If the propeller shaft has to pass through any holds it has, of course, to be enclosed in a tunnel which must allow enough room for an engineer to walk along to inspect the shaft. This tunnel takes up considerable space in any hold, which is one argument for putting the engines right aft. Another is that it leaves the middle of the ship with its large square holds available for cargo. (Holds at the ends of the ship tend to be V-shaped.) There were two traditional reasons for having engines amidships. It was essential when ships were fitted with paddle wheels, and also vessels with the weight in the middle were on an even keel when completely empty and thus the minimum of ballast was required. With modern ships, adequate ballast is seldom a problem, as seawater can easily be pumped in as needed.