In a post-post-apocalyptic world with adequately schizophrenic tech, a tinkerer decides to build a submarine engine around a lump of radium, or some other reliable source of heat. But instead of using some complicated setup to drive a propeller, the engine simply evaporate water in a chamber, then uses the pressure to push water and/or vapour at the rear in order to create thrust.

Assuming there is no problem of evaporation residues or material strength, what would be the efficiency of such an engine? And assuming problems with evaporation residues, how long would the engine be expected to run before needing to scrape the chambre?

edit: For this question, assume stealth is not a problem. The goal is not to build a SSBN!

  • 2
    $\begingroup$ You do realize that a submarine leaving a trail of boiling, bubbling water behind isn't very well hidden, right? $\endgroup$
    – Elmy
    Apr 5, 2019 at 9:51
  • 1
    $\begingroup$ @Elmy On the other hand, think of the intimidation factor! :) $\endgroup$
    – Eth
    Apr 5, 2019 at 9:54
  • $\begingroup$ If you don't specify the "problems with evaporation", how can we estimate the mean time to trouble? $\endgroup$
    – L.Dutch
    Apr 5, 2019 at 9:56
  • 6
    $\begingroup$ @Eth Or the effect of having your opponents incapacitated because they're lying on the floor, laughing at the farting whale... $\endgroup$
    – Elmy
    Apr 5, 2019 at 9:58
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    $\begingroup$ "Residues": submarines and surface ships desalinate the water taken from the outside before putting it in the boiler. A regular nuclear submarine works by using a reliable source of heat to boil (that is, "evaporate") water and then use the vapor under high pressure to drive a turbine. This has the advantage that the vapor does not need to expand against external pressure, does not need to operate at insanely high tempeatures, allows for much greater efficiency, and works in a closed circuit so that the submarine does not need to have open pipes to the surrounding medium. $\endgroup$
    – AlexP
    Apr 5, 2019 at 10:03

3 Answers 3


the engine simply evaporate water in a chamber, then uses the pressure to push water and/or vapour at the rear in order to create thrust.

If you just pick water from the depth you are and evaporate it, you will not get any additional pressure. The vapor bubble will just last until it cools down and collapse. This will have a very low efficiency. It's called a pop boat engine, and you might have seen it in Studio Ghibli's Ponyo.

If you want to pressurize the water before evaporating it, you need to add a compressor. But at that point close the cycle and use a conventional Rankine cycle like it is done in all nuclear submarines. That will allow you to keep the advantage of submarines: their low visibility.

A tail of bubbling water is really a poor way to hide a ship. If you accept being visible, save the struggle of going under the water surface and stay above it.

  • $\begingroup$ Interesting, so what kind of efficiency can we expect? 10%? How does it vary with pressure? $\endgroup$
    – Eth
    Apr 5, 2019 at 10:42
  • $\begingroup$ @Eth if the pressure is too high you will not get steam no matter how much you heat the water. I suspect this would have to be just beneath the surface in order to work at all. $\endgroup$
    – N. Virgo
    Apr 5, 2019 at 17:09
  • $\begingroup$ To improve upon "pop boat", we don't need to pressurize - just provide a separate water intake. $\endgroup$
    – Alexander
    Apr 5, 2019 at 23:19
  • $\begingroup$ After looking at pop-pop boats, it looks like that's what it would actually be. With at best 0.1% efficiency at the surface, I think we can safely say the submarine, if it moves at all, will never be anything more than a toy. $\endgroup$
    – Eth
    Apr 7, 2019 at 20:06

This sounds like you would be creating a form of Pulsejet. Fill a chamber at rear of submarine with water, superheat it, then open the chamber for a 'pulse' as the water expands. Flush the chamber with cold water, close it, and repeat.

Unfortunately, this is likely to be slower, less stealthy, and more complicated than just having a basic, continuous, closed-cycle steam-engine turn a driveshaft, and a gearbox connecting that to your propeller.

  • $\begingroup$ The big advantage of the (air) pulsejet is that it is incredibly simple: in some cases, nothing more than a fun-shaped tube and a fuel injector. If this can be made to work like a pulsejet, wouldn't it be simpler than a conventional system, then? $\endgroup$
    – Eth
    Apr 5, 2019 at 10:44
  • $\begingroup$ @Eth Sure, if you were using fuel instead of superheated water - no need for the additional valves, et cetera. At which point, the "using evaporation" part of the question has been completely ignored $\endgroup$ Apr 5, 2019 at 10:51
  • $\begingroup$ Even if a pure valvless design is impossible, wouldn't a one-way valve still be simpler than a complete closed-cycle steam engine? $\endgroup$
    – Eth
    Apr 5, 2019 at 11:09
  • $\begingroup$ @Eth For the pulsejet, you will require at least 2 valves (in and out), a pump - and the "out" valve needs to be mechanically actuated, otherwise it opens before you have built up enough pressure for propulsion. A closed-cycle condensing steam engine requires 0 valves and 1 pump $\endgroup$ Apr 5, 2019 at 11:31
  • $\begingroup$ Even with two valves, how is that simpler than a closed circuit, a cooling heat exchanger, a turbine and a propeller? $\endgroup$
    – Eth
    Apr 5, 2019 at 13:56

Skip using vapor. If you are going nuclear, first separate hydrogen from the water, then heat the hydrogen. You will have incredibly higher pressure, and will have one-upped the world's space agencies by beating them to the first usable nuclear thermal rocket. Such rockets are very efficient in a vacuum - I don't know about their efficiency in water, but hey, as long as you can electrolyze water and your uranium lasts, you'll be able to keep going.

Provide enough thrust, and your submarine will not swim - it will fly underwater, by creating a bubble of vapor around it due to supercavitation. Your submarine will need wings. Seriously!


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