Technically, this is a raised fore deck, rather than a lowered aft-deck. Called a 'forecastle', it was typically added to ship designs for two reasons: to improve seakeeping and to add accommodation space. Increasing the height of the forecastle helps a ship move through 'head' seas, where the ship is moving into the direction of the waves. Early British destroyer designs had a 'turtleback' forecastle (seen here); this tended to dig in to head seas, making the ships very wet and slow in heavy weather. Succeeding designs had raised forecastles, making them much more seaworthy in bad weather. Raising the forecastle adds more space, which was generally used for accommodation. However, there are flaws with it. When ships steam across waves, they experience both 'hogging', where a wave crest is amidships, and 'sagging', where there are two wave crest at each end. These motions create stresses, because some parts of the ship are effectively unsupported. Having a sudden change in the depth of the ship concentrates these stresses; the 'break' of the forecastle is just such a change in depth. This creates a considerable weak point in the ship; many British ships with such features suffered cracking as a result. It could also exacerbate the effects of battle damage. In 1939, HMS Belfast broke her back when she struck a mine, with the break coming at the end of the forecastle. Similarly, in testing after the end of the war, the German destroyer Z38 (in British hands as HMS Nonsuch), would be sunk by a charge that she should theoretically have survived, as poor welding and poor calculations of the stresses caused her to break apart at the break of the forecastle. To avoid this, strengthening could be added, but this was heavy. Following WWII, during the design of the Tribal-class frigates in the 1950s, the Royal Navy calculated that the weight of the strengthening was actually greater than simply extending the forecastle deck all the way to the stern. As a result, the RN moved towards 'flush deck' designs, where there was no sharp break in the deck level. Instead, it was kept at the same level as the old forecastles throughout the length of the ship.