Kdjykuj

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!

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.

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.

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!