The vessels of the "Olympic" class also differed from "Lusitania" and "Mauretania" in the way in which they were compartmented below the waterline. The White Star vessels were divided by transverse watertight bulkheads. While "Lusitania" also had transverse bulkheads, it also had longitudinal bulkheads running along the ship on each side, between the boiler and engine rooms and the coal bunkers on the outside of the vessel. The British commission that had investigated the sinking of "Titanic" in 1912 heard testimony on the flooding of coal bunkers lying outside longitudinal bulkheads. Being of considerable length, when flooded, these could increase the ship's list and "make the lowering of the boats on the other side impracticable" — and this was precisely what later happened with "Lusitania". The ship's stability was insufficient for the bulkhead arrangement used: flooding of only three coal bunkers on one side could result in negative metacentric height. On the other hand, "Titanic" was given ample stability and sank with only a few degrees list, the design being such that there was very little risk of unequal flooding and possible capsize.