Here are 6 closeup pictures of the ROK navy ship [Photo Gallery: The Wreckage of the Cheonan] from Der Spiegel.
DPRK operates two classes of submarines, the Larger Romeo class and the midget submarine Yugo class. Specs are from WIKIPEDIA.
Romeo Class submarine Displacement:
1,475 tons surfaced 1,830 tons submerged Length: 76.6 m (251 ft 3 in) Beam: 6.7 m (22 ft) Draught: 5.2 m (17 ft 1 in) Propulsion: Two diesels delivering 2.94 MW (4000 shp) with two electric motors driving two shafts. Speed: 15.2 knots surfaced 13 knots submergedThe Romeo is too big and too noisy to operate in the shallow waters,
45 metres depth, where the Cheonan was hit.Yugo Class submarine Type: midget submarine Displacement: 90 tons (submerged) Length: 20m Beam: 2m Propulsion: Single-shaft MTU diesel engine Speed: 10 knots (19 km/h) surfaced
4 knots (7.4 km/h) submerged Range: Unable to reach the southern extreme of South Korea Capacity: 4-6 Special forces troops Armament: Some armed with two 21-inch (530 mm) torpedoes, possibly in drop collars. Notes: First successful indigenous midget submarineA sub at 20m length is very chancy operating and staying submerged in
45m water and still remain undetected or, once detected, to escape. Its too small to house internal torpedoes and having external drop torpedoes makes noisy maneuvering. In any case at 4 knots submerged its only good for ambush and useless for pursuit.The damage to the hull is very interesting. A below hull non contact explosion crates a big air bubble that expands as it rises. This lifts the whole ship up snapping the hull or at least bend the plates or bend the prop shafts, shake up the engines and sensitive electronic equipment. The Cheonan snapped in half. Shouldn't the superstructure plates be drawn into the middle instead of being spalyed outwards?