If people have been distilling spirits for the last thousand years, why did this concept never appear on ships for desalinization? Did they simply not make the mental jump?

One of my historical passions is ancient refrigeration technology (one of my highest rated posts in this sub is about ancient ice-making), and distillation is sort of related to it. Fair warning, this is more of an engineering question, but I sprinkled some ~~crack~~ history on it.

I once saw Michael Wood's The Story of India, in which he showed how Indians made ice using radiative transfer. This got me into a long hunt for ancient technologies related to this, such as refrigeration and condensation (water catching). The ancients were fairly advanced with respect to refrigeration and humidity control (air conditioning). The Persians were particularly adept at cooling technology with the use of the bagdir or wind towers, combined with the quanat or underground aqueduct (which acts as a heat exchange). They were able to store ice year round, even in ancient times, and were even able to produce ice using yakhchal ^1. Pot-in-pot evaporative cooling was used throughout the Indus valley, and the technology existed to create ice from wind-driven evaporative cooling, and radiative transfer (using a clear cold sky as a heat exchange) using hay as insulation as well ^2. While this may seem off topic, the reason I'm providing this context is to get across a point–the ancients were very aware of condensation, evaporation, radiative transfer, and similar physical processes that are related to distillation. So, as you point out, it was known (alcohol distillation in a vessel was recorded in 1CE) for quite some time, but these technologies simply lack the efficiency necessary to make them practical.

Seawater distillation not efficient enough to hydrate even a small crew of a boat using pre-industrial means.

A human needs approximately 2 liters of water per day. That doesn't account for heavy exertion or high temperatures (such as on the deck of a boat plying the Mediterranean Sea) either. Phoenician transport ships of 1200 BCE are called gauloi or 'round' ship. The gauloi were common throughout the Mediterranean as Pheonicians traded and moved goods around ^3. Typically the gauloi had a crew of approximately 20 men; that's about 40 liters a day, minimum. While boat technology changed greatly through the centuries, the size and crew of typical Mediterranean cargo ships is pretty similar, even today in some places.

The best modern multi-stage flash distillation technology requires 25 kWh per cubic meter of distilled water. So, it would take about one kWh to distill the crew's minimum water ration using the very best modern technology. An iron-age distillation vessel would be much smaller and much less efficient. It would not be efficient enough to use solar power; it would definitely require a fire. A fire can produce about 3 kWh. So, it would have required a typical cargo ship to carry a very large distillation vessel (boiler and condenser), made of some of the finest pre-industrial engineering available, and to have a rather sizable fire burning under it for hours at a time (on a very flammable ship). That's why the Phoenicians came up with a much simpler strategy: to go somewhere, take many short hops of a day or three in length, and at each stop, get more water.

By the 1800s, vessels did carry distillation equipment, in particular, when steam-power was adopted (as they go hand in hand). All modern navies use this technology. It would be great if there were a naval historian familiar with R. M. Rippon's work on the Royal Navy, because I believe he covers their long experimentation with freshwater condensation beginning in the 1770s with Jame's Cooks employment of an evaporator on the HMS Resolution.

(1)

I have many interesting sources about Persian cooling architecture, but I thought this account really brings to life that ice-making was a real thing for ancient Persians.

An Autumn Tour in Western Persia By Ella R. Durand

One year, in the beginning of 1895, we had skating in the open. The ice was more than a foot thick and like a deep green mirror. When we began, before the ice got cut up, you saw the reflections of the skaters gliding under them. General the sun is too hot to let ice form in the open, but any year one could get skating in the shade. The persons are very fond of ice in the summer, and to ge tit they what they call 'yakhchals'. These are long walls thirty or forty feet high, facing south. On the north side is dug a shallow trench, perhsps ten yards broad, and this is filled with water, which getting no sun freezes hard. Every fortnight or so the ice is broken up and stored in pits under the wall. By hiring a yakhchal for the winter it is possible to keep very fair ice.

(2)

The Process of Making Ice in the East Indies Sir Robert Barker, 1775

Following is the method that was used to make ice in India as it was performed at Allahabad and Calcutta. On a large open plain, 3 or 4 excavations were made, each about 30 feet square and two deep; the bottoms of which were strewed about eight inches or a foot thick with sugar-cane, or the stems of the large Indian corn dried. Upon this bed were placed in rows, near to each other, a number of small shallow, earthen pans for containing the water intended to be frozen. These are unglazed, scarce a quarter of an inch thick, about an inch and a quarter in depth, and made of an earth so porous, that it was visible, from the exterior part of the pans, the water had penetrated the whole substance. Towards the dusk of the evening, they were filled with soft water, which had been boiled, and then left in the afore-related situation. The ice-makers attended the pits usually before the sun was above the horizon, and collected in baskets what was frozen, by pouring the whole contents of the pans into them, and thereby retaining the ice, which was daily conveyed to the grand receptacle or place of preservation, prepared generally on some high dry situation, by sinking a pit of fourteen or fifteen feet deep, lined first with straw, and then with a coarse king of blanketing, where it is beat down with rammers, till at length its own accumulated cold again freezes and forms one solid mass. The mouth of the pit is well secured from the exterior air with straw and blankets, in the manner of the lining, and a thatched roof is thrown over the whole.

(3)

The Phoenicians By Sabatino Moscati

and

Herodotus 4.42:

So the Phoenicians set out from the Red Sea and sailed the southern sea; whenever autumn came they would put in and plant the land in whatever part of Libya they had reached, and there await the harvest; then, having gathered the crop, they sailed on, so that after two years had passed, it was in the third that they rounded the pillars of Heracles and came to Egypt.

It did through most of western history in fact. The problem was without modern technology it was an unreliable and difficult source to maintain a steady supply of water. Once your fuel source runs out you need to go get more and if you're somewhere with lots of trees there's a good chance you're somewhere with a source of freshwater and at that point might as well cut out the middle man. So while it was certainly used onboard ships dating back to antiquity it very rarely acted as the primary or only source of water. As far as actual examples of distillation we have Pliny the Elder saying this "Because those at sea often suffer from failure of fresh water, I shall describe ways of meeting this difficulty. If spread around a ship, fleeces become moist by adsorption of evaporated sea water, and from them can be squeezed water which is fresh. "

And another chronicler from 200 AD “sailors at sea boil sea water and suspend large sponges from the mouth of a bronze vessel to imbibe what is evaporated. In drawing this off the sponge, they find it to be sweet water" Both these describe a rudimentary form of distillation which is simply the process of heating the liquid to cause evaporation and the removal of unwanted compounds.

Later on in 1662 Sir Richard Hawkins reported that he was able to keep his men hydrated through an onboard distillation system.

And as one last example Thomas Jefferson as Secretary of State wrote a report on the advances and successes of ship board distillation methods and recommended that the distillation methods be printed on the back of all permits issued for sailing vessels. And congress passed that into law one year later in 1792. The full text of his report can be found here: https://founders.archives.gov/documents/Jefferson/01-22-02-0296

Edit: and as /u/skytomorrownow pointed out by the time coal and steam power rolled around it became very common place

Other sources used: http://www.desware.net/Sample-Chapters/D01/01-003.pdf https://www.nap.edu/read/12184/chapter/4