One of the challenges of ship design is the propulsion system. The traditional propeller and drive shaft present problems such as:
- Complicated gearing systems which are an operating and maintenance challenge
- Long drive shafts which take up precious space in the ship
- Multiple points of watertight seals that have proven to be less than watertight in damage situations
- Shaft bearings that are prone to overheating and require lubrication and cooling
- Complex lubrication systems for shafts and gears with sea water influx being a common problem
One potential solution is the pod system in which an electric drive motor and propeller are mounted in a pod external to the ship’s hull. This eliminates the shaft, many bulkhead penetrations and seal points, complicated gearing, etc. The pod rotates via a small steering motor mounted in the ship’s hull directly above the pod’s mounting flange. The only penetration of the hull is the electric cables from the power generating unit to the electric motor in the pod. Large commercial pods may have maintenance access hatches built into the pod mount.
|Typical Propulsion Pod Concept|
Pods can be either push (traditional rear mounted propeller) or pull (front mounted, like an airplane) and can be either fixed or rotating (azimuthing). Some ships have used a combination of fixed and rotating pods. Front mounted (pull) pod props have been claimed to reduce cavitation compared to conventional push propulsion systems due to operating in less disturbed water flow.
The features of pod systems include:
- No gear systems required
- Space saving
- Pods can rotate 360 deg
- 2-25 MW per pod
- Eliminates need for a rudder
Pods have a range of power outputs from 2-25 MW per pod. As a point of comparison, the Burke’s propulsion system consists of 4x LM2500 turbines connected via gearing to two propeller shafts producing a total of 78 MW of power. Two large pods would produce 25 MW and three would produce 75 MW which equals the conventional Burke output.
|Various Types of Pods|
Pods have been extensively fitted to icebreaking vessels and cruise ships, among others. Some examples include:
Rolls Royce Mermaid Pod
The QUEEN MARY 2 has four Rolls Royce Mermaid podded propulsion systems, the first four-podded installation to date and the largest output for any podded-driven vessel. The system comprises two fixed and two azimuthing pod units delivering a combined output of more than 85 MW, which gives the ship a service speed of 30 kts. 
ABB azipods are produced in a range of sizes and power outputs. For example,
The luxury polar expedition cruise ship, due delivery for the French cruise company Ponant in 2021, will feature two Azipod VI propulsion units, designed for the most challenging ice conditions.
Upon delivery from Norway’s Vard Søviknes, a Fincantieri Company, the cruise ship will take passengers to unexplored destinations in Arctic and Antarctic waters. In the harsh conditions of ice-covered seas, Azipod units with six-meter propellers and combined power of 34 MW will help the vessel achieve greater maneuverability and efficiency, as well as lower the environmental impact. Due to minimal noise and vibration, Azipod propulsion will also improve passenger and crew comfort. 
On a larger scale, the Royal Caribbean cruise ship, Oasis of the Seas, uses 3x ABB 20 MW (60 MW total) rotating azipods.
Some naval examples of pod propulsion include:
- French Mistral LHD class uses 2x 7 MW pods. The ship is 650 ft long, 20,000 tons displacement, and can achieve 18 kts.
- Australian Canberra LHD class uses 2x Siemens Navantia 11 MW pods. The ship is 757 ft long, 27,500 tons displacement, and is rated at 20+ kts.
Pods seem like a worthwhile possibility for naval vessels. The reduction in complexity, equipment, and bulkhead/hull penetrations make pods an attractive alternative to convention systems. In particular, eliminating the complex combination gearing systems that the Navy seems completely unable to operate and maintain would significantly reduce the number of ship breakdowns the Navy has encountered of late. How pod systems would fare in combat and damage control situations is, of course, unknown and would have to be tested.
At the moment, I see a lot of upsides and no real downsides to this technology.