Why Don’t Nuclear Submarines Rest on the Sea Floor?

You’ve seen it in the movies about WWII, a US navy submarine spends its day resting on the sea floor waiting for nighttime to surface, recharge its electric batteries and begin hunting for Japanese warships or a big fat oil tanker.

These days, US submarines are nuclear-powered and huge by comparison to those used in WWII. and they no longer spend any time sitting on the bottom of the ocean floor. Here is what that is all about.

Before nuclear submarines were developed, they ran on a combination of diesel and electric power and operated mostly on the surface at night and submerged during the day running on electric power stored in banks of lead-acid batteries.  Submarines in the pre-nuclear power age trying to conserve electricity would often sit on the bottom in relatively shallow waters of less than 500 ft and then pop to the surface at night to resume their patrols on the surface.

These submarines were also much smaller in terms of length, and displacement. A US sub of the Gato class in WWII displaced about 2,000 tons of water when submerged while the modern Seawolf class of fast attack subs displaces more than 9,000 tons.  WWII submarines lacked a great deal of the electronic sophistication of modern subs which had numerous openings in the hull to intake seawater to convert it to fresh water and to draw oxygen from it for the crew to breathe. Unlike WWII submarines, modern subs have sensors fitted to various places on the hull including the bottom.

Additionally, the outer hulls of modern subs are also coated with rubber called the Special Hull Treatment to reduce the amount of sound they make in the water.

Submarines in WWII and even later didn’t have all these sensors and coatings on the hull so they could rest on the bottom in shallow water without too much concern about damaging the hull or its fittings.

This is not to say that sitting on the bottom was something easy to do or without its perils.  The sea floor is not uniformly flat. In WWII, submarines were essentially blind underwater and could not know what was beneath them when they set down.  Later in the war they had sounding sonars than could tell them how close they were to the bottom, but they mostly relied upon charts with soundings taken by hand that were not very accurate at all.

The current standard for nuclear subs is to not sit on the sea bed, but there are still “special cases” where they are required to sit on the bottom of the ocean, according to former US Navy veteran John Jones.

“Submarines of the WWII era had the capability of resting on the sea bottom, but that surmises a sandy bottom in shallow water. Modern Submarine hulls aren’t built like the old boats, and in any event, for any operation requiring a boat to be stationary, it’s a simple matter of hovering, or station-keeping, to maintain depth and position if and when required.”

WWII submarines had rather simple control surfaces to operate with underwater, believe it or not they were designed to operate mostly on the surface and only dive to escape or avoid detection.  For this reason, they could be hard to trim out even and just hold a depth underwater.  There was also the problem of navigation fixes.  A 2,000-ton sub hovering at 400 ft would drift with the ocean currents and when it surfaced 8-10 hours later it could be many miles off its plotted location, requiring the navigator to re-establish its position on the charts.  Not much of a problem in the open ocean, but if close to land you could drift into shallow water and run aground.  A modern sub can not only hold its position stationary in the water but can also judge the speed of currents over the hulls and adjust itself for drift.

That being said, modern submarines are still mostly blind underwater and have to come shallow fairly often to update the positions on charts to navigate safely underwater.

A modern submarine also has a rounded hull and not a flat bottom, at the sea floor it might roll over on its side unless its ballast tanks were carefully trimmed to keep it upright.  The third submarine named Seawolf(SSN-575) decommissioned in the late 1980s actually had retractable legs that allowed it to sit on the sea floor for prolonged periods and be stable and upright. This is because it operated as a spy sub, and test bed for new reactor designs and passive sonar systems. Though she was nuclear-powered sitting on the bottom outside an enemy harbor would allow her to listen to and record the sound of every ship entering or leaving.  It could then classify those sounds as to exactly what kind of ship they were listening to.  Sitting on the bottom like a rock and giving off no noise from her own propellers also meant she could avoid detection by patrolling destroyers above her.  Even if they turned on their active sonar she would appear to be a reef or sea mound on the bottom.

So in closing, we suppose modern submarines could still rest on the bottom if they wanted to, but they really no longer need to to save battery power as they did in the pre-nuclear age of submarines.