 15 Canadian frigates were initially fitted with a single 4-inch gun forward but with the exception of , they were all eventually upgraded to the double mount.For underwater targets, the River-class frigate was equipped with a Hedgehog anti-submarine mortar and depth charge rails aft and four side-mounted throwers.River-class frigates were the first Royal Canadian Navy warships to carry the 147B Sword horizontal fan echo sonar transmitter in addition to the irregular ASDIC.