The previous Valasca class layout was kept but a few changes were made. The open 4" mountings were replaced with twin 4" turrets. The new turrets featured faster loading and loading at higher angles for better use in the AA mode. This was a major advance for the next three destroyer classes.
The last Hylla class was laid down in late 1939 and completed in 1941. There were supposed to have been a further four laid down, but these were replaced with a War Emergency destroyer class. Smaller and cheaper to build. The Hylla class were mixed with Iphito class cruisers to form Squadrons. The Squadron would then be assigned duties as needed.
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Late 1944 and the Bellona (last completed of the class) was taken in hand as the trials ship for the new twin 3" automatic weapon system and the new 24" Rocket Assisted Torpedo System (RATS) which could be used against surface or subsurface targets, The 24" torpedoes had a target homing system that allowed them to home on acoustic sounds from the propeller cavitation noise. The rocket assistance part got the torpedo section closer to the target, faster. It would be visible to surface targets as it arrived, then entered the water. The work was completed in June 1945 and the ship was sent to Okinawa where the systems could be live fired. The 3" performed well, the greater knockdown power shot down the Japanese Kamikaze aircraft where the 40mm just knocked bits off them. The 3" had new, more precise, radar predictors mounted on either side of the funnel. The RATS system had its own surface radar set between the mountings. A new, more powerful. Asdic unit was mounted under the bows to detect submarines as distant as possible. Then the rocket/torpedo combo could be fired out to as close as possible to the contact so that when the torpedo entered the water it would only have a few hundred yards to run. Hopefully catching the submarine unawares. Unfortunately the live firing trials off Okinawa provided no targets for the RATS.
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Eleven survived the war. The one lost was sunk by aerial torpedo in the Barents Sea, escorting a Russian bound convoy. The oldest four were offered for sale in 1948, and they were purchased by the new Israeli State. Four more were sold during the 1950's to South American customers. The last three (including the Bellona) were kept and were converted to Rapid Frigates with updated equipment. The Frigates were discarded and scrapped in the early 1970's.
