a trunk deck ship is a type of merchant ship with a hull that was stepped inward in order to obtain more favourable treatment under canal toll rules then in effect. as those tolls were set by net tonnage, a measure of volume, and as the tonnage rules did not account for all of the cargo space of such vessels, trunk deck ships incurred lower tolls than more conventional ships of equivalent capacity. when the measurement rules were changed, the type was no longer built.