Motors, power electronics and cabling sit close together in this installation from Helix’s work with Magic Carpet e
(All images courtesy of Helix)
Helix has a clear explanation for why electric pod drives have largely belonged to megawatt-scale vessels. Below that size, a pod’s drag takes a growing share of the power it delivers, and the UK maker of power-dense motors and inverters believes a Pod Cartridge combining motor, reduction gear and inverter changes the sums. We asked James Edwards, Chief Engineer for Marine at Helix, how that works in practice.
It starts with drag, because unlike inboard or outboard motors, where the propulsor sits within the vessel or high at the rear, “a pod drive architecture means that the propulsor becomes a drag-generating element of a ship or boat”. Edwards explains that a pod’s drag is directly determined by its frontal cross-sectional area.
That is where the square-cube law bites. For a perfect cylinder, the area of the front face grows with the square of the radius while volume, and with it the propulsor mass doing the work, grows with the cube, so the lower the power of the system, the greater the share of its power lost to drag.
Why power density matters for electric pod drives
To show the scale of the problem, Helix maps what happens to a pod built around the motor of a 2004 Toyota Prius, at 1.1kW/kg, and how much of its power would be cannibalised by drag. On Helix’s numbers, it is only at megawatt scale that the layout becomes worthwhile in practice.
The answer, Edwards says, is power density. By bringing pods into line with what advanced e-motor engineering can achieve today, Helix can “dramatically shift the drag-per-kW curve down and suddenly make pods a superior propulsor for many sub-MW applications”. Modelled with the motor from a 2022 Lucid Air, at 16.1kW/kg, Helix’s model puts drag per kilowatt at about one-sixth of the Prius-density level at every size.
A reduction gearbox capable of delivering the high torque marine propulsion requires then reduces the torque demanded of the motor and downsizes the package further, and integrating the inverter produces what Edwards describes as “a drop-in unit which OEMs can integrate into their own package with minimal effort and external connections”.
Continuous power changes the engineering brief
The density that matters here is continuous, and Edwards points out that sustained marine use changes a lot of basic engineering priorities. Helix’s marine motors draw on its experience in Formula E and E1 powerboat racing. A motor can exceed its continuous rating for short bursts because “the steel and aluminium of the structure can absorb some of the losses as the temperature increases”, though that heat sink has limits and peak power only lasts for short bursts before exceeding material temperature limits.
Designing for continuous output is a different job, one “about designing a system that can indefinitely run at a given power level”. In practice that means minimising losses, maximising the effectiveness of the cooling system to raise the continuous rating, and monitoring internal temperature through sensors or a thermal model so that derating can protect the motor if it hits a limit.
Marine duty adds a layer of its own, which Edwards sums up as “waterproofing, certification and corrosion-proofing”. Helix’s standard is IP6K9K and it also supports IP67, testing to IP67 on site during development and using local test houses for the other ingress levels. For applications with additional requirements, such as marine, where immersion depth can be significant, Helix works with the customer to tailor the motor, which avoids the risk of adding cost when the customer already has marine sealing in place.
Silicon carbide and higher voltage in electric pod drives
Cutting losses is the inverter’s job too, and silicon carbide switches sit at the heart of Helix’s inverter technology. They offer “significantly lower switching losses than the more conventional IGBT solution”, which reduces cooling requirements and supports higher frequency operation that improves motor efficiency, with the payoff being greater range from the vessel or potential cost reductions through reduced energy storage on board.
Voltage follows power, because current climbs with it in the DC bus and in most cases within the motor. Thicker cables and a larger motor and inverter can absorb that, but Edwards says this is usually detrimental for cost and certainly for weight, so “by increasing the voltage, we can reduce the current requirement and keep components and cables small”.
The changeover point depends on the application, and Edwards puts the ceiling for 48V systems at around 50kW, because that already means “over 1000A DC”. Beyond that point, he says, “the legislation changes”, and 800V “has become the standard up to the MW region”.
Integration and the route to 3MW
How much of this Helix provides depends on the customer. Some want an off-the-shelf solution and handle wider integration in-house, while others work with Helix on their electrical architecture, battery placement or propulsor layout. “While our responsibility is the propulsion system behind the pod, we know that the complexity of modern vessels means that we need to be available to help ensure system integration is successful,” Edwards says.
Helix and New Eagle offer a package spanning power electronics, propulsion management, shore charging and energy co-ordination, and what a boatbuilder takes up varies with their needs. According to Edwards, the range runs from “the pure propulsion motor through to a full system with integration”.
The initial Pod Cartridge range spans 100kW to 300kW, with larger versions from 480kW to 3MW planned. For the sub-MW end, Edwards says “we can apply our existing motors to slightly larger pod geometries to offer an efficient solution in a relatively short timeframe, and we don’t anticipate any major roadblocks”.
Above 1MW the market drives additional considerations, starting with operating hours between services and the desire to repair in situ. Helix is very likely to look at voltages above 800V, which would mean revising the motor’s internal configuration to increase creepage and clearance and changing the inverter technology to suit. Cooling needs further review as well, because Edwards anticipates that the efficient through-the-wall cooling concept of the smaller pods will not be sufficient above a few MW.
Service intervals and in-situ repair are the areas Helix would look to develop with a trusted partner, Edwards says, so that the larger cartridges meet the needs and expectations of the commercial market.
James Edwards, Chief Engineer for Marine at Helix