The need for navies arose from the weakness of merchant ships and the corresponding necessity for assuring them safe voyages and proper treatment even in time of peace; while in time of war they have always been an anxious care, and have needed and received the protection of fighting ships that have been taken away from the fleet to act as convoys.
If commercial sea power was not the power meant by Mahan, then he must have meant naval power.

And if one reads the pages of history with patient discrimination, the conviction must grow on him that what really constituted the sea power which had so great an influence on history, was _naval_ power; not the power of simply ships upon the sea, but the power of a navy composed of ships able to fight, manned by men trained to fight, under the command of captains skilled to fight, and led by admirals determined to fight.

Trafalgar was not won by the merchant service; nor Mobile, Manila, or Tsushima.
If sea power be essentially naval power, it may be interesting to inquire: In what does naval power consist and what are its principal characteristics?
If one looks at a fleet of war-ships on the sea, he will be impressed consciously or unconsciously with the idea of power.

If he is impressed consciously, he will see that the fleet represents power in the broadest sense--power active and power passive; power to do and power to endure; power to exert force and power to resist it.
If he goes further and analyzes the reasons for this impression of power, he will see that it is not merely a mental suggestion, but a realization of the actual existence of tremendous mechanical power, under complete direction and control.
In mechanics we get a definition of power, which, like all definitions in mechanics, is clear, definite, and correct.

In mechanics, power is the rate at which mechanical work is performed.