Navies undoubtedly will play important roles in the 21st century. These roles will differ for various navies, and the numbers of ships, submarines, and aircraft and their capabilities will be important, but there will be a need for a more meaningful criteria for measuring the effectiveness of major fleets.
The principal criteria for measuring warships from the 1400s onward was firepower—how many guns, how many decks of guns, and the weight of their shot. By the late 1700s—the era of Nelson—ships-of-the-line mounted 100 guns or more. With the introduction of steam propulsion and steel to warship construction in the 1800s, however, ships were able to mount rifled guns in rotating turrets. The new centerline turret with two large guns was able to challenge successfully the broadside batteries of warships with larger numbers of small guns. Thus, gun caliber or bore diameter became the principal measurement of major warships.