Antares Yachts uses premium components for all the systems aboard the Antares 44i – and the autopilot drives are no exception. The JEFA electro-mechanical autopilot drive unit uses less energy and still provides more power and speed control. Here is what the engineers at Jefa steering systems have to say:
It really does matter which autopilot drive unit you choose as there are big difference between an electro-mechanical drive unit and a hydraulic cylinder. Electro-mechanical JEFA drive units are much better suited for sailboats than hydraulic drive units for the following reasons:
Drag / Friction:
Much less drag while not in use. Especially on small boats it’s very important to have the minimum force required to back drive the autopilot. Some of these boats use a tiller and the others a very sensible mechanical steering system. Lots of money is invested nowadays by the yards in roller bearings for the rudder shaft to achieve the maximum in feedback and the minimum in drag. Adding a hydraulic cylinder to the rudder/steering system is like adding a big damper. Hydraulic autopilot rams tend to back drive very difficultly as the cylinder has high pressure seals that brake the piston heavily and the oil has to be pressed from one side to the other through a small bypass valve causing a high friction.
Much higher efficiency. On a power boat (where hydraulic autopilots were designed for) the autopilot efficiency is not important as you have a continuous source of energy. On sailboats you don’t have this source. Autopilots are continuous power users as often they work round the clock. In an electro-mechanical drive you have one energy transformation. You have electrical energy and it’s transformed to mechanical energy. In a hydraulic drive you have three energy transformations. First you have electrical energy and you transform it to mechanical energy in the electro motor. The electro motor drives a hydraulic pump and transforms the energy into hydraulic energy. The hydraulic energy is transformed to mechanical energy in the cylinder. Every energy transformation has it’s specific efficiency and loss. Due to three energy transformations the losses in the hydraulic unit are much bigger than in the electro mechanical unit. Even when you would have 75% efficiency per transformation (which is not realistic) you get 0.75*0.75*0.75=0.42 = 42% efficiency.
Another big difference between the hydraulic and electro-mechanical unit is the type of electro motor used. The hydraulic unit uses a standard brushed DC electro motor (small diameter, long shape) which are cheap but have a limited efficiency (45-55% efficiency). The electro-mechanical unit uses a flat wound (pancake) motor with more than 80-90 % efficiency. This results in a total efficiency of hydraulic units to be between 25 and 35 % and electro-mechanical units between 70 and 80 %. In practice this will mean you need up to 3 times more power to get the same output out of the hydraulic unit. The differences are even bigger on larger (>55 foot) boats. You can’t use a reversible type hydraulic drive any more (not available as the motor currents would be too big to control). The hydraulic option on bigger boats is a “continuously running power pack”. This power pack keeps a hydraulic reservoir continuously under pressure and the autopilot uses solenoid valves to pressurize each side of the cylinder. Due to the continuous running power pack motors, this system consumes even more electrical power and could use up to 10 times more energy than an electro mechanical solution.
Much better speed control. Modern autopilots use pulse width modulation to vary the power sent to the drive unit from zero to hundred percent. This way they can accurately control the speed of the drive unit. JEFA electro-mechanical drive units use flat wound (pancake) electric motors, which have a thin but very large diameter rotor, have a very high start torque (due to the large rotor diameter) and a very small inertia as there is no iron in the rotor, just windings. Due to these features, they follow exactly the speed curve sent by the autopilot.
Normal standard brushed DC electro motors, used in hydraulic drive units, are not made to start and stop and reverse all the times. They are made for continuous and constant rpm rotation. They have a low starting torque (due to the small diameter rotor) and a high inertia (due to the heavy rotor). The result of this is a cut off of the ideal autopilot speed curve below 50-60% speed as these motors tend to need at least 50% of the maximum voltage to even start rotating. Flat wound motors start rotating at 5-10% of the maximum voltage.
The result of this all is a near binary (on-off) reaction of the hydraulic unit to the speed control of the autopilot and a smooth full speed control with soft start and soft stop by the electro-mechanical unit with a fraction of the power consumption.