It's all about torque. The rotation of the propeller against the water drives the engine by turning it on its steering axis. If you let go of the tiller of a speedboat at full throttle, it will turn on itself. I advise you not to try this, the turn is violent. It is the torque that drives the engine in this turn.
Below 40 hp, it is usual to consider that the power is still tenable with a tiller. Above that, it is more problematic and requires a good physique from the pilot to maintain the course.
Manufacturers like Suzuki offer a power tiller (up to 140 hp). This tiller is assisted by a gas cylinder to help support the torque. The tiller is even equipped with a multi-function LCD display that provides information on rpm, hours and alarms. The trim is also accessible from this tiller.
This type of mounting is appreciated by professionals (oyster farmers on their barge for example), but can also be adopted by the boater trolling for example.
From 40 to 140 hp: the cable to drive
Up to 140 hp, a cable steering system can be used. This is a cable that connects the steering wheel to the engine. The movement is therefore mechanical. By turning the steering wheel, you push (or pull) the cable which acts directly on the engine. To prevent the engine torque from being transmitted back to the flywheel through the cable, a mechanical anti-torque system is added to the cable. This anti-torque system avoids having to hold the steering wheel when driving in a straight line.
Cable steering is the cheapest when you want a steering wheel. It is also the most constraining, because the cable does not like curves. It must therefore find the most linear passage possible between the steering wheel and the engine. This is not always easy on small boats or to pass through a console.
150 hp and more: Here are the hydraulics
For power levels above 150 hp, the cable is no longer sufficient. It is not powerful enough. You have to switch to hydraulic steering. This is a cylinder that is operated by a hydraulic pump in the steering wheel hub. It is the pressurized oil that transmits the movement to the cylinder. Non-return valves prevent the engine torque from being transmitted to the steering wheel.
The engine manufacturer specifies the power of the cylinder to match the engine power. This also defines the pump and oil tank capacity. The hoses to guide the oil are easy to install in a boat. There is no problem of routing to connect the flywheel to the engine.
It is also possible to install this type of steering in bimotorisation. The 2 motors are then coupled with a bar and a single cylinder makes them pivot together. If 3 motors are to be mounted, 2 cylinders are needed (one on each side). A single cylinder would be too big to support the torque of the 3 motors.
Above 250 hp, a hydroelectric steering assist can be added. This is an electric pump that supplements the steering wheel pump to add steering comfort.
The must, the electric steering
The electric steering allows to connect the steering wheel with the hydraulic steering pump only by electric cables. It is therefore a much simpler (but also more expensive) installation. This system eliminates any resistance at the steering wheel. The steering remains as smooth as ever, regardless of the engine torque. To avoid road deviations when speed increases, some systems are connected to a GPS that hardens the steering wheel according to the speed. This type of steering is recommended for large boats, as it remains very comfortable and easy to use.
