Autopilot for Full Keel Sailboats: Sizing Tips

Why a full keel asks more of the autopilot

A full keel is a keel that runs a large part of the hull's length, usually with the rudder hung at the aft end or on a skeg. This is the classic long-keel layout on older cruising sailboats and on many modern bluewater designs. The rudder works on a longer lever arm and has a large area, so it produces more force when turned. That is why full keel boats feel steady at the helm, and it is also why the autopilot has to work harder to move the rudder.

The part of an autopilot that actually turns the steering is the drive unit: a tiller drive, a wheel drive, a linear drive, a rotary drive or a hydraulic pump. The right drive for a full keel boat depends on the steering that is already aboard, not on the hull shape alone. The maker's page for the drive or the pack states the steering type and displacement it is built for. That stated range is the fit detail a buyer compares with the boat's displacement. For a wider look at how drives differ, see autopilot drive types.

Torque and rudder authority in plain terms

Torque is the twisting force that moves the rudder. Rudder authority is how strongly the rudder changes the boat's heading at a given angle. A full keel rudder has high authority because it is large and sits far aft, which is good for steering feel but demands a drive with enough torque to overcome the water pressure on the blade.

Autopilot listings rarely print a torque number. Instead they state the drive type, sometimes the steering type the drive is built for, and sometimes a displacement range. On this site, those stated details are the basis of comparison. A full keel boat whose displacement sits near the top of a drive's stated range is a sign to look at the next size up in the same line. The torque requirement page explains how to think about this without inventing numbers.

What the listing says about drive size

The Raymarine drive unit page is one example of how makers state fit. It gives the Cockpit Tiller Drive for tiller-steered sailing boats up to 6,000 kg (13,000 lbs) displacement. The Type 1 Rotary Drive is stated for vessels up to 11,000 kg (24,000 lbs), the Type 2 Rotary Drives for vessels up to 20,000 kg (44,000 lbs), and the Type 1 Linear Drive Unit for boats up to 11,000 kg (24,000 lbs). These figures are the maker's stated range, not a promise about how a hull behaves.

For a full keel boat, displacement is the most useful stated number. A boat whose displacement is near the top of one drive range and below the next range is a case for checking the larger drive's stated limits. The same logic applies to a linear drive on a mechanical steering system. Compare the listing's stated range with the boat's actual displacement, and let the maker's page be the final word.

Sail pilot names versus power pilot names

Pack names carry the steering type in the name. Raymarine's pack page lists the EV-100 Tiller Autopilot for small sailboats, the EV-100 Wheel Pilot for small to medium sailboats, the EV-200 Sail Pilot for mid-sized sailboats with mechanical steering, and the EV-400 Sail Pilot for large sailboats with mechanical steering. On the power side, the EV-100, EV-150, EV-200 and EV-400 Power Pilots are stated for boats with hydraulic steering.

The "Sail Pilot" name signals mechanical steering on a sailboat, not a hull shape. A full keel sailboat with wheel steering and mechanical cable fits the Sail Pilot name. One with hydraulic steering looks at the power pilot names or a hydraulic pump drive. Tiller-steered full keel boats look at the tiller pilot names. The difference between sail and power autopilots is explained in more depth on the sailboat versus powerboat autopilot page.

Displacement as a stated fit detail

The maker's page decides fit. The catalogue has no data on how a drive holds a course on a particular hull, and no article should promise that a given unit is right for a given keel. What the catalogue can do is compare stated details: the drive type, the steering type, the displacement range, and whether the pack includes the drive and control head.

For a full keel boat, the buyer's job is to find the boat's displacement and steering type, then to read the maker's stated range for the drive or pack. When the two do not match, the cautious approach is to ask a dealer or a qualified installer. The maximum boat length and sailboat autopilot guides give more context on how these details are read.

Core pack or complete pack for a keel boat

A core pack includes the heading sensor and the actuator control unit or course computer, with no drive in the box. A complete pack includes the drive and, often, a control head. For a full keel boat that already has a suitable hydraulic pump or a mechanical drive, a core pack may be the simpler route. For one that needs everything, a complete pack matches all parts in one listing.

The listing states which parts are in the box. Read the title and bullet list for "core pack", "complete pack", "drive included" or "no drive". Choose the route that fits the steering already aboard. The core packs without a drive page and the complete pack versus custom page explain the trade-offs in more detail.

Questions to ask before you order

These questions help a buyer read a listing for a full keel boat without turning to guesswork:

• What steering system is on the boat: tiller, wheel and cable, or hydraulic?
• What does the maker's page state for the drive you are looking at: steering type and displacement range?
• Does the listing name a drive in the box, or is this a core pack without a drive?
• Does the pack include a control head, or is the head sold separately?
• Who will do the installation: a qualified installer, a dealer, or the owner following the maker's manual?

What to pick for your situation

Questions

What is a full keel?

A full keel is a keel that runs a large portion of the hull's length. The rudder is often attached to the aft end of the keel or mounted on a skeg. This layout gives strong rudder authority and steady steering, but it also means more force is needed to turn the rudder.

Why does a full keel need more torque in the autopilot?

The rudder is large and sits far aft on a long lever arm, so water pressure on the blade is high. The autopilot's drive must supply enough torque to move that rudder through the steering system. Listings state the drive type and sometimes displacement, which are the details to compare with the boat.

Will a tiller pilot work on a full keel boat?

A tiller pilot is stated for tiller-steered sailboats within a maker's displacement range. Some full keel boats are small enough to fit; larger ones exceed the range. The maker's page decides fit, not an article.

Do I need a rudder reference for a full keel boat?

A rudder reference or feedback unit tells the autopilot the rudder angle. Many complete packs include one, and some packs name it in the listing. Ask the maker or dealer what the pack includes; installation of the rudder reference is a job for a qualified installer following the maker's manual.

What does a displacement figure on a listing mean?

It is the maker's stated range for that drive or pack, usually the upper displacement the unit is built for. It is a fit detail, not a performance claim. Compare it with the boat's actual displacement.

Does an autopilot replace a lookout?

No. An autopilot steers while the crew remains responsible for keeping watch. The U.S. Coast Guard Boating Safety page is the general reference for safe boating practices and education.