On ships, propeller shaft has to pass through holds it and must be enclosed in a tunnel which is called Shaft Tunnel. Shaft Tunnel must allow enough room for an engineer to walk along to inspect the shaft. Shaft Tunnel takes up considerable space in any hold, which is one argument for putting the engines in the aft. Another argument is that Shaft Tunnel 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. In the old days, there were two traditional reasons for having engines amidships:
- It was essential when ships were fitted with paddle wheels
- Ships with the weight in the middle were on an even keel when completely empty and thus the minimum of ballast was require
With modern ships, adequate ballast is seldom a problem, as seawater can easily be pumped in as needed.