USS carrier "sunk" by a single Swedish diesel sub

How could a single diesel sub (Swedish some more) manage to sneak in and “fire” several torpedoes into the side of USS Ronald Reagan, “sinking” it?:

Clever strategy! I suppose that this is a real problem/threat to surface vessels such as carriers which are a hard to miss target. Thinking about it there probably exists a rather large thermal footprint of such a sub. However, infrared does not propagate very well under water. What does propagate very well is sound, the slightest can be detected at large distances. Perhaps there exists a unique acoustic footprint of such a sub. Interesting. On the other hand why not ping around because the enemy will know where you are anyway, that is a carrier with a fleet of support vessels such as destroyers that cannot remain undetected.

Not always effective due to submarines staying beneath “the layer”:

From https://en.wikipedia.org/wiki/Thermocline

“In the open ocean, the thermocline is characterized by a negative sound speed gradient, making the thermocline important in submarine warfare because it can reflect active sonar and other acoustic signals. This stems from a discontinuity in the acoustic impedance of water created by the sudden change in density.”

Plus a carrier battle group cruising along at 25+ knots is making a lot of noise themselves which hampers their ability to accurately use sonar.

The Swedes are obviously better at hiding their subs than to discover others:

The Norwegian navy did an exercise to test their ability to detect submarines hiding under thermoclines in the Sognefjord some years back.
In this deep fjord there are also layers of water with different salinity that distort acoustics.

They attempted to detect the known submarine hiding in the fjord without luck, until they invited some fishing skippers on board.
They are used to detect schools of caplin in the Barents Sea in similar conditions and had no problem spotting the sub by sonar.

PS> The best skipper claim that they can not only spot the schools, but determine whether it is mostly females with roe. (What they cannot detect is whether the roe is mature enough for the Japanese market)

this is real, but its pretty easy to hear through the thermocline with a little bit of data processing. Marine geophysicists hardly even think about the thermocline when they’re imaging through the water, its done in the background in real time once its set up. I’m not a submariner, but the tech that we use in seismic originates with submarine warfare, so I don’t understand why they would have technical difficulties that we don’t seem to have. We calibrate weekly or so with a T/S Dip, which is a small boat mission to lower a sensor to get the temperature/salinity profile. Its just a can on a rope, I guess the military could do something more sophisticated.

Very possible this is not a thing now…it’s been 15 years since I was on a boat…

The T/S dip is one way of predicting sound propagation in a water column that is quick and convenient for seismic purposes. When surveying a velocimeter is generally employed. This emits a sound pulse which reflected back to a receiver. The speed of sound is recorded over the known distance. The system requires a high speed winch with the electrical wiring inside the winch cable and up to 15,000 metres of wire on the winch.
Submarines carefully monitor the sea temperature and take advantage of any thermocline.
In the early days we used a bathythermograph which was a torpedo like object about 4 or 5 feet long operated from a winch at the stern of a frigate.
Temperature against depth was scratched on a piece of smoked glass and a slide viewer was used to determine the results.
I did some work for NOAA where we used expendable bathythermographs which sink at a known depth and the temperature is recoded on a graphic display of depth against temperature.
The results were forwarded to NOAA by a dedicated satC.
This more modern unit is used by surface warships to detect thermoclines

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