Likely a sudden gust of wind blew her off her line at the worst possible time. Something Gills Bay is notorious for.
Ships watertight integrity was not compromised by that gash which was well above the waterline and for'ard of the watertight seal which on that ship is built into the back of the visor.
I should also add the fact that if you look at the footage at the end, showing her backing into SMH, if you look at the way the crew are moving ( rushing ) about, like an old silent movie, you will realise the film was speeded up at that point.
Stewart.
