e-hovercraft Design

Is the craft really powered by solar panels?

No – the craft is currently powered by 10kwh of LIPO batteries mostly recharged from solar panels at home or mains electricity. At best the onboard panels provide around 400w of charging on a bright sunny day in the UK (not that often!). The cruising consumption is about 8kw at 12-20 knots so in effect 20 minutes of onboard charging could give 1 minute of cruising.

Whats the range?

Allowing a reasonable safety margin for higher thrust power levels when going over hump, turning sharp corners, headwinds etc the current range currently is about one hour ie 12 – 20 miles. The current prototype thrust duct is around 50kg heavier than it should be so the plan this winter is make a lighter one which should hopefully enable the craft to carry another 10kwh of battery packs – thus doubling the range.

What’s the battery voltage?

The craft uses 14 Lithium Polymer (LIPO) cells in series (known as 14s) which means the fully charged battery is 58.8 volts – which is defined as non-lethal and safe. Currently the craft operates with 4 battery packs each with a capacity of 2.5kwh.

Each pack was handbuilt in a waterproof case using 154 Molicel P42A LIPO cells in a 14S x 11P configuration using spot welding techniques learned from the Vortechs YouTube channel.

However future craft are likely to use second hand batteries from electric cars. They promise the same power and weight of the earlier home assembled battery packs but at lower cost and without weeks of self assembly work.

What’s the thrust motor?

It’s an MP202150 (44kv version) normally used for large drones. Currently it’s providing about 19kw (25bhp) at 1700 rpm directly driving a 1.03m diameter multiwing type 5Z fan with 6 blades (set at 45 degree pitch) in a 6 blade hub producing about 70kg of static thrust.

What’s THE Lift motor?

It’s an MP15470 (84kv version) used for more modest drones. Maximum power is 5kw (6.6bhp) at 2700 rpm although it normally cruises at about 2kw. It directly drives a 450mm diameter Zehl & Abegg 450mm diameter Zpro centrifugal fan.

Why is ‘Getting over the hump’ so important?

When a hovercraft is on hover but stationary over water the air cushion displaces water – essentially it’s digging a ‘hole’ in the water. The depth of that ‘hole’ depends on the air pressure in the cushion. The heavier the craft, or the smaller the area (length x width) of the cushion the deeper the hole. To gain speed the craft has to climb out of that hole, but while it’s doing that it’s using a lot of force to push back waves. So the lower the pressure, and the longer the craft (which makes the ‘slope’ you’re climbing less steep) the less thrust is needed to ‘get over the hump’. Our craft ‘Sound of Silence’ is 4 metres long by 2.1 metres wide and currently weighs 280 kg unladen.

Once over hump speed (for Sound of Silence that’s 7.5 knots) you’re not making waves, you need less thrust, and can throttle back on thrust, saving battery power, and making less noise. In shallow water hump drag can increase by up to 50%. In this VIDEO you can see the craft start to move forward initially subhump thus creating waves until the waves pass backwards along the sides of the craft until it’s overhump and thrust power can be reduced.

There are simple formulae to calculate this, which will be included in future upgrades on this website. For an electric hovercraft conserving power is critical. More power means more batteries and thus more weight – which makes getting over hump even more difficult. There’s a spiral here which can rapidly lead to a ‘non viable’ design.

Is an electric hovercraft automatically quieter?

No – it’s perfectly possible to build a noisy electric hovercraft. Obviously an electric motor does not have the exhaust and air intake noise of a petrol engine, but if you run the motor at high rpm or drive the motor with a square electrical waveform rather than a sine wave the motor will emit a high frequency screaming noise, similar to a jet engine.

Fan noise has been shown in previous studies to be a considerable source of noise mostly caused by struts, belts, pulleys and splitter plates in front of and behind the fan. These cause turbulence which the fan blades hit as they rotate. This techtalk gives a recipe for a quiet petrol hovercraft which uses a long shaft drive, belts behind the fan and a better aerodynamic curved splitter to achieve a full power noise level of 71dba at 25 metres (using the World Hovercraft Federation measurement method – para 9.4). This compares favourably with typical cruising craft at 80-90 dba and racing craft at 90-100 dba. However Sound of Silence achieves 64 dba by using a separate centrifugal lift fan and using a hub mounted electric motor to eliminate all the splitter, strut, pulley etc noise completely. It is the quietest craft by a considerable margin of over 150 craft measured worldwide and the only one where the measured fan noise is the same as the manufacturers predicted noise level.

Aren’t batteries too heavy?

A typical petrol cruising hovercraft will use a Briggs and Stratton 37bhp 4 stroke engine for thrust – which weighs about 54kg on it’s own plus the metal in the belt transmission system needed to connect the engine to the fan – and a tank of fuel. The MP202150 electric thrust motor weighs 12kg and because the thrust fan is directly connected to the motor there’s no speed reduction belt drive. A 10kwh LIPO battery weighs 50kg. So basically an e-hovercraft can be about the same weight as a similar petrol craft.

Why don’t electric hovercraft need a reduction drive?

Petrol craft use thrust engines which produce their peak power at 3600 rpm (eg that B&S engine) to 10000 rpm (for a top racing 2 stroke) and need a reduction drive, often a toothed belt system, to reduce down to the optimum speed for the fan (1200-3000 rpm). Electric motors can produce a lot of torque even at low rpm so it’s possible, with the right motor and controller, to directly connect a fan to the motor at 1200 – 1500 rpm. This saves weight, cost and reduces noise.

Which Electronic switch controllers (ESC)?

The ESC (or Invertor as it is known in some applications) is a crucial piece of electronics which takes up to 58 volts DC from the batteries and converts it into a form of ‘three phase AC’ which is fed into the Brushless DC (BLDC) motor. The ESC has been a major problem area with 6 ESCs being burnt out during the development, and one probably causing a motor to catch fire. We learned you need an ESC whose software can be adjusted to suit our particular fans, and that reports in realtime data such as amps, volts, temperatures etc. The ESC must use a sinewave to control the motor, rather than the noisier square wave and most of all it needs to reliably handle a high number of motor or phase amps. We’re now using the open source VESC family – a VESC 75/300 for lift and and in 2025 a RFLX from Dynamesys for thrust. Great technical support from Trampa Boards in Nottingham UK has been critical to the project. There may be other good ESCs out there but we know these work for us.

Why do you need high motor or phase amps?

In a BLDC motor the volts input to the motor control the rpm and the amps control the torque. A good ESC can independently control volts and amps separately. We’re currently controlling the amps, and thus the torque fed to the fan, directly from the throttle. Our ‘low noise means direct drive’ philosophy means we need a lot of torque and thus motor amps. To get 19kw of thrust power at 1700pm we need ….nm of fan torque. In our experience these motors are typically 80-90% efficient so that means we need to budget for ….nm of ‘motor torque’. A 44kv rated motor will produce 0.22nm of torque for every motor amp put in. So …nm requires … x 0.22 = 500 motor amps. But 500 motor amps does not mean the current drawn from the battery is 500 amps. When an electric motor rotates it acts as a generator, making electricity that opposes the voltages being injected into the motor, so the effective motor voltage is much smaller than the battery voltage. Conversely the motor current is larger than the battery current. So for example with a 44kv motor our 500 amps motor current requires just … of current from the battery {needs more work!}

How many computers on board?

There are currently 8. The dashboard computer is a small Windows 11 PC with a sunlight readable touchscreen. It runs a custom program (which will be open source) written in Python which displays information to the driver which is collected via usb links to the 7 other computers. Each of the 4 battery packs has a Battery Management System (BMS) computer; and there’s the lift and thrust VESC’s. Both the BMS and VESC’s come professionally programmed but need various settings adjusted before use. We’ll publish the settings.

The seventh computer is an adafruit metro M4 with a custom program, written in micro-python, which monitors the onboard solar panels and provides some overall switch on/off functions.

Isn’t all this electrical stuff unreliable?

In the first 2 years of development this was certainly true. But once we sorted out what worked and what didn’t the craft has proved highly reliable. Much more than the petrol craft alongside us with their broken belts, cracked rubber couplings, failed exhausts, blocked carburettors etc – all things that don’t exist on our e-hovercraft. Over the past 2 years of operation at a variety of sites in the UK and France it’s proved to be very reliable.

Why wasn’t the craft built in a mould?

Apart from the challenges of building a plug, making a mould from it and then taking a grp moulding from that our experience of hulls made this way are that they are heavy. ‘Sound of Silence’ is based on the excellent Osprey 3 petrol hovercraft from the early ’90s. We weighed the bare hull (no duct, no motors etc) of one of these at 200kg. The equivalent hull on the new craft weighs 100kg using foam sandwich construction. The latter is also much easier to make for a one off prototype built in your home garage. The quality of our finish is not as good as a professional moulded craft but that wasn’t our focus – we wanted to see if it would work! If finish is your thing it’s perfectly possible to improve it on a foam sandwich craft built without a mould – just spend a lot more time filling and smoothing the top layer of grp! But use micro balloons as a thickener for your filler. Conventional fillers from a car shop can add a lot more weight!