Farm Ship

Oceans cover some 75% of the earth's surface. Most of the oceans are, in biological terms, a vast desert. The nutrition for plant growth to start off the biological cycles has for the most part sunk to the bottom. Near the shores, though the opposite problem happens, as farm runoff arrives in the ocean, triggering a vast bloom of algae, which quickly turns into a suffocating oxygenless muck. The sun shines upon the ocean making rain, some of which powers the land-based photosynthesis with water, but most of which falls back into the ocean again. The availability of sun and water suddenly gave me an interesting idea. We take a discarded cargo ship, and convert the large surface area to a vast farm. Equipment is installed beneath the ship to suck up water and desalinate it, and have it bubble up beneath the dirt at the surface. The ship sails around the world, slowly growing tons and tons of food. Periodically the ship arrives at harbors where food prices are highest to sell the food, buy fertilizer, and change crews. (The laboring farmers would be mostly employees, and this system could hopefully offer some nice wages, enough for a plane ride back home every so often.)

If there's any runoff from this, there would be a temporary bloom running behind the ship's wake, but not severe enough to cause any sort of red tide or harmful eutrophication, and this causes a temporary fish spawning point. The benefits of ocean feeding occur provided the ship remains in motion at all times.

In addition, the ship could remain in constant spring or summer by constantly sailing back and forth between the northern and southern hemispheres, maintaining beneficial conditions for the crops at all times.

This would take on the large scale a considerable amount of energy. For best results, this should be done after fusion power is available, which would allow for the boat to operate pollution free. In practice, though, the boats would probably be coal fired, or diesel driven, with all the problems that attend that.

For feeding an increasingly hungry world, this might make at least a small difference.