If a mage with the ability to generate heat, used this to heat the water around an ocean vessel, would they be able to do it in such a way that propels the ship? I was thinking something similar to the gulf stream.


  • A Mage able to Generate large amounts of heat nearby, up to a hundred feet or so beneath the ship
  • Said mage can also manipulate heat, move it, and make an area colder
  • The ship in question is of any "medieval" size, within the range of a small rowboat to a large Cog etc.

I'm also specifically asking about in an ocean setting, but input on using this on rivers, or other bodies of water would be appreciated.

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    $\begingroup$ A mage capable of generating lots of heat up to a hundred feet beneath a medieval ship would leave the vessel in hot water. This wouldn't create a current. Useful for ships in cold latitudes. The mage-powered heat needs to be put to work in an engine of some kind. $\endgroup$ – a4android Feb 3 '18 at 11:42
  • $\begingroup$ What if they could cause differences in temperature in the water (cooling an area while heating another) like you said below? Is there no way to use this to create forward momentum? $\endgroup$ – Heron Rain Feb 3 '18 at 19:49
  • $\begingroup$ Forward momentum might be created created by going to extremes. The mage freezes a mass of seawater on the aft of the vessel. The seawater behind & below the ice-mass is overheated instantaneously to cause an explosive eruption of steam. And does so repeatedly. The ice is a thermal buffer and momentum absorber. A spectacularly dangerous way to propel a ship. $\endgroup$ – a4android Feb 4 '18 at 3:28
  • $\begingroup$ An alternative method would be if the mage heated a mass of air. The hot air could fill a traditional hot-air balloon would could then act as a sail to pull the vessel before the prevailing winds. This does not involve ocean currents & depends on the technical ability of medieval shipwrights to fabricate a suitable balloon. Doing this should be withing the capacity of your mage. Sorry if this is not what you're looking for. $\endgroup$ – a4android Feb 4 '18 at 3:31

Run a set of pipes from underthe stern onto the deck and coil them to make a heat exchanger, then back into the water (Piot steam jet or Piot heat engine). You have a mage powered steam jet boat, with nothing newer than bronze age technology.

Wikipedia has a good introduction to the physics that make it work.



  • $\begingroup$ Has this process ever been scaled up that you know of? I can't seem to find any examples of this process larger than a toy. $\endgroup$ – Heron Rain Feb 3 '18 at 5:04
  • $\begingroup$ It's not as efficient as a screw propeller, which had been invented by the time this toy was invented, so there has never been a reason to scale it up. A process piping engineer would be the right person to answer definitively. $\endgroup$ – pojo-guy Feb 3 '18 at 13:45
  • $\begingroup$ I take it back ... the V1 engine of 1944 was a scaled up Piot engine. The overriding principle is that you must have sufficient back pressure to maintain a siphon on the cold (water intake) side. Once the stream is released on the hot side, a negative pressure is available in the boiler section to pull water in for the next cycle. Suction can only lift water a short distance (14 feet) so the boiler must be close to the water. Introducing a simple flapper valve between the cold piping and the boiler to ensure one way flow of water would improve the efficiency of the engine. $\endgroup$ – pojo-guy Feb 3 '18 at 14:23
  • $\begingroup$ +1 for things I learned today ... also here is a vid of someone scaling it up youtube.com/watch?v=nVBk7v6sn90 $\endgroup$ – CaffeineAddiction Feb 4 '18 at 5:33

Possibly a fire mage could make a current by doing two things simultaneously. Firstly, cooling the near surface water in front of the vessel to close to freezing. Four degrees centigrade for preference, because is at its densest. Secondly, heating a mass of water at depth behind the vessel. This will cause the hotter, less dense water to rise.

The dual action will have sea water sinking in front the vessel and while sea water rises aft. This might create an action similar to that of an ocean current. A sort of Fire Mage powered bootstrap mechanism.


Water currents - eh.

Air currents, absolutely. Manipulating air temperature also manipulates air pressure: aka wind. A constant warmer + lower pressure zone of air would keep the sails full.


There are two specific problems that I see here- one is range and the other is energy.


Ocean currents are big. Like, ocean-size big. The Gulf Stream is about 2,000 miles long and 60 miles wide. If you want truly want to create an ocean current,your range needs to be in the hundreds of miles, not hundreds of feet.


Ocean currents also transfer huge amounts of energy across the planet. An ocean current like the Gulf Stream transports 32 sverdrup of water from the equator to the poles, powered by a thermal gradient of maybe 20 degrees C. Given that the specific heat of the water is ~1 calorie per cubic centimeter, we're talking about

$$\frac{32*10^6 m^3\ \text{water}}{second}*\frac{10^6\ cm^3}{1\ m^3}*\frac{1\ calorie}{1\ cm^3*1\ degree}*20\ degrees = 6.4*10^{14}\frac{calories}{second}$$

That's 640 trillion calories per second that powers the Gulf stream. In other terms, that's the energy released by a superbolide meteor, the power output of a hurricane, or ten times the energy of the nuclear bomb dropped on Hiroshima. Your mage would also have to consume about a billion Big Macs per second, which is slightly above the average production rate of 17.

Fear the ocean. It is not to be trifled with.

  • $\begingroup$ I didnt mean recreate the gulf stream, just recreate the process that drives it locally. You say the gulf stream is driven by a thermal gradient of 20 degrees C, so could the mage create that gradient on a smaller scale to move ONE ship? $\endgroup$ – Heron Rain Feb 4 '18 at 5:22

How strong is their ability to generate heat? If it's strong enough, they could boil seawater in a holding tank in a way that causes it to jet out a tube, generating thrust and pushing the vessel forward. It's the same idea as those water jetpack things.

  • $\begingroup$ strong enough to boil water, but I was meaning more along the lines of creating something like a warm water current, like the gulf stream, and not so much something like an engine. $\endgroup$ – Heron Rain Feb 3 '18 at 21:00

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