How blockade changes the threat to convoys

Earlier convoy systems were designed with enemy surface fleets in mind. Once blockade confines the opposing battle fleet, the more likely danger becomes a frigate-sized raider, privateer, or pirate able to slip out quietly or at night. Escort plans can then rely on a large frigate, brigs, and the merchants' own weapons rather than another battle fleet. Armed East Indiamen complicated an attacker's calculation because their two-deck appearance could suggest a fourth-rate warship was present. Buying and converting some East Indiamen into fourth rates multiplied that uncertainty, making every similar merchant hull more costly for a raider to approach.