Self-steering Gear - Mechanical

Mechanical

Mechanical or "wind vane" self-steering started out as a way to keep model sail boats on course. The first time that it was used to cross an ocean was on a motorboat. The most widespread form of self-steering, the servo pendulum, was introduced to cope with the power required to operate a larger rudder and was a successor to the servo trim tab principle (introduced by Herbert "Blondie" Hasler). Both methods use the power derived from the motion of the boat through the water to hold a constant angle to the wind with the use of the boat's main rudder. Despite the popularity of servo systems, one manufacturer has successfully developed a system that uses an auxiliary rudder direct from the windvane; the picture of the windvane shown uses this principle with the previously used large fabric vane on a vertical axis (the use of wind vanes with a horizontal axis is now predominant used). Offshore, the wind direction is relatively stable, so over a number of hours this results in a reasonably constant compass course, and also means that the sails need not be adjusted. Mechanical self-steering can be complicated to set up, so it is typically used only for long-distance sailing where the same course is maintained for long periods. Many boats fitted with mechanical self-steering also carry an electrical autohelm for use over shorter periods where it is not worth setting up the wind vane.

Mechanical self-steering gear is made by a number of manufacturers, but most share the same principle. A narrow upright board, the wind vane, is mounted on a horizontal axis carrier that is itself rotated so that with the boat traveling in the desired direction the vane is vertical and edge-on to the wind. The wind vane is held upright by a small weight below the pivot, but if the boat turns so that the board is no longer edge-on to the wind it will be blown over to one side as the extra surface area is revealed. This movement is transmitted by a series of linkages to a blade (or oar) in the water. In the simplest devices this blade acts directly as a secondary rudder, and steers the boat back onto the proper course however most designs of windvane self steering find that the force provided by the wind vane alone is not sufficient to make this system work and hence a so-called servo pendulum system is used.

As the blade described above turns, the pressure of water moving past it causes it to swing out sideways on the end of a pivoted rod. The length of this rod and the speed of the water means that a considerable force is available at the top end of it, sufficient to change the course of much larger boats. This is achieved either by a connection to the main wheel or tiller (typically involving a complex arrangement of lines and blocks rigged around the stern of the boat) or by fixing the main steering in place and equipping the self-steering gear with its own rudder. Once the boat has moved back to its correct course, the wind vane stands up again as it is no longer blown over by the wind.

To make wind vane self steering work well it is essential to have the vessel's sails balanced with little load on the rudder before any attempt is made to engage the self steering. With the sails are trimmed correctly wind vane self-steering is very effective. Some experimentation and judgement is usually needed, however, to determine the proper settings for a given vessel and steering mechanism. In addition, wind vanes perform poorly in very light winds, as the forces needed to operate them are much reduced. The same applies when travelling downwind, as the apparent wind speed is reduced by the speed of the boat.

As well as their requirement for power, many long-distance cruisers observe that electronic self-steering machinery is complex and unlikely to be repairable without spare parts in remote areas. By contrast, the often agricultural-looking mechanism of a wind vane gear offers at least the possibility of an improvised repair at sea, and can usually be rebuilt on land using non-specific parts (sometimes plumbing parts) by a local welder or machinist.

Another version of wind vane self steering on sail boats is known as the vertical axis vane and usually, because of the inferior power it has to its Servo Pendulum cousins it makes use of a trim tab hung off the rudder to control the course of the boat. The vane spins at right angles to the ground and can lock to the trim tab in any desired position, as the boat falls off the wind the vane will be turned by the wind and will take the trim tab with it which in turn causes the rudder to move in the opposite direction and thus corrects course. Generally self steering like this, with a trim tab can only be used on boats with transom (or aft hung double enders) rudders as the trim tab needs to be mounted directly to and aft of the rudder to produce the desired effect, and of course has to be controlled even as the rudder swings side to side. This is typically accomplished by use of a slotted bar in which the connection to the vane assembly can slide in as the rudder turns. These self steering systems are generally simpler and are thus easier to set and adjust course as they don't make use of lines controlling the rudder but control it more directly through solid linkages.

A related device has been used on some windmills, the fantail, a small windmill mounted at right angles to the main sails which automatically turns the heavy cap and main sails into the wind, (invented in England in 1745). (When the wind is already directly into the main vanes, the fantail remains essentially motionless.)

A popular source on contemporary windvane technology is The Windvane Self-Steering Handbook. One particularly valuable contribution of Morris's book is his coverage of the variety of alloys used in vane gear manufacturing. Morris admits to his practice of setting a kitchen timer for a half hour at a time and sleeping while the windvane steering device controls the helm, even in head winds of 25 to 35 knots. In a recent interview, he said he once narrowly missed being hit by a huge freighter while sleeping on his sail up the Red Sea. Morris points out, "An autopilot wouldn't have made any difference in this case. If I had been using an electronic autopilot, that freighter still would have been there. I made a choice to sail two-thirds of my circumnavigation single-handed, and I accepted the risks that came with that decision. I guess fate was on my side."