Leonardo da Vinci Sketches the Armored Vessel
This armoured assault vessel is probably one of the projects that Leonardo presented to Ludovico il Moro, “the Moor”, Duke of Milan, in order to obtain a commission as a military engineer.
The drawing on which it is based is for a light vessel fitted with a prow armoured with metal and used to ram the enemy. A rotating covering shield, which opens during the boarding phase of the attack, is also fitted.
In Leonardo's notebooks there is an array of war machines which includes a tank to be propelled by two men powering crank shafts. Although the drawing itself looks quite finished, the mechanics were apparently not fully developed because, if built as drawn, the tank, with a lot of effort, might be made to rotate on the spot, but would never progress in a forward direction. In a BBC documentary, a military team built the machine and found it not working, until they changed only one of the gears. It is believed that Da Vinci deliberately left this error in the design, in order to prevent it from being put to practice by unauthorized people.
Leonardo da Vinci: The Complete Works