Spin-Offs From the Suction Sail
The suction sail is an innovative design that uses a small amount of
energy to redirect a large flow of crosswind to assist ship propulsion,
reducing the amount of thrust required from ship propellers. Spin-offs
from the suction sail concept can be applied to other maritime
transportation applications.
Introduction
The concept of the suction sail begins with conventional boat sails
that converts the kinetic energy of crosswinds to vessel propulsion,
applying Newton’s law of motion that there is a kinetic reaction for
every kinetic action. Designers of early sails, kite makers and builders
of early airplane wings focused on the interaction between wind on the
upwind side of the sail, not the shadow side. The early aviation sector
discovered the important role of the upper shadow side of a wing
sustaining ‘lift’ as compared to the wing underside.
Developers of yachts experimented with adapting an aeronautical wing
or airfoil sail to vessel propulsion, by redirecting crosswind kinetic
energy. Airfoil construction and ‘angle-of-attack’ in relation to
crosswind direction made greater use of the shadow side of the airfoil
to provide propulsive force, as long as air flowed over the shadow side
as water flows down the side of a tilted mug of water. Airfoil design
produced a low-pressure zone near the forward edge, diverting a large
amount of crosswind rearward around the airfoil shadow side to produce
greater propulsive force.
Suction Sail
The suction sail is a deck-mounted airfoil with an extractor fan
installed at the upper end, to pull air in through slits in the airfoil
to develop a low-pressure zone across the airfoil shadow side. That
modification greatly increases the amount of crosswind that is diverted
rearward around the shadow side of the airfoil, greatly increasing
propulsive force by several orders of magnitude. The concept has
potential to be adapted to other areas of maritime propulsion, including
below the waterline involving hydrofoils and even Flettner Rotors.
Suction Hydrofoils
The ability of suction sail technology to greatly increase the
equivalent of ‘lift’ along the shadow side of an airfoil-sail provides
the basis to adapt the concept to operate underwater, in the form of
suction hydrofoils. When operating submerged, a small propeller would
pull a small volume flow rate of water through narrow slit-type inlets
built into the hydrofoil upper surface. Water would flow through the
interior of the hydrofoil and out through an outlet installed below the
hydrofoil or at its far end, potentially increasing the low-speed ‘lift’
of the hydrofoil.
Using suction technology to increase ‘lift’ along the top surface of a
hydrofoil increases potential to raise a vessel hull above water at
lower sailing speed, also increasing the amount of weight that a vessel
could carry on its hydrofoils. Raising the vessel hull at lower speed
reduces drag when sailing through severely choppy water, allowing the
vessel to sail at low-speed over extended distances with hull above
water. While most hydrofoil vessels are designed to sail at speed, there
might actually be a market for low-speed hydrofoil vessels capable of
sailing smoothly through choppy water.
Suction Rotor
The success of suction sail technology during real world operation
provides a basis to combine it with a competing technology, the
vertical-axis spinning cylindrical Flettner Rotor. A hollow rotor with
inlet slits and an extraction fan installed at its upper end offers the
concept of a suction rotor. Reversible blades would allow the rotor and
extraction fan to spin in either clockwise of counter-clockwise
directions while pulling air through the rotor. A planetary overdrive
gear would spin the extractor fan at extreme rotational speeds,
sustaining a low-pressure zone inside the cylinder while diverting air
inward through the inlets.
The moving boundary layer of a conventional spinning Flettner rotor
develops low-pressure zone in the crosswind shadow, diverting crosswind
energy toward the low-pressure zone and changing its direction to
produce propulsive thrust. Air flowing into the inlets at sonic speed
would restrict air mass flow rate involving wind blowing directly at the
inlets, allowing air to flow into inlets on the downwind shadow side. A
rotary valve that momentarily closes inlets on the upwind side while
keeping shadow side inlets operational, would theoretically divert a
greater volume of crosswind kinetic energy rearward, producing greater
propulsive force.
Conclusions
The suction sail is the ultimate development of airfoil-sail
technology, to develop propulsive force from crosswind kinetic energy.
In wind-assisted ship propulsion, it outperforms all previous
airfoil-sail designs. It is a proven concept based on a flow dynamic
that has potential application below water, in hydrofoils intended to
raise a vessel hull above water at low sailing speed and carry greater
weight at higher sailing speed. There is also scope to adapt suction
sail air flow dynamic to a competing wind-assisted ship propulsion
technology, the vertical-axis spinning rotor.
In both suction sail application and potentially with spinning rotor
application, the air flow dynamic offers the ability to divert a greater
proportion of crosswind kinetic energy to vessel propulsion, using a
small input of energy. The concept can achieve the same result as an
extremely tall wind technology using less height and a lower center of
gravity. Adapting suction rotor dynamics to a cylindrical rotor spinning
on a vertical axis will need to be the focus of future research, to
develop greater propulsive thrust from a greater proportion of crosswind
kinetic energy.