Drive Units | Simrad

Below deck, the steering mechanism and size of your boat decide what sort of autopilot computer and drive system you’ll need –the hardware that actually moves your rudder and keeps track of its position.

There are two main types of drive unit.

A reversible hydraulic steering pump is used to add autopilot capabilities to outboard or sterndrive (inboard/outboard) vessels with existing hydraulic steering systems.
Larger boats need larger pumps; your choice of pump will decide whether you need a standard NAC-2 or high-current NAC-3 autopilot computer.

Helm drives and linear rams are used to add autopilot capabilities to vessels with mechanical (non-hydraulic) steering systems.

Electronic helm drive units suit smaller cable steered runabouts.
Linear rams operate rudders on power-driven boats, or steering quadrants on sailing vessels.

Helm drives and smaller rams work with a standard NAC-2 autopilot computer; more powerful rams for larger vessels are driven by a high-current NAC-3 autopilot computer.