The submarine was designed to accomplish a clear and definite purpose--a secret under-water attack on an enemy's ship in the vicinity.

It has succeeded so well in its limited mission that some intelligent people declare that we need submarines only--ignoring the fact that, even if submarines could successfully prevent actual invasion, they could not carry on operations at a distance from their base of supplies.

It is true that submarines may be made so large that they can steam at great speed from place to place, as capital ships steam now, carry large supplies of fuel and food, house their crews hygienically, and need no "mother ship" or tender.

But if submarines achieve such size, they will be more expensive to build and run than battleships--and will be, in fact, submersible battleships.

In other words, the submarine cannot displace the battleship, but may be developed and evolved into a new and highly specialized type of battleship.
The necessity for operating at long distances from a base carries with it the necessity for supplying more fuel than even a battleship can carry; and this means that colliers must be provided.