Rick Willoughby's free offset shaft drive system
Rick Willoughby is an outstanding Australian pedal boat designer and builder. His V12 may be the fastest log distance pedal boat in existance at the moment.
His flex drive systems, shown in this image, implements a novel solution, that Rick brillianty designed built and tested, the unsupported shaft with self stabilizing propeller.
His offset shaft. also shown, solves the hull-shaft interference problem, allowing easiest modularization, important for the Open Waterbike.
Rick presented his flex drive system at boatbuilding forum as an anwer to Mark Drela proposal for the Open Waterbike. Here is Rick's contribution:
That shaft operates without any support other than at the gearbox. A pushing prop is self stabilising so as soon as it starts to turn the prop just rises and aligns with flow.
I pushed the OC1 to 15kph with just that shaft. The aluminium section is 1.2m long.
I normally use a tension strut to enable going in reverse. With this and the prop beside me I can reach down and pull it up to remove fouling. Also for beaching or obstacles the prop just bounces over.
There is a balance between torsional rigidity and fatigue life. With good spring steel you can get a good compromise. I design for prop speed of 300rpm at design boat speed and 1/4" steel is just a bit thin. For sprinting I go up to 10mm spring steel and live with the extra weight to get a tolerable stress range. There is very little loss due to shaft torsional constant once you get to this thickness.
This system does result in very low appendage drag for the prop.
Rick's shafts are thicker than this proposed by Mark Drela; while characterized by a slightly higher drag they are stronger and torsionally stiffer, alowing thus also a different less rubbery feeling while pedalling.
Please give us your feedback on this brilliant solution
Our boat is fater than mine