The
fact of having successfully sailed a rotor boat of this size does not
conclude the project by any means, given that I envisage rotors as
environmentally sound drivers of large ships. It may seem that having
proved the idea on one scale, one can simply apply some clever
conversion factors to derive the effect of putting rotors on a ship. In fact, although 'Flow round a circular
cylinder (rotating circular cylinder. . .oscillating circular
cylinder. . .etc)' is a classic topic in fluid dynamics, most of the work done
relates to very low Reynolds numbers, non-turbulent flow and infinitely long cylinders. There is
everything to learn about real world applications of the principle.
Even the
self-evident fact of Anton Flettner's ship, the Buckau, having worked
well does not provide conclusive answers: the Buckau was said to have
been markedly superior to her conventional-sail sister ships, but they were mainly consuming inexhaustibly
renewable energy - the wind. The Buckau had one or two 45hp Diesel
engines in her belly.
Furthermore, I have heard that the Flettner rotor did have serious
problems in its design. Since almost all I know about the
Flettner ship is from internet sources with, often, questionable
degrees of authority, I don't propose to elaborate on these problems
here, but as the designer of a rotor myself, I feel well placed to
understand them, and I'm pleased to say that none of them have hampered
my design. I feel I should state this because of the body of
'unconventional rig' enthusiasts who consider that all spinning rotors
are Flettner rotors, and that Flettner rotors have been demonstrated to
possess insuperable problems.
For the rotor to be useful, it must be efficient,
and there are too many unknowns and complexities between aerodynamic
and hydrodynamic factors to make all but the broadest assumptions about
scale effects. As hinted above, available theoretical data fall well
outside the physical ranges at issue here. Established techniques for
scale translation are often misleading because they cannot account for
the critical properties of spanwise flow, induced drag and tip/root
losses, vortex shedding, boundary-layer behaviour and lift/drag ratio,
windshear above water surface, best freestream/surface speed ratio at
real-world Reynold's numbers. . .all of these can only be investigated
in the open atmosphere and may (as in fixed-wing aerodynamics) confound
predictions based on even the more sophisticated computer models
used currently.
You will gather from the above that the project
needs to be scaled up. I am currently able to
demonstrate a viable rotor boat with obvious development
potential. There are however steps I can take to further the
project even on the current scale.
I hope to find funds to solar-power
the boat and so render it visibly self-sustaining. This is an
important step because whereas the boat's energy consumption is well
within what a solar panel of 0.5 square metre or so can provide, this
is less than evident to any observer who hasn't suffered my
indoctrination spiel. Judging from people's comments on watching
the boat sail, it is far from obvious to them what they are seeing, and
so I can hardly expect them to guess the power source. I have
learnt again and again that it's demonstration, not explanation, that
captures people's imagination.
I hope to build a simple dynamic test rig to measure the
rotor's properties in all winds. This will inevitably provide
only approximate values, but I expect these figures to be more
meaningful than those derivable from the theoretical models mentioned
above. As well as trying to establish curves of lift and drag
against wind speed, and various related properties, a very useful
product of this test rig will simply be to observe the rotor's
behaviour in winds high enough to discourage me from setting sail on
open water.
Having sailed this rotor boat since 2004 however, I really feel it is
time to put a bigger rotor on a bigger boat, as a step to validating
the device as a minimally-polluting power source for large vessels.