U.S. patent application number 14/439890 was filed with the patent office on 2015-10-15 for planing hydrofoils for marine craft.
The applicant listed for this patent is Ian James DUNCAN. Invention is credited to Ian James Duncan.
Application Number | 20150291257 14/439890 |
Document ID | / |
Family ID | 47429093 |
Filed Date | 2015-10-15 |
United States Patent
Application |
20150291257 |
Kind Code |
A1 |
Duncan; Ian James |
October 15, 2015 |
PLANING HYDROFOILS FOR MARINE CRAFT
Abstract
A hydrofoil (20) for mounting to a high speed surface vessel
arranged for planning operation at least at higher craft speeds and
in which the trailing edge (202) remain substantially fully wetted
at all craft speeds when operating under calm conditions.
Inventors: |
Duncan; Ian James; (Bedford,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DUNCAN; Ian James |
|
|
US |
|
|
Family ID: |
47429093 |
Appl. No.: |
14/439890 |
Filed: |
November 4, 2013 |
PCT Filed: |
November 4, 2013 |
PCT NO: |
PCT/EP2013/072947 |
371 Date: |
April 30, 2015 |
Current U.S.
Class: |
114/278 ;
114/274 |
Current CPC
Class: |
B60V 1/22 20130101; B63B
1/18 20130101; B63B 1/248 20130101; B60V 1/08 20130101; B63B 71/10
20200101; B60V 3/065 20130101 |
International
Class: |
B63B 1/18 20060101
B63B001/18; B63B 9/00 20060101 B63B009/00; B63B 1/24 20060101
B63B001/24 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 2, 2012 |
GB |
1219778.6 |
Claims
1. A hydrofoil for mounting to a high speed surface vessel arranged
for planing operation at least at higher craft speeds and in which
the trailing edge remains substantially fully wetted at all craft
speeds when operating under calm conditions.
2. A hydrofoil according to claim 1 in which the ratio of the
wetted chord at the tip to the wetted chord at the centre of the
hydrofoil is in the range of 15% to 75% in the design condition and
preferentially under all normal high speed operating conditions due
to the fact that the drag coefficient rises substantially outside
this range.
3. A hydrofoil according to claim 1 in which the planing surfaces
are prismatic and are arranged such that the angle of attack, the
angel of dihedral and the forward spray root line ensure that the
underside planing surface of the hydrofoil extends out to
substantially the full span of the hydrofoil under all normal
operational conditions.
4. A hydrofoil according to claim 1 in which the dihedral angle
.beta. is zero such that the root spray angle is ninety
degrees.
5. A hydrofoil according to claim 1 being a rectangular hydrofoil
with no dihedral.
6. A hydrofoil according to claim 1 comprising a cambered planing
surface.
7. A hydrofoil according to claim 1 comprising a new hydrofoil
section which comprises a forward downwards inclined generally
straight section, an intermediate upwards cambered section, an aft
downwards cambered section and a downwards inclined trailing edge
section.
8. A hydrofoil according to claim 1 comprising a new hydrofoil
section which comprises a forward generally straight section, an
aft downwards cambered section and a downwards inclined trailing
edge section.
9. A hydrofoil according to claim 6 in which the spray root line is
adapted to the cambered section and in which such modified spray
root line extends out to substantially the full span under all
desirable operating conditions.
10. A hydrofoil according to claim 1 in which any part of the
hydrofoil is constituted by any fixed part of the hull.
11. A hydrofoil according to claim 1 in which any part of the
hydrofoil is constituted by any part attached to the hull.
12. A hydrofoil according to claim 10 in which the adjacent hull
surfaces serve to increase the effective aspect ratio of the
hydrofoil.
13. A hydrofoil according to claim 1 which further comprises at
least one trailing edge flap.
14. A hydrofoil according to claim 13 in which any trailing edge
flaps is arranged with a control mechanism and a controller for
adaptive lift control.
15. A hydrofoil according to claim 1 which is adapted to provide a
significant amount of lift due to the principle of ground
effect.
16. A hydrofoil according to claim 15 which is adapted to provide
at least 10% of the lift required due to the principle of ground
effect.
17. A hydrofoil according to claim 1 in which the relationship
between the root spray angle, the angle of attack and the dihedral
angle for prismatic surfaces is given by the expression below
.gamma. - tan - 1 ( .pi. 2 tan .tau. tan .beta. ) ##EQU00002##
18. A hydrofoil according to claim 7 in which the spray root line
is adapted to the cambered section and in which such modified spray
root line extends out to substantially the full span under all
desirable operating conditions.
19. A hydrofoil according to claim 8 in which the spray root line
is adapted to the cambered section and in which such modified spray
root line extends out to substantially the full span under all
desirable operating conditions.
20. A hydrofoil according to claim 11 in which the adjacent hull
surfaces serve to increase the effective aspect ratio of the
hydrofoil.
Description
SUMMARY OF THE INVENTION
[0001] This invention relates to a new form of hydrofoils for
high-speed marine craft. The hydrofoils are configured such that
they provide both high lift coefficients and high ratios of lift to
drag over a wide range of craft speeds, whether running submerged
in close proximity to the free surface of the water at lower speeds
or planing thereon at higher speeds.
[0002] The invention has particular application to the use of high
speed catamarans and other surface craft which can benefit from the
greatly reduced power consumption and improved ride and handling
provided.
[0003] Whilst the majority of applications are likely to be for
faster craft, the lift and drag characteristics of the new
hydrofoils are such that significant reductions in hull resistance
have been recorded at displacement Froude numbers only a little
above 1.0 such that the hydrofoils also have application to
relatively heavy commercial and workboats.
[0004] Once fully planing the lift to drag ratio increases steadily
with speed such that the power requirement remains relatively
constant over a wide speed range, essentially only increasing due
to the increasing wind resistance.
[0005] If the operational speed range is high enough the hydrofoils
may pass through the four defined states of shallow immersion,
planing, skating and ground effect with increasing speed.
[0006] The displacement Froude number Fn.gradient. is given by the
following expression:
Fn.gradient.=V/ (g.DELTA..sup.1/3)
where V is the velocity of the craft, .DELTA. is the volume of
water displaced by the hull when it is at rest and g is the rate of
acceleration due to gravity (all in consistent units)
[0007] A number of prior art technologies have addressed the issue
of the application of hydrofoils to high speed craft but few have
achieved commercial success. A number of craft with
surface-piercing hydrofoils have been successfully operated,
particularly in inland lakes and waterways typically achieving
about 38 knots and a displacement Froude number of around 3.0. A
relatively small number of craft with deeply submerged foils have
been operated. Details of such craft are disclosed in U.S. Pat. No.
4,159,690 and U.S. Pat. No. 5,404,830. Such craft tend to have
optimum ride comfort but suffer from a a limited speed range and
demonstrate a considerable `hump` at on-to-foil speeds. They are
also relatively complex and require special lifting arrangements
and protection systems. At speeds above about 44 knots it becomes
difficult to manage cavitation and the lift/drag ratio starts to
deteriorate significantly. A few prototype craft of this type have
been fitted with super-cavitating hydrofoils but lift/drag ratios
have been poor. A significant number of hydrofoil assisted
catamarans using shallowly-submerged hydrofoils have been
successfully operated. Details of a number of such craft are
disclosed in WO2008007249, U.S. Pat. No. 4,606,291, EP0051073,
EP0094673, EP0352195, US2003029370, U.S. Pat. No. 5,520,137 for
instance. Such craft tend to have ride qualities intermediate
between surface-piercing hydrofoils and deeply-submerged hydrofoils
and to give improved performance, particularly in the 30/40 knot
range and to demonstrate lift/drag ratios in the 8:1 to 11:1 range.
Above this speed the lift/drag ratio tends to increase
substantially as the hydrofoils get closer to the water surface and
as the cavitation number falls away.
[0008] With increasing emphasis on fuel efficiency and the
increasing demand for both good ride comfort and speed new
technologies are required. High aspect ratio planing hydrofoils
mounted to catamarans or other suitable hulls in accordance with
this invention have been shown to operate at much higher lift drag
ratios than any other systems at displacement Froude numbers above
about 2.4, and to demonstrate remarkable lift/drag ratios at higher
speeds.
[0009] The primary object of this invention is to provide means
which enable a significant increase in top speed, cruising speed
and cruising range with a reduction or at least no increase in
power or fuel capacity.
[0010] It is a further object of this invention to provide means to
enable the effective design of hull surfaces adjacent to the
described planing hydrofoils such as to optimise system performance
of the hydrofoil and hull system.
[0011] It is a further object of this invention to provide improved
ride comfort to the craft to which the hydrofoils are
integrated.
[0012] In a first preferred embodiment simple hydrofoils without
trailing edge flaps or other control means are described.
[0013] In a second preferred embodiment the hydrofoils comprise
controllable flaps to control the craft trim and roll attitude.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Preferred examples of the new hydrofoils will now be
described with reference to the accompanying drawings.
[0015] FIG. 1 shows the variation of the lift coefficient of
typical sub-cavitating hydrofoil sections designed for operation at
a depth below the water level operating at a constant angle of
attack at varying depth of submergence;
[0016] FIG. 2 shows the variation of the lift coefficient of
cavitating hydrofoil section operating at a constant angle of
attack at varying depth of submergence;
[0017] FIG. 3 shows the variation of the ratio of the lift
coefficient at various depth/chord ratios relative to the lift
coefficients at infinite depth of typical hydrofoil sections;
[0018] FIG. 4 shows the variation of the chord Froude Number
F.sub.C with craft speed for a hydrofoil section of unit chord;
[0019] FIG. 5 shows the variation of the lift/drag ratios with the
lift coefficient based on the span of planing hydrofoils of aspect
ratios varying between 2.5 and 12 operating at a constant angle of
attack and with varying camber;
[0020] FIG. 6 shows the variation of the lift/drag ratios with the
lift coefficient of planing hydrofoils of aspect ratios of 5 and 10
having constant section operating at varying angles of attack;
[0021] FIG. 7 shows a typical variation of the lift coefficient and
the lift drag ratio for a planing hydrofoil at varying angles of
sweep back of the 50% chord line;
[0022] FIG. 8 shows the variation of the lift coefficient for
typical planing hydrofoils of rectangular platform at various small
angle of dihedral;
[0023] FIG. 9 shows the variation of the lift coefficient and the
lift drag ratio for typical planing hydrofoils of rectangular
platform at various deflections of a 10% trailing edge flap;
[0024] FIG. 10 shows a planform view of a hydrofoil with swept-back
leading and trailing edges and a taper ratio fitted with trailing
edge flaps;
[0025] FIG. 11 shows a frontal view of a hydrofoil with
dihedral;
[0026] FIG. 12 shows a side view of prismatic planing surfaces of a
hydrofoil with swept-back leading and trailing edges and with
dihedral;
[0027] FIG. 13 shows a planform view of a hydrofoil with swept-back
leading and trailing edges and a taper ratio fitted with trailing
edge flaps with an alternative trailing edge profile;
[0028] FIG. 14 shows a planform view of a rectangular hydrofoil
fitted with trailing edge flaps;
[0029] FIG. 15 shows a frontal view of a planing hydrofoil without
dihedral and with a supporting strut fitted to a catamaran hull.
The figure includes a sectional view of the water flow around the
hydrofoil and adjacent hull profiles;
[0030] FIG. 16 shows a frontal view of a planing hydrofoil with
dihedral fitted to a catamaran hull in which the local deadrise
angle of the hulls is similar to the dihedral angle of the
hydrofoil. The figure includes a sectional view of the water flow
around the hydrofoil and adjacent hull profiles;
[0031] FIG. 17 shows a frontal view of a planing hydrofoil with
dihedral fitted to a catamaran hull in which the local deadrise
angle of the hulls is similar to the dihedral angle of the
hydrofoil in which the hydrofoil is fitted with struts such that it
lies below the hulls. The figure includes a sectional view of the
water flow around the hydrofoil and adjacent hull profiles;
[0032] FIG. 18 shows a frontal view of a planing hydrofoil with
dihedral fitted to a catamaran hull in which the local deadrise
angle of the hulls is similar to the dihedral angle of the
hydrofoil in which the hydrofoil extends over the full width of the
catamaran hulls and is fitted with struts such that it lies below
the hulls. The figure includes a sectional view of the water flow
around the hydrofoil and adjacent hull profiles;
[0033] FIG. 19 shows an underside isometric view of a preferred
embodiment of a planing hydrofoil with trailing edge flaps fitted
to a hull with two forward sponsons have a step immediately aft of
the hydrofoil;
[0034] FIG. 20 shows an isometric view the hydrofoil of the
preferred embodiment of FIG. 21 in which the surface area of the
hydrofoil in contact with the water surface at in the condition at
which it becomes fully planing is shown;
[0035] FIG. 21 shows an isometric view the hydrofoil of the
preferred embodiment of FIG. 21 in which the surface area of the
hydrofoil in contact with the water surface at the design condition
is shown;
[0036] FIG. 22 shows an underside isometric view of a preferred
embodiment of a forward planing hydrofoil fitted to a hull with two
forward sponsons have a step immediately aft of the hydrofoil which
also comprises two aft planing hydrofoils;
[0037] FIG. 23 shows a preferred section for a planing hydrofoil
with indications as to the water surface under various operating
conditions;
[0038] FIG. 24 shows an alternative section for a planing
hydrofoil;
[0039] FIG. 25 shows the pressure distribution around a preferred
hydrofoil section operating deeply immersed in water or air;
[0040] FIG. 26 shows the pressure distribution around a preferred
hydrofoil section operating in ground effect.
[0041] FIG. 27 shows the lift generated by particular hydrofoils
comprising the section of FIG. 26 operating in ground effect
mode.
DETAILED DESCRIPTION
[0042] The following describes the general principles governing the
invention.
[0043] Referring to FIG. 1 curves 1 show a rapid reduction in lift
coefficient for sub-cavitating sections as the hydrofoil nears the
water surface. Although not shown on this figure the lift/drag
ration also falls away due to the an increasing effect of the
friction drag. Initially this reduction is quite slow, but as the
value of d/c approaches 0.25 the reduction in the lift/drag ratio
becomes increasingly marked. Curve 11 shows the variance of the
lift coefficient with the depth/chord ratio for an efficient
hydrodynamic section with a slightly concave under surface. Curve
12 shows the variance for a more classic aerofoil section which a
slightly convex under surface. The difference is due to the
increasing reliance on the pressure distribution on the lower
surface as a cavitation bubble increasingly grows on the upper
surface which becomes fully ventilated at some point. Both sections
have a 2D lift coefficient of 0.63 when deeply immersed.
[0044] Referring to FIG. 2 the opposite effect is evident for
cavitating sections. For the flat plate shown by curve 2 the lift
coefficient doubles between deep immersion and zero immersion with
most of this occurring when the hydrofoil is very close to the
surface. The curve for more efficient cavitating sections follows
the same trend although the overall increase in lift coefficient is
reduced from 100% to generally 25% to 50%. The lift/drag ratio for
a cavitating section tends to improve as the surface is approached.
The frictionless value tends to be little changed but the friction
coefficient has a reducing effect as the lift coefficient increases
close to the surface.
[0045] Referring to FIGS. 3 and 4 it is evident that the lift
coefficient is increasingly reduced as the depth/chord ratio
reduces, particularly in the region around a chord Froude number of
1, where the chord Froude number is given by the expression
F.sub.C=V/ (gC)
where V is the velocity of the craft, C is the chord of the
hydrofoil section and g is the rate of acceleration due to gravity
(all in consistent units).
[0046] Line 4 of FIG. 4 shows the variation of F.sub.C with craft
speed for a chord of one metre from which it can be seen that a
significant reduction in lift coefficient is to be expected as the
hydrofoil section approached the surface, particularly in the range
of speeds from 5 to 10 m/sec at which a high lift coefficient may
be required to get a craft foil borne.
[0047] It will be evident from FIGS. 1, 2, 2 and 4 that the design
of a suitable section is highly dependent on the range of immersion
depth intended, particularly if operation within the range of
immersion depths between 0.5 and zero is expected.
[0048] Referring to FIGS. 5 and 6 the benefit is shown of using the
a planing hydrofoil section arranged for constant wetted span in
which the chord reduces and by consequence the aspect ratio
increases as the speed increases. FIG. 5 shows the rapid
improvement in the lift/drag ratio as the aspect ratio is improved.
It also shows that the camber and associated value of the lift
coefficient based on span must be carefully selected to lie within
a desired range of lift/drag values. FIG. 6 shows values of CL and
the lift/drag ratio for hydrofoils having aspect ratios or 5 and 10
with the same section with the lift coefficient varied by changing
the angle of attack. These curves show the importance of
maintaining an optimum angle of attack with the performance
dropping away rapidly as the angle of attack is increased. The
aspect ratio is equally of key importance.
[0049] Referring to FIG. 7, line 6 shows the variation in the lift
coefficient for a typical planing hydrofoils with varying angles of
sweep back at 50% chord. The 50% chord point is taken as the centre
of sections adapted to such hydrofoils typically have centres of
pressure at around 50% of the chord. Preferred sections for use in
this invention typically have a centre of pressure at around 75% or
even further back in the design condition. Line 6 shows the
variation of the lift/drag ratio for the same hydrofoil. This
Figure shows that no significant difference in the lift coefficient
is to be expected at sweep back angles below 35 degrees and that no
reduction in the lift/drag ratio is to be expected at sweep-back
angles below 45 degrees.
[0050] Referring to line 7 of FIG. 8 it is evident that a
significant increase in lift coefficient may be achieved by
arranging the hydrofoil with a dihedral angle. It will be seen
later that this implies the application of an appropriate
sweep-back angle depending on the angle of attack. The variation in
the lift/drag ratio has been found not to follow any such simple
variation with dihedral angle and needs to be optimised for
specific conditions.
[0051] Referring to FIG. 9 in which line 8 shows the variation in
the lift coefficient and line shows the corresponding variation in
the lift/drag ratio. Whilst the figures are only applicable to a
particular hydrofoil it is generally the case that increasing the
flap deflection provides an increased lift coefficient and a
decreased lift/drag ratio. Sections of the preferred type generally
show much more desirable characteristics if such a variation in
lift coefficient is achieved by variation of the sectional camber
indicating that use of flaps should be limited as much as possible
during operation.
[0052] Referring to FIG. 10, a planform view of a hydrofoil 20
having both sweep back and a taper ratio is shown. The hydrofoil
has a leading edge 201, a trailing edge 202 and tips 203. It also
has a span b, a root chord C.sub.R and a tip chord C.sub.R and a
fully wetted area S. The geometric aspect ratio A for the fully
wetted hydrofoil is given by the expression:
A=B.sup.2/S
[0053] The taper ratio TR is defined by the expression:
TR=C.sub.T/C.sub.R
[0054] Hydrofoil 20 preferentially embodies trailing edge flaps 205
extending preferentially over the majority of the span b.
[0055] The preferred trailing edge 202 is arranged to be generally
normal to the centreline 204 as it crosses the centreline. An
alternative straight trailing edge form is shown in FIG. 13. The
centreline of FIG. 10 results in smoother flow conditions which is
particularly important if the flow would otherwise impinge on an
after hull body or if an additional hydrofoil or other lifting
surface is fitted aft of hydrofoil 20. When planing, a spray root
is formed at the intersection of the waterplane and the hydrofoil
under-surface. Such spray-root defines the forward edge of the
pressure surface and by consequence defines the forward end of the
wetted chord. For prismatic planing surfaces the spray root line is
approximately straight and is shown by line 208 at an angle .gamma.
to the centreline 204
[0056] The basic relationship between the root spray angle, the
angle of attack and the dihedral angle for prismatic surfaces is
given by the expression below
.gamma. = tan - 1 ( .pi. 2 tan .tau. tan .beta. ) ##EQU00001##
[0057] Where: .gamma. is the angle in radians between the spray
root line and the centre line in plan view for prismatic planing
surfaces, .tau. is the angle of attack in radians and .beta. is the
dihedral angle in radians
[0058] For the preferred cambered sections of this invention the
spray root line becomes curved as shown by line 209. Although
approximate expressions are available for the computation of such
lines it is generally sufficient to compute the straight line and
prior to computing a more precise geometry using computational
fluid dynamics tools.
[0059] Referring to FIG. 11 the hydrofoil 20 is shown having a
lower surface 206 which is generally wetted in both planing and
non-planing conditions and an upper surface 207 which is generally
wetted in the submerged condition but generally dry in the planing
condition. For the purposes of the planing hydrofoil a dihedral
angle .beta. is defined by the inclination of the under lifting
surface 206 to the horizontal.
[0060] Referring to FIG. 12 showing a prismatic planing surface 206
bounded by the spray root line 208, the trailing edge 202 and
proportions of the tip chords 203 and the centreline chord 204, the
centreline chord is at an angle .tau..sub.C and the tip chord at an
angle .tau..sub.T to the horizontal wherein a median value of .tau.
is used in the expression above for the angle .gamma..
[0061] Referring to FIG. 13 showing an alternative swept planform
with a line between the 50% tip and root chord points describing a
sweepback angle .DELTA..sub.50%.
[0062] Referring to FIG. 14, a hydrofoil 20 with a simple
rectangular planform is shown with a span b and a chord C is shown
with one or more trailing edge flaps 205.
[0063] Referring to FIG. 15, a catamaran hull 30 is shown with a
hydrofoil 20 without dihderal. The hydrofoil 20 is attached to the
inner walls 3013 of the hulls 301a and 301b such that the lower
face of hydrofoil 20 is approximately aligned to the inner edges of
the two hull surfaces 3012. A strut 211 may be required to stiffen
hydrofoil 20. A tunnel is defined by the inside walls 3013 of the
hulls, the under face of the bridge structure 302 and the water
surface 40
[0064] The lower surfaces 3012 of hulls 301a, 301b are arranged
with a deadrise angle .beta..sub.H. At sufficient speed the spray
root angle for the hydrofoil will be 90 deg whereas the spray root
angle for the hulls will be lower than this depending on angle
.beta..sub.H and the angle of attack of the hulls. Some form of
complex spray jet pattern will result in this region as indicated
by 402. The water surface at the hydrofoil will thus be represented
by line 401 and patterns 402.
[0065] Referring to FIG. 16, a catamaran hull 30 is shown similar
to that of FIG. 15 together with a hydrofoil 20 with a dihderal
angle .beta.. The hydrofoil 20 is attached to the inner walls 3013
of the hulls 301a and 301b such that the lower face of hydrofoil 20
is approximately aligned to the inner edges of the two hull
surfaces 3012. A tunnel is defined by the inside walls 3013 of the
hulls, the under face of the bridge structure 302 and the water
surface 40
[0066] The lower surfaces 3012 of hulls 301a, 301b are arranged
with a deadrise angle .beta..sub.H similar to the dihedral angle
for the hydrofoil 20 At sufficient speed the spray root angle for
the hydrofoil and the hulls will be similar and will generally
result in improved flow conditions and a significant increase in
the effective aspect ratio will result. The spray jet 402 will be
directed upwards and the sides of the hull 3011 will tend to form
some form of fence or tip further improving the hydrodynamic
efficiency of the lifting surfaces. The water surface at the
hydrofoil will thus be represented by line 401 and patterns
402.
[0067] Referring to FIG. 17, a catamaran hull 30 is shown similar
to that of FIG. 16 together with a hydrofoil 20 with a dihedral
angle .beta.. The hydrofoil 20 is attached to the inner walls 3013
of the hulls 301a and 301b my means of fences or struts 212. A
tunnel is defined by the inside walls 3013 of the hulls, the under
face of the bridge structure 302 and the water surface 40
[0068] The lower surfaces 3012 of hulls 301a, 301b are arranged
with a deadrise angle .beta..sub.H similar to the dihedral angle
for the hydrofoil 20. However, in this case the hydrofoil is
arranged below the bottom extremity of the hulls 301a, 301b which
will consequently ride clear of the water at sufficient speed. In
this case the hydrofoil foil area S and/or the aspect ratio of the
hydrofoil will be reduced due to the reduced span. The spray jet
402 will be directed upwards and the sides of the fences or struts
212 will tend to form some form of fence or tip which may make up
some of the deficiency due to the lower aspect ratio of the
hydrofoil 20. At sub-planing speeds the hydrofoil 20 will be more
deeply immersed such that by reference to FIG. 1 it can be seen
that it will generate more lift, however, once planing its
performance can be expected to be inferior to a full width
hydrofoil. The water surface at the hydrofoil will thus be
represented by line 401 and patterns 402.
[0069] Referring to FIG. 18, a catamaran hull 30 is shown similar
to that of FIG. 17 together with a hydrofoil 20 with a dihedral
angle .beta.. The hydrofoil 20 is attached to the outer walls 3011
of the hulls 301a and 301b my means of fences or struts 212. A
tunnel is defined by the inside walls 3013 of the hulls, the under
face of the bridge structure 302 and the water surface 40
[0070] The lower surfaces 3012 of hulls 301a, 301b are arranged
with a deadrise angle .beta..sub.H similar to the dihedral angle
for the hydrofoil 20. However, in this case the hydrofoil is
arranged below the bottom extremity of the hulls 301a, 301b which
will consequently ride clear of the water at sufficient speed.
Compared to the arrangement of FIG. 17 the hydrofoil 20 can be
arranged with increased span qnd will have improved performance.
The spray jet 402 will be directed upwards and the sides of the
fences or struts 212 will tend to form some form of fence or tip to
further increase the hydrodynamic efficiency. At sub-planing speeds
the hydrofoil 20 will be more deeply immersed such that by
reference to FIG. 1 it can be seen that it will generate more lift.
It will also have improved performance compared to the other
arrangements in the planing configuration due to the continuous
hydrofoil. The water surface at the hydrofoil will thus be
represented by line 401 and patterns 402.
[0071] Referring to FIG. 19, a craft hull 30 is shown with forward
sponsons 401a, 401b. A full width flapped hydrofoil according to
the present invention straddles the two sponsons. This preferred
embodiment is in accordance with FIG. 16.
[0072] Referring to FIGS. 20 and 21, a hydrofoil 20 of FIG. 19 is
shown at the point at which it starts to plane in FIG. 20. The
whole under face 23 of the hydrofoil shown shaded is wetted and
subject to a generally upwards positive pressure. FIG. 21 shows the
same hydrofoil at higher design speed such that the wetted surface
23 is now of smaller area. For a hydrofoil according to this
invention the span remains generally constant such that the aspect
ratio of the planing surface is increased. FIG. 21 also shows a
preferred arrangement in which the ratio of the wetted chord at the
tip to the wetted chord at the centre of the hydrofoil is in the
range of 15% to 75% in the design condition and preferentially
under all normal high speed operating conditions. This is due to
the fact that the drag coefficient rises substantially outside this
range.
[0073] Referring to FIG. 22 a craft 30 comprising forward sponsons
301a, 301b stepped at 309 comprises both a front hydrofoil 20 and
rear hydrofoils 206a, 206b according to the present invention.
Propulsion units 215 are provided aft of the rear hydrofoils. At a
design speed only an aft portion of the front and rear hydrofoils
will be wetted as shown in FIG. 21. It will be evident that unlike
a planing hull operating in a seaway, the overall surface area
which may be expected to come into contact with the water is
limited to the area of the hydrofoils and the forward sponsons so
that slamming forces are relatively limited.
[0074] Referring to FIG. 23, a preferred hydrofoil section
demonstrates improved lift and lift/drag ratios compared to
existing sections. The hydrofoil section 20 comprises a leading
edge 201, a trailing edge 202, a lower surface 206 and an upper
surface 207 and preferentially comprises a trailing edge flap with
a pivot 205a. The lower surface 206 comprises a generally straight
forward section 206a, a cambered "belly" section 206b, a pocketed
concave section 206c and a downwards inclined trailing edge section
206d.
[0075] At slow speeds hydrofoil 20 is fully submerged at a depth d
below the undisturbed water surface 403. Although the undisturbed
water surface is shown immediately ahead of the leading edge of the
hydrofoil the flow of the water past the hydrofoil will actually
influence the water level some way ahead of the leading edge and a
long way aft of the trailing edge. The disturbed water level is
shown pictorially at 4031. At some increased speed "the planing
speed" the leading edge will break through the surface and a spray
jet with an associated spray root will become established at point
410 just behind the leading edge 201 of the section corresponding
to a water surface 404. At this point the effective planing chord
of the hydrofoil is shown as C. As the speed increases the surface
area required to generate the required lift will reduce. At some
speed the water surface will be situated at 405 and the
corresponding spray root will be located at point 411 corresponding
to an effective chord C.sub.E. As the speed is further increased to
some design speed corresponding to a design displacement the water
surface will be situated at 406 and the corresponding spray root at
point 412
[0076] An aspect of this new section when used for a planing
hydrofoil is that as the hydrofoil rides up over the water surface
the chord reduces and progressively the angle of attack also
decreases. By ensuring the dihedral angle, the spray root angle and
angle of attack continue fit the expression for the spray root
angle, the full hydrofoil span will remain wetted and the aspect
ratio will increase in the proportion to the reduction in the
effective chord. Using the new section, the effect of reducing the
chord results in an increase in both the section lift coefficient
and the section lift/drag ratio The combined effects of increasing
the aspect ratio and increasing section performance results in a
rapid reduction in drag or hull resistance as indicated in FIG.
5.
[0077] At some speed, which is normally taken as the design speed
for a particular displacement, the chord will reduce to C.sub.DES.
The ratio C.sub.DES/C largely determines the ratio between the
design speed and the planing speed for the hydrofoil. This chord
ratio is shown as about 50% in FIG. 23, but can be arranged to be
of the order of 25% or less. Assuming a constant lift coefficient
this lower ratio results in a speed ratio of about 2.0 between the
planing speed and the design speed Assuming a typical displacement
Froude number of 2.4 for the planing condition the resulting design
value of F.sub.nv will be 4.8.
[0078] At yet higher speeds the incoming water surface 405 largely
misses the underbelly 206b of hydrofoil 20 and a spray jet is
created just forward of the trailing edge 202. Under these
conditions a pressurised mixture of air, water and water vapour is
created in the under pocket 206c. Under such conditions the chord
is not substantially reduced from the value C.sub.DES but the
pressure reduces such as to decrease both the lift coefficient and
the drag coefficient. The lift/drag ratio becomes extremely high
due to the almost equal areas over which the pressure acts in the
sense of the longitudinal axis. At the same time the gaseous
mixture reduces the skin friction such that the craft effectively
skates on a gas/water film. At yet higher speeds the hydrofoil acts
increasingly as a high aspect ratio airfoil under ground effect at
virtually zero height with the high pressure under the section
depressing the water surface. Under these conditions the lift/drag
ratio could increase to 250 and the lift coefficient to 1.3 for the
new section and with hydrofoils according to the present
invention.
[0079] Referring to FIG. 24, an alternative preferred hydrofoil
section also demonstrates improved lift and lift/drag ratios
compared to existing sections. The hydrofoil section 20 comprises a
leading edge 201, a trailing edge 202, a lower surface 206 and an
upper surface 207 and preferentially comprises a trailing edge flap
with a pivot 205a. The lower surface 206 comprises a generally
straight forward section 206a and a downwards inclined trailing
edge section 206d.
[0080] As for foil 20 of FIG. 23 the hydrofoil 20 of FIG. 24 is
fully submerged at slow speeds. At the planing speed a spray jet
with an associated spray root becomes established at point 410 just
behind the leading edge 201 of the section corresponding to a water
surface 404. At this point the effective planing chord of the
hydrofoil is shown as C. As the speed increases the surface area
required to generate the required lift will reduce. At some speed
the water surface will be situated at 405 and the corresponding
spray root will be located at point 411 corresponding to an
effective chord C.sub.E. As the speed is further increased the
effective chord will continue to decrease.
[0081] As for the section of FIG. 23 this new section when used for
a planing hydrofoil the chord reduces as the hydrofoil rides up
over the water surface. By ensuring the dihedral angle, the spray
root angle and angle of attack continue fit the expression for the
spray root angle, the span will remain wetted and the aspect ratio
will increase in the proportion to the reduction in the effective
chord. Whilst using the new section of FIG. 24 results in an
increase in aspect ratio with increasing speed, there is little
variation in either the section lift coefficient or the section
lift/drag ratio as both the angle of attack and the maximum camber
remain sensibly constant such that the reduction in drag or hull
resistance will still be generally as indicated in FIG. 5, but will
be less advantageous that results for a hydrofoil using the section
of FIG. 23. None-the-less the fact that the section of FIG. 24
exhibits no minimum value for the effective chord CE can be
beneficial in certain circumstances.
[0082] Referring to FIG. 25, a pressure distribution around the
section of FIG. 23 is shown at deep immersion. This shows a
relatively high negative pressure coefficient of between about 0.6
and 0.8 extending over a large majority of the upper surface and an
high positive pressure coefficient extending over a large majority
of the lower surface.
[0083] Referring to FIG. 26, a pressure distribution for the
section of FIG. 24 is shown for a condition in which the section is
just fully airborne, riding over the top of the water surface. For
this case the negative pressure over the top of the section is
reduced to between about 0.25 and 0.45 extending over a large
majority of the upper surface and an exceptionally high positive
pressure coefficient approaching 1.0 extending over virtually the
whole of the lower surface. In this condition the lift coefficient
for a typical hydrofoil is around 1.37 and the lift/drag ratio is
well in excess of 200.
[0084] Referring to FIG. 27, the lift relative to the lift required
for the craft to become airborne based only on the areas of the
hydrofoils is shown, the craft effectively flying in very close
proximity to the water surface at a displacement Froude number of
about 17.5. In this condition the list/drag ratio for the
hydrofoils is substantially in excess of 200:1.
* * * * *