What was the cause for the invention of submarine?

Answer 1

The NEED to sink ships without being sunk itself (the need to sink ships without being seen).

Prior to the sub, in order to sink an enemy vessel (other than my mines) was to "build another ship just like the enemies-and slug it out with them." To do battle; thus the "battleship."

But, battleships were very expensive and took many months (or years) to build; and they consumed a very large amount of material (resources). A nation could build dozens of subs from the material required to build just one battleship; and at less cost! Which is why Germany in both World Wars chose that route.

Then, after a country (nation) went through the trouble and finances of building a battleship, it took (in less there was a lucky shot) HUNDREDs of main gun shells to sink another battleship. Each one of those shells was equal to one aerial bomb dropped by an airplane; AGAIN, very expensive!

But a sub, with just one $1,000 to $6,000 dollar torpedo, could sink a battleship...and do it WITHOUT being seen (thus not struck by enemy shells). Although, in reality, it often took more than one torpedo, usually two or three ($2,000 to $12,000 to sink one battleship). But still, a lot cheaper than 100 shells (each worth about a grand).