Autopilot Drive Sizing: Match Torque and Stroke

What a drive does in an autopilot

An autopilot drive is the part that physically moves the rudder or the boat's steering. The course computer and the heading sensor decide where the boat should point; the drive provides the motion.

Sizing a drive means choosing one that suits the steering already aboard and the force that steering needs. Three ideas do most of the work: rudder torque, stroke length, and boat displacement.

Steering type comes first. A hydraulic steering system needs a hydraulic pump; a mechanical cable system needs a linear or rotary drive; a tiller boat takes a tiller pilot. See the autopilot drive types explained article for the categories. Then the maker's stated size guidance, not a general rule, decides which model within that category fits your boat.

Rudder torque: the force behind the turn

Rudder torque is the turning force the rudder needs against the water to change the boat's heading. Heavier, faster, and larger boats generally need more torque, but no safe rule of thumb can put a number on your boat.

Makers do not publish a torque figure that a buyer can look up against their own hull. They publish guidance by displacement and steering system size. The site's marine autopilot picks are compared on stated details, and the maker's page is the source that decides fit. See the autopilot torque requirement article for how listings refer to it.

When a listing names a drive type, that name is the maker's own size category. Read it as a category, not as a torque value.

Stroke length: how far the drive must push

Stroke is the travel distance of a linear drive's rod or, for a hydraulic cylinder, the volume of fluid the system moves. The drive must push the steering linkage far enough to move the rudder from one side to the other.

A drive with too little stroke cannot turn the rudder far enough. One with too much may push the rudder against its stops. Both hurt steering. The needed travel is a property of your boat's steering linkage, so the boat's manual and the maker's guidance are the place to confirm it.

For linear drives, stroke is part of the model design. For hydraulic systems, the steerable volume is stated by the maker. Read hydraulic pump flow and linear drive stroke to see how each is named on a listing.

Displacement and steering size: the two numbers on the maker's page

Displacement is the weight of water a hull pushes aside, which for a floating boat is its weight. It is the closest practical proxy for the force a drive must produce, so makers use it in size guidance.

Raymarine's drive unit page states displacement limits for its drives. The cockpit tiller drive is for tiller-steered sailing boats up to 6,000 kg (13,000 lbs.) displacement. The Type 1 rotary and linear drives are for vessels up to 11,000 kg (24,000 lbs.), and the Type 2 rotary drives for vessels up to 20,000 kg (44,000 lbs.).

These figures come from the maker's page and are the kind of detail a drive name on a listing signals. Compare them with your boat's displacement before ordering.

Why one autopilot line comes in several drive versions

The same control head, heading sensor, and course computer can serve many boats. The drive is the part that changes with the steering and the boat size, which is why packs are named by drive. Raymarine's pack page lists EV-100 Tiller Autopilot, EV-100 Wheel Pilot, EV-100 Power Pilot, EV-200 Power Pilot, EV-200 Sail Pilot, and EV-400 Power Pilot as examples.

Furuno does the same in its NavPilot line: pack titles such as NavPilot 300/PG-M and 300/PG-S name the pump in the box, per the Furuno autopilots page. See autopilot for hydraulic steering for how the steering type is identified.

The title of a pack tells you which drive is in the box. A complete pack delivers the drive; a core pack leaves it out for boats that already have one. Read core pack versus complete pack to see how the contents differ, or compare complete packs and core packs on stated details.

What this site can and cannot tell you

This site compares autopilots on stated details only: the steering type, the drive in the box, the control head in the box, and the model line. It does not give installation steps, and it makes no fit claim beyond what a maker's page states.

Sizing a drive is a question for the maker's published guidance and an authorized dealer. Ask the dealer which drive type your steering needs and confirm the displacement range from the maker's page.

The autopilot is a steering aid, not a lookout. The operator keeps watch and remains responsible for the boat, as the U.S. Coast Guard Boating Safety page describes in its guidance for recreational boaters.

What to pick for your situation

Questions

What is rudder torque in an autopilot listing?

Rudder torque is the turning force the drive must produce to move the rudder against the water. Listings rarely give a torque number. They name the drive type, and the maker's page states the size guidance, so you compare on stated details.

What does stroke length mean for a linear drive?

Stroke is the travel distance of the drive rod. The drive must push the steering linkage far enough for the rudder to move fully side to side. The needed stroke comes from your boat's steering linkage, so confirm it with the boat's manual and the maker's guidance.

Does boat displacement decide the drive I need?

Displacement is the main practical guide. Raymarine, for example, states displacement limits for its drive units on its drive unit page. Compare your boat's displacement with the maker's stated range before ordering.

What does a hydraulic pump type number mean?

A type number such as Type 1 or Type 2 is the maker's size category for the pump. Raymarine states each type's range in cubic centimeters of steering system volume on its drive unit page.

Is a rudder reference unit part of drive sizing?

A rudder reference unit tells the autopilot where the rudder is, which is a separate function. It does not size the drive. The steering type and the maker's size guidance decide the drive.

Who decides the right drive size for my boat?

The maker's published guidance and an authorized dealer decide the fit. The site compares stated details only and gives no installation steps.