A shaft tunnel is a waterproof compartment, encasing a vessels shafts. In most modern vessels with aft machinery arrangements, or where diesel-electric installations limit shafting requirements, shaft tunnels are not needed. For vessels with midships or fore machinery arrangements where drive shafts would, by necessity, pass through multiple subdivision bulkheads, shaft tunnels will be used. They will be sized so as to allow access for bearing inspections, dissassembly, etc. Traditionally, they had an inverted horse shoe shape.