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.

2022 Rudder

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.

Mini rudder V1

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)

New main drive unit

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.

Mini rudder V2

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. ```