Op-Ed: The promise of modern pod drives

James Edwards

Historically, mariners have had to choose between outboard and inboard engines. Each of these options have come with trade-offs. An outboard sacrifices valuable deck space and produces a lot of noise, while also raising a vessel’s center of gravity. An inboard sacrifices compartment space to accommodate engine rooms and prop shafts, which also crates challenges around ventilation, fuel lines, and exhaust.

A third option that’s occasionally been explored have been azimuth thrusters, drive systems that place a propeller underneath that can be rotated along a vertical axis. A great advantage offered by azimuth thrusters is that a propeller can pivot and apply force in the direction needed, which eliminates the need for rudders and boosts fuel economy. 

However, azimuth thrusters can be awkward to install. They need their engine to be placed above the propeller in an “l-drive” configuration, or connected through a propshaft and “Z-drive.” This means that they’ve historically combined some of the most significant disadvantages of both outboards and inboards.

This is where the idea, and advantages, of an electric pod drive come to the fore. These iterate on the azimuth thruster by integrating an electric motor directly inside the submerged “pod” attached to the propeller. The big advantage of this is that an electric motor doesn’t need fuel, oxygen, or a drive shaft connection—only an electrical connection to a generator or battery inside the hull. 

As a result, the electric pod drive offers the benefits of the azimuth thruster without the need for an l-drive or a Z-drive. And since a motor doesn’t need to idle like an engine, pod drives are often far more fuel-efficient and have a reduced maintenance burden.

Despite these benefits, there’s a challenge to rolling out pod drives for all but the largest vessels: drag. Since a pod drive is underwater, it introduces drag in proportion to its cross-sectional area – and that area is determined by motor power.

To illustrate this, we can take one of the most powerful production motors of 2004 as an example: the one that powered the Toyota Prius. That motor’s power density—its power per unit of mass—was 1.1kW per kilogram. In turn, this means that if we wanted 100kW of power, we’d need 90.9kg of motor. If we assume this 100kW motor was as dense as an equivalent block of iron and was shaped like a cube, its cross-sectional area would be around 500cm².

This undermines the usefulness of pod drives for smaller vessels, since many of their efficiency benefits are cancelled out by this significant extra drag. But it’s not so much of a problem for larger ships, since motor power is subject to a square-cube law: the drag-producing cross-sectional area increases with the square of the motor’s radius, but its total power scales with the volume—the cube of the radius.

As a result, a 10x more powerful motor (1,000kW) at Prius-levels of power density would experience 4.6x the drag of a 100kW unit for ten times the power. A 20x more powerful motor (2,000kW) would produce 7.4x the drag. As a motor gets bigger, less power is cannibalized by drag increases. This makes pods a great solution for larger vessels like cargo ships, tankers, or cruise ships that need ultra high-power thrusters, which has led to their widespread adoption.

However, the power density of motors is improving—particularly due to the degree of investment and innovation taking place in the EV space. As more power dense motors trickle down from automotive to marine, the range and viable size windows for ships using pod drives is widening dramatically. 

For example, we could take the motor from the 2022 Lucid Air, which has a power density of 16.1kW per kilo—nearly 16x that of the 2004 Prius motor. That same 100kW motor that once weighed 90.9kg now weighs just 3.1kg. Using the same assumptions for the Prius-quality motor—all-iron density and shaped like a cube—the Lucid-quality motor has a cross-sectional area of 85cm². That’s just 17% of the area of the Prius-type motor. 

In short, the same power can now be delivered with 83% less cross-sectional area and drag. By slashing the amount of drag produced by pod thrusters in this way, they suddenly become far more appealing for smaller vessels and also impose less of a range penalty on larger vessels. As a result, electric pod drives may soon finally be in a position to offer the bulk of the marine market a superior alternative to many applications that currently rely on inboards and outboards.

James Edwards is a marine chief engineer at Helix, a supplier of high-end electric motors and drives in the U.K.