Shaft drive has the benefit of delivering the lift inherent in a carefully considered shaft angle, while an inboard engine is easy to inspect regularly to head off problems. Inboard engines also allow simplified maintenance and all-around access, as well as being relatively protected from the elements.
When an Aspen Power Catamaran is configured with an inboard engine, a few different benefits come into play. Because of the proa design, only a single engine is required, in the larger, starboard hull. This reduces the need for another engine, saving the weight of the engine and reducing appendage drag by having only a single set of running gear (prop, shaft rudder and strut). Additional efficiencies are gained by eliminating the space required and construction weight needed to rig a second engine and by eliminating additional fuel capacity to run multiple engines. Inboard engines also simplify maintenance and are protected from the elements.