In the movie it went for over half an hour. These kind of patrol boats would have sunk in minutes.
It would have been good if the movie had a stop watch running from he start of the battle and superimposed on every scene. it would have helped to realise how ferocious this battle was.
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I larf in you general direction.
A timer would have been hugely distracting.
At 10:25, the vessel that first crossed the line opened fire with its 85 mm gun and scored a direct hit on the wheelhouse...
...Now heavily outnumbered and taking casualties, the North Korean vessels retreated back across the Limit Line at 10:59.
So, first to last shot took around 34 minutes. So, the movie was pretty close to exactly real time it seems. Nice.
Bullets don't do a great job of penetrating water. Most damage to PKM357 was well above the waterline, and only a few low down that I assume caused enough flooding it caused the sinking when under tow some hours later.