Why are submarines not shaped like fish? After all, billions of years of evolution have probably resulted in fish having the perfect shape. Or has it? We’re specifically talking about the nose of the submarine. Or, more correctly, the submarine’s Bow.
Why is it round and not pointy like fish? Looking at various classes of modern submarines compared to multiple fish species, it’s clear that fish have pointy noses while submarines have rounded bows. Have you ever wondered why is it so?
Before the Cold War, most submarines had a pointy bow, and that’s because older submarines were more like ships as they spent more time on the surface than underwater, especially when travelling long distances. The reason for it was straightforward- Older submarines could not stay submerged for an extended period as they needed fresh air intake for their diesel engines to work. In addition, once submerged, subs were running on batteries which powered electric Motors.
The thing is, the batteries wouldn’t last very long. Modern nuclear-powered submarines can stay submerged for about 90 days, primarily due to limited food supplies. So theoretically, a nuclear submarine can run for almost 20 years without refuelling. Conversely, older submarines could only stay submerged for about 12 hours or so, with late war German designs achieving the capability of staying submerged for a few days while travelling at a speed of about five knots.
Submarines were very slow underwater. For instance, an American Balao-class submarine had a submerged top speed of just under nine knots, while it could go over 20 knots when travelling on the surface. But modern submarines have top speeds above 30 knots while travelling underwater; this is why travelling underwater for long distances for older submarines was just not feasible. And that’s why they would move to the surface like ships travelling long distances; submerging was only for attack or escape runs. For this reason, older submarines had ship-shaped bows, as they functioned like a ship for the most part.
When these older submarines were designed, the focus was always on the surface performance and not on the submerged performance. But then came nuclear power, which made submerged performance a thing. And as often happens, submarine designers had to learn the hard way.
In the early 1950s, the US Navy was building two unique submarines which proved to be game-changers, USS Nautilus and USS Albacore. Launched in 1954, USS Nautilus was the first nuclear-powered submarine globally. Nuclear power meant that submarines could travel long distances underwater from then on. As a result, both speed and endurance were significantly increased. However, while Nautilus did not have a proper ship-shaped bow, it had a semi-pointed bow, which was a huge problem.
The Bow was shaped around its ultra-modern sonars. However, the sonars were more or less useless, as Nautilus was a very noisy submarine. Whenever the boat travelled as faster as seven knots, her sonars would be deafened by the noise of the sub itself, which rendered the sonars unusable. This issue was traced back to serious design flaws in her Bow and sail, that resulted in extreme vibrations at high speeds. By the time Nautilus had reached the end of life, travelling at just four knots would cause enough noise to deafen the sonar.
There was some design work to be done, but that design work was already underway. It just happened concurrently as Nautilus was being built. The diesel-powered USS Albacore was a research submarine whose design was derived from extensive hydrodynamic and wind tunnel testing. It was the first attempt to design a submarine with a focus on submerged performance compared to surface performance. One of the goals was to minimize submerged resistance.
Albacore’s new Hull shape was known as a teardrop hull. Launched in 1953, USS Albacore underwent several overhauls where design elements were modified and tested. During early sea trials, it was found out that she could operate at maximum speed at half shaft power compared to the older Guppy type submarines. Albacore’s design proved revolutionary compared to the older ship-shaped designs because she was much faster and more manoeuvrable. Her maximum speed was over 25 knots, and even though Albacore was diesel-powered, she was much quieter than Nautilus.
Albacore’s hull design was eventually implemented on the Skipjack class, the first class of American nuclear attack submarines in 1959 and in all modern American submarines since. But there was a downside to all this. She was not very stable on the surface. Around the same time, the Soviet Union also commissioned its first nuclear submarine, the K-3 Leninsky Komsomol, which had a round bow which proved to be much more efficient and spacious than older ship-based bows. So what is it about the round Bow that makes it such a good design choice?
The first thing to understand is that submarines are under extreme water pressure during dives. As a result, modern submarines can dive much deeper than before, with most able to dive over 1000ft with a record of 3350ft. To withstand such extreme pressures, the boat’s geometry makes a huge difference. Generally, the curvier, the better.
If you’ve heard of the de Havilland Comet, you’d know that the airplane had problems with square windows generating three times the stress around fuselage joints. After one fatal disaster, all future airplanes switch to windows with round corners to eliminate stress concentration. This is because round shaped things tend to be stronger than angular frames. Similar logic applies to submarines, with most of the body being cylindrical with a curved bow.
Modern submarines typically have two hulls, a pressure hull and a light hull. The pressure hull provides an atmospheric pressure for the crew and other devices that need to function at a specific pressure, humidity and temperature. The light hull offers the space for equipment that can handle the water pressure at depth, such as ballast tanks, sonars, and torpedo tubes. The Bow of most submarines is part of the light hull, since it gives the sub its shape for the best possible performance.
Another thing to consider is how noisy submarines can be. As the water flows around the submarine, the turbulent pressure fluctuation in the flow radiates, a noise known as flow noise. As was learned from USS Nautilus, the shape of the Bow makes a big difference in the amount of flow noise generated. If there’s too much noise, it can render the boat sonar unusable. Moreover, it would be much easier to detect a noisy submarine, which could be fatal in combat situations.
Generally speaking, curvier bows produce less drag, which results in less flow noise. With that said, flow noise is also affected by the positioning of diving planes, the size of the sail, and the overall shape of the submarine as a submarine travels submerged and encounters resistance from the water ahead.
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Even though many modern submarines are nuclear-powered, there is still a cap on the amount of power they can generate, so minimizing submerged resistance is crucial for submarine performance. In addition, the amount of curvature in the Bow impacts resistance, and specific shapes will produce less resistance than others.
But the question is, would a fishlike bow produce less resistance compared to the round shape bow of the modern submarines? A paper in the Journal of Scientific and Engineering Research analyzed the effects of the various bow shapes on the submerged resistance. One of the bows studied was the “ogive shaped” Bow, which almost resembles a fish.
Other bow shapes analyzed included ship-shaped Bows, elliptical, Conic, and hemispherical. The ship-shaped Bow has pretty high resistance, as it was not designed for underwater performance. The hemispherical Bow has the highest resistance, while fish shapes and Conic bows have the lowest resistance. Elliptical and Conic elliptical sit in the middle of the spectrum. So why is it that most modern submarines don’t have fishy bows or conic bows but instead have bows that vary from elliptical to hemispherical?
Other than minimizing flow noise and submerged resistance, the shape of the Bow is also dependent on the internal architecture of the boat. Besides the ballast tanks, the submarine bows usually features a giant spherical sonar and torpedo tubes, neither of which would fit in a conical or a fish-shaped bow. So, that’s the reason why the submarines’ bow are round and not conical or fish-shaped.
Here is a super-interesting video on this topic:
When sub’s are forced to cruise on the surface, why don’t they submerge to just the tower being above water so that their drag is reduced by most of the hull being underwater? With the air intake way above this, there should be no problems with ventilating the Sub.