How to Choose a Marine Autopilot: Steering Type, Drive Unit, and Sizing

A marine autopilot is a closed-loop steering system that holds a heading or follows a route by driving your tiller, wheel, hydraulic pump, or steer-by-wire actuator from a course computer and heading sensor. When you are figuring out how to choose a marine autopilot, start with steering type, then size the drive to laden displacement or cylinder volume, then pick the computer and compass that match that drive.

Best-of lists are useful for brand shopping. This guide is the decision tree we walk customers through at the counter and on the shop floor: what is on your boat now, what force the drive has to fight, and which sensors keep the boat from hunting in a chop.

Quick answer: how to choose a marine autopilot

1. Identify your steering: tiller, mechanical cable/rod, hydraulic, or steer-by-wire / power-assisted.
2. Size the drive: for mechanical systems use fully laden displacement (add about 20% if you only have design displacement); for existing hydraulic steering match pump flow to ram volume in cc or cubic inches.
3. Match the computer to the drive current: small low-current packs vs high-current computers such as Simrad NAC-2 vs NAC-3.
4. Pick a modern heading sensor: 9-axis / rate-gyro cores beat basic fluxgate in following seas.
5. Confirm network and voltage: NMEA 2000 or SeaTalk NG, plus 12 V vs 24 V for the drive itself.

If you want brand shortlists after you know the drive size, see our related roundup: Best Marine Autopilot Systems of 2026.

The four parts of a marine autopilot system

Every working system has the same four blocks. Confusing them is how people buy a nice control head and then discover they still need a pump, a clutch cable, and a compass.

On Raymarine Evolution, the EV-1 is both sensor and brain; the ACU is the power amp for the drive. On Simrad Continuum, the NAC is the brain and power stage, and the Precision-9 is the compass. On Garmin Reactor 40, the ECU carries the 9-axis AHRS and talks to a SmartPump or mechanical drive. Same physics, different packaging.

Step 1: Identify your steering type before you shop

Short answer: the drive has to match how the boat already steers. Wrong drive family will not “almost work.”

Tiller steering

Common on smaller sailboats. You want a cockpit tiller drive that clamps to the tiller and pushes/pulls it. Raymarine rates its cockpit tiller drive for boats up to about 6,000 kg (13,000 lb) displacement on its drive-unit pages. Above that, you are usually into below-decks linear drives with a tiller arm on the rudder stock.

Mechanical wheel steering (cable, rod, or quadrant)

Most mid-size sailboats and some powerboats. Options:

• Cockpit wheel drive for lighter sailboats (fits on the wheel rim).
• Below-decks mechanical linear drive pushing a tiller arm or quadrant — quiet, strong, and our usual recommendation once the boat gets heavy.
• Rotary drive that engages the steering chain with a sprocket — useful when linear geometry will not fit.

Raymarine’s own guide is clear: the linear or rotary drive must be able to back-drive the steering so you can hand-steer when the clutch releases. If your steering will not back-drive, you need a different mechanical solution or a hydraulic linear add-on.

Hydraulic steering

Standard on most powerboats over about 25 feet and many modern sailboats. You tee a reversing pump into the existing hydraulic lines (or use a solenoid valve on a constant-running pump). You size by cylinder volume, not by LOA. More on that below.

Steer-by-wire and power-assisted systems

Volvo IPS, Yamaha Helm Master, some Mercruiser/Volvo sterndrives, and other electronic helms need a brand-specific gateway or actuator pack — not a generic reversing pump. Raymarine sells stern-drive actuators and Volvo DBW Evolution packs; Simrad sells Helm Master and Volvo IPS steer-by-wire kits. Confirm the exact engine/helm software version before you order.

How to choose a marine autopilot drive by boat size (laden displacement)

Short answer: size mechanical drives to fully laden displacement. If you only know design/light-ship displacement, add about 20%, then size up if you are near the top of a drive’s range.

Raymarine’s Evolution documentation and drive pages use fully laden displacement for mechanical linear and rotary drives. That means fuel, water, gear, dinghy, and people — the weight you actually go to sea with. An undersized drive hunts, runs hot, and dies early. An oversized drive is usually fine; an undersized one is not.

Raymarine Type 1 / Type 2 / Type 3 mechanical ranges

From Raymarine’s current drive-unit product pages:

Rule we use in the shop: if your laden weight is within about 10% of the top of a Type rating, buy the next size up. A 10,500 kg boat on a Type 1 will work on paper and feel soft in a following sea.

Browse current Evolution computers, EV cores, and drives in our Raymarine Autopilots collection.

Simrad NAC-2 vs NAC-3: computer current, not just length

Simrad rates the computer by boat length and drive current:

• NAC-2: recommended for boats up to 10 m (35 ft); built for low-current pumps and mechanical drives such as RPU80, SD10, and HLD350-class units.
• NAC-3: recommended for boats 10 m (35 ft) and up; rated for high-current drives such as RPU160 / RPU300 and DD15. Drive output is 30 A continuous, 50 A peak (1 second) on 12/24 V DC per Simrad’s NAC-3 specs.

A 34-foot boat with a heavy hydraulic ram can still need NAC-3 current. Length is a starting filter; amp draw of the pump is the real gate. The Simrad NAC-3 Autopilot Core Pack (NAC-3 + Precision-9 + RF25N + N2k starter kit) is the usual core for bigger Continuum installs. More packs and pumps live in our Simrad Autopilots collection.

How to choose a marine autopilot hydraulic pump (cylinder volume)

Short answer: read the ram volume off the cylinder plate or steering manual, then pick the pump whose range covers that volume. Do not “size up for safety” on hydraulic pumps the way you do on mechanical drives.

Raymarine’s hydraulic drive guide says matching the wrong pump makes the boat steer too fast or too slow, and moving up or down outside the published range does not improve performance. Dual rams in parallel: add the volumes. Rams in series: use a single capacity.

Raymarine Evolution hydraulic pump ranges

From Raymarine’s “Selecting the Right Hydraulic Drive” guide:

Cannot find a volume on the ram? Note the manufacturer and model; dealers cross-reference Teleflex, SeaStar, and similar cylinders. Raymarine also notes Type 0.5 suits some Volvo D4/D6 sterndrive applications.

Garmin SmartPump approach

Garmin’s Reactor 40 hydraulic corepack takes a different path: one SmartPump v2 with a configurable flow rate from 0 to 2.4 L/min, so the installer tunes response to the steering volume instead of stocking five discrete pump sizes. The Garmin Reactor 40 with SmartPump v2 + GHC 50 is the full hydraulic kit we sell most often for mid-size powerboats. It is hydraulic-only — cable/mechanical boats need a different Garmin drive family.

Heading sensors: 9-axis / rate gyro vs basic fluxgate

Short answer: buy a 9-axis or rate-gyro sensor unless you only day-cruise in flat water and hate spending money.

A fluxgate compass reads the Earth’s magnetic field. It is fine in flat water and confused by heel, pitch, and yaw. A rate gyro / 9-axis AHRS measures angular rate and attitude directly, so the computer corrects before the heading error grows.

• Raymarine EV-1: 9-axis solid-state sensor core; the heart of Evolution.
• Simrad Precision-9: solid-state compass outputting heading, rate of turn, roll, and pitch on NMEA 2000 (heading/ROT at 20 Hz on the NAC-3 pack description).
• Garmin Reactor 40 AHRS: 9-axis sensor in the ECU; mount in any orientation — the calibration figures out which way is up.

Practical tradeoff: the modern sensor costs more upfront and saves you from the “autopilot drunk walk” in a quartering sea. If you already own a good NMEA 2000 satellite compass, some systems can use it as a heading source — check the manufacturer’s supported-sensor list before you assume.

Network and power: NMEA 2000, SeaTalk NG, 12 V vs 24 V

Almost every current recreational pilot speaks NMEA 2000 (Raymarine’s SeaTalk NG is the same backbone with different connectors and a simple adapter). That is how the autopilot follows a route from your chartplotter, takes wind data for sail modes, and shares heading with radar and AIS.

Power is a separate decision:

• The electronics (computer, compass, control head) are usually happy on a clean 12 V supply.
• The drive must match boat voltage. Many Raymarine and Simrad pumps/drives are offered in both 12 V and 24 V. Garmin Reactor 40 ECU accepts 11.5–30 V DC, so one ECU covers both house systems.
• Run the drive on a dedicated breaker sized to the manufacturer’s fuse rating (NAC-3 uses an internal 30 A slow fuse; Reactor hydraulic peaks are covered with a 40 A blade fuse in Garmin’s kit docs). Voltage drop on a shared circuit makes pumps chatter.

Rudder feedback: inboards and most sailboats want a real rudder reference unit. Outboards and sterndrives with planing hulls can often use virtual rudder feedback (Simrad VRF, some Raymarine/Garmin modes). When in doubt on a displacement hull, install the feedback unit.

Sail vs power extras that actually matter

Powerboat features worth paying for

• Shadow Drive–style takeover (Garmin): turn the wheel and the pilot drops out, then re-engages on the new heading when you release. Requires the compatible Shadow Drive valve.
• SteadySteer / similar hydraulic bypass kits (Simrad fitting kits): let you override without fighting the pump.
• No-drift / track modes tied to GPS so the boat holds a ground track in current, not just a compass heading.

Sailboat features worth paying for

• Wind mode (apparent or true) so the pilot holds a wind angle instead of a compass course.
• Auto tack / jibe when a wind sensor is on the network.
• Heel-aware response from a 9-axis sensor so the pilot does not treat heel as a heading error.

Pair the pilot with a working wind instrument and a chartplotter on the same backbone. AIS on that same network is smart insurance when the pilot is steering and you are off the helm — see our AIS systems collection.

Common mistakes when choosing a marine autopilot

• Shopping by boat length alone. A light 38-foot express and a heavy 38-foot trawler are different animals. Use laden displacement or ram volume.
• Undersizing the mechanical drive “to save money.” You will buy twice.
• Oversizing a hydraulic reversing pump outside the published cc range. Raymarine specifically warns against this.
• Skipping rudder feedback on boats that need it. The pilot overshoots and hunts.
• Mounting the compass next to speakers, wipers, or a lithium bank. We have seen multi-degree heading errors from bad placement.
• Ignoring 12 V vs 24 V on the drive. The computer may be dual-voltage; the pump often is not.
• Buying a hydraulic corepack for a cable-steered boat (or the reverse). Confirm steering type before you click buy.
• Expecting DIY on a complex hydraulic or DBW install. Plumbing, bleeding, and commissioning still matter more than the brand logo.

Buying checklist: how to choose a marine autopilot without second-guessing

1. Write down steering type: tiller / mechanical / hydraulic / DBW.
2. Write down laden displacement or hydraulic ram volume (cc or in³) and voltage (12 or 24).
3. Pick the drive family that matches that steering and size.
4. Pick the computer/ACU rated for that drive’s current.
5. Pick a 9-axis / rate-gyro heading sensor (or a pack that includes one).
6. Decide control head vs MFD-only control; flybridge boats usually want a dedicated head at each station.
7. Confirm NMEA 2000 / SeaTalk NG plan, including drop cables and terminators.
8. Add rudder feedback unless your hull/drive explicitly supports VRF.
9. For sail, budget a wind sensor; for power, consider Shadow Drive / SteadySteer if you want grab-and-go overrides.
10. Plan commissioning time: compass swing, hard-over timing, and a sea trial in real chop.

Still stuck between brands after the drive is sized? Compare finished packs in our Raymarine and Simrad autopilot collections, or start from the 2026 best-of guide once the physics are settled.

FAQ: how to choose a marine autopilot

What should I decide first when choosing a marine autopilot?

Steering type. Tiller, mechanical, hydraulic, and steer-by-wire each need a different drive. Everything else — computer, compass, control head — follows the drive.

How do I size an autopilot drive for my boat?

For mechanical drives, use fully laden displacement and compare it to the manufacturer’s kg/lb rating (Raymarine Type 1 linear tops out at 11,000 kg; Type 2 short at 15,000 kg; Type 2 long at 20,000 kg). Add about 20% if you only know design displacement, and size up near the limit. For hydraulic steering, match pump type to ram volume in cc or cubic inches.

What is the difference between Simrad NAC-2 and NAC-3?

NAC-2 is aimed at boats up to 10 m (35 ft) with low-current drives. NAC-3 is aimed at boats 10 m and up and can run high-current pumps and mechanical drives at 30 A continuous (50 A peak). Choose by drive current as much as by length.

Do I need a rudder feedback unit?

Usually yes on inboards and displacement hulls. Outboards and many sterndrives can use virtual rudder feedback. If the pilot hunts without a feedback unit, install one.

Is a 9-axis heading sensor worth it?

Yes for anything beyond flat-water day trips. Rate-gyro / 9-axis sensors hold course in chop and following seas far better than a basic fluxgate.

Can my autopilot follow a chartplotter route?

Yes, when the autopilot and chartplotter share a NMEA 2000 (or SeaTalk NG) network and you engage track/route mode. Same-brand MFDs usually give the cleanest controls; cross-brand still works for standard NMEA 2000 navigation data.

12 V or 24 V — which autopilot drive do I need?

Match the drive to your boat’s DC system. Many pumps are sold in both voltages. Some computers (for example Garmin Reactor 40 ECU and Simrad NAC-3) accept 12/24 V input, but the motor/pump itself must still be the correct voltage variant.