudder shaft. The emergency tiller can be faced in any direction.
Pendulum/Trimtab on Main Rudder
The Saye's Rig is a patented unique hybrid pendulum/Trimtab gear. A tiller arm that looks like a hair pin is attached, facing aft, to the rudder. On deck is an airvane which is connected to a pendulum oar which is "harnessed" at the aft end of the tiller arm. This system is often the ideal gear for large (45 to 70 feet) boats, which often have hydraulic steering, midship cockpit and high freeboard. On such boats an auxiliary rudder is often too small to control the boat and because of the hydraulic steering a servo pendulum gear cannot be hooked up to the wheel (too many turns and slippage in the system).
The boat should preferably have a conventional design with its rudder located at least a couple feet forward of the stern. The longer the tiller arm is, the more power the gear will have.
Double Servo Systems
A unique windvane principle is the double servo system. The intention is to improve the sensitivity and power of the self-steering by aiding the air vane with more than one servo device. Only one selfsteering device of this type has been commercially produced. A servo-oar is used to turn the main rudder of yacht. However, the vane does not control the servo-oar directly. Instead, the signal from the vane turns a very small trim-tab on the trailing edge of the servo-oar which in turn forces the servo-oar to rotate. This trim-tab provides an increase in power used to control the servo-oar. Since less is asked of the air vane it can in theory be smaller. It is often used by smaller boats in the 20-30 foot range, of light to medium displacement.