By Lieutenant E E Goulaeff, Imperial Russian Navy. Published in 1876
One of the most important conditions for the defence of the Black Sea coasts limited the draught of the vessels to about 13 feet. The only then existing type of vessel which fulfilled this condition was that of unarmoured gunboats, since any armoured ship, if built of ordinary form, and if designed to carry heavy guns and thick armour, required much great immersion. Unarmoured gun boats, however, were not considered efficient, because a single shot from an insignificant gun is sufficient to penetrate their sides and sink them with all hands on board. Therefore an entirely new class of vessel was requisite to admit of the heavy guns and efficient armour protection, combined with the very small draught of water. To satisfy those conditions, no type of vessel could have been better adopted than the circular, because with no hull of any other form and of the same weight could so great a displacement upon the same draught of water have been obtained.
The head of the Navy, His Imperial Highness the Grand Duke Constantine, with his usual wisdom, appreciated the value of such considerations, and accordingly two circular ships of 96 feet in diameter, and 12 feet 6 inches draught, were ordered to be built. The displacement which such small dimensions gave has yet enabled them to carry two 28-ton guns mounted in a turret, protected, as well as the sides at the water-line, by armour equivalent to 11 inches in thickness. They were meant strictly for coast defence purposes, and therefore, no great speed was required from them.
They have open turrets, because being intended for the defence of certain narrow straits and entrances they can, when in action, occupy positions behind some defensive protection, such as submerged torpedoes; and, when attacked, can choose their own distance from the enemy, placing themselves always beyond the reach of rifle-fire. The precision of firing from guns so mounted, and other reasons, have settled the adoption of open turrets on board these circular ships.