M81138 Standard Stern Drive Autopilot Actuator

The Raymarine Standard Stern Drive Actuator (M81138) automates the steering of boats with stern drive (Z-drive) propulsion systems that feature hydraulic power steering. This unique drive unit ties into the boat’s steering hydraulics – on MerCruiser and Volvo Penta setups, this means connecting to the hydraulic power steering valve. When the autopilot is engaged, the actuator operates the hydraulic valve just as the helm cable does, redirecting fluid to the steering cylinder to pivot the outdrive left or right.

The standard sterndrive actuator is engineered to be compatible with the widely-used steering systems on MerCruiser drives (notably models from 1997 onward, which often include an autopilot interface port) and Volvo Penta outdrives. These systems run on 12V, and the M81138 hooks into them, drawing power from the boat’s 12V supply and being commanded by the ACU-200 autopilot controller. In operation, the autopilot sends signals to M81138, which in turn meters the hydraulic flow in the steering system’s valve – effectively turning the outdrive automatically without input from the helm.

One of the key advantages of this actuator is that it does not interfere with manual steering when not active. As soon as you switch the autopilot to standby or go back to manual mode, the M81138’s electro-magnetic clutch disengages and isolates the unit from the steering system. The boat’s steering wheel once again directly controls the hydraulic circuit with no added friction or play, which is absolutely critical for safety at high speeds or during sudden evasive maneuvers.

Benefits and use cases: This standard sterndrive drive unit makes controlling fast boats – such as sport cruisers, RIBs with I/O engines, or workboats – much easier for the autopilot. During straight-line high-speed runs, the autopilot (via M81138) makes fine adjustments to keep the boat on a steady heading without the typical “hunting” or constant corrections a human might have to make. During turns, especially at speed, the autopilot can smoothly initiate the maneuver following a set course or navigation route.

Example: Imagine a 30-foot speedboat with MerCruiser sterndrives – with M81138 and an autopilot, the captain can set a GPS course and the boat will automatically steer, even at 30+ knots, maintaining stability. If suddenly an obstacle appears ahead, the captain simply grabs the wheel – since the unit is disengaged, he has immediate and full control to dodge the obstacle.

The Raymarine M81138 is durable and built for the marine environment. Its housing and moving parts are made of materials resistant to saltwater and corrosion. Since it often mounts in the engine bay or near the motor, the unit is constructed to withstand high temperatures and exposure to engine fumes. Also, given that it interfaces with the hydraulic system, it is engineered to be leak-free – no hydraulic fluid bypass is acceptable.

Installation typically involves special brackets and tee fittings into the existing sterndrive hydraulic lines. Raymarine provides a kit and instructions tailored to MerCruiser and Volvo Penta models – in many cases, the installation procedure is straightforward with the provided components. It’s also possible to adapt this actuator to other brands of sterndrives as long as there’s a way to tie into their hydraulic steering (some may require additional adapters or custom fittings).

Conclusion: The M81138 standard stern drive actuator is a key enabler for adding autopilot to boats with outdrive engines. It brings the convenience of automatic steering to a realm where previously a skipper had to keep a constant hand on the wheel in fast boats. Now, with this actuator, long cruises, patrol operations, or even just enjoying a sunset cruise become easier – the autopilot handles the helm, while you relax, knowing you can instantly take control if needed. This device is designed to work hand-in-glove with your engine’s steering, providing the same steering response you’d have manually, but with the precision and consistency of electronic control.