In most countries, the merchant service is so large that colliers can be taken from it, but in the United States no adequate merchant marine exists, and so it is found necessary to build navy colliers and have them in the fleet.

The necessity for continuously supplying food and ammunition to the fleet necessitates supply ships and ammunition ships; but the problem of supplying food and ammunition is not so difficult as that of supplying fuel, for the reason that they are consumed more slowly.
In order to take care of the sick and wounded, and prevent them from hampering the activities of the well, hospital ships are needed.
Hospital ships should, of course, be designed for that purpose before being constructed; but usually hospital ships were originally passenger ships, and were adapted to hospital uses later.
The menace of the destroyer--owing to the sea-worthiness which this type has now achieved, and to the great range which the torpedo has acquired--has brought about the necessity of providing external protection to the battleships; and this is supplied by a "screen" of cruisers and destroyers, whose duty is to keep enemy destroyers and (so far as is practicable) the submarines at a safe distance.
We now see why a fleet must be composed of various types of vessels.
At the present moment, the battleship is the primary, or paramount type, the others secondary, because the battleship is the type that can exert the most force, stand the hardest punishment, steam the farthest in all kinds of weather, and in general, serve her country the best.
Of course, "battleship" is merely a name, and some think not a very good name, to indicate a ship that can take the part in battle that used to be taken by the "ship of the line." The reason for its primacy is fundamental: its displacement or total weight--the same reason that assured the primacy of the ship of the line.

For displacement rules the waves; if "Britannia rules the waves," it is simply because Britannia has more displacement than any other Power.
The fleet needs to have a means of knowing where the enemy is, how many ships he has, what is their character, the direction in which they are steaming, and their speed.

To accomplish this purpose, "scouts" are needed--fast ships, that can steam far in all kinds of weather and send wireless messages across great distances.

So far as their scout duties go, such vessels need no guns whatever, and no torpedoes; but because the enemy will see the scout as soon as the scout sees the enemy, and because the enemy will try to drive away the scout by gun and torpedo fire, the scouts must be armed.