Ollie Cooper and Matt Venner, senior naval architects at Lateral, investigate whether traditional shaft lines are becoming obsolete, with developing electronic propulsion systems offering a compelling alternative that aligns with the industry’s trajectory towards more optimised, comfortable and manoeuvrable yachts.

In the ever-evolving world of superyacht design, a quiet revolution is taking place beneath the waterline. Electric azimuthing propulsion devices – or PODs as they’re known in the industry  – are challenging the supremacy of traditional shaft-driven systems. Already a mainstay in the cruise-ship sector for many decades, these innovative propulsion units are gaining significant traction in yacht design with promises of enhanced manoeuvrability, improved space utilisation and possible efficiency benefits that align with the industry’s push towards sustainability.

But is there any validity to these claims? And what exactly does this mean for the future of superyacht design? We’ve conducted extensive research into the implications of this technology adoption, examining everything from performance metrics to structural considerations, as well as publishing a technical paper. The findings reveal a complex picture of trade-offs and opportunities that every yacht owner and industry professional should understand.

The rise of the POD
At their core, PODs represent a fundamental reimagining of marine propulsion. Unlike traditional shaft systems, these units can rotate 360 degrees, combining propulsion and steering in a single package. They come in various configurations, from compact 0.2mW units to powerful 7.0mW systems and above, each suited to fit specific vessel requirements.

The technology behind these units falls into two main categories. The first uses mechanically gear-driven propellers, with electric motors mounted above in either a Z-drive (horizontal motor alignment) or L-drive (vertical motor alignment) configuration. The second, more integrated approach places electric motors directly in the propulsion module.

Propeller configurations vary too. Direct-mounted motors typically employ fixed-pitch propellers, while mechanical systems can accommodate controllable-pitch propellers for enhanced efficiency. Some units even utilise contra-rotating propellers, where a rear propeller works in harmony with the energised flow from the front propeller’s swirl, maximising power density and minimising losses.

Figure 2: Speed vs length, diesel mechanical with shafts vs diesel electric with PODs.

The performance question
Perhaps the most pressing question is how PODs compare to traditional shaft systems in terms of performance, and our research reveals some interesting trade-offs. From our Large Yacht Statistics database, and using recent new-build data and experience, Figure 2 shows that POD-equipped yachts typically achieve maximum speeds about two knots slower than comparable shaft-driven vessels.

Although we are not POD designers, we postulate the lower speed stems from a reduced power density at the propeller disc, affected by factors such as strut strength requirements, larger propeller hub sizes and higher RPM operations. However, it must also be considered that across the fleet, superyachts are generally transiting at slower speeds due to the associated environmental benefits, so the speed limitations of PODs are not necessarily a concern in the modern industry.

Raw speed isn’t the whole story, though. When comparing resistance and efficiency between podded and shaft-line vessels, multiple factors come into play. A case study conducted on an 88-metre yacht reveals that POD-driven vessels can demonstrate up to 8 per cent less appendage resistance at top speed, thanks to the elimination of shaft brackets and rudders. Wouldn’t it be great for PODs if the story ended here?

Figure 3: Draught design loop.

The concept of efficiency becomes more complex when considering the entire propulsion chain. While some POD configurations showed delivered power requirements ranging from 14 per cent less to 6 per cent more than traditional shaft systems at top speed, the final brake power demand at the power source typically ended up being 0 to 17 per cent higher for POD systems once all transmission and electrical losses were considered. This variation highlights that the question of efficiency can be argued in more ways than one, and with different outcomes. In the example we investigated, it was concluded that electric pods have a higher power demand than a mechanical shaft-line solution.      

However, an electric energy architecture does offer great opportunities when it comes to power supply. So although the power demand may be higher, an electric power supply presents many cases for efficiency or sustainability gains. These include batteries, alternative fuels, hybridisation and power diversification optimised to a specific yacht’s operational profile.

Figure 4: Varying POD arrangements.

Reimagining hull design
Integrating PODs into a superyacht design requires a fundamental rethinking of hull design principles. The underwater profile must balance multiple competing factors, from resistance and seakeeping performance to efficiency and comfort. Five key areas demand particular attention:

First, draught considerations become crucial. The lower power density of PODs means that larger units often require deeper draughts as power requirements increase, creating a design loop that can impact vessel specifications. When operational requirements restrict draught, designers may need to compromise on contract speed.

Second, transom design takes on new importance. PODs favour U-shaped sections (Figure 4, arrangement A) with shallow deadrise angles and outboard bilges, while curved or rounded sections can complicate mounting and may require tilting that reduces interior space (arrangement B). Fairings can help accommodate excessive tilt but they typically increase resistance by 1 to 2 per cent (arrangement C).

It’s also less complex to mount PODs on flatter underwater section shapes but it’s key to avoid shallow flat sections to minimise stern slamming. Making the transom deeper would reduce the slamming risk but could come at the cost of increased resistance. A fine balance must be struck.

Third, the transverse positioning of PODs requires careful consideration to enable full rotation while staying within the vessel’s beam waterline. This positioning must balance manoeuvrability with practical considerations such as whether we should discuss access to stern between the PODs rooms.

Fourth, longitudinal positioning affects the entire aft ship design. PODs need dedicated technical spaces with adequate maintenance headroom, often pushing the units further forward in the hull. This affects buttock line design and can influence the vessel’s overall balance.

Finally, weight distribution presents its own challenges. Diesel-electric propulsion systems with PODs typically increase the lightship displacement compared to conventional diesel-mechanical systems, requiring careful consideration in the impacts to overall weight distribution and hull design.

Figure 5: Polar plot comparison.

The comfort factor
One area where PODs truly shine is in noise and vibration control. By concentrating thrust loads at the aft end of the vessel, PODs significantly reduce vibration transmission through the hull.
This isn’t the same for traditional shaft lines, where propulsion-related disturbances can be heightened due to mechanical configurations.

PODs, with their adaptable strut lengths, allow for optimised propeller tip clearance during design which, coupled with their lower power density, reduces pressure pulses on the hull. Combined with the elimination of shaft alignment issues and wear-related vibrations, these factors can contribute to a notably quieter and more comfortable on-board experience.

Masterful manoeuvrability
Perhaps the most compelling advantage of POD systems lies in their superior manoeuvrability. Underway, full-scale tests have shown that PODs can reduce the turning circle diameter by up to 38 per cent compared to traditional shaft-rudder systems. And when undertaking close-quarters operations in harbour at slow speeds, improved dynamic positioning and station-keeping capabilities outshine shafted vessels, with POD systems able to maintain position in wind speeds up to 30 per cent higher than conventional arrangements.

Design freedom and future prospects
The adoption of PODs, particularly in conjunction with diesel-electric systems, opens up new possibilities in superyacht design. Without the arrangement constraints of traditional shaft lines, designers gain greater flexibility in engine-room placement and capability to distribute the power sources.

However, as with all change, this freedom comes with its own constraints. PODs require dedicated ‘POD rooms’ above their steering modules, which can affect beach-club areas and stern access – key locations for owners to enjoy their yacht.

Our solution to this issue is to integrate POD propulsion without impacting the general arrangements, what we call the ‘Free From Bulkheads’ concept. By increasing the freeboard of the vessel, all technical spaces, including POD rooms, can be kept below the lower deck while allowing a full-length guest experience above.

Looking ahead
While traditional shaft lines remain the dominant propulsion method in superyacht design, the industry’s growing focus on adopting electric architecture makes POD systems an increasingly attractive option. Their benefits in manoeuvrability, comfort and design flexibility often outweigh their limitations in top speed.

The key to successful POD implementation lies in early consideration during the design phase, allowing for optimal integration of all systems. As technology continues to advance and electrical efficiencies improve, we can expect to see POD systems playing an increasingly important role in the future of superyacht design.

For owners and designers considering POD propulsion, the decision ultimately depends on specific project requirements and priorities, with speed being the dominant factor. While they may not entirely replace traditional shaft lines in the immediate future, PODs represent a compelling alternative that aligns well with the industry’s trajectory towards more optimised, comfortable and manoeuvrable yachts.

This article first appeared in The Superyacht Report – New Build Focus. With our open-source policy, it is available to all by following this link, so read and download the latest issue and any of our previous issues in our library.

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