This also made it more difficult to board by force, as the ships would come to contact at their widest points, with the decks some distance apart. The narrowing of the hull above this point made the boat more stable by decreasing the weight above the waterline, which is one of the reasons it remained common during the age of sail. In addition, the sloping sides of a ship with an extreme tumblehome (45 degrees or more) increased the effective thickness of the hull versus flat horizontal trajectory gunfire (a straight line through a sloped hull faced more material to penetrate) and increased the likelihood of a shell striking the hull being deflected—much the same reasons that later tank armour was sloped.