U-Boats, Density Currents, and the Strait of Gibraltar

Thanks goes out to Lt. Wallace.

Since ancient times, the narrow Straits of Gibraltar separating the Southern most tip of Spain from North Africa have been known as the "Gateway to the Mediterranean Sea." From a geographic standpoint, one can easily see why the Straits would be a strategic military "choke point" from which traffic and trade between the Mediterranean and the Atlantic could be controlled. However, a closer look at the Straits from an Oceanography point of view reveals many more unique and significant characteristics that have played important roles in conducting warfare in that region.

1. During World War II, what methods did we use to detect U-boats moving through the Strait?

Over the course of World War II, several methods were developed for finding U-boats traversing the Strait. Because the area is so narrow, visual spotting was the most common method used. As a general rule, U-boats preferred to remain surfaced if possible not only for visual navigation purposes near the coasts but also because once submerged their oxygen supply was limited. Safe navigation on the surface, even with good land marks or by using the stars, was treacherous enough without having to worry about the varying depth and irregular coastline. By 1941, the strait was so heavily patrolled by ships and watched by spotters on the land that most U-boat commanders preferred to cross under the cover of darkness with the incoming tide on the surface. Some U-boats using this method were even reportedly spotted by lighthouses along the coast.

Aircraft were also being increasingly used throughout the war to spot and find the U-boats both on the surface and sometimes even when they were submerged due to the clarity of the waters in the Mediterranean. Aircraft could cover large search are as quickly and were armed with an array of weaponry including guns, depth charges and torpedoes. By 1943, aircraft were launched out of Gibraltar, the Azores, other small airfields on the coast and off of "hunter-killer" escort aircraft carriers throughout the area.

One of the most significant detection methods implemented and improved during World War II was ASDIC (originally developed by the Allied Submarine Detection Investigation Committee), also known as SONAR. This was a transmitter attached to the underside of a ship that sent out a directional sound wave through the water. When this wave hit an object such as a submarine, it was reflected back to the receiver. The bearing, range and whether or not the submarine was opening or closing on the ship could further be determined. The SONAR could also be used in a passive role to "listen" for the sound of the submarine engine and propeller. Even the release of air out of the ballast tanks on the U-boats could be heard by the experienced SONAR operator. To combat the SONAR, U-boats would try to dive deep and rig for "silent running" by slowing down or shutting down the engine entirely. The Germans also developed chemical pellets that could be released from the U-boat and formed large clouds of bubbles to reflect active SONAR pulses.

The unique environment of the Strait of Gibraltar challenged submerged U-boats and the SONAR operators hunting them alike. The submarines submerged in the relatively shallow strait had to be very familiar with the varying depth of the sea floor and were very limited in their ability to maneuver evasively. However, the echoes from the SONAR could be reverberated by the close proximity to the bottom and cause confusion for the operator. The amount of ambient noise due to the high volume of shipping through the confined straits also limited SONAR operators as they tried to isolate the sound of the U-boat.

2. How many U-boats were able to enter the Mediterranean by "riding" the current and how many were able to leave?

Between 1941 and 1944, a total of 62 U-boats were reportedly sent into the Mediterranean via the Strait of Gibraltar. It is unknown how many of these U-boats were able to pass through the straits by relying on the currents alone while submerged. However, it is known that casualties in the strait overall were high. 9 of the 62 were sunk while attempting the passage. 21 others were forced to abort either because of damages or because the risk was deemed too high by their commanders. None of the U-boats that successfully entered the "mousetrap," as the Mediterranean became to be known by U-boat commanders, ever made it back to the Atlantic. All were either destroyed by the allies or scuttled by their own crews.

After the war, more research was done on currents below the surface in other areas of the world. In 1952, a current below the South Equatorial Current that stretches for about 8,000 miles from the coast of New Guinea to Ecuador was discovered by a member of the U.S. Fish and Wildlife Service. The Cromwell Current, named after Townsend Cromwell, flows in the opposite direction of the surface current from about 100 feet to 750 feet below the surface at a speed of approximately 3 knots. Submarines could utilize this current for movement much like airplanes use the jet stream. Another similar current exists below the South Equatorial Current in the Pacific and flows at about half the speed of the Cromwell Current.

3. How deep is the Strait of Gibraltar? How deep could the U-boats safely be?

Due to a ridge running from Spain to Morocco along the sea floor, the Strait of Gibraltar is only about 300m (1000 ft) deep towards the center. In a few places the depth is less than 100m. This is quite shallow compared to the average depth of the Atlantic Ocean and the Mediterranean Sea on either side of it.

Submarines are limited in their maximum dive depth by their ability to withstand the water pressure on their hulls that increases as they descend. The type VIIC German U-boat was built from 1941 to the end of the war and became the "workhorse" of the U-boat force. Its maximum dive depth was about 220m (722 ft). Dives this deep were normally done as a last ditch effort to exceed the depth at which depth charges were set to explode and reportedly were not uncommon from 1942 onward. However, to dive to such a depth in the Strait of Gibraltar with the varying depth of the sea floor required good knowledge of the area in addition to a great deal of courage.

4. How fast are the currents through the strait?

The surface water in the strait consists of eastward flowing Atlantic water. At depth, there is dense, high-salinity westward flowing water. The water between these two layers is a mixing zone. Depending on the winds, the non-tidal water at the surface flows between 2 - 4 knots (2.3 - 4.6 mi/hr). The non -tidal currents below the surface flow to the west with speeds decreasing with depth.

The overall speed of the current can be affected by the tide as well. The tides in the region follow a semidiurnal pattern (two high tides and two low tides per 24 hrs). The strongest tidal currents occur midway between the high and low tides. The maximum current in the center of the strait can reach 5 knots and is typically stronger in the autumn and weaker in the winter. The strength and direction of the current, on and below the surface, is also slightly affected by the varying geography and depth along the coast.

5. How long would it take U-boats to pass through?

The length of the Strait of Gibraltar is approximately 25 - 30 miles long. Depending on where a U-boat commander decided to submerge, a passage of this length of time would take 5 hours or longer if relying only on the current to carry the boat. The most reasonable approach would have been to catch the tide when it was flowing the fastest and rig the U-boat for "silent running" to slightly increase speed without too much engine noise. Without good knowledge of the tides and currents a U-boat could easily lose a few precious knots in speed, which could make the difference between life or death. On the surface the U-boats could travel up to 17 knots and drastically cut down the amount of time they spent in the area.

6. Do subs still sneak through the strait by shutting things down and riding the currents?

Since the Second World War, technology has greatly enhanced submarines, the doctrine by which they operate, and the methods by which they are detected. However, the oceanography of the strait has not changed and it still challenges the naval forces that operate in the area. In addition to the challenges that remain from the unique oceanography of the area, the strait has become one of the busiest shipping lanes in the world. This added traffic has tremendously increased the amount of ambient noise in the region and has not made detecting submarines any easier.

In the mid 1980’s, a Soviet submarine trying to utilize this heavy shipping traffic to remain undetected demonstrated how valuable a thorough knowledge of the oceanography in the strait can be. The submarine attempted to pass through the narrow straits while staying relatively close to the underside of a heavy tanker. By hiding within the acoustic signature of the louder tanker, it intended to pass through the area unheard by any listening devices in the region. This plan backfired, however, when an internal wave caused the submarine to rise up unexpectedly and collide with the bottom of the tanker. Needless to say, heavy damage was inflicted on the submarine and it was forced to surface in the strait, much to the chagrin of the Soviet Navy.

The internal wave the submarine encountered is believed to have been caused by the interaction between the water of the Atlantic and the Mediterranean Sea. The Mediterranean Sea is very restricted in its circulation with other bodies of water due to the fact that it is virtually landlocked. The only outlets to ocean water are through the Suez Canal and the Strait of Gibraltar. The sea is also relatively shallow, with an average depth of approximately 1500 m. This in addition to the high rates of evaporation due to its location near the equator causes the Sea to be much saltier and more dense than the water in the Atlantic Ocean. The east setting Atlantic water interacting with the more dense, west setting Mediterranean water below can create waves with amplitudes hundreds of feet high in the mixing layer between the two.

7. Do you know of any articles written about this topic?

I found two articles pertaining to currents in the Strait of Gibraltar but not specifically pertaining to the U-boats during WWII. The "Strait Waves" article has an interesting satellite picture that clearly shows the internal waves in the strait. They are as follows:

1. "Strait waves." Discover Magazine. v.14 (Dec. ’93) p.22

2. Garret, Chris. "Sea-level Flips: at the Strait of Gibraltar." Nature Magazine. v.348 (Nov. 22, ’90) p.292