Also known as Naysmyth's Submarine Mortar and the Steam Ram, the Anti-Invasion Floating Hammer was a semi-submerged naval ship design conceived and published by inventor James Nasmyth in 1853.

The walls of the mortar were 10 feet (3.0 m) thick, protecting it against potential enemy gunfire of that period.

[2] At the front of the boat was a hollow brass cap shaped like a ram that was 9 feet thick.

Inside the ram was a case filled with a heavy charge of explosive powder.

[3] Because of potential dangers associated with its means of attack, the Anti-Invasion Floating Mortar was never built.