Steel-Fish S104 Micro Sub: 2021 Onwards







The all new S104 Micro which I have been designing in my odd moments over the last 10 years, Covid finally giving me the chance to start the build. Single seat and ultimate performance, speeds up to 12kn and range over 100 miles. X planes give superb manoeuvrability at all speeds. Fighter style cockpit and flown with responsive aircraft style side-sticks. Full variable ballast system to noiselessly hover close to the sea bed. The overall length is 5m and will float in just 725mm of water, mid-thigh to an average person.

At 1500 kg the Micro-sub is easily launched from a trailer or yacht. Generous freeboard, stability and reserve buoyancy allows entry and egress on the surface. Very easy to operate with intuitive controls. Any capable person can by taught to operate this vehicle with a short training course.

These were the design objectives which took me quite some time to resolve. Low cost was the other important factor.  Composite construction for both pressure hull and fairings has been chosen to provide complete freedom to shape of the critical components and a super smooth exterior finish. The main pressure hull is built on a rotating former and is 24mm thick resulting in a crush depth of 800m, 8 times the operating depth of 100m. The thermoformed acrylic hatch hemisphere has been used in vehicles with a 200 m operating depth but has a 2,500m crush depth. The choice of composite over steel means maintenance inside ballast tanks for example is never going to be a problem.

The ellipsoidal pressure hull centre section is a roomy 860 diameter with generous leg room and plenty of seat height adjustment. The two control sticks fall easily to hand fitted to the two side control consoles. One side is air and ballast with all the required displays and the other side is electrical for propulsion control and monitoring. The navigation display sits directly in front of the pilot and slides forward to make entry and exit easier. There is a full HD camera fitted in the bow pointing forward and down feeding into a tablet computer to cover the area visually occluded by the bow section.

Aft of the pilot compartment behind a bulkhead is the battery. The maximum size battery that can be fitted is 30kW hr made up of prismatic LiFePO4 cells. These are the super safe variety. The battery fitted in the prototype is 22 kW hr which due to extremely low water resistance will provide more power than most people will ever use. This allows almost complete autonomy enabling adventurous expeditions far from the launch point with only minimal surface support.

At 5kn the 22kW hr battery will give a range of 100 miles. The 30kW hr battery will give the same range at 6kn, or put another way, in an 8hour day you could cover 60 miles at 8kn. This is military performance. 

Power is provided by a direct drive super quiet brushless DC motor in the tail driving a 5 blade GRP propeller. The maximum motor speed is 800 RPM, power 10kW continuous, 20kW for short bursts. Most of the time the motor will be turning at less than 400 rpm. Also found in the tail section is the motor control unit and the electric servos for the aft planes. This space is accessed if required by removing the O ring sealed after tail section complete, but if all is well there is nothing in here that needs routine attention. The tail is fixed securely in place with 6 bolts accessed from inside. 

Below the pilot is a 30L variable ballast tank to compensate for different pilot weights (in addition to fixed weights) and to provide fine control of buoyancy. The brass 3-cylinder piston pump fitted behind the pilot runs at 8L/min and is directly driven by a DC motor. There is also a basic hydraulic system fitted to operate the emergency buoy release, drop-weight release and the vent valves. The small pump is manual. A weight can be released off the bottom to gain an instant 100kg of buoyancy and the tower fairing can be released to float up on a line if the vehicle is unable to surface. 

Operation is made easier by placing the HP air cylinders inside the removable nose fairing. These are standard 12.5-litre SCUBA cylinders which can be detached and refilled at the local dive shop. In this space is also the day and emergency oxygen supply.  The CO2 scrubber is fitted aft the pilot's seat. 2 kg of Sofnolime 1025 will last 8 hours with sufficient refills stowed either side of the main battery. Emergency life support is a minimum of 72 hours.

Dynamic control is by a single joystick on the right side operating a pair of electrical servos in the tail through a PID control loop. On the left is the throttle. There is a motor driven trim weight in the aft compartment underneath the battery. This is very useful to control the height above the sea bed when moving slowly. In addition, there is a Blue Robotics thruster fitted in a transverse duct in the bow to aid low speed manoeuvring. 

Surfacing is achieved by blowing the two ballast tanks, one forward and one aft providing approximately 250 kg of reserve buoyancy. The blister tanks at the waterline greatly enhance the surfaced stability. There is enough HP air on board to blow the tanks 20 times. To dive, the air is released from the tanks through two hydraulically operated vent valves similar to those I have developed for the Triton subs and almost every submersible built in the UK in the last 20 years. There are float switches in the tanks to indicate when the blow is complete and to sound an alarm if any attempt is made to open the hatch too soon.

As of Feb 2023, the pressure hull and battery assembly is complete and virtually all the components on the shelf. Next is the outer GRP fairing and complex GRP tail. However, submarine and habitat design commissions have now returned with some intensity which is bound to slow progress.