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I am having a discussion on why we rarely recover ships that have sunk, while we have repeatedly landed on the moon, which is also a tremendous engineering challenge.

The discussion boiled down to whether:

  • a) The engineering of salvaging shipwrecks is not more challenging than space exploration, but we simply don't put the money into it, or
  • b) That the attributes and aspects of the (deep) seas are more challenging, and it isn't purely a function of how much money is thrown at it- it is simply easier to go to space than to the bottom of the deep ocean.

Support for the cost-theory: The Apollo Program cost around \$280 billion inflation-adjusted to 2020[1,2], (meaning more than \$310b in 2025), whereas the most expensive maritime salvage operation of the Costa Concordia amounted to an inflation-adjusted mere \$1 billion, with the second-most-expensive trailing behind with less than $400m [3].

Support for the more-challenging-theory: Whenever the yet-unsuccessful search for Malaysia Airlines MH370 is discussed, experts emphasise just how limited our technologies are when it comes to mapping the ocean, explaining the limitations of our modern communication technologies. [4]

So I thought that looking at research budgets would be an approach, but I am not sure which specific areas would require funding for, say, sonar-based technology to advance or perhaps a completely different approach to be developed.

Perhaps this is the wrong SE for this question, but I had hoped to find someone from the maritime engineering perspective that can offer more insight.

Edit after reading comments: I am aware that salvaging for parts is not worth the cost. But there are plenty of ship/submarine-wrecks that have/had human remains on them, something people usually go to great lengths to recover for burial. Then there are the cases such as of the K-278, a nuclear submarine that is still at the bottom of the sea and has needed repeated visits and maintenance to ensure that no radioactive material leaks: "The wrecked submarine is on the floor of the Barents Sea, about 1.7 km (1 mile) deep, with her nuclear reactor and two nuclear warhead-armed torpedoes still on board." [5] Surely 35 years of repeated visits and monitoring by both the Soviet/Russian and Norwegian Agencies must be more expensive than lifiting the sub?

marts
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I'm an electrical engineer and know little about shipping - although I did learn my scuba diving theory. So ...

One answer is that the cost of salvage is greater than the value of the salvaged ship and therefore uneconomical. Exceptions will be where for environmental reasons, hazard to other shipping, beauty / tourism spots, etc., make it a requirement.

In shallow water a ship might be refloated using underwater parachutes filled with compressed air. These have the advantage that as the ship rises and the air in the parachutes expands due to decompression that it will self-vent around the edge of the parachute and bursting due to over-pressure is not likely to be a problem.

For air-assisted floatation 1 m3 of air will give a bouyancy of 1000 kg or 1 tonne (varying with salinity). The problem is that the pressure required will increase by 1 bar for every 10 m of water (again, varying with salinity). So the air pressure required will be $p = \frac {depth}{10}$ (bar). (That's gauge pressure which starts at atmospheric pressure.)

While that may seem easy enough, attachment of straps, slings, anchor points, etc., can be extremely difficult and control of the lift very challenging.

Transistor
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