As a platform, Wirruwana is a break through design, integrating features appearing on my earlier vessels along with new ideas worked up from design studies. Converting my successful outboard designs into an inboard diesel vessel of any size is not trivial and has been the focus of considerable research. Initially I focused on fitting in a conventional shaft line while maintaining the hull shape and weight distribution which makes these vessels what they are. However one quickly gets mugged by the reality of fitting in a commercially practical shaft line.
Surface piecing propellers, from a mechanical point of view, offer a good solution; the engine is down the back of the boat, preserving the good characteristics of the design. The propeller shaft extends through the transom; the propeller operating in some supernatural half immersed manner at almost the same efficiency as a normal propeller. While not a new idea, the first successful vessel being built in 1914, surface piercing propellers consumed a single paragraph of my naval architectural textbook on propeller design. There is still an inadequate amount of material available for practical design. However what hard data that is available has a little explored corner of the useful design space where, my catamarans could live well.
The tender requirements were to say the least challenging, a low-power, high speed landing craft, less than 12 m, required to carry a 10 t truck and various bits of unspecified machinery on and off unprepared beaches. This last requirement was met with a sliding trunnion ramp design which extends the length of the ramp. The ramp is controlled by the C-zone electrical system which allows a simple rocker switch to coordinate the pump and valving required operating the ramp. Survey requirements included a drencher system the vehicle deck.
When launched, the vessel made all of 6 kn causing angst for all involved. The most efficient operating points for surface piecing propellers are often perilously close to those which make them little more than a giant food processor. Sorting out the problem required more engineering analysis in this case, some computational fluid dynamics code, typically used by academic researchers, left to run on a spare computer for a couple of weeks. Our propulsion supplier Rolla and Twin Disc came to the party with new propellers which although the same diameter and pitch, took the boat from 6 kn to 22 kn