When it came to wartime machines, Leonardo often seemed to look at both sides of the coin. He would invent offensive equipment and then he would design defensive equipment just in case the enemy had similar ideas to him. A good example of this is in the double-hulled and semi-submersible (sometimes termed Leonardo's submarine) boat designs.

The 'submarine' was simply a shell with room enough for one person to sit inside. It was topped with a conning tower which had a lid and pre-dated the true submarine by over one hundred years. Leonardo was to describe it as a "ship to sink another ship."

Leonardo considered that the best way to defend against underwater attack by ships similar in design to his 'submarine' was to have double-hulled boats. This would not only solve the problem of ramming, but also that of divers interfering with the vessel. By this time he had already devised a method by which divers could separate the planks of ships.

He considered how lost ships could be recovered, and designed air-filled tanks which divers could attach to the hulls of a sunken ships in an attempt to re-float them. And once your ship is once more on the surface you need to remove the excess water. Leonardo then designed a machine which would extract the water and then dry the holds of ships.

His designs included a one-man battleship, and in considering ships for wartime use Leonardo realised the importance of making guns easier to load and fire. This same drawing displays two remote-control guns with rapid-firing mechanisms.