"the wind does the work"
A sailing ship seems to obtain all its motive power
from one inexhaustible source - the wind.
In fact, a power input is necessary
over
and above that required to navigate the vessel. It requires
manpower to work the sails. In other words the classic
sailing vessel
consumes energy
in order to exploit the much
greater energy potential of the wind. The rotor vessel is no
different. The argument over whether the rotor boat
is
generically a sailing
boat or a motor boat is diverting, but unimportant. The
important
consideration is
that by arranging the power input and control of the rotor to
be as simple as
possible, the operator
input required will remain the same irrespective of the scale of the
system.
In other words, the spinning rotor is viable as powerplant for
large commercial vessels. On a small scale, the power input
the
rotor requires is
within that which solar panels can
provide, making a rotor boat self-sustaining.
Solar panels are the ideal power source as they add
the least drag penalty, no drive train losses, and least complexity as
a whole. At some as-yet unknown hull size, presumably
the greater energy density of hydrocarbon fuels would need to
be
resorted to; but bearing in mind that 'the wind does the
work', the fuel consumption would be a fraction of that
required to propel the
same vessel by water screw.
The phenomenon at issue here is known as the Robins effect or
the
Magnus effect. A spinning body in a viscous flow will
experience a
force normal to the freestream direction. The phenomenon is
in fact
quite mundane, and explains the ability of sportsmen to impart a
curving flight to balls of various types, from soccer balls to
baseballs. My rotor exploits this force along the length of a
cylinder.
A wing - any wing - is a device for
creating a reaction normal to its surface against a fluid passing round
it. A conventional boat sail is a type of wing, as is my
rotor.
Both sail and rotor create this reaction by compelling the
fluid
under consideration - air - to flow
round them asymmetrically. The aerodynamics are really much
the
same in both cases; the two devices only differ
in practical considerations of complexity, efficiency and
adaptability.
I explain this because many people on seeing the boat for the first
time see the rotor as some arcane means of driving a propeller or
somesuch underwater. There is nothing under the water except
the
keel
surface and the rudder!
What is it?
My
rotor is a single, cantilever shell driven through a full range of spin
speed by a small electric motor. The speed can be varied to
provide more thrust in light winds, or to achieve the best ratio of
spin speed to wind speed (though noone seems very clear what this ratio
should be). The main fact is that the current rotor, 3.6
metres long, uses in the region of 20 watts of electrical
power to drive the boat to hull speed even in light winds.
The rotor is home-made, and there is no doubt that at many
spin speeds you notice the slight noise and vibration it generates,
when you're in the boat. People who've been
watching the boat from quite close quarters though seem unaware that
they've been watching a spinning rotor. A rotor made
professionally say, using precision engineering, should produce very
little noise and vibration indeed.
Why
a rotor?
People
reasoned long ago that a cantilever, rigid,
smooth-surfaced wing
would be more efficient than fabric sheets strung on masts and booms,
as a means of propelling a boat in any given wind. However,
conventional rigs
have evolved to carry large amounts of fabric, countering basic
inefficiency with sheer surface area, and using principles like washout
and the slot effect to make the best of that area. A fixed
wing large
enough to produce the same lift could be a liability in circumstances
where a conventional craft could reef, or lower sail altogether.
The
fixed wing would bring a weight penalty too unless it was externally
braced, and whereas in the analogous move in aviation (from
strut-and-wire-braced biplanes to cantilever monoplanes) the weight
penalty was more than offset by the benefits, more weight aloft is
always a disadvantage at sea. Additionally, all the most
efficient
fixed-wing aerofoils possess positive camber, meaning they only provide
their best lift when air flows past them from the flatter side. This is
alright for an aeroplane, which, by and large, only ever wants the lift
force to work in one direction - away from the ground - but a sailing
craft needs to tack from side to side in order to progress to
windward.
This necessitates either a symmetrical aerofoil, at a loss in
efficiency, or a mechanically complex means of varying camber.
A rotor
offers different pros and cons. Although my
rotor is no more nor less than a wing, it differs crucially
from a cantilever fixed wing in that it is active not
passive. A fixed
wing uses its shape to induce the necessary asymmetry in the fluid
passing over it: my rotor is itself entirely symmetrical in
cross-section, and instead uses spin to achieve the same effect.
The
immediately obvious shortcoming of this approach is that the rotor
needs a power source to make it spin, making the system complex, and an
absorber of energy over and above the absorbed energy (induced
drag) of a conventional rig. That's why right from the start,
I
thought only of mechanical simplicity and aerodynamic
refinement,
believing that only thereby could I produce something both technically
reasonable and valuably more efficient than what has gone before.
From
experience gained with my prototype I estimate that not
only would
the rotor
for a large boat be lighter than a conventional sailing rig
producing
the same thrust, but that the centre of gravity would be
correspondingly lower. A common early objection to a
rotor is
windage (excess
of it), but by observation of my prototype's behaviour I suspect the
windage to be in the same realm as that of a conventional rig with
bare poles (listen to the wind singing through a yacht's rigging).
Further, by spinning the rotor at some
low rate, the turbulent wake (synonymous with windage) can be much
reduced. Inherently, this tactic should also minimise any
possible
effects of standing vortex sheets. I have waited until
my rotor is oscillating under the impetus of a standing vortex
sheet, spun it at a low rpm, and watched the oscillation reduce or
vanish.
Advantages
of rotor v.
conventional sail (small scale)
Extreme simplicity of use
Reduction in weight aloft/lower c.g.
No gybing danger
Low maintenance
Clear decks, superb visibility
Windage possibly less than conventional
Self-reefing quality*
Less heel due to higher L/D ratio and reduced weight
Extreme manoeuvrability with >1 rotor
Brake allows rapid de-powering
Self-righting, full capsize impossible
Potential for emergency speed **
Development potential - motor and photovoltaic R&D***
Large
scale
advantages
Hugely reduced fuel costs/sulphur emissions
Added manoeuvrability and stability - reduced roll
Zero underwater noise pollution
notes
*Lift of conventional sail increases as the square of the wind speed,
so
when sailing at full power, a gust can overpower the
sail and cause a capsize. Sails are reefed to avoid this hazard.
Lift of rotor increases much more in line with wind speed,
reducing sensitivity to gusts.
**Although renewable power sources, e.g. solar, inevitably limit the
continuous power able to be fed to the rotor, there is the ability
greatly to increase rotor power in the short term to escape danger e.g.
lee shore.
***electric motors, electrical storage, photovoltaics and associated
technology are areas of intensive research and development, with
advances reported frequently.
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