G and H-class destroyer

The G- and H-class destroyers were a class of twenty-four destroyers of the Royal Navy (two later transferred to the Royal Canadian Navy and one to the Polish Navy) launched in 1935–1939. They served in World War II and sixteen were lost, with a seventeenth being written off as a constructive total loss. Other ships were built for the navies of Argentina, Brazil, and Greece.

G class
The G class were ordered as part of the 1933 naval construction programme, the H class following in 1934. They were generally repeats of the preceding F class, with a slight reduction in dimensions by the reduction of cruising turbines. Armament in the Gs was same as the Fs, based on the 4.7 inch QF Mark IX as the main gun. The 40° elevation of the guns in the CP (central pivot) Mark XVII mounting was achieved by building a lowered section of the deck, the "well", that allowed the breech of the gun to be lowered below deck height. Glowworm trialled the new quintuple PR Mk. I torpedo tubes, her sisters retaining the quadruple Mk.VIII. All G-class ships had tripod foremasts and pole mainmasts.

H class
The H class were repeats of the Gs, but a more satisfactory solution was found to achieve the gun elevation, by rearranging the breech end of the guns, the mountings CP Mk.XVIII could elevate to 40° without the need for the deck wells. Despite the availability of the quintuple tubes as trialled in Glowworm, these were not fitted due to topweight concerns. Design improvements and the increased use of welding resulted in a decrease in displacement of some 50 tons.

Hero and Hereward saw the introduction of a new style of bridge that would become standard on all Royal Navy fleet destroyers from the I class through to the Battle class of 1944. This was necessary as Hereward carried a prototype twin 4.7 in gun mounting CP Mark XIX that was to be fitted to the Tribal and the J, K and N classes. This weapon had a trunnion height 13 inches higher than the previous weapons, therefore it was necessary to raise the wheelhouse to allow the helmsman to see over the top. Raising the wheelhouse meant it had to be placed in front of, rather than underneath, the bridge, and it was given sloping, armoured faces, resulting in a characteristic wedge shape, with a sloping roof to give the bridge a view over the fo'c'sle. Internally, the H class were as per the G class, except Hyperion, which received a single Johnson type boiler that was both smaller, lighter, and more economical than the Admiralty design. All ships had pole masts fore and aft and were fitted with TSDS (Two Speed Destroyer Sweeps) minesweeping gear.

Flotilla leaders
As per the E and F class, the flotilla leaders were built to an enlarged design, incorporating a fifth 4.7 inch gun in 'Q' position, between the funnels. They were based on the F-class leader, Faulknor. Grenville was slightly shorter as she used compact Yarrow-type side fired boilers. Hardy could be identified by having a tripod foremast. Both ships were early wartime losses and consequently received no modifications.

Havant class
The Havants were laid down in 1938 for Brazil. They had pole masts forward and tripods aft, were completed without 'Y' gun on the quarterdeck, allowing an increase in depth charge stowage, and with the funnels cut down to improve the field of fire for A/A weapons. They were completed with rangefinders only, but later shipped the designed combined rangefinder-director on the bridge, as opposed to the separated functions of their half sisters. After commissioning into the Royal Navy, Handy and Hearty were renamed Harvester and Hesperus respectively to avoid confusion with Hardy. The six Havant-class destroyers initially formed the 9th Destroyer Flotilla of the Home Fleet assigned to anti-submarine protection of Scapa Flow.

In late 1940, 9th Destroyer Flotilla was transferred to the Western Approaches Command and re-designated 9th Escort Group. In March 1942, the remaining five Havant-class destroyers were designated group leaders of the Mid-Ocean Escort Force through the winter of 1942-43. HMS Hurricane was leader of Escort Group B-1 for convoys HX-187, ONS-96, HX-193, ONS-108, SC-92, ON119/HX-201, ONS-124, HX-206, ONS-134, SC-105, HX-215, ON-151, SC-114, ON-162, SC-119, ONS-171, HX-230, ON-178 and HX-236. HMS Hesperus was leader of Escort Group B-2 for convoys SC-81, ON-97, SC-86, ON-107, HX-198, ONS-118, HX-203, ONS-128, HX-208, ONS-138, HX213, ONS-148, HX219, ON-159, SC-118, ON-170, SC-123, ONS-4, SC-129 and ONS-9.

HMS Harvester was leader of Escort Group B-3 for convoys HX-188, ONS-98, HX-194, ONS-110, SC-93, ON-121/HX-202, ONS-126, HX-207, ONS-136, SC-106, ONS-146, HX-218, ON-157, SC-117, ONS-167, and was sunk while escorting HX-228.

HMS Highlander was leader of Escort Group B-4 for convoys SC-82, ON-99, SC-87, ON-109, HX-199, ONS-120, HX-204, ONS-130, HX-209, ONS-140, HX-214, ONS-150, HX-220, ON-161, ONS-169, HX-229, ON-176, HX-234, and ON-183. HMS Havelock was leader of Escort Group B-5 in the Caribbean until escorting convoys ON-168, SC-122, SC-126 and ONS-7.

Havant class
These six ships were ordered by the Brazilian Navy but on the outbreak of World War II they were requisitioned by the Royal Navy. They are usually included with the H class.

Argentine Navy
Seven ships were built for the Argentine Navy as the Buenos Aires class, they were delivered in 1938. They were built by Vickers Armstrongs (Barrow), Cammell Laird and John Brown & Company (Clydebank).

Brazilian Navy
Brazil ordered six Jarua-class ships from Britain in 1938. These ships were purchased by Britain on the outbreak of war in 1939 and are described above. The Brazilians decided to produce indigenous destroyers, the Acre-class destroyer, at the Ilha das Cobras shipyard, Rio de Janeiro. The design was based on the H-class plans supplied by Britain, but with guns and machinery supplied by the USA. Although laid down in 1940, the ships were not completed until 1949–51.

Royal Hellenic Navy
Two ships, modified versions of the G class, were built for the Greek Royal Hellenic Navy by Yarrow. The ships were fitted with German-made 12.7 cm SK C/34 naval guns and 37 mm AA guns. The installation of the armament was carried out in Greece as the Germans refused to ship the weapons to Britain. Two further ships, the Vasilefs Konstantinos and Vasilissa Sofia, named after King Constantine I and Queen Sofia respectively, were to be built in Greece, but construction halted due to the outbreak of the Second World War.
 * Vasilefs Georgios: Named after King George I, she served with the RHN during the Greco-Italian War. Damaged by German aircraft, she managed to reach the Salamis Navy Yard and was put in drydock for repairs, where after further damage during German air attacks, she was finally scuttled to prevent capture. The Germans raised and repaired her and she was commissioned into the Kriegsmarine as Hermes (ZG3) on 21 March 1942. Hermes was heavily damaged off Cape Bon on 30 April 1943 and scuttled on 7 May 1943.
 * Vasilissa Olga: Named after Queen Olga, she served with the RHN during the Greco-Italian War. Along with other ships, escaped to Alexandria in May 1941 and joined the Allied forces. She was lost to German aircraft while anchored in Lakki Bay, Leros on 26 September 1943.