Closed Loop vs Open Loop Autopilot Control

What an open-loop autopilot does

An open-loop autopilot sends a steering command to the drive and does not check what happens next. You set a heading, the controller tells the drive to move, and the drive moves the rudder, wheel or tiller. No sensor reports back whether the boat actually turned, so the system has no way to know if wind, current or waves pushed the boat off the heading you asked for.

The name open loop describes that one-way path. The command goes from controller to drive, and nothing returns. In general terms, an open-loop system can move the steering gear, but it cannot verify that the boat is heading where you set it.

What a closed-loop autopilot does

A closed-loop autopilot adds a return path. A heading sensor, usually a compass of some kind, continuously measures the boat's actual heading. The course computer compares that reading with the heading you set on the control head. When the two differ, the computer sends a corrective command to the drive until the measured heading matches the heading you asked for.

The loop is closed because the information flows in a circle: set a heading, measure the actual heading, compare, correct, then measure again. In general terms, that return path is what lets an autopilot respond when the boat is pushed off its course. The difference between what you set and what the boat actually does is sometimes called follow-up error; see follow-up error for how that term is used.

The parts that close the loop

Three parts matter when you read a listing: the heading sensor, the rudder reference, and the course computer. Raymarine describes the Evolution sensor as the central processor and 3D heading sensor at the heart of every Evolution autopilot system. Furuno's NavPilot page names a Rudder Reference Unit (RRU) as a required part for some features, and notes that with Fantum Feedback, outboard installations no longer require a physical rudder feedback unit.

A heading sensor tells the controller where the bow is pointing. A rudder reference or feedback unit tells the controller where the rudder actually is. The course computer, often called an actuator control unit, compares the desired heading with the measured heading and decides what command to send to the drive.

Some pack titles name these parts, others do not. A title that says no drive means the drive is left out, but the pack may still include the sensor and the control head. Read the stated details to see which parts are in the box. See what a core pack without a drive includes for how the drive fits into a pack name.

Why feedback matters when you buy

For a boat owner, the practical difference between open and closed loop is simple: a closed-loop system can react to what the boat actually does. That reaction is what allows an autopilot to hold a course when conditions change. How well it does that depends on the whole system, the boat's steering, and the installation, not on any single part.

When you compare packs, look at the stated details: is a heading sensor in the box? Is a rudder reference included? Is there a control head? Those are the parts that make the loop closed. The maker's product page is the source for what the pack contains, so check it before you assume a listing is complete.

If you are deciding between a complete pack and a core pack, the sensing parts are a key difference. A complete pack usually includes the control head and the sensor, while a core pack may leave the control head out. See complete autopilot packs and core packs for how this site compares them on stated details.

What a listing can and cannot tell you

A listing title names the model line, such as Raymarine Evolution or Furuno NavPilot, and the pack type. It may say tiller, wheel, hydraulic or drive-by-wire, which tells you which steering the pack is designed for. Raymarine's Autopilot Packs page lists names like EV-100 Tiller, EV-100 Wheel Pilot, EV-200 Power Pilot and EV-DBW Pilot.

The title alone does not tell you which sensing parts are included. You need the stated details and the maker's page. The catalogue cannot tell you how a system will behave on your boat; that depends on the boat's steering, the drive and the installation, which is the job of the maker's manual and a qualified installer. For a general comparison of model lines on stated details, see best marine autopilot.

Questions to ask before you buy

Before you order a pack, write down the questions that decide whether the loop is closed for your boat. For more on the parts behind these questions, see heading sensor types, rudder feedback sensors, and control head options.

• Is the heading sensor in the box, or do you need to buy it separately?
• Does the pack include a rudder reference or feedback unit?
• Does the listed model use a physical rudder feedback unit, or does the maker's software do without one?
• Which control head is named in the pack title, and is it in the box?
• Does the listing say no drive, or is the drive included?

What to pick for your situation

Questions

What does open loop mean for an autopilot?

Open loop means the controller sends a steering command without measuring the result. In general terms, an open-loop autopilot can move the drive but cannot verify that the boat is on the heading you set.

What does closed loop mean for an autopilot?

Closed loop means a heading sensor measures the boat's actual heading and feeds it back to the controller, which compares it with the heading you set and corrects the drive when they differ.

Is a rudder reference the same as a heading sensor?

No. A heading sensor measures where the boat is pointing. A rudder reference measures the rudder's position. Furuno's NavPilot page notes that with Fantum Feedback, outboard installations no longer require a physical rudder feedback unit.

Do I need a closed-loop autopilot?

Modern autopilot packs for recreational boats include a heading sensor, which makes the loop closed in general terms. The practical question is whether the pack includes the sensing and control parts you expect: sensor, control head and drive.

Where can I find out what is in a pack?

Read the stated details on the listing and the maker's product page. The maker's page is the source for what the pack contains and which sensing parts are included.