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HMS Portisham
Class overview
Operators	Royal Navy  Royal Australian Navy  French Navy  Ghana Navy  Libyan Navy  Indian Navy  Royal Malaysian Navy  Royal Saudi Navy  Yugoslav Navy
Built	1954–1959
Completed	93
General characteristics
Type	Minesweeper
Displacement	120 long tons (122 t) standard 164 long tons (167 t) full load
Length	100 ft (30 m) p/p 106 ft 6 in (32.46 m) o/a, except third sub-group 107 ft 6 in (32.77 m)
Beam	26-group: 21 ft 4 in (6.50 m) 27-group: 22 ft (6.7 m)
Draught	26-group: 5 ft 6 in (1.68 m) 27-group: 5 ft 9 in (1.75 m)
Propulsion	2 shaft Paxman 12YHAXM diesels 1,100 bhp (820 kW)
Speed	14 knots (26 km/h; 16 mph)
Range	15 tons diesel fuel, ?
Complement	2 officers, 13 ratings
Armament	1 × Bofors 40 mm L/60 gun or Oerlikon 20 mm cannon

The Ham class was a class of inshore minesweepers (IMS), known as the Type 1, of the British Royal Navy. The class was designed to operate in the shallow water of rivers and estuaries. All of the ships in the class are named for British place names that end with -"ham". The parent firm that was responsible for supervising construction was Samuel White of Cowes, Isle of Wight.

The class consisted of 93 ships, launched between 1954 and 1959. HMS Inglesham was the first. They were built in three slightly different sub-groups, the first sub-group, the 26-group, is distinguished by pennant numbers 26xx, and the second and third sub-groups, the 27-group, are distinguished by pennant numbers 27xx. The 26-group was of wood and non-ferrous metal composite construction and the 27-group was of all-wood construction. The third sub-group is distinguished by a prominent rubbing strake around the hull and slightly larger dimensions.

Unlike traditional minesweepers, they were not equipped for sweeping moored or magnetic mines. Their work was to locate individual mines and neutralise them. This was a then-new role, and the class was configured for working in the shallow water of rivers, estuaries and shipping channels.

The vessels displaced 164 long tons (167 t) fully laden and were armed with one 40 mm Bofors or 20 mm Oerlikon gun. They were 32.5 metres (107 ft) long overall with a 6.4-metre (21 ft) beam. The construction was of wood to minimise the magnetic signature. The crew complement was 15, rising to 22 in wartime.

The engines of this class were Paxman diesels, some of which were built under licence by Ruston and Hornsby of Lincoln. Each vessel had: two 12YHAXM (intercooled) for main propulsion, rated at 550 bhp (410 kW) at 1,000 rpm, plus one 12YHAZ for pulse generation. Maximum speed was 14 knots (26 km/h; 16 mph) dropping to 9 knots (17 km/h; 10 mph) when mine hunting.

The class shared the same basic hull as the Ley-class minehunter and the Echo-class inshore survey craft.

In 1964 Ten of the vessels were allocated to the Royal Naval Auxiliary Service

• Warships of the Royal Navy, Captain John. E. Moore RN, Jane's Publishing, 1979