The hull below the waterline, along the main armour belt, formed the Side Protection System (SPS). It was subdivided into a series of longitudinal compartments in a void-liquid-void layout; the outer and inner were filled with air, and the middle compartment with liquid (fuel or water). The outer hull plating in the region of the SPS was thin to reduce potential splinter damage from a torpedo. The outer compartment of the SPS was normally an empty or "void" space (containing only air) and this allowed the initial explosion from a torpedo to expand while minimizing damage to the ship. The centre compartment was filled with oil or seawater and this spread the pressure pulse over a larger area while the liquid contained any metal splinters that were created from the torpedo explosion. The inboard compartment was another void space and served to contain any liquid leaking from the liquid layer and any remaining pressure pulse from the torpedo explosion. Inboard of the final void space was an armoured bulkhead which varied in thickness from 1.5 in (37 mm) over the machinery spaces to 1.75 inch (44 mm) abreast of the magazines. This bulkhead formed the "holding bulkhead" and it was designed to resist the residual blast effects from the torpedo hit. If this final inner bulkhead was penetrated a further set of subdivided compartments would contain any leaks; inboard of the holding bulkhead the ship was highly subdivided into small compartments containing auxiliary machinery spaces.