U.S. patent application number 14/494318 was filed with the patent office on 2015-03-26 for hydrofoil watercraft.
The applicant listed for this patent is Jonathan Sebastian Howes. Invention is credited to Jonathan Sebastian Howes.
Application Number | 20150083034 14/494318 |
Document ID | / |
Family ID | 52689819 |
Filed Date | 2015-03-26 |
United States Patent
Application |
20150083034 |
Kind Code |
A1 |
Howes; Jonathan Sebastian |
March 26, 2015 |
HYDROFOIL WATERCRAFT
Abstract
A hydrofoil section comprises first and second faces that
create, in operation at speeds above a ventilation speed, a
ventilated cavity defined by a first cavity face which departs from
the first hydrofoil face and a second cavity face which departs
from the second hydrofoil face. Each cavity face represents a free
surface and each face separating from the said free surface at a
discontinuity on that surface, the separated faces forming a
continuation of the faces of arbitrary shape and enclosed by the
free surfaces without contacting the said free surfaces. Below the
speed at which full ventilation occurs the arbitrarily shaped
portion of each face is configured to provide a modified flow
configuration resulting in changed lift and or drag and or pitching
moment under partial, or unventilated operation.
Inventors: |
Howes; Jonathan Sebastian;
(Bolney, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Howes; Jonathan Sebastian |
Bolney |
|
GB |
|
|
Family ID: |
52689819 |
Appl. No.: |
14/494318 |
Filed: |
September 23, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12935065 |
Dec 10, 2010 |
8863681 |
|
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PCT/GB2009/000615 |
Mar 6, 2009 |
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14494318 |
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Current U.S.
Class: |
114/278 |
Current CPC
Class: |
B63B 32/60 20200201;
B63B 1/242 20130101; B63B 1/248 20130101 |
Class at
Publication: |
114/278 |
International
Class: |
B63B 1/24 20060101
B63B001/24; B63H 25/38 20060101 B63H025/38 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2008 |
GB |
0805623.6 |
Jul 21, 2008 |
GB |
0813297.9 |
Claims
1-27. (canceled)
28. A hydrofoil watercraft having a normal pitch attitude, the
watercraft comprising: at least one buoyant body, at least one
strut extending below the or each buoyant body, a hydrofoil secured
to the or each strut beneath the buoyant body, the hydrofoil being
for lifting the buoyant body of the watercraft above the surface of
the water and means for driving the watercraft forwards; wherein
the hydrofoil is adapted to plane when the watercraft is driven
above a planing speed and has: a leading edge, a trailing edge, a
first, lower surface between the leading edge and the trailing
edge, the lower surface being: smoothly curved and shaped: to plane
on the surface of the water when the watercraft is driven forwards
at or above the planing speed with the lower surface wetted at
normal pitch attitude and to generate lift when submerged with the
watercraft being driven at less than planing speed at normal pitch
attitude and a second, upper surface between the leading edge and
the trailing edge, the upper surface having: at least one
discontinuity between the leading edge and the trailing edge,
providing a fore portion between the leading edge and the
discontinuity and an aft portion between the discontinuity and the
trailing edge and being shaped: at the fore portion to generate
negligible lift in comparison with that of the lower surface when
submerged at normal pitch attitude and at the aft portion to slope
down towards the trailing edge when submerged at normal pitch
attitude, whereby: the aft portion generates lift when the
watercraft is driven at a speed lower than a super-ventilation
speed with the aft portion wetted and the aft portion generates
less lift when the watercraft is driven at a speed above the
super-ventilation speed to allow formation of super-ventilation
over at least part of the aft portion.
29. The hydrofoil watercraft according to claim 28, wherein the
strut is provided with ventilation means for atmospheric air
communication to the discontinuity.
30. The hydrofoil watercraft according to claim 28, wherein the
discontinuity is an aft facing step.
31. The hydrofoil watercraft according to claim 28, wherein: the
fore portion of the second upper surface is substantially flat, the
aft portion of the second upper surface is substantially flat the
aft portion is angled down with respect to the fore portion from
the discontinuity to the trailing edge.
32. The hydrofoil watercraft according to claim 28, wherein the aft
portion of the second upper surface has at least one second
discontinuity dividing the aft portion into facets.
33. The hydrofoil watercraft according to claim 32, wherein: the
fore portion of the second upper surface is substantially flat, a
fore one of the facets of the aft portion is substantially flat and
is angled down with respect to the fore portion and an after one of
the facets is substantially flat and is angled down with respect to
the fore one of the facets.
34. The hydrofoil watercraft according to claim 32, wherein the
strut is provided with ventilation means for atmospheric air
communication to the second discontinuity.
35. The hydrofoil watercraft according to claim 32 where the second
discontinuity is an aft facing step.
36. The hydrofoil watercraft according to claim 28, wherein the
hydrofoil has a swept profile such that the discontinuity has a
spanwise component.
37. The hydrofoil watercraft according to claim 28, wherein the aft
facing step has a depth tapering towards a tip of the
hydrofoil.
38. The hydrofoil watercraft according to claim 29, wherein the
strut is substantially vertical and has a forward strut portion an
aft strut portion, and an intersection between the forward and aft
strut portions extending from the buoyant body to the hydrofoil,
the intersection providing the ventilation means for atmospheric
air communication to the discontinuity.
39. The hydrofoil watercraft according to claim 38, wherein the
intersection is an aft facing step.
40. The hydrofoil watercraft according to claim 28, wherein: the
aft portion of the second upper surface has at least one second
discontinuity dividing the aft portion into facets, the strut is
substantially vertical and has a forward strut portion an aft strut
portion, an intersection between the forward and aft strut portions
extending from the buoyant body to the hydrofoil, the intersection
providing ventilation means for atmospheric air communication to
the discontinuity and a second intersection between fore and aft
facets of the after strut portion, the second intersection
providing ventilation means for atmospheric air communication to
the second discontinuity.
41. The hydrofoil watercraft according to claim 40, wherein the
second intersection is an aft facing step.
42. The hydrofoil watercraft according to claim 28, wherein the
leading edge of the hydrofoil has an armour surface.
43. The hydrofoil watercraft according to claim 28, including flow
fences on the second upper surface.
44. The hydrofoil watercraft according to claim 28, including: a
rudder and a stabilising, submerged hydrofoil is attached to the
rudder.
45. The hydrofoil watercraft according to claim 28, including: a
rudder and a secondary, surface running, ventilated hydrofoil is
attached to the rudder at a foil-borne waterline.
46. The hydrofoil watercraft according to claim 28, wherein the
hydrofoil is located close to the longitudinal centre of gravity of
the watercraft and the watercraft includes a submerged stabilising
foil positioned aft, the combination forming a stable, surface
following combination.
47. The hydrofoil watercraft according to claim 28, wherein the
hydrofoil is located close to the longitudinal centre of gravity of
the watercraft and the watercraft includes a surface-running
stabilising foil positioned fore, the combination forming a stable,
surface following combination.
48. The hydrofoil watercraft according to claim 47, wherein the
surface-running stabilising foil has an aspect ratio of less than
two.
49. The hydrofoil watercraft according to claim 47, wherein the
surface-running stabilising foil is swept by more than 45
degrees.
50. The hydrofoil watercraft according to claim 28, wherein the
first lower surface of the hydrofoil has a dihedral angle when
viewed in a vertical plane transverse to the longitudinal axis of
the watercraft.
51. The hydrofoil watercraft according to claim 28, wherein the
strut extends below the hydrofoil to provide stabilisation against
leeway.
52. The hydrofoil watercraft according to claim 51, wherein the
strut extends behind the hydrofoil as well as below the hydrofoil
to provide stabilisation against leeway.
53. The hydrofoil watercraft according to claim 28, wherein the
watercraft is a sail board.
54. The hydrofoil watercraft according to claim 28, wherein the
watercraft is a sail boat.
55. The hydrofoil watercraft according to claim 28, wherein the
watercraft is a motor boat.
56. A hydrofoil watercraft comprising: a buoyant body, a strut
extending below the buoyant body, a hydrofoil secured to the strut
beneath the buoyant body for lifting the buoyant body above the
surface of the water when travelling at a surface running speed and
means for driving the watercraft forwards; wherein the hydrofoil
has: a first, lower surface shaped to generate a distributed
pressure for supporting the watercraft with the hydrofoil running
at the surface of the water at surface running speed and a second,
upper surface having: at least one discontinuity between a leading
edge and a trailing edge, dividing the upper surface into fore and
aft portions and being shaped at the fore portion to generate
negligible lift and at the aft portion to provide lift tending to
raise the watercraft when travelling at speeds at which the aft
portion is wetted by water flow in contact with it and to allow
ventilation at intermediate speeds higher than the wetted speeds
and below surface running speeds and wherein both the strut and
hydrofoil are provided with ventilation means for atmospheric
communication from above the surface of the water to the
discontinuity in the upper surface of the hydrofoil.
57. The hydrofoil watercraft according to claim 56, wherein the
ventilation means are aft facing steps in the strut and the second,
upper face of the hydrofoil.
58. The hydrofoil watercraft according to claim 56, wherein: the
fore portion of the second upper surface is substantially flat, the
aft portion of the second upper surface is substantially flat the
aft portion is angled down with respect to the fore portion from
the discontinuity to the trailing edge.
59. The hydrofoil watercraft according to claim 57, wherein the aft
portion of the second upper surface has at least one second
discontinuity dividing the aft portion into facets.
60. The hydrofoil according to claim 59, wherein: the fore portion
of the second upper surface is substantially flat, a fore one of
the facets of the aft portion is substantially flat and is angled
down with respect to the fore portion and an after one of the
facets is substantially flat and is angled down with respect to the
fore one of the facets.
61. A hydrofoil watercraft having lift inducing lower surface and a
top surface which is substantially flat from a leading edge to a
first discontinuity and substantially flat and angled down from the
first discontinuity to a trailing edge.
62. The hydrofoil watercraft according to claim 61, wherein the
discontinuity is an aft facing step.
63. The hydrofoil watercraft according to claim 61, wherein the
strut is provided with ventilation means for atmospheric air
communication to the discontinuity.
64. The hydrofoil watercraft according to claim 61 further
comprising a second discontinuity, wherein the top surface which is
substantially flat from the leading edge to the first discontinuity
and further is substantially flat and angled down from the first
discontinuity to a second discontinuity and substantially flat and
angled further down from the second discontinuity to the trailing
edge.
65. The hydrofoil watercraft according to claim 64, wherein the
first and second discontinuities are aft facing steps.
66. The hydrofoil watercraft according to claim 64, wherein the
strut is provided with ventilation means for atmospheric air
communication to the discontinuities.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part application of
Ser. No. 12/935,065 filed on Dec. 10, 2010, which is a U.S.
National Phase application under .sctn.371 for International
Application No. PCT/GB2009/000615 having an international filing
date of Mar. 6, 2009, and from which priority is claimed under all
applicable sections of Title 35 of the United States Code
including, but not limited to, Sections 120, 363 and 365(c), and
which in turn claims priority under 35 USC .sctn.119 to U.K. Patent
Application No. 0806523.6 filed on Mar. 28, 2008 and to U.K. Patent
Application No. 0813286.9 filed on Jul. 21, 2008.
FIELD OF THE INVENTION
[0002] The present invention relates to an improved hydrofoil
watercraft. More particularly, the present invention relates to the
design, configuration and construction of improved wind and motor
driven watercraft having ventilated hydrofoils.
BACKGROUND OF THE INVENTION
[0003] Hydrofoils are widely used in both motor and wind powered
water craft to reduce drag and/or improving passenger comfort by
lifting the hull of the craft out of the water. However, it can be
difficult to lift and maintain the body of the craft at a specified
distance above the water surface, known as ride height. Hydrofoils
can suffer inconsistencies in performance resulting from cavitation
around the surface of the hydrofoil. Further, hydrofoils often have
a very narrow operating speed range unless moving parts such as
flaps are introduced to maintain optimum conditions.
[0004] It is known to control ride height by use of a ladder of
hydrofoil lifting surfaces. As the speed of the craft increases,
lift is generated and the craft rises. As lift increases, the upper
lifting surface is lifted clear of the water as the water craft
rises. The operational loss of a lifting surface results in reduced
lifting area, producing a reduction in total lift. This continues
as the craft rises until equilibrium is reached and the craft rises
no further. In addition, each of the plurality of foils produces
additional drag at low speed in a fully immersed condition.
[0005] An alternative known solution is use of an inclined
hydrofoil that pierces the surface of the water. Again, as the
speed of the craft increases, lift is generated and the water craft
rises. As the craft rises, a portion of the inclined foil rises out
of the water thus resulting in a reduced lifting area and a
reduction in total lift. A further consequence of the lifting
surface rising above the surface of the water is the resulting
unwanted ventilation and spray drag.
[0006] In the case of the inclined hydrofoils, sections that are
optimal when fully immersed, are sub-optimal at the water surface
and produce undesirable characteristics as they pass through
it--such as unwanted ventilation and spray drag. For ladder foils
as well as the above difficulties the multiple small hydrofoils and
junctions produce additional drag at low speed in a fully immersed
condition.
[0007] An alternative approach to height control is use of fully
immersed hydrofoils that control ride height by varying the amount
of lift generated via a mechanical or electrical surface sensor.
However, such systems struggle to control height accurately in the
presence of large waves and varying loads resulting from variations
in operating conditions. Further, such systems add complexity and
require long vertical legs between the buoyant body and the lifting
hydrofoils.
[0008] It is difficult for mechanical systems to control height
accurately in the presence of large waves and varying loads.
Additionally sections with high lift to drag ratios cavitate at
high speeds so reducing their lift.
[0009] Super ventilating surface running hydrofoils have been used
to control ride height directly as they run on the surface.
However, such hydrofoils tend to have high drag and low lift at low
speed and undesirable pitching moment characteristics. The
transition from unventilated to ventilated operation is often
associated with undesirable non-linear lift behaviour with
hydrofoil sections with high lift to drag ratios tending to
cavitate at high speeds thus reducing the amount of lift generated.
Since ventilated foils often require sharp or very thin leading
edge sections they are also vulnerable to damage and erosion. This
mitigates against the use of simple fibre-composite construction
adding both expense and complexity.
[0010] The present applicants have identified the need for a simple
robust hydrofoil system together with constructional techniques and
configurations for deploying it advantageously on water craft such
that the said craft inherently maintains an appropriate ride
height, has good lift-drag characteristics at low speed and
transitions smoothly between non-ventilated and ventilated
operation and further, does not suffer any adverse effects when the
hydrofoils ventilate and does not suffer any significant
degradation of performance at high speed due to cavitation.
SUMMARY OF THE INVENTION
[0011] The object of the present invention is to provide a robust
hydrofoil of simple construction configured for use with water
craft such that the craft inherently maintains an appropriate ride
height, has good lift-to-drag characteristics at low speed,
transitions smoothly between non-ventilated and ventilated
operation and does not suffer significant degradation of
performance at high speed due to cavitation.
[0012] In accordance with the present invention there is provided a
watercraft comprising: [0013] at least one buoyant body, [0014] at
least one strut extending below the or each buoyant body, [0015] a
hydrofoil secured to the or each strut beneath the buoyant body,
the hydrofoil being for lifting the buoyant body of the watercraft
above the surface of the water and [0016] means for driving the
watercraft forwards; wherein the hydrofoil is adapted to plane when
the watercraft is driven above a planing speed and has: [0017] a
leading edge, [0018] a trailing edge, [0019] a first, lower surface
between the leading edge and the trailing edge, the lower surface
being: [0020] smoothly curved and shaped: [0021] to plane on the
surface of the water when the watercraft is driven forwards at or
above the planing speed with the lower surface wetted at normal
pitch attitude and [0022] to generate lift when submerged with the
watercraft being driven at less than planing speed at normal pitch
attitude and [0023] a second, upper surface between the leading
edge and the trailing edge, the upper surface having: [0024] at
least one discontinuity between the leading edge and the trailing
edge, providing a fore portion between the leading edge and the
discontinuity and an aft portion between the discontinuity and the
trailing edge and being shaped: [0025] at the fore portion to
generate negligible lift in comparison with that of the lower
surface when submerged at normal pitch attitude and [0026] at the
aft portion to slope down towards the trailing edge when submerged
at normal pitch attitude, whereby: [0027] the aft portion generates
lift when the watercraft is driven at a speed lower than a
super-ventilation speed with the aft portion wetted and [0028] the
aft portion generates less lift when the watercraft is driven at a
speed above the super-ventilation speed to allow formation of
super-ventilation over at least part of the aft portion.
[0029] Preferably, the buoyant body is a sailing boat or ship but
can also be a sailboard or other vessel or body having a preferred
ride height.
[0030] Non-active surfaces of the hydrofoil are allowed to
ventilate and hence, as long as a supply of air (or other gas) is
available, hydrofoil behaviour is consistent across a very wide
speed range. The hydrofoil can either run fully submerged with air
delivered via a channel or along the exterior of a suitably
designed strut or other foil, or at the surface in which case it
planes at the water surface. Performance degradation caused by
impact with waves is not significant since air is entrained on
immersion and hydrofoil behaviour is largely unaffected.
[0031] When applied to a sailing craft the ventilated hydrofoil may
serve as the primary lifting foil which runs at the water surface,
a conventional foil may then be applied aft to operate as a
stabilising foil. In this way the height control of the vessel is
provided by the surface following tendency of the main ventilated
surface and the aft foil finds a natural level of submersion at
which to operate. Optionally, the stabilising foil may also be of
surface running form in which case both surfaces will plane on the
surface.
[0032] In another form the aft stabilising foil may be mounted on
the rudder. In yet another form the aft stabilising hydrofoil may
comprise two hydrofoils, one of ventilated and surface running form
and a conventional, non-ventilated, or ventilated hydrofoil
positioned below the surface running hydrofoil. In this way
ventilation down the rudder may be controlled by the presence of
the surface running hydrofoil resulting in more reliable rudder
operation. The surface running foil may also provide a
discontinuity of lift with immersion depth and so provide a
reference for maintenance of the correct running angle for the
vessel, and hence the primary lifting hydrofoil angle of
attack.
[0033] A ventilation path may be provided by a strut or struts that
attach the hydrofoil to the vessel by making the strut or struts of
wedge cross section such that the base of the wedge forms the
trailing edge of the strut or struts. In this way the pressure on
the base (base pressure) will, in operation, be reduced below that
of the free stream and will entrain air from the water surface and
conduct it down to the low pressure regions on the second face of
the hydrofoil and so provide an air source for ventilation of the
hydrofoil.
[0034] Preferably, the strut is substantially vertical.
[0035] If the attachment strut base is configured to coincide with
the aftmost second face discontinuity the ventilation air flow will
first reach the aftmost facet and air will then reach the facets
ahead of the aftmost discontinuity in a sequential manner with
increasing speed.
[0036] In another embodiment, the attaching struts may be of
conventional i.e. non-cavitating or non-ventilating hydrofoil cross
section with the trailing edge truncated to provide a base area. In
this way the pressure on the base (base pressure) will, in
operation, be reduced below that of the free stream and will
entrain air from the water surface and conduct it down to the low
pressure regions on the second face of the hydrofoil and so provide
an air source for ventilation of the hydrofoil.
[0037] In yet another embodiment, the attaching struts may carry a
second base area in the form of an aft facing step positioned ahead
of the strut trailing edge and meeting the second face of the
hydrofoil ahead of the strut trailing edge. In operation this
allows an additional air path to more forwardly located facets. If
the top of this aft facing step is below the point at which the
strut meets the surface of the hull the step may be prevented from
conduction air to the more forwardly located facets until the hull
has been lifted some distance above the static rest waterline. This
allows a higher degree of ventilation to be established before the
hydrofoil reaches the water surface resulting in a smaller change
in performance as surface running is established.
[0038] In another configuration the primary lifting hydrofoil may
be placed behind the stabilising, secondary foil in which case it
will be beneficial for both surfaces to be of ventilated form. A
configuration where both hydrofoils are of similar size and of
ventilated form may also be found to be beneficial in that it will
give a wide, stable centre of gravity position range. Although the
board may be rolled to generate a lateral component of force to
resist the lateral rig loads, a vertical hydrofoil would be
beneficial in a similar manner to the vertical fins normally used
under sailboards to ensure that lateral resistance is always
available to react the rig loads. This vertical fin may either be
attached directly to the board or to the primary lifting hydrofoil.
If necessary, for the purposes of directional balance against sail
loads, the vertical fin may be positioned ahead of, or behind the
main lifting hydrofoil by means of a boom extending ahead or behind
the hydrofoil.
[0039] If the primary lifting hydrofoil is positioned behind the
secondary hydrofoil the secondary hydrofoil may be configured to
provide some directional stiffness by means of dihedral, i.e. the
tips are raised above the root. This dihedral may take the form of
a vee foil, which may then be surface piercing, or a highly tapered
planform such that the tip section is significantly thinner than
the root and the dihedral is then on the lower surface only. The
dihedral then provides a small keel area to the secondary hydrofoil
which generates some lateral force in response to side slip.
[0040] In another embodiment the lateral resistance of the
secondary hydrofoil may be provided by a fin or fins below the
hydrofoil.
[0041] The secondary hydrofoil may have a section in accordance
with the present invention. It may also be of low aspect ratio,
typically less than two, to provide a high stalling angle and make
the board less prone to uncontrollable divergences in pitch due to
stalling, particularly in rough water.
[0042] The hydrofoil of the watercraft is in the fully ventilated
condition. The load is carried by the first lower pressure face and
the second face is designed to carry a zero pressure differential.
Air is admitted to the flow around the foil such that the aft
portion of the upper surface are geometrically defined by a free
surface, i.e. if the foil surface was locally removed, the flow
pattern would match the removed surface and hence other than the
first face, all surfaces are defined by the natural free-surface of
the fluid.
[0043] As all load is carried on the first lower face flow
separation is rarely a concern and the maximum mean pressure
coefficient can approach unity although the pressure drag in this
case would be excessively high. Having selected a pressure
distribution, a camber line is developed to produce a desirable
chordwise load distribution.
[0044] A symmetrical thickness distribution produces, on both first
lower and second upper surfaces, half the pressure loading for the
chordwise load distribution on each surface.
[0045] Adding the thickness distribution to the camber line
produces a cambered section with a zero pressure coefficient on the
second upper face and the designed pressure coefficient
distribution, and hence full chordwise loading on the first face.
The second surface takes the form of a free surface.
[0046] A practical way to design hydrofoil sections of this form is
to define an array of vorticity across the chord of the foil where
the vortex strengths are set to develop the intended chordwise
loading at free stream velocity across the chord. This is
sufficiently accurate for a thin, lightly loaded foil although
corrections to the free stream velocity will become necessary if
very high pressure coefficients are sought as the camber will be
increased and streamwise direction velocity increments induced by
the vorticity become significant. Effective designs have been
developed using this method up to positive pressure coefficient
values of around 0.5. Solving the flow vectors across the chord in
the presence of this array of vorticity provides the slope of the
camber line across the chord which, in turn, allows the camber line
to be developed. The thickness distribution is developed using a
chordwise array of sources, the strengths of these sources being
solved to develop half the intended chordwise loading across the
chord, in this case, symmetrically and on both faces. The addition
of the thickness form to the camber line results in the pressure
loading on each face being additive, hence the second face becomes
zero-loaded and the first face then carries the full load at the
design condition.
[0047] As sections with very sharp, thin leading edges are
vulnerable to damage and can have handling risks the section is
preferably modified by the addition of slight thickening, or
armour, at the leading edge. This will give a small rounding to the
leading edge and will result in increased strength in the region of
the main force production and positive load on the first face. The
applicant has been surprised to ascertain that a small thickening,
typically one percent of section chord or lower, will not affect
the overall performance of the section to any significant degree
despite some localised cavitation.
[0048] The aft section of the second upper face of the hydrofoil
section may be truncated between the discontinuity and the trailing
edge. In this way the low speed (unventilated or partially
ventilated) characteristics of the foil may be modified. This may
also be used to allow adjustment of the structural capabilities of
the section.
[0049] To ensure clean separation of the free surface from the
second face the second face must diverge from the free surface at a
discontinuity, this discontinuity may take the form of a sharp
chine, i.e. a local, sudden, angular change in direction away from
the free surface, or an aft facing step. Under lower speed
operation, i.e. operation in which the reduction in pressure
coefficient after the chine or step is insufficient for ventilation
to overcome hydrostatic pressure at the level of immersion of the
hydrofoil, the flow will now remain attached to the second face
and, if the second face is so configured, will result in a greater
deflection of the flow and a negative pressure coefficient on the
second face. In this way the lift coefficient of the section may be
increased at lower speeds without any significant change in
geometric incidence and without the addition of moving parts (e.g.
flaps).
[0050] Further, the aft portion of the second upper face is divided
into a series of facets by further discontinuities. Each facet may
be defined by a straight or curved line when considered as a
two-dimensional section, the precise profile being defined by the
desired flow characteristics when operating with the flow attached
to that facet.
[0051] A progressive ventilation may be achieved with increasing
speed such that the aftmost facet ventilates first, followed by
ventilation of the next most aft facet until the second free
surface departs the second face at the most forward discontinuity
and fully ventilated operation is established. This results in a
series of lift coefficient steps with increasing or decreasing
speed as each facet ventilates and the flow geometry is modified
providing a progressive reduction in lift coefficient with
increasing speed and a corresponding progressive increase in lift
coefficient with decreasing speed. The first lower surface
generates useful force at lower speeds as the water craft
accelerates such that a very wide operational speed may be
used.
[0052] Advantageously, since all load is carried by surfaces with a
positive pressure coefficient, cavitation is entirely eliminated or
reduced to a limited area adjacent the leading edge.
[0053] Advantageously, since the hydrofoil requires ventilation in
use, the foil will naturally tend toward a running position at the
water surface when sufficient speed is achieved simultaneously
raising the craft to a corresponding level. In a suitable foil
configuration this gives a craft so fitted a natural surface
following capability.
[0054] Non-active parts of the hydrofoil are allowed to ventilate
and hence, as long as a supply of air (or other gas) is available,
the foil behaviour is consistent across a very wide speed range.
The foil can either run fully submerged with air delivered via a
channel or along the exterior of a suitably designed strut or other
foil, or at the surface in which case it planes at the water
surface. Impact with waves is not significant since air is
entrained on immersion and the foil behaviour is largely
unaffected.
[0055] When applied to a sailing craft the ventilated foil may
serve as the primary lifting foil which runs at the water surface,
a conventional foil may then be applied aft to operate as a
stabilising foil. In this way the height control of the vessel is
provided by the surface following tendency of the main ventilated
surface and the aft foil finds a natural level of submersion at
which to operate. Optionally, the stabilising foil may also be of
surface running form in which case both surfaces will plane on the
surface.
[0056] In another form the aft stabilising foil may be mounted on
the rudder. In yet another form the aft stabilising hydrofoil may
comprise two hydrofoils, one of ventilated and surface running form
and a conventional, non-ventilated, or ventilated hydrofoil
positioned below the surface running hydrofoil. In this way
ventilation down the rudder may be controlled by the presence of
the surface running hydrofoil resulting in more reliable rudder
operation. The surface running foil may also provide a
discontinuity of lift with immersion depth and so provide a
reference for maintenance of the correct running angle for the
vessel, and hence the primary lifting hydrofoil angle of
attack.
[0057] A ventilation path may be provided by a strut or struts that
attach the hydrofoil to the vessel by making the strut or struts of
wedge cross section such that the base of the wedge forms the
trailing edge of the strut or struts. In this way the pressure on
the base (base pressure) will, in operation, be reduced below that
of the free stream and will entrain air from the water surface and
conduct it down to the low pressure regions on the second face of
the hydrofoil and so provide an air source for ventilation of the
hydrofoil.
[0058] If the attachment strut base is configured to coincide with
the aftmost second face discontinuity the ventilation air flow will
first reach the aftmost facet and air will then reach the facets
ahead of the aftmost discontinuity in a sequential manner with
increasing speed.
[0059] In another embodiment, the attaching struts may be of
conventional i.e. non-cavitating or non-ventilating hydrofoil cross
section with the trailing edge truncated to provide a base area. In
this way the pressure on the base (base pressure) will, in
operation, be reduced below that of the free stream and will
entrain air from the water surface and conduct it down to the low
pressure regions on the second face of the hydrofoil and so provide
an air source for ventilation of the hydrofoil.
[0060] In yet another embodiment, the attaching struts may carry a
second base area in the form of an aft facing step positioned ahead
of the strut trailing edge and meeting the second face of the
hydrofoil ahead of the strut trailing edge. In operation this
allows an additional air path to more forwardly located facets. If
the top of this aft facing step is below the point at which the
strut meets the surface of the hull the step may be prevented from
conduction air to the more forwardly located facets until the hull
has been lifted some distance above the static rest waterline. This
allows a higher degree of ventilation to be established before the
hydrofoil reaches the water surface resulting in a smaller change
in performance as surface running is established.
[0061] The hydrofoil may be provided with sweep such that the
hydrofoil tips are positioned behind the hydrofoil root. If
sufficient sweep is provided and ventilation paths are provided to
the hydrofoil root area, the flow over the hydrofoil will have a
component along each second face discontinuity from root to tip.
This can assist the spanwise spread of ventilation along each
discontinuity.
[0062] The hydrofoil may also be provided with sweep such that the
hydrofoil tips are positioned ahead of the hydrofoil root. If
sufficient sweep is provided and ventilation paths are provided to
the hydrofoil tip area, the flow over the hydrofoil will have a
component along each second face discontinuity from tip to root.
This can assist the spanwise spread of ventilation along each
discontinuity.
[0063] Another means of controlling the spanwise development of
ventilation is by means of upper surface fences as is well known in
the art of conventional hydrofoils, however, their application to
ventilated hydrofoil is not found in the art. This is advantageous
if, for example, the tip sections are designed to ventilate at a
higher speed than the root sections or that ventilation must be
inhibited on a part of the hydrofoil until surface running is
established, or that the tips may break the water surface first as
ride height is increased and the additional ventilation path
resulting from this breaking the surface must be limited to avoid a
sudden loss of lift.
[0064] If the second face discontinuities are configured as aft
facing steps some control of spanwise ventilation rate may also be
achieved by varying the step depth across the span, for example, if
root ventilation is desired the steps may be configures to be of
greater depth close to the root and lesser depth towards the
hydrofoil tips. In another embodiment the step may be tapered out
to zero depth at a partial span location and the discontinuity may
then continue as a simple chine.
[0065] A ventilated hydrofoil that may achieve a surface running
condition may also be furnished with a second, conventional
hydrofoil positioned beneath the ventilated hydrofoil. In this way
the ride height of the assembly may be set by the position of the
surface running hydrofoil whereas the conventional hydrofoil may
provide a substantial part of the total lift. This will be found
advantageous in that ride height may then be controlled without
moveable components or surface following mechanisms or sensors.
[0066] If applied to a sailboard, the main lifting hydrofoil may be
positioned ahead of, but close to the centre of gravity. A
conventional trailing submerged foil, or another surface running
ventilated foil may then be attached to the rear of the board as a
stabilising surface.
[0067] In another configuration the primary lifting hydrofoil may
be placed behind the stabilising, secondary foil in which case it
will be beneficial for both surfaces to be of ventilated form. A
configuration where both hydrofoils are of similar size and of
ventilated form may also be found to be beneficial in that it will
give a wide, stable centre of gravity position range. Although the
board may be rolled to generate a lateral component of force to
resist the lateral rig loads, a vertical hydrofoil would be
beneficial in a similar manner to the vertical fins normally used
under sailboards to ensure that lateral resistance is always
available to react the rig loads. This vertical fin may either be
attached directly to the board or to the primary lifting hydrofoil.
If necessary, for the purposes of directional balance against sail
loads, the vertical fin may be positioned ahead of, or behind the
main lifting hydrofoil by means of a boom extending ahead or behind
the hydrofoil.
[0068] If the primary lifting hydrofoil is positioned behind the
secondary hydrofoil the secondary hydrofoil may be configured to
provide some directional stiffness by means of dihedral, i.e. the
tips are raised above the root. This dihedral may take the form of
a vee foil, which may then be surface piercing, or a highly tapered
planform such that the tip section is significantly thinner than
the root and the dihedral is then on the lower surface only. The
dihedral then provides a small keel area to the secondary hydrofoil
which generates some lateral force in response to side slip.
[0069] In another embodiment the lateral resistance of the
secondary hydrofoil may be provided by a fin or fins below the
hydrofoil.
[0070] The secondary hydrofoil may have a section in accordance
with the present invention. It may also be of low aspect ratio,
typically less than two, to provide a high stalling angle and make
the board less prone to uncontrollable divergences in pitch due to
stalling, particularly in rough water.
[0071] Construction of hydrofoils in accordance with the present
invention may be of any suitable material, however, as the leading
edges tend to be extremely thin they can be vulnerable to damage,
accordingly it may be found to be beneficial to place a metallic
amour around the leading edge. This may be applied within a
moulding process such that the armour becomes a part of the mould,
or it may be attached after moulding. The aft facing steps arising
from the edges of the armour do not adversely affect the
performance of the foil since the first face operates under a
highly stable, positive pressure coefficient environment and the
edge on the second face will act as a natural break point for the
free surface to separate the flow from the surface of the foil.
[0072] In a further embodiment, fences may be applied to the upper
surface of the hydrofoil. Fences are small fins placed to prevent
ventilation air from migrating along a hydrofoil. The fences are
attached to the hydrofoil running in a fore-aft orientation to be
parallel to the direction of fluid flow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0073] To help understanding of the invention, a specific
embodiment thereof will now be described by way of example and with
reference to the accompanying drawings, in which:
[0074] FIG. 1 shows an example hull with a typical hydrofoil
installation;
[0075] FIG. 2 shows a hydrofoil section designed according to the
basic principles of the present invention;
[0076] FIG. 3 shows a hydrofoil section designed according to the
principles of the present invention with indicative streamlines
representing ventilated and non-ventilated operation;
[0077] FIG. 4 shows a hydrofoil section designed according to the
principles of the present invention with the second surface
discontinuity in the form of an aft facing step;
[0078] FIG. 5 shows a hydrofoil section designed according to the
principles of the present invention with sequential second-surface
discontinuities and associated indicative streamlines;
[0079] FIG. 6 shows a hydrofoil section designed according to the
principles of the present invention with sequential second-surface
discontinuities configured as aft-facing steps and associated
indicative streamlines;
[0080] FIG. 7 is a table of co-ordinates for a hydrofoil;
[0081] FIG. 8 is a perspective view of a hydrofoil;
[0082] FIG. 9 is a perspective view of a hydrofoil assembly with
multiple second surface discontinuities;
[0083] FIG. 10 is a three-view drawing of a foil installation on a
sail board;
[0084] FIG. 11 is a three view drawing of the forward foil of the
board;
[0085] FIG. 12 is a pressure plot of an advantageous pressure
distribution for an hydrofoil for a watercraft;
[0086] FIG. 13 is an hydrofoil cross-sectional shape generating the
pressure distribution of FIG. 12;
[0087] FIG. 14 is an hydrofoil cross-sectional shape for a
watercraft of the invention;
[0088] FIG. 15 is a pressure plot for the hydrofoil of FIG. 14 at
slow, non-ventilated, fully wetted speed;
[0089] FIG. 16 is a cross-section similar to FIG. 14 showing the
hydrofoil operating at ventilated speed;
[0090] FIG. 17 is a pressure plot for the hydrofoil of FIG. 14 at
ventilated speed of FIG. 16;
[0091] FIG. 18 is an alternative hydrofoil cross-sectional shape
for a watercraft of the invention;
[0092] FIG. 19 is a pressure plot for the hydrofoil of FIG. 18 at
slow, non-ventilated, fully wetted speed;
[0093] FIG. 20 is a cross-section similar to FIG. 18 showing the
hydrofoil operating at partially ventilated speed;
[0094] FIG. 21 is a pressure plot for the hydrofoil of FIG. 18 at
partially ventilated speed of FIG. 20;
[0095] FIG. 22 is a cross-section similar to FIG. 18 showing the
hydrofoil operating at fully ventilated speed;
[0096] FIG. 23 is a pressure plot for the hydrofoil of FIG. 18 at
fully ventilated speed of FIG. 22;
[0097] FIG. 24 is a cross-section similar to FIG. 18 showing the
hydrofoil operating at surface running/planing speed; and
[0098] FIG. 25 is a pressure plot for the hydrofoil of FIG. 18 at
surface running/planing speed of FIG. 24.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0099] By reference to FIG. 1 a typical installation of a hydrofoil
in accordance with the present invention may be described. From the
hull means (1) depends a substantially vertical strut (4) to which,
at the foil-borne water surface is attached a substantially
horizontal hydrofoil means (2), below this hydrofoil (2) is a
further substantially vertical surface (7) which provides lateral
resistance when the vessel is foil-borne with the foil (2) at the
water surface. A further foil(6) is mounted on the rudder (3) to
act as a stabilising surface. This may be of conventional
non-ventilating and non-cavitating form and configured to operate
fully submerged. To provide a means of keeping the primary foil (2)
at the correct incidence a further foil (5) may be attached to the
rudder at a height setting coinciding with the foil-borne water
surface. This foil (5) may advantageously be of ventilated form to
allow consistent surface running operation. The primary foil (2)
and the rudder surface running foil (5) have a section designed in
accordance with the present invention. A cross section of the
primary foil (5) is shown on FIG. 1 as a section through A-A
(8).
[0100] FIG. 2 illustrates the significant features of a hydrofoil
section (8) designed in accordance with the present invention.
There is provided a hydrofoil section comprising a first face (9)
and a second face (10) which creates, in operation at speeds above
the ventilation speed, a ventilated cavity(11) defined by a first
cavity face (12) which departs from the first hydrofoil face (9)
and a second cavity face (13) which departs from the second
hydrofoil face (10), each cavity surface representing a free
surface and each face (9,10) separating from the said free surfaces
at a discontinuities on those faces (15), the separated faces
forming a continuation of the first and second faces of arbitrary
shape (14) and enclosed by the free surfaces (12,13) without
contacting the said free surfaces (12,13). Below the speed at which
full ventilation occurs the arbitrarily shaped portion of each face
(14) is configured to provide a modified flow configuration
resulting in changed lift and or drag and or pitching moment under
partial, or unventilated operation.
[0101] FIG. 3 illustrates the two flow conditions representing
ventilated and unventilated operation. The two states of operation
are governed by behaviour at the second surface discontinuity (16).
If the flow remains attached after this point a negative pressure
coefficient will be generated over the aft region (14) of the
second face (10) and the flow will generate the stagnation
streamlines shown in the FIG. 17). If the pressure over this region
(14) falls below the hydrostatic pressure arising from the depth of
foil immersion and an air path is provided (e.g. by a duct or other
means) the aft region (14) will ventilate and flow will separate at
the discontinuity (16), the free surfaces (12,13) will then become
established around the ventilation cavity (11). The ventilated
cavity surfaces are similar to those shown in FIG. 1 (12, 13). In
non-ventilated operation the leading and trailing stagnation
streamlines are shown (17) and the greater deflection of,
particularly the trailing stagnation streamline, indicates that the
lift coefficient will be significantly increased over that of the
ventilated state.
[0102] FIG. 4 shows the discontinuity (16) replaced by an aft
facing step (18) to ensure more positive separation of the
free-surfaces (12, 13) when ventilation conditions are present. The
streamline patterns are only minimally affected by this shape and
are shown as being similar to FIG. 3 in both the ventilated
condition with the free surfaces (12, 13) and the non-ventilated
condition with the stagnation streamlines (17) depicted.
[0103] FIG. 5 illustrates the subdivision of the continuation of
the second face (14) into a series of facets (20, 21), each with a
discontinuity (16, 22) at the leading edge of the facet. When in
non-ventilated operation the stagnation streamlines (17) apply and
the maximum lift coefficient occurs. As speed increases the
pressure on the aftmost facet (20) reduces until ventilation occurs
and flow separation is established at the aft discontinuity (22)
resulting in cavity surfaces (19) deflected through a lesser angle
than the stagnation streamlines (17) and a reduced lift
coefficient. Further increase in speed results in the pressure
reducing over the next most forward facet (21) which may then draw
air forwards from the cavity defined by the two free surfaces (19)
to the discontinuity at the leading edge of the facet (16). This
results in a further reduction in lift as evidenced by the
deflection angles associated with the new cavity shape defined by
cavity surfaces (12, 13). A facetted foil is illustrated below, for
operation at three different lift coefficients, namely, 0.2 fully
ventilated, 0.48 with ventilation initiating from the aft
discontinuity (22) and 0.77 for non-ventilated operation.
[0104] FIG. 6 shows a similar situation to FIG. 5 but with the
discontinuities replaced by aft facing steps (18). There is only a
minimal impact on the flow patterns associated with this
replacement, however, separations at the discontinuities become
more positive and reliable due to the more severe discontinuity
caused by the steps.
[0105] FIG. 7 presents a table of co-ordinates for a hydrofoil with
a design lift coefficient of 0.2. The "upper surface" is the second
surface (10), the "lower surface" is the first surface (9). The
upper surface follows the free surface cavity shape (13) and
discontinuities may be placed at any location on this surface, the
lower surface (9) is the force generation surface and the lower
cavity surface (12) continues from this face at the 100% chord
location.
[0106] FIG. 8 provides a perspective view of a primary lifting
hydrofoil assembly. The substantially vertical strut (4) comprises
a forward portion (27) and an aft portion (26). The forward portion
may be of a straight sided wedge section or with cambered faces,
the aft portion may also be of a straight sided wedge section or
with cambered faces. The intersection between the forward and aft
portions is a discontinuity such as a sharp change in angle or an
aft facing step as shown. In operation this discontinuity (25)
generates a local drop in pressure which entrains air from the
surface to feed the discontinuity (18) of the substantially
horizontal hydrofoil (2). The substantially horizontal hydrofoil
(2) is shown with an aft facing step at the second surface
discontinuity. This is an example of a single discontinuity, more
discontinuities may be beneficial if more steps in lifting
performance are desired. This step is tapered in depth from the
root (23) to the tip (24) with the depth of the step being less at
the tip than the root. This feature provides an air path at the
discontinuity in operation of greater cross-sectional area at the
root than the tip. Since air reaching the tip (24) must first
travel from the root (23) but must also ventilate the root it is
clear that the spanwise flow rate must be higher at the root than
the tip and so a greater spanwise flow path cross-section will be
advantageous. A small amount of sweepback of the hydrofoil (2) is
shown this is advantageous to ventilation as the water flow across
the foil is, by this means, given a spanwise component along the
discontinuity (18) and this assists the airflow in the spanwise
direction and hence aids the establishment of full ventilation.
Generally greater sweep will give a greater benefit in this regard
although the drag performance of the hydrofoil may then be impaired
and hence an engineering compromise is implicit.
[0107] FIG. 9 illustrates a foil assembly with multiple second
surface discontinuities (18, 29), in this case two discontinuities
are shown although more could be used. The substantially vertical
strut (4) comprises a forward (27) and an aft part (26) separated
by a discontinuity (25) in the form of an aft facing step. This
discontinuity (25) meets at its lower end the aftmost discontinuity
(29) on the substantially horizontal hydrofoil (2). Since the
aftmost region of the hydrofoil (2) ventilates first with
increasing speed the cavity so created is a source of air for
ventilation of the forward discontinuity (18) as the pressure
around this discontinuity (18) decreases with increasing speed. As
speed increases, with no change in ventilation lift will also
increase and the vessel may then be lifted above the water surface
by the hydrofoil. Ventilation of the forward discontinuity (18) may
then occur when the vessel is partially lifted and so there is
provided a second discontinuity (28) on the vertical strut (4)
which extends partially over the length of the vertical strut (4)
and communicates with the water surface only when the foil has
lifted sufficiently to expose the upper end of the discontinuity
(28). By this means an additional air path is created for
ventilation of the forward hydrofoil (2) discontinuity (18) as
speed and lift are increased ensuring that when the hydrofoil (2)
reaches the water surface substantially full ventilated operation
has been established and no sudden lift loss occurs.
[0108] FIG. 10 presents a three view drawing of a hydrofoil
assembly configured for a sail board. Two substantially vertical
struts (31, 33) are attached below the board (30) in forward (31)
and aft (33) locations and carry at their respective lower ends two
substantially horizontal hydrofoils (32, 34). Below the aft
hydrofoil (34) is a further substantially vertical surface (36)
which resists lateral forces when in operation and provides
directional stability. This surface (36) is attached to the aft
hydrofoil (34) via a boom (35) extending in an aftwards direction
from the root of the hydrofoil (34). The primary lifting surface is
the aft hydrofoil (34) and pitch stability and a small amount of
lift is provided by the forward hydrofoil (32) which also provides
tactile surface reference feedback to the operator to assist in
keeping the board trimmed at the optimum incidence angle to the
water surface. The forward foil (32) is of low aspect ratio
(typically less than two) and may also incorporate significant
leading edge sweepback (typically greater than 45 degrees), both
features leading to high stalling angles of attack and rendering
the behaviour of the board more consistent in rough water and when
manoeuvring. Another feature of the forward foil (32) is that the
lower surfaces form a shallow vee when viewed from the front. This
provides a small keel effect, i.e., some lateral resistance is
created when the foil, and hence the board, is side slipped. This,
in combination with the substantially vertical surface (36)
provides some directional stiffness and damps directional
stability.
[0109] FIG. 11 shows a three view of the forward foil (32). The
front view (right hand side of figure) shows the shallow vee angle
in the lower surface (37) which provides a small keel effect. The
upper surface has two discontinuities (38) which operate in the
same manner as already described. These discontinuities assist with
lifting the front of the board at lower speeds and help to avoid a
nose-diving tendency as the board accelerates.
[0110] Referring to FIG. 12, its graph shows an advantageous
pressure distribution for a hydrofoil. For a typical leading to
trailing edge chord, the upper surface has a substantially zero
pressure distribution 101, generating negligible lift, whereas and
substantially constant for the lower surface has a substantially
constant pressure distribution 102, generating substantially evenly
distributed lift for supporting the weight of the watercraft to be
supported by the hydrofoil. Normally this will be the entire weight
of a hydrofoil watercraft of the invention. In other words, this
graph indicates zero lift suction over the upper surface and
substantially constant lift pressure over the lower surface.
[0111] FIG. 13, is a typical hydrofoil shape capable of exhibiting
this pressure distribution. Its upper surface 103 is substantially
flat and horizontal in use, whereby flow over the top surface is
flat and aligned with free stream flow past the hydrofoil. Such
flow generates no suction on the upper surface. Its lower face 104
is angled down and slightly concave from the leading edge 105 at a
steadily increasing angle. Flow along the lower surface is
progressively deflected downwards giving rise to lift pressure on
the lower surface. Whilst the leading edge of this foil shape is
sharp, its trailing edge 106 is a vertical face. This latter
creates vortices and reduced pressure. It results in drag on the
hydrofoil, which is disadvantageous.
[0112] The invention provides a practical foil shape shown in FIG.
14 benefiting from advantageous pressure distribution, in that a
front portion 1031 of the top surface remains substantially flat
from the leading edge 105 to a discontinuity step 107 beyond which
an aft portion 1032 of the top surface slopes down to the trailing
edge in the same position as that of FIG. 13. The pressure plot for
this shape at slow speed, FIG. 15 has a top surface suction hump
108 from the discontinuity back to the trailing edge, whilst the
lower surface retains the same plot for the same lower surface
shape and speed. It should be noted that the quantum of upper
surface suction and lower surface lift is less than that of FIG. 12
and not enough to support the full weight of the watercraft which
still requires some buoyancy support at this speed. When speed
increases, the top surface flow detaches at the discontinuity as
result of ventilation show diagrammatically as 109, resulting in an
air bubble 110 below the water surface 111 forming over the rear
portion 1032 and a substantial reduction of the top surface hump
108', as shown in FIGS. 16 and 17. However the lower surface
pressure increases, with result that the watercraft rises
slightly.
[0113] It should be noted that the pitch attitude of the watercraft
has an effect on the lift of the hydrofoil. The Figures show the
lift for the normal pitch attitude of the watercraft, namely
parallel to its normal floating attitude.
[0114] As shown in FIG. 18, the hydrofoil shape can be further
improved by provision of two discontinuities 1071,1072. The
pressure coefficient plot for this shape of hydrofoil at slow
speeds has two humps, FIG. 19. As the speed increases to the point
where the flow detaches from the rear discontinuity, the pressure
coefficient plot changes with the reduction of the rear hump, FIGS.
20 and 21. On further speed increase, the flow detaches at the
front discontinuity, with reduced suction behind the front
discontinuity, FIGS. 22 and 23.
[0115] In this situation, the lift pressure generated by the lower
surface is nearly enough to support the full weight of the
watercraft. A small increase in speed causes the hydrofoil to rise
to the surface, FIG. 24. At this point the hull of the watercraft,
also referred to as its buoyant body, is clear of the water and the
lower surface pressure supports the entire weight of the
watercraft, as shown in FIG. 25. A trough 112 in the water surface
extends behind the foil immediately behind it.
[0116] The advantages of these two foils are that at lower speed
through the water, they generate increasing lift as speed increases
to come close to being able to support the weight of the craft as
the flow detaches at the discontinuities. Below the detachment
speed, the rear portion of the upper surface generates suction
lift, without significant drag. A small further increase in speed
generates added lift from the lower surface alone for the hydrofoil
to lift the craft for the foil to rise to the surface.
[0117] The absence of suction lift from the front portion of the
upper surface results in there not being a reduction in overall
lift as the foil breaks the surface, with the absence of the foil
hunting above and below the surface.
[0118] The twin discontinuity foil has advantage over the single
discontinuity in that the reduction of lift on flow detachment at
the single discontinuity is smoother with increase in speed.
[0119] Although the invention has been described above with
reference to one or more preferred embodiments, it will be
appreciated that various changes or modifications may be made
without departing from the scope of the invention as defined in the
appended claims.
* * * * *