Ships – Hull or hull adjunct employing fluid dynamic forces to... – Having airfoil
Reexamination Certificate
2001-05-02
2004-01-13
Avila, Stephen (Department: 3617)
Ships
Hull or hull adjunct employing fluid dynamic forces to...
Having airfoil
C114S274000, C114S280000, C114S039240, C114S039310
Reexamination Certificate
active
06675735
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved sail craft. In particular, the invention relates to a wind powered sailing craft with improved speed performance as compared with the prior art.
BACKGROUND OF THE INVENTION
Sail powered craft have been well known for many years and have been used for many purposes including commercial and military applications. In more recent times, with the advent of active propulsion systems, wind powered sail craft have generally been restricted to leisure activities.
Popular forms of modem day sail craft include yachts, catamarans and sail boards. Whilst the applications for this type of craft have become more restricted in recent times, there is still a great deal of,interest for leisure applications. The leisure market is substantial and the competition for new and improved designs is significant.
In particular, there is substantial competition to produce a sail craft with superior speed performance as compared with prior art designs. In this regard, the competition to produce sail craft of ever improved speed performance is similar to the competition to produce solar powered or man powered vehicles of greater performance than their predecessors. A notable example of this type of craft that has been designed to produce the best known speed performance is the Australian designed “Yellow Pages Endeavour” which is a wind powered sail craft that has recorded an average top sailing speed of 46.52 knots in a 19 knot true wind speed with minimal wave height.
However, the “Yellow Pages Endeavour” is restricted in that it has a reduced handling capability as compared with generally available craft. The most significant of these is that the craft can only sail on one tack.
The present invention is intended to provide a wind powered sail craft with superior speed performance as compared with the prior art. In addition, it is also intended to provide a wind powered sail craft that provides an improved speed performance without sacrificing handling capabilities as generally occurs in the prior art.
Some of the basic nomenclature used throughout the specification is introduced with reference to
FIG. 1
that sets out the fundamental principles of craft velocity in relation to true wind velocity. In particular,
FIG. 1
diagrammatically represents the theoretical maximum craft velocity that can be achieved with any type of craft. An analysis of
FIG. 1
produces a number of relationships that are plotted in FIG.
2
.
FIG. 1
is a vector diagram detailing a locus of all possible velocities of a sail craft, designated V, for a given true wind velocity, designated V
T
, and apparent wind angle, designated &bgr;. The velocity of the craft can be projected into a downwind and an upwind component with the maximum downwind and upwind velocities achievable designated V
D
and V
U
respectively.
The apparent wind velocity is designated V
A
. For a given value of true wind V
T
and apparent wind angle &bgr;, the range of all possible craft velocities comprises an arc of a circle with the true wind velocity being a chord. The arc representing all possible craft velocities is designated V
POSS
. The maximum possible craft velocity occurs when the velocity V intersects the centre of the circle V
POSS
and extends over the diameter of the circle. At this position, the maximum velocity achievable is designated V
max
. As the circle V
POSS
designates the range of all possible craft velocities it can be readily seen that the maximum upwind component of velocity V
U
and downwind component of velocity V
D
, are projections from the circle of V
POSS
parallel to the true wind velocity V
T
.
From the vector diagram of
FIG. 1
, it can be readily derived that the maximum speed, V
max
, is given by:
V
max
=
V
T
sin
β
The maximum velocity made good to windward, that is upwind component, V
U
, is given by
V
U
=
V
max
-
V
T
2
The maximum velocity made good downwind, V
D
, is given by
V
D
=
V
max
+
V
T
2
The boat speed associated with V
U
is given by
V
=
sin
(
π
4
-
β
2
)
sin
β
V
T
The corresponding apparent wind is given by
V
A
=
sin
(
π
4
+
β
2
)
sin
β
V
T
The corresponding ratio of boat speed to apparent wind speed is given by
V
A
V
=
1
+
sin
β
1
-
sin
β
These relationships are plotted for varying apparent wind angle &bgr; and appear in FIG.
2
. The vertical axis of the plot in
FIG. 2
represents units of true wind speed with one unit representing the true wind speed. The horizontal axis represents varying apparent wind angle from 0 degrees to 90 degrees.
As can be seen from the plots in
FIG. 2
, the plot representing the maximum velocity of the craft, V
max
, has a value approaching the limit of the true wind speed as the apparent wind angle approaches 90 degrees and that the maximum velocity increases with a decreasing apparent wind angle. The “Yellow Pages Endeavour” achieved a top speed of approximately 2.5 times the true wind speed on the day of the test, and as can be seen from the plot, this represents an apparent wind angle of approximately 25 degrees.
The true wind angles, designated &ggr;, for maximum velocity and maximum up wind and down wind components are as follows:
V
max
is achieved at
γ
=
π
2
+
β
VU is achieved at
γ
=
π
4
+
β
2
VD is achieved at
γ
=
3
π
4
+
β
2
However, it is very difficult to obtain low values of apparent wind angle with a sail craft whilst at the same time being able to sail and control the craft.
The analysis presented above and the diagrammatic representations of
FIGS. 1 and 2
are applicable to all types of sailing craft. They effectively represent the theoretical principles that apply irrespective of the structure of the craft.
Whilst the above analysis is generally applicable to any type of craft, the following discussion will focus upon the general principles relating to the structure of crafts and leads to a detailed discussion of the specific structure of the craft of the present invention.
In the design of high performance sailcraft it is necessary to consider three principle classes of force, namely aerodynamic, hydrodynamic and gravitational. Hydrostatic forces may be considered to be negligible once the craft has sufficient speed. The resultant of the gravitational forces is a single force acting through the centre of mass. The aerodynamic forces can be reduced to a single resultant force and possibly a residual torque with an axis parallel to the line of action of the resultant force. A similar reduction also applies to the hydrodynamic forces. Ideally the residual torques will be negligible, leaving just the resultant aerodynamic, hydrodynamic and gravitational forces to consider. If three non parallel forces act on a rigid body, then for equilibrium the forces must sum to zero, must be coplanar and must be concurrent.
Additionally, it has been recognised for some time that the analysis of the operation of sail craft can be considered from the perspective of considering the water and the air as two interfacing fluids of substantially different density. As such, sailing craft reside at the interface of the two fluids and impinge into the fluids; the hydrofoil extending into the water and the aerofoil extending into the air. Exploiting this interface is effectively the basis of the operation of sailing craft.
In most conventional sailing craft designs, the hydrofoil and the aerofoil are in generally vertical alignment. In the case of a yacht, the keel forms the hydrofoil and the sail forms the aerofoil. In this instance, the analysis of the various forces acting upon the vessel to produce the motion of the vessel is relatively straightforward as most of the forces acting upon the hydrofoil and aerofoil lie substantially parallel to the horizontal plane of the interface between the two fluids. As will be appreciated by those with a basic understanding of vector addition, the task of analysing re
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