Oar+Some - Prototype story so far

The most important thing to try out is the novel (and key) propulsion idea. So the first prototype step was to make a simple timber chassis on which to mount an experimental pedal bar and oar assembly.

The first challenge ...

The cross beams have insufficient torsional stiffness. They don't deflect downwards problematically, but they twist forwards excessively - due to the rider's weight being significantly forwards of the structure.

Of course the timber chassis bears little resemblence to the triangular crosss section in the more detailed design - but that too is unlikely to be stiff enough in torsion and a re-think is needed.

The optimum structure to bear twisting moments is a circular cross section tube. So how do we integrate this with a structure that can also cope with the vertical loading - while remaining in our space envelope and being as light as possible?

Here's my idea - see picture. This is a minature cardboard model - intended to quickly and cheaply explore the stiffness and strength of this structure. It combines an I-beam and a circular tube co-axially, with the loads being presented via flanges with annular bonding to the outside of the tube.

I've been pleasantly surprised at the performance of this model. Take a look at this picture. The minature model is only cardboard, and weighs just 3 oz = but it can cope with the considerable weight of 5 litres of oil - with no perceptible deflection.

So the next step is to make a full size prototype chassis based on this structure - which is the point I've reached - and the status quo!