Third-rate

In the British Royal Navy, a third rate was a ship of the line which from the 1720s mounted between 64 and 80 guns, typically built with two gun decks (thus the related term two-decker). Years of experience proved that the third rate ships embodied the best compromise between sailing ability (speed, handling), firepower, and cost. So, while first rates and second rates were both larger and more powerful, the third-rate ships were in a real sense the optimal configuration.