E1 series is an inshore powerboat series using bespoke battery-powered boats that will see competitors racing in identical boats powered by identical powertrains. The boat itself, the E1 Racebird, has a highly unusual design. At racing speeds it is designed to foil – to lift itself clear of the water. That reduces drag on the boat and means it can travel far faster for a given level of power than a conventional boat that has much of its hull in the water.
The E1 Racebird, seen here testing off Venice, Italy is a visually striking boat. Hydrofoiling allows greater speeds with a given level of power by reducing drag (Photos courtesy of E1 Series)
It is worth noting that all kinds of wind-powered watercraft, from windsurfers to record-breaking yachts, have used the concept of foiling to extract greater speeds from a given level of wind. Indeed, the wind-powered world record was held by a foil-board (a foiling windsurfer) until 2009. Foiling is also used commercially for passenger ferries.
It makes sense therefore to use the same low-drag design feature to reduce the amount of power (and therefore the size and weight of the battery for a given race duration) required for the desired level of performance. It also adds another compelling visual aspect to the racing.
I discussed the powertrain for this new series with Josh Wesley and Jack Ferryman, from Seabird Technologies, which is managing the design and build process for the Racebird. The drive is supplied by a single motor integrated into the outboard along with the inverter (power/propulsion control electronics). Engines or motors mounted inside the boat are inboard, while those outside are outboard. The propulsion drive is supplied by Mercury Racing, and the battery is from Kreisel, both familiar names in motorsport. Mercury has long been involved in watersports, and Kreisel is involved in several forms of electrified motorsport (notably RX1E Rallycross). Seabird Technologies is also responsible for the control system software.
The motor itself is pretty standard as far as motorsport e-machines are concerned. It is a conventional three-phase, radial flux, inner rotor design, with permanent magnets. The use of permanent magnets is common where high torque and power density are required, but they need their operating temperature to be kept within certain limits if they are not to suffer a persistent loss of performance owing to demagnetisation. The rotor has six pole pairs and turns at 7000 rpm at maximum power. An e-motor of this type supplies constant torque until a certain speed, beyond which it is controlled to maintain constant power. The Seabird engineers say the motor produces 150 kW (201 bhp) at 7000 rpm. By some simple maths, we can work out that the motor’s maximum torque is 204.6 Nm (151 lb-ft).
The E1 Racebird’s outboard motor, mounted with the shaft axis vertical and the power electronics mounted in the blue box at the top. The green ‘wing’ below the propellor is part of the hydrofoil system, which raises the boat out of the water at speed
Seabird has not elaborated on the type of windings used or the number of stator slots. The engineers have not been very specific about the DC voltage either, stating simply that it is in the region of 400-600 V. It does weigh 30 kg though. It is cooled by a water-glycol mixture, and is slightly unusual in having the rotor actively cooled in order to extract heat from the rotor’s magnets. The motor’s construction uses high-grade aluminium in the motor casings, with additional marine coatings applied for durability, given some of the concerns with galvanic corrosion of aluminium components. The motor shaft spins at a higher speed than the propeller, and the speed reduction is 2.07:1.
The Racebird’s battery
The battery sits directly behind the driver and has a capacity of 37 kWh. It has a mass of 240 kg, giving an overall energy density of 0.154 kWh/kg. That compares favourably with the battery in the 2021 Voxan electric land speed record motorbike, whose equivalent figure is 0.052 kWh/kg. For reference, the batteries supplied by Balance (detailed in RET 136, January 2022) have an energy density of greater than 0.2 kWh/kg. In terms of power density though, the Racebird’s is 0.625 kW/kg compared with the Voxan, which is 3.75 kW/kg (see RET 137, February/March 2022). 0.625 kW/kg is a perfectly respectable power density, but the Voxan is exceptional in having been optimised for a very unusual use case.
The battery consists of 3312 lithium-ion cells, of the popular 18650 type (18 mm diameter, 65 mm long). By some basic calculations, there are either 23 parallel strings of 144 cells in series or 24 parallel strings of 138 cells in series. Fewer strings, each containing a greater number of cells, would give a voltage higher than the range stated while more strings, each containing fewer cells, would produce a voltage lower than the range stated.
Battery cooling is always one of the major design concerns, as it can be a limiting factor in terms of performance. The Racebird’s battery uses Kreisel’s patented dielectric direct cell cooling design whose precise control over battery temperatures allows them to be used at high rates of charge and discharge. The battery modules are said to have undergone a robust development programme in terms of cooling and thermal behaviour so that the temperature can be assumed to be equal at each cell, which means the number of individual cells requiring temperature monitoring is reduced to a minimum. Compare that with the Electroflight air speed record battery (detailed in RET 141, August/September 2022) where every one of the 6480 cells is continuously monitored for temperature.
The inverter (power electronics) is also situated in the outboard, and has a volume of 7 litres and a mass of less than 10 kg. Power electronics cooling always focuses on removing heat from the power switches (IGBTs or MOSFETs) and, similar to the motor, these are cooled by a waterglycol mixture, although it has its own dedicated cooling circuit.
The entire E1 Racebird has been designed around the use of an electric powertrain. The battery and motor combination of energy storage and an energy conversion device has a fundamentally different distribution of mass to a fuel tank and IC engine. In order to get the centre of gravity and handling dynamics right requires detailed calculation, and these depend significantly on the distribution of mass within the vessel.
The E1 Racebird needs to be safe and stable in operation while offering good performance and excellent handling. The constant and instantaneous torque provided by the electric drivetrain means the boat can perform in a way that could only be achieved by an IC engine powertrain of greater performance.