Understanding the Range Differences in Submarines: Surface vs. Submerged

One of the most intriguing aspects of submarine design and operation is the significant difference in range between surfaced and submerged modes of propulsion. This article explores the reasons behind these differences using the example of HMS Aeneas, a diesel-electric submarine from the Amphion class.

The Propulsion System of HMS Aeneas

HMS Aeneas was an Amphion class submarine that entered service in 1945. However, only two of these submarines were completed, and neither saw active service during the war. The design showcased a traditional diesel-electric propulsion system, which is similar to the systems used in World War II submarines but not yet adapted to nuclear power.

Diesel and Electric Propulsion Systems

The propulsion system of HMS Aeneas included both diesel and electric motors. The diesel engine drove the submarine on the surface, providing a maximum range of 10,500 nautical miles (NM) at a speed of 11 knots. While the submarine could travel much shorter distances submerged, the electric motors, which powered the submarine underwater, could only cover 16 NM at a speed of 8 knots or extend to 90 NM at 3 knots.

The Diesel Engine's Limitations

One of the key reasons for the significant difference in range between surfaced and submerged modes is the requirement of a diesel engine to receive air for combustion. Diesel engines, including those in submarines, need air to function, which is why they can only be run while surfaced. Underwater, the electric motors, powered by pre-charged batteries, are the only means of propulsion.

The batteries on the HMS Aeneas had a limited capacity, providing enough power for submerged operation for only a short duration—about 16 NM at reduced speeds. This means that even though submerged, the submarine could only travel a few hours before needing to surface to recharge the batteries. A single load of fuel allowed the submarine to travel much farther on the surface, enabling long-distance voyages.

The 16 NM limit refers to the contiguous submerged endurance, not the amount of fuel the submarine could carry. The submarine could submerge multiple times with a single load of fuel but could only travel 16 NM at a time before surfacing to recharge the batteries. After surfacing, the diesel engine would recharge the batteries, allowing for another 16 NM of submerged travel.

Historical Context and Evolution

HMS Aeneas was a pioneer in its time, but it was not the first to face these challenges. Many World War II submarines operated with similar limitations. In the post-war era, modifications were made to some classes of submarines, such as the addition of a snorkel attachment, which allowed the diesel engines to run while partially submerged, thereby extending the submerged range.

Modern Submarines: A Leap Forward

Modern nuclear-powered submarines have significantly extended the range and submerged endurance of submarines. These modern vessels can remain submerged for several months at a time, as they do not need to surface to recharge their batteries. Additionally, they can create their own air and fresh water from the sea, making them far more independent and capable than their predecessor models.

An Example: Submarine Journey from Bermuda to UK

A notable example illustrating the potential of submarines is the journey of a modern submarine that traveled 2,500 miles from Bermuda to the UK entirely submerged in just 15 days. This voyage demonstrated the vast improvements in submarine technology over the decades.

Conclusion

Understanding the range differences between surfaced and submerged modes is crucial for comprehending the operational dynamics of submarines. While the diesel-electric propulsion system of HMS Aeneas offers a historical perspective, it also highlights the continuous evolution and advancement of submarine technology. Modern nuclear-powered submarines have revolutionized the field, providing unparalleled endurance and capability.