Fundamentals · 7 minute read
Marine autopilot drive types
Compare marine autopilot drive types: tiller, wheel, linear, rotary, hydraulic and solenoid. Learn how steering design determines drive sizing and installation.
The drive is the part of the autopilot that meets the boat, and it is the part most likely to be wrong. Course computers and heads are interchangeable within a brand; drives are chosen for the steering gear, the rudder loads and the space available. Get the drive right and almost any modern computer will steer the boat well. Get it wrong and no amount of tuning helps.
Key facts
- Tiller push-rod
- Cockpit socket to tiller pin; to about 4.5 t
- Wheel belt drive
- Clamps to wheel spokes; to about 7.5 t
- Linear ram
- Tiller arm on the rudder stock; 6–35 t
- Rotary drive
- Sprocket in chain-and-cable steering
- Hydraulic pump
- Tees into hydraulic steering; sized by cylinder
- Solenoid
- Controls a constant-running pump on power-steered vessels
Tiller push-rod drives
The simplest drive: a motor in a tube extends and retracts a rod between a cockpit socket and a pin on the tiller. In self-contained tiller pilots the compass and computer share the tube; in the Raymarine EV-100 Tiller and the Pelagic the drive is separate from the brain. Thrust is modest, 57–77 kg on the mainstream units, and the geometry must give full rudder travel within the stroke. Mounting is everything: a strong socket base, a pin at the right height, the rod level.
Wheel belt drives
An above-deck wheel drive clamps a large sprocket or ring to the wheel spokes and turns it with a belt from a motor housing on the pedestal or cockpit floor. The Raymarine EV-100 Wheel and the CPT are the common examples. They are quick to fit, easy to move between boats and adequate for coastal use on boats to about 7.5 tonnes. Belt tension, clutch wear and salt exposure are the maintenance items; a cover doubles the belt's life.
Linear drives on a tiller arm
The workhorse of the below-deck world. An electric ram (mechanical, or hydraulic-linear on Raymarine's larger units) bolts between a foundation on the hull and a on the rudder stock, and pushes the rudder directly. It is quiet, powerful, out of the weather and disengaged by a clutch. Raymarine's Type 1, 2 and 3 and Jefa's range for B&G, Garmin and Simrad cover boats from 6 to 35 tonnes. Rudder stops that limit the drive's travel are essential; the drive is strong enough to bend a tiller arm at the end of its stroke.
- Foundation glassed or bolted to a structural surface
- Tiller arm keyed or clamped to the rudder stock
- Drive stroke matched to the arm length for full rudder angle
- Rudder reference on the same arm, linkage parallel
- Mechanical rudder stops inside the drive's travel
- Clutch accessible for manual override
Rotary drives
Boats with chain-and-cable pedestal steering can take a : a motor turning a sprocket inserted in the chain run, so the pilot turns the wheel from below deck without a belt in the cockpit. Raymarine's Type 1 and 2 rotary and Jefa's rotary units are the usual choices. The installation needs a rigid mounting and correct chain alignment, and the system inherits the backlash of the cable steering.
Hydraulic pumps
Hydraulically steered boats take a reversible pump plumbed into the steering circuit with tee fittings, so that the pump moves the same cylinder the helm does. Pumps are sized by cylinder volume and the you want: small pumps for outboard cylinders, 1.2 L and 2.0 L pumps for cruisers, larger for heavy vessels. Air in the system after installation is the commonest fault; bleed it properly. Garmin's and Furuno's add a sensor that hands the wheel back when you turn it.
Solenoid and constant-running systems
Larger vessels with power-assisted hydraulic steering already have a pump running whenever the engine does. The autopilot then controls a solenoid valve block that directs flow to the cylinder. Raymarine's ACU-300 and ACU-400, Simrad's NAC-3 and Furuno's 711C all support solenoid control. It is an installer-configured arrangement common on displacement cruisers and commercial vessels.
Windvanes: the non-electric drive
A windvane replaces the whole electronic chain. gears use a water blade to pull lines to the wheel or tiller; auxiliary-rudder gears steer with their own rudder. Neither draws power; both steer to the wind only. On ocean passages from Australia they are the backup that never runs flat.
Match the drive to loaded displacement
Questions
Frequently asked
Can I fit a linear drive to a boat with a wheel?
Yes, if there is a tiller arm or room for one on the rudder stock. It is the preferred below-deck solution for wheel-steered yachts.
Is a hydraulic linear drive better than a mechanical one?
Raymarine's hydraulic-linear drives suit larger boats and give a softer feel; mechanical linear drives are lighter and simpler. For most yachts under 15 tonnes a mechanical linear drive is the right choice.
What is hard-over time?
The time the drive takes to move the rudder from full one side to full the other. Cruising boats aim for 10–15 seconds; planing hulls faster; heavy displacement vessels slower.
Can I reuse an old drive with a new course computer?
Often, within a brand and sometimes across brands with an installer's help. A sound ten-year-old Type 1 linear drive with a new ACU and EV-1 is a common and sensible upgrade.
Practise autopilot operation with an instructor
Habits are learned at the helm with someone watching. The training section explains what a good instructor covers and where to find one in your state.