Why did david bushnell invent the submarine?

Answer 1

Bushnell didn't invent the submarine - Bushnell was responsible for creating the first American submarine (the Turtle) and the first submarine used in combat, but long after others had designed and built a working model.

Credit for the invention of the submarine is given to William Bourne, a British mathematician and ex-Royal Navy gunner who published a design in 1578. The first navigable submarine for which reliable construction data exists was built from Bourne's design in 1620 by Cornelius Jacobszoon Drebbel, a Dutch inventor working for British King James I.

The first submarine credited with a wartime sinking is the Confederate Army vessel H.L. Hunley (it was never a commissioned Naval warship), which sank the Union blockade ship USS Housatonic in Charleston Harbor in 1864. Recent findings by those working on the Hunley (raised intact several years ago) shows that the crew didn't die from flooding damage caused by its Spar Torpedo as has long been believed, but rather by a lack of oxygen. Had they made it to shore, it likely would've altered the course of the war.