Rudder
Propulsion System History and Overview
The main propulsion system is housed inside the rudder. The rudder is mounted at the rear of the boat, approximately 440 mm from the stern. Over the years several different versions of the propulsion system have been developed. A brief overview of the previous systems is given below.
2022 – 2025: Underwater Mounted Inrunner
This system consisted of a carbon-fiber-wrapped 3D-printed rudder design. The rudder was mounted to the boat using a carbon fiber tube attached through a so-called “well”. The well itself was made using a slightly larger carbon fiber tube integrated into the hull structure.
The propulsion system used a Lehner 30100/12 three-phase inrunner motor combined with a Neugart PLE-6 series single-stage planetary gearbox. Over the years 4:1, 5:1, and 7:1 gearbox ratios were used. The complete drive assembly was mounted inside an aluminium housing, with the motor and gearbox thermally coupled to the housing using thermal paste for cooling.
The propeller was mounted in pulling configuration, meaning the driveshaft protruded through the front side of the housing. The inside of the motor tube was sealed from the surrounding water using a shaft seal, while the rear end of the tube was closed off using a custom threaded cap.
A magnetic encoder was mounted at the rear end of the motor. The motor tube itself was connected to the rudder using aluminium interface pieces and rubber O-rings. Power cables were routed through the rudder to the three-phase motor controller located in the rear compartment of the boat.
Two major versions of this drivetrain were developed, with V2 featuring improved structural reinforcement and mounting features.
Although the system performed well initially, it was ultimately scrapped due to continuous water ingress into the motor tube. The resulting corrosion of the gearbox and motor caused repeated failures and required frequent replacement of expensive components.
2025: Mini Rudder V1
During the 2025 season a temporary drivetrain was constructed using a Flipsky 65111 waterproof inrunner motor. This system was designed as a backup solution for the heavily troubled main drivetrain and was used during the 2025 Amsterdam Solarboat Race.
The rudder featured a carbon-fiber-reinforced 3D-printed body and focused mainly on reliability and ease of deployment rather than outright performance.
Development of the New Propulsion Platform
Following the 2025 season, development started on a completely new propulsion platform. The main design goals of the new system were:
- Move expensive and sensitive electronics away from direct water exposure
- Improve the drivetrain mounting system for future development
- Increase structural rigidity for future hydrofoil implementation
- Add integrated features required for hydrofoil compatibility
- Implement a counter-rotating propeller system for improved wake alignment and efficiency
This redesign required significant modifications to the rear of the boat, including:
- Replacement of the drivetrain well
- Replacement of the entire steering system
- Replacement of the well support structure
Construction started in late 2025 and was completed in early 2026, with the first on-water trials taking place in May 2026.
Current Drive Systems
At the time of writing two propulsion systems are in active use:
- New Main Propulsion Unit
- Mini Rudder V2
New Main Propulsion Unit
The new main propulsion unit is powered by a Lehner Torqstar 3 7050/10 water-cooled outrunner motor. The motor drives a set of counter-rotating propellers mounted in pulling configuration.
Power is transferred through an 8 mm steel shaft to an angled gearbox mounted at the bottom of the rudder. The rudder itself uses an aluminium clamshell construction housing the following systems:
- Helical bevel gearbox using Graessner P054 2:1 gears
- Hydrofoil trim system
- Cooling water inlet system
The transmission consists of an inner and outer shaft, each driving one propeller.
Bearings
The drivetrain uses the following bearings:
- 2× 63804 (20×32×10 mm) for the outer shaft
- 1× 6002 (15×32×9 mm) for the inner shaft
- 2× 688 (8×16×4 mm) for inner/outer shaft support
- 1× 698 (8×19×6 mm) at the bottom of the main driveshaft
- 2× 638/8 (8×16×6 mm) for the remaining driveshaft support points
Seals
Three shaft seals are used throughout the drivetrain:
- 1× NBR R 20×28×7 between the housing and outer propeller shaft
- 2× NBR R 8×16×7 between:
- the inner and outer propeller shafts
- the housing and the upper main driveshaft
Keys
The following keys are used throughout the transmission:
- 2×2×10 mm keys at both ends of the driveshaft
- 3×3×25 mm key for the front propeller
- 6×6×30 mm key for the rear propeller
- 4×4×10 mm keys for both drive gears
The motor shaft is connected to the main driveshaft using a KTR ROTEX 14 steel coupling.
The clamshell housings are machined from 7021 aluminium and bolted together using low-profile Torx screws according to ISO 14580-1 in various diameters and lengths.
At the time of writing the transmission is lubricated using 40–60 mL of Kroon Oil ATF A gearbox oil.
Transmission Specifications
- Maximum motor power: 10 kW
- Maximum motor speed: 7200 RPM
- Maximum motor torque: 6 Nm
- Maximum gearbox torque: 18 Nm
Hydrofoil Trim System
The hydrofoil trim system uses a custom aluminium hinge assembly actuated through a carbon fiber pushrod.
The hinge mechanism uses:
- 8×12 mm stainless steel dowel pins
- 4×8 mm stainless steel dowel pins
The pushrod is actuated using a brass threaded interface driven by a geared stepper motor (StepperOnline 11HS12-0674S-PG5). The rotational axis is redirected using a miniature universal joint.
The system allows hydrofoil adjustment over a range of approximately 12 degrees.
Hydrofoil System Bearings and Seals
- 1× NBR shaft seal 6×16×4 mm
- 4× 6701 bearings (12×18×4 mm)
Rudder-controller
Full documentation: Rudder controller — board function, connector pinouts, CAN messages, servo behaviour and firmware updates.
The new rudder design needs a controller of its own to run the integrated cooling system and the back-foil trim. That controller is now a purpose-built board, the Rudder controller (Altium project AKD-Rudder_Controller): a 4-layer STM32L471 design that hangs off the CAN-bus and drives the trim stepper, the cooling pump and the cooling instrumentation. It replaces the 3D-printer tool boards that were used for prototyping up to late 2025.
It takes commands and reports everything over the CAN-bus — there are no local controls.
| Feature | Description | Electrical connections | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Power | 24 V from the CAN-bus cable, with an on-board step-down module for the logic rail.
A design requirement was that the inrush of the connected motors must not drag the CAN-bus voltage down; this is handled by 150 µF bulk capacitors local to each motor driver rather than by current limiting. |
Two 5-pin connectors carrying CAN-bus, 24 V and the safety line | ||||||||||||
| Communication | CAN 2.0A at 1 Mbit/s, 11-bit identifiers. Firmware can be updated over the bus without a debug probe. | Shares the two 5-pin CAN-bus connectors; wired in parallel so the board sits in a daisy chain | ||||||||||||
| Back-foil control | A TMC2209 stepper driver, using StallGuard™ to find the mechanical end stop during homing. Position is open-loop step counting against that stop. | On board | ||||||||||||
| Back-foil motor | The Nema 11 stepper motor (11HS12-0674D-PG14, 13.73:1 gearbox) drives the back-foil. Commanded as a 1000–2000 setpoint over the full travel, roughly 12 degrees of foil movement. | Four coil pins on the shared 10-pin connector
| ||||||||||||
| Water pump | The ZC-A210 (24 V version), driven through an H-bridge with a firmware-set current limit of 1.0 A. It runs only while the battery reports that it is discharging. | Two pins on the shared 10-pin connector
| ||||||||||||
| Flowmeter | Two DWS-MH-02 flowmeters, inlet and outlet. Each gives a pulse output, counted in hardware, and an NTC. The TDS pair is not connected on the board. Only the inlet meter is read by the current firmware — the outlet meter's NTC shares a microcontroller pin with the motor NTC. | Inlet on four pins of the shared 10-pin connector; outlet on its own 5-pin connector
| ||||||||||||
| Steering angle | A potentiometer on the steering column, calibrated on the vehicle and reported as a normalized left/centre/right position. | 4-pin connector: 3.3 V, wiper, presence detect, GND | ||||||||||||
| Motor and board temperature | An NTC on the motor, plus an I²C sensor measuring the board's own temperature. | Motor NTC shares the outlet flowmeter connector |
See Rudder controller for the exact pin numbering of each connector, the CAN message formats, and the outstanding issues.
Mini Rudder V2
The second-generation mini rudder consists of an aluminium structural skeleton with a 3D-printed hydrodynamic outer shell.
The system is powered by a Flipsky DC6374 140KV 3600W waterproof outrunner motor and uses a custom aluminium propeller.
Because future plans include water cooling for additional electrical systems, the Mini Rudder V2 also includes a cooling water inlet similar to the main propulsion unit.
Rudder Mounting System
Both rudders are mounted to the steering tube in a similar manner.
The steering tube features M6 mounting holes at the bottom, allowing the propulsion units to be mounted from underneath the boat. The steering tube also contains an integrated cooling water pass-through, making it possible to connect the cooling circuit without installing additional hoses through the hull.
The Mini Rudder can be installed without modification to the steering tube. Only the power cables need to be routed through the tube, allowing quick replacement of propulsion units and ensuring continued operation in case of drivetrain issues. ```