Model yachts with their wind vane boat steering system inspired a whole series of ingenious systems for steering yachts automatically. These systems have been replaced by electrically-powered marine autopilots.
Wind vane boat steering system systems or windpilot systems are able to steer yacht to windward extremely well, but are less capable of steering a straight course downwind as the pressure of the apparent wind on the vane is much less. This is true of lightweight yachts that surf down the waves and there is almost no apparent wind and a powered compass-controlled marine autopilot is only option.
Some extremely ingenious designs of the windpilot system were produced prior to the powered systems. The aim was to amplify the weak force felt on a vane when the yacht went off course into a stronger one capable of operating the boat steering. The effective type uses a wind vane to turn a deep, narrow servo rudder harnessing the boats speed through the water to produce a strong force that is transferred via lines and pulleys to the main rudder.
Added to their disadvantage of steering downwind, the windpilot system consists of delicate mechanisms projecting over the stern of the yacht, vulnerable to damage from severe weather.
Because they steer with reference to wind direction rather than compass course a wind change may end up making radical change in course. If sailing to windward and not wanting the yacht to respond to wind shifts it is therefore better to have wind vane sensing.
Compact tiller pilot systems are popular automatic boat steering systems because of their ease to install and to use. The tiller pilot is a self-contained unit consisting of a push-pull ram, one end of which is attached to a hinged bracket on the side of the cockpit and the other to a fitting on the tiller. The motor, controls and compass are all built in and connection to the boat’s electrical supply is all the installation needed.
Raymarine manufactures a wide range of tiller pilots which are inexpensive. A feature of these tiller pilots is the ability to remove the complete unit enabling it to be secured in the cabin or taken home. The installation of a Tiller Pilot
Wheel boat steering is a different type of marine autopilot, motor driven and connected to the wheel hub via a toothed rubber belt. These systems can be simple and removable, but on large wheel-steered cruising yachts the system is incorporated into the wheel pedestal and the motor is fully protected from sea spray and the weather.
Tuning the marine autopilot to suit the boat is important so it steers a good course with the minimum of effort. If the marine autopilot constantly makes small corrections of the helm, the battery drain will be excessive and the system suffers premature wear.
The primary adjustment is the sensitivity which is the amount the boat is permitted to wander off course before the marine autopilot responds. When sailing in waves, the boat weaves from side to side and the aim is to achieve the correct average course without too much steering. Because of this, sensitivity needs to be progressively reduced as the sea-state increases, preventing an overload of the autopilot.
A further control is amplitude, or response, which is the amount of helm the marine autopilot applies to achieve the required correction of course. Older types of boat with rudders attached to the keel tended to need a firm application of rudder before they responded, whereas modern lightweight designs with spade rudders react more precisely.
An unwanted feature with marine autopilots is when the big wave that knocks the boat off-course also makes the compass spin, further confusing the mechanism. One answer is the use of a fluxgate compass which has no moving parts. More sophisticated systems incorporate a gyro that can differentiate between drifting off-course and being violently spun by a wave.
Most boats exhibit what is known as 'weather helm' when sailing to windward, which requires a small but constant pressure in one direction on the helm to keep on course. There should be a bias adjustment in the marine autopilot to compensate for weather helm and for a tacking switch so that the same bias can be applied in the other tack.
The marine autopilot can be incorporated into other navigation system such as GPS sets, which can work out 'cross-track error' and then give the autopilot the correct course adjustment to bring the yacht back onto the correct track.
A useful accessory to the marine autopilot is a remote control keypad allowing the watchkeeper to try small adjustments of course while standing at the mast studying the set of the sails, or in an emergency to take over manual control. <