Electric auxiliary power – a new approach

Since launch, in 2014, Arpex used a small Torqeedo T1003 electric outboard as auxiliary propulsion. It worked well and proved something important: a boat like Arpex does not need much power. The 1 kW motor was normally used well below full throttle.

After the Torqeedo finally failed, I started thinking about a replacement. Rather than simply buying another electric outboard, this became an opportunity to develop a solution better integrated into the boat.

The idea is quite simple: a 48 V brushless electric motor mounted inside the hull, driving a 2.2 m long-tail shaft. The motor I am currently considering is an industrial 2.5 kW BLDC unit running at 3,000 rpm. The additional power is not really intended for normal operation, but provides a useful reserve when pushing against wind or current.

The interesting part is the installation.

The long-tail will pivot around a point close to the motor. In use, the shaft swings down and places the propeller well below and behind the hull, in clean water. When sailing, the complete motor-and-shaft assembly swings upward until the shaft lies almost flush against the underside of the boat. No propeller or lower unit remains permanently in the water.

There is a small geometric complication. To allow the shaft to retract completely, its axis needs to sit slightly below the underside of the deck. This means that part of the motor will also project below it. A small fairing will therefore be needed to protect the motor from spray. The controller, battery and other electronics will remain separately mounted in a dry area inside the boat.

The system should use a relatively small 48 V LiFePO₄ battery, since experience with the Torqeedo has shown that carrying a large battery simply isn't necessary. Solar panels will recharge the battery while sailing.

There are still details to solve — propeller choice, balance around the pivot, motor control, cooling and the exact geometry of the installation. But that is also the enjoyable part of the project.

Rather than adding an electric outboard to the boat, the objective is to make the auxiliary propulsion part of the boat itself.