The HSY-55-class gunboat is a class of naval vessel designed by the Hellenic Navy and built by Hellenic Shipyards (HSY). This class of ship uses the modular concept so that weapons and sensors can be changed as required. These vessels are similar in appearance to Osprey 55-class gunboat. The first pair was ordered on 20 February 1990, but completion was delayed by the shipyard's financial problems. Pyrpolitis (P57) was launched on 16 September 1992, and Polemistis (P61) on 21 June 1993. Each ship can carry 25 fully equipped troops. Alternative guns and Harpoon SSM can be fitted as required.

Polemistis (P 61)
Class overview
Operators	Hellenic Navy
Built	1990–1993
In commission	1993–present
Completed	2
Active	2
General characteristics [1]
Type	Patrol boat
Displacement	595 t (586 long tons) full load
Length	56.5 m (185 ft 4 in)
Beam	10 m (32 ft 10 in)
Draught	2.7 m (8 ft 10 in)
Propulsion	2 × Wärtsilä Nohab 16V25 diesels, 9,200 hp (6.9 MW) sustained; 2 shafts
Speed	24 knots (44 km/h; 28 mph)
Range	2,470 nmi (4,570 km) at 15 kn (28 km/h; 17 mph) 900 nmi (1,700 km) at 24 kn (44 km/h; 28 mph)
Troops	23
Complement	36 (6 officers)
Sensors and processing systems	Thomson-CSF Triton; I-band surface search radars Selenia Elsag NA 21 weapons control
Armament	4 × McDonnell Douglas Harpoon SSM 1 × OTO Melara 3 in (76 mm)/62 compact gun 1 × Bofors 40 mm 2 × 20 mm guns 2 × Mine rails

Pyrpolitis was renamed Kasos on 11 November 2005, after the island of Kasos in the Dodecanese. The ship is based in the area of Kasos, and the name recognises the island and the Kasos Massacre during the Greek War of Independence.^[2]