Sailboats Steering Systems
Fall into 3 main categories:
- Tiller Steering: Simplest type of steering; very few parts involved
- Hydraulic Steering: Same principal as for power boats with the distinction that the system does NOT include a lock out valve. This enables the helmsman to ‘feel’ the steering loads, which is desirable. When adding a Drive Unit (hydraulic pump) to these systems the addition of a lock out valve is required between the Drive Unit pump and the manual steering helm pump.
- Mechanical Steering: There numerous types of mechanical steering systems used on sail boats; they all tend to have one or two pedestal mounted steering wheel(s) with various mechanisms mounted below decks to connect to and activate the rudder stock. Some common systems are: pull-pull cable onto a quadrant; drag link onto a tiller; push-pull cable onto a tiller.
Drive System Categories
There are 3 main categories of autopilot drive systems used in sail boats. They are - Cockpit Pilot – Octopus Cable Drive and Inboard Pilot. Cockpit pilots are considered entry-level autopilot systems. Inboard pilots are the pilot of choice of the more serious boater in every category of user. The Octopus Cable Drive offers the below decks installation convenience and security while priced competitively with Cockpit pilots. All sail boat autopilot Drive Units must provide a way for the helmsman to hand steer with minimal drag from the drive unit. Minimal power consumption and operating noise are considered to be major factors when selecting a drive.
- Cockpit Pilots: The Drive Units are mechanical and are either attached to the tiller arm (“tiller pilot”) on smaller vessels or are attached to the wheel (“wheel pilot”) on small to medium vessels equipped with a pedestal type steering wheel. The drives for both types of cockpit pilot are customized proprietary devices that are only available from 1 or 2 of the major marine electronics manufacturers. Octopus does NOT offer any drive systems for this category.
- Octopus Cable Drive: The Octopus Type RS Sailboat Drive System uses a push-pull cable and linear actuator connected to a powerful motor and clutch mechanism; its flexible installation options allow it to be installed below deck in cramped areas as a more robust alternative to cockpit pilots, at a competitive price.
- Inboard Pilots: The drives used on “Inboard Pilots” or “Below Deck Pilots” are mounted below decks and are permanently attached to the steering mechanism. The steering systems are usually mechanical, with 1 or 2 large diameter steering wheel(s) mounted on a pedestal. The most common Drive Units for this application, more accurately known as linear Drive Units, can be either mechanical or hydraulic. In unusual situations these drives are based upon a rotary shaft and sprocket arrangement; they are a small minority and will not be discussed here.
Linear Drive Unit Types
- Linear Drive Units (Mechanical): The mechanical linear Drive Units are a screw and ball nut device, driven by a dc motor through a reduction gear train and clutch. The extension/retraction motion is linear and when the clutch is de-activated it is possible to back drive the unit enabling hand steering. The clutch and motor are controlled directly by the Autopilot Course Computer. This type of drive tends to be very smooth, quiet and efficient in operation, but tend to suffer from reliability problems due to wear in the ball nut and screw mechanism. Octopus does NOT make this type of Drive Unit.
- Linear Drive Units (Hydraulic): The hydraulic linear Drive Units are an assembly of a reversing hydraulic pump, a bypass valve and steering ram, driven by a DC motor. The extension/retraction motion is linear and when the bypass valve (clutch) is de-activated it is possible to back drive the unit enabling hand steering. The bypass valve (clutch) and motor are controlled directly by the Autopilot Course Computer. This type of drive tends to be very smooth, reasonably quiet and efficient in operation, and has proven to be the most durable; it is the drive of choice of the more serious boater in every category of user, and available in a number of models from the Octopus product line.
- Linear Drives (Cable): Octopus offer a unique range of linear drives that are designed to fit into smaller spaces than the traditional types of linear drives as discussed above. The drives comprise of a compact linear actuator attached to the steering quadrant or rudder mechanism, which is operated by a push pull cable connected to a powerful motor and clutch mechanism. These drives are suitable for yachts up to 38ft, and provide an effective solution where space is at a premium.
Linear Drive Sizing
The size of the linear drive must be matched to the size of the vessel or rudder torque requirements, and the power supply capability of the autopilot must also be considered. Care must be taken when using theoretical formulas to determine rudder torque requirements, as conservative formulas can result in grossly oversized drives. In addition the power consumption and thrust specifications for some manufacturer’s linear drives can be misleading, as the power consumed is often quoted as an average and the peak thrust is not sustainable for long periods.
Octopus has developed a 2 step guide for sizing a linear drive: by using the length of the vessel as a general guide and the displacement of the vessel as the main criteria for selecting the correct drive. Refer to our Drive Selection tool for Octopus Drive Unit selection.
Autopilot Electrical Output Requirements
The autopilot Course Computer must have an output capability to match the power requirements of the drive unit, and a clutch output that is capable of activating the solenoid on the drive unit. As a general rule, the autopilot system should be capable of supplying enough power on a sustained basis to run the drive at 2 x normal steering amperage and on a momentary basis at 3 x normal steering amperage. In all cases a fuse as specified by the autopilot manufacturer should be fitted to the system power supply.
Note that in larger linear drive systems that use a continuous running pump, a separate starter relay is used for supplying power to the pump motor. In these cases the autopilot need only supply milliamp power to switch the starter relay, operate the bypass valve and effect steering corrections thru a solenoid valve.