U.S. patent application number 13/388206 was filed with the patent office on 2012-07-26 for towable buoy.
This patent application is currently assigned to ULTRA ELECTRONICS LIMITED. Invention is credited to Ian Nock.
Application Number | 20120186507 13/388206 |
Document ID | / |
Family ID | 41171465 |
Filed Date | 2012-07-26 |
United States Patent
Application |
20120186507 |
Kind Code |
A1 |
Nock; Ian |
July 26, 2012 |
TOWABLE BUOY
Abstract
A towable buoy having a tow point for connection to a tow cable
of a vessel, wherein the position of the tow point is
translationally moveable with respect to the remainder of the buoy.
Also, a method of operating the towable buoy.
Inventors: |
Nock; Ian; (Gloucestershire,
GB) |
Assignee: |
ULTRA ELECTRONICS LIMITED
Greenford
GB
|
Family ID: |
41171465 |
Appl. No.: |
13/388206 |
Filed: |
August 6, 2010 |
PCT Filed: |
August 6, 2010 |
PCT NO: |
PCT/GB2010/001490 |
371 Date: |
April 16, 2012 |
Current U.S.
Class: |
114/280 ;
114/125; 114/140; 114/244; 114/253; 114/274 |
Current CPC
Class: |
B63G 8/42 20130101; B63G
8/38 20130101 |
Class at
Publication: |
114/280 ;
114/253; 114/125; 114/140; 114/274; 114/244 |
International
Class: |
B63B 21/56 20060101
B63B021/56; B63G 8/42 20060101 B63G008/42; B63B 1/24 20060101
B63B001/24; B63B 43/06 20060101 B63B043/06; B63B 3/38 20060101
B63B003/38 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 14, 2009 |
GB |
0914314.0 |
Claims
1. A towable buoy comprising a tow point for connection to a tow
cable of a vessel, wherein the position of the tow point is
translationally moveable with respect to the remainder of the
buoy.
2. A towable buoy according to claim 1, wherein the position of the
tow point is moveable in a fore/aft direction and/or vertically
with respect to the remainder of the buoy.
3. A towable buoy according to claim 1, wherein the position of the
tow point is selectable based upon the speed and/or depth of the
towing vessel.
4. A towable buoy according to claim 1, further comprising a
passive tow point adjustment mechanism for moving the position of
the tow point.
5. A towable buoy according to claim 4, wherein the passive tow
point adjustment mechanism is operable to move the position of the
tow point according to loads in the tow point.
6. A towable buoy according to claim 1, further comprising an
active tow point adjustment system for moving the position of the
tow point.
7. A towable buoy according to claim 1, comprising a significant
metacentric height.
8. A towable buoy according to claim 1, further comprising means to
vary the centre of mass of the buoy.
9. A towable buoy according to claim 1, further comprising means to
vary the buoyancy of the buoy.
10. A towable buoy according to claim 1, further comprising an
internal ballast tank adjustable to alter the hydrostatic
characteristics of the buoy.
11. A towable buoy according to claim 1, further comprising a
keel.
12. A towable buoy according to claim 11, wherein the keel is
moveable.
13. A towable buoy according to claim 1, further comprising a
lifting hydrofoil.
14. A towable buoy according to claim 13, wherein the hydrofoil is
adjustable to alter its shape, surface area and/or pitch angle.
15. A towable buoy according to claim 1, further comprising a
control system for adjusting the hydrodynamic and/or hydrostatic
characteristics of the buoy.
16. A towable buoy according to claim 15, wherein the control
system is operable to act according to the speed, depth and/or
orientation of the towing vessel.
17. A method of operating a towable buoy having a tow point which
is translationally moveable with respect to the remainder of the
buoy, the method comprising: towing the buoy behind a moving vessel
by attaching a tow cable of the vessel to the tow point; and
adjusting the position of the tow point with respect to the
remainder of the buoy.
18. A method according to claim 17, wherein the position of the tow
point is adjusted according to the speed and/or depth of the
vessel.
19. A method according to claim 17, wherein the buoy is towed
submerged, preferably just beneath surface water waves.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a towable buoy,
particularly, though not exclusively, to a communications buoy to
be towed by a submarine.
BACKGROUND OF THE INVENTION
[0002] Towed communication buoys have been used for many years to
provide communications to submarines. Communications antennae and
electronics mounted within the buoy can provide a wide range of
frequency coverage. Non-communication type sensors can also be
mounted in the buoy. The buoy is physically, electrically and/or
optically connected to a submersible vessel (platform) by a
retractable tow cable. Modern towed communications buoys need to be
operable at a large range of platform speeds and depths and to
maintain stability whilst close to the sea surface over a range of
sea state conditions.
SUMMARY OF THE INVENTION
[0003] According to a first aspect of this invention, there is
provided a towable buoy having a tow point for connection to a tow
cable of a vessel, wherein the position of the tow point is
translationally moveable with respect to the remainder of the
buoy.
[0004] According to another aspect of this invention, there is
provided a method of operating a towable buoy having a tow point
which is translationally moveable with respect to the remainder of
the buoy, the method comprising towing the buoy behind a moving
vessel by attaching a tow cable of the vessel to the tow point, and
adjusting the position of the tow point with respect to the
remainder of the buoy.
[0005] The invention is advantageous in that the tow point
adjustment helps to balance hydrodynamic and hydrostatic forces on
the buoy. At slow speeds the tow cable orientation is dominated by
the positive trim of the buoy resulting in an almost vertical link
profile between the vessel and the buoy. At medium to high speed,
buoy and tow cable drag takes dominance and so a more horizontal
profile of the link results. To improve buoy stability across the
operating range, the tow point can be moved such that it can help
balance hydrostatic forces on the buoy at low speeds, and
hydrodynamic forces at high speeds. As tow speed increases, the
offset between the tow point and the buoy centre of gravity is
preferably increased both vertically and in the fore/aft direction
(horizontally) by translational movement of the tow point with
respect to the remainder of the buoy.
[0006] Movement of the tow point can be achieved using either a
passive or active control mechanism. The passive method could
harness changes in tow cable tension and/or drag forces to
translate the tow point. The simplest passive method would use a
spring to oppose the tow cable tension and locate the tow point
accordingly. An active mechanism could use a control loop system to
translate the tow point position according to the submarine's speed
and or tow cable tension levels, with the tow point movement being
under the action of a servomotor, for example.
[0007] It is intended that the buoy is stable in pitch and roll
when towed a few metres, typically less than 10 m, below the sea
surface by a submerged vessel at a wide range of speeds and depths,
in sea states ranging from 0 to at least 6. More particularly, the
buoy may pitch only slightly (typically less than +/-10 degrees at
sea state 6 at 6 m/s) in response to wave perturbations.
[0008] To achieve the required stability, the buoy may have a
significant offset between its centre of gravity (CoG) and centre
of buoyancy (CoB), i.e. a significant metacentric height. The buoy
may further comprise means to vary the centre of mass and/or
buoyancy of the buoy. The significant metacentric height may be
achieved using a keel for the buoy. The keel may be moveable. In
particular, the keel may be movable between a deployed position and
a retracted position such that, when retracted, the buoy has a
small space requirement for stowage. The keel may also be moved in
its deployed position so as to alter the centre of mass as
required. The buoy may also have an internal ballast tank
adjustable to alter the hydrostatic characteristics of the buoy.
The buoy may further include a lifting hydrofoil for increasing the
buoyancy of the buoy. The hydrofoil may be fixed, or may be
actively controlled so as to alter its shape, surface area and/or
pitch angle for balancing the buoy in trim as the towing vessel
speed and/or depth changes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Embodiments of the invention will now be described with
reference to the accompanying drawings, in which:
[0010] FIGS. 1 and 2 illustrate side and front elevations of a
first example of a buoy of this invention;
[0011] FIG. 3 illustrates the orbital particle motion that occurs
within wave structures;
[0012] FIG. 4 illustrates a buoy performance simulation setup;
[0013] FIG. 5a illustrates a buoy depth profile and FIG. 5b a pitch
profile for a 6 m/s tow at SS6;
[0014] FIG. 6a illustrates a buoy depth profile and FIG. 6b a pitch
profile for a 3 m/s tow at SS6;
[0015] FIG. 7a illustrates a buoy depth profile and FIG. 7b a pitch
profile for a 0 m/s tow at SS6;
[0016] FIG. 8a illustrates a buoy depth profile and FIG. 8b a pitch
profile for a 6 m/s tow at SS3;
[0017] FIG. 9a illustrates a buoy depth profile and FIG. 9b a pitch
profile for a 3 m/s tow at SS3;
[0018] FIG. 10a illustrates a buoy depth profile and FIG. 10b a
pitch profile for a 0 m/s tow at SS3;
[0019] FIG. 11 illustrates the depth keeping performance of the
buoy;
[0020] FIG. 12 illustrates a side elevation of a second example of
a buoy of this invention; and
[0021] FIGS. 13a to 13d illustrate various elevations of a third
example of a buoy of this invention.
DETAILED DESCRIPTION OF EMBODIMENTS
Buoy Set-Up
[0022] A first example of a buoy is shown schematically in FIGS. 1
and 2. The buoy 1 has a hydrodynamic body 2, a hydrofoil 3, and a
moveable tow point 4. The buoy 1 houses a payload, an adjustable
ballast tank 5, a tow point actuator and a hydrofoil actuator. The
tow point actuator is operable to move the tow point 4 between a
high, forward position for high speed tow, and a low, aft position
for low speed tow. The hydrofoil actuator is operable to adjust the
shape, surface area and/or pitch angle of the hydrofoil 3. The
adjustable ballast tank may include a chamber having an opening to
the surrounding water, the volume of the tank being adjustable by
moving the position of a piston within the chamber such that the
tank contains a variable mass of ballast water. The remainder of
the chamber may be filled with compressible gas. The buoy is
connectable to a towing vessel by a tow cable (not shown) attached
to the tow point 4.
[0023] The buoy is configured as follows: [0024] Tow Point
Translation--acting to balance hydrodynamic and hydrostatic forces.
At slow speeds the tow cable orientation is dominated by the
positive trim of the buoy resulting in an almost vertical link
profile. At medium to high speed, buoy and cable drag takes
dominance and so a more horizontal profile of the link results.
This can be achieved using either a passive or active control
mechanism. The passive method could harness changes in tow cable
tension and or drag forces to translate the tow point. A simple
passive method could use a spring to oppose the tow cable tension
and locate the tow point accordingly. An active mechanism could use
a control loop system to translate the tow point position according
to the submarine's speed and or tow cable tension levels. [0025]
Buoy Buoyancy--Nett buoyancy (buoyancy subtract mass) is varied by
changing the mass alone, through the ballast tank. This is
important for low tow speeds when the buoy function has to support
variable vessel depths and thus changes in tow cable mass (assuming
tow cable is not neutrally buoyant). [0026] Metacentric
Height--directly affecting roll and pitch stability. This is
important when maintaining control in the dynamic wave zone.
Stability improves with increasing vertical separation of CoG and
CoB.
Orbital Particle Motion
[0027] Orbital particle motion is the movement of particles in the
water column below a passing surface wave. Its effect on the buoy 1
will be apparent from the following sections, and a brief
introduction to this motion will now be described. As each wave
passes along the water's surface the particles in the water column
below complete an orbital path; moving upwards on the leading edge
of the wave, then horizontally in the direction of the wave at the
top, then downwards at the back edge of the wave. In deep water
this effect is experienced for many meters below the surface,
eventually decaying to less than 5% at a depth approximately half
the wavelength. An illustration of the particle movement is shown
in FIG. 3 along with a graph of the typical orbital velocities that
occur in sea state 6 at a depth 5 m below the mean surface
elevation. As can be seen, this orbital particle motion does
include a significant vertical and horizontal velocity.
Buoy Performance
[0028] To evaluate the ability of the buoy 1 to maintain a shallow
depth across the operational envelope a series of simulations were
performed, using the setup shown in FIG. 4.
[0029] Performance of the buoy 1 has been evaluated for 0, 3 and 6
m/s submarine speeds (V) and in sea states (SS) 3 and 6. Submarine
depth (D) was maintained at 102 m. The link length (l) was varied
according to the submarine speed and sea state conditions.
[0030] The following two sections present the results of the two
sea state conditions evaluated.
Sea State 6
[0031] To maintain a depth of less than 3 m means that the buoy
must attain a position that will be above the trough of many of the
larger waves. Extracts of the resulting buoy depth and pitch
profiles from the 6 m/s tow in sea state 6 are shown in FIGS. 5a
and 5b, respectively. The vertical translation of the buoy is
generated purely by the orbital motion of the water particles.
[0032] This helps to minimise depth variation of the buoy as it
passes beneath the waves. The buoy however will react to the
localised particle flows at a slower rate due to its size and the
tow cable constraint, and so relative depth will change according
to the wave height and period.
[0033] Buoy pitch was limited to .+-.15.degree. and was generated
by the variable particle vertical velocity acting across the length
of the buoy and variations in tow cable tension. The stability of
the buoy is very good, with the relatively small variations in both
pitch and depth being directly linked to wave motion.
[0034] At 3 m/s the buoy still attains vertical motion, caused by
the wave particle motion but the increasing depth motion, occurring
at the back of the waves, appears to have a larger influence than
that driving it up. When each wave passes over, the buoy gets
pushed below the trough of the wave and it takes several seconds
for the buoyancy to bring the buoy back up to the optimum
depth.
[0035] The effect of the down-flow being more influential than the
up-flow is also evident during the 0 m/s case (see FIGS. 7a and
7b). Here the limited tow cable prevents the buoy from climbing
above 2 m depth and so the advantage of the up-flow is lost. The
effect of the down-flow however is not restricted and so the buoy
experiences this for a longer time period.
Sea State 3
[0036] With its much smaller wave elevations, it is not necessary
for the buoy to enter the wave structure so the buoy maintains a
much safer distance from the surface with little chance of
broaching.
[0037] FIGS. 8a and 8b (6 m/s case), FIGS. 9a and 9b (3 m/s case),
and FIGS. 10a and 10b (0 m/s case) show the reduced effect of the
particle motion upon both the depth and pitch of the buoy at sea
state 3. With the slight adjustments to trim and tow point position
the buoy is able to maintain depth at all speeds evaluated.
Tow Cable Study
[0038] The submarine pulls the tow cable and buoy through the water
but it is up to the buoy to lift the tow cable towards the sea
surface. The natural lie of a cable being pulled through the water
is directly behind the submarine due to the drag forces acting on
it (depending on the tow cable trim).
[0039] To achieve the same climb height (H) for a fixed length (350
m) of tow cable at a tow speed of 6 m/s requires as much as four
times more lift than at 3 m/s. This is tow cable length dependent,
reducing to a factor of three for longer tow cables.
[0040] To double the height (H) of the buoy from 100 m to 200 m
without changing submarine speed or cable length (350 m) requires
an increase in lift demand by a factor of four. This reduces
slightly to a factor of three for longer tow cables.
[0041] Tow cable weight has a considerable impact on the amount of
lift needed to raise it. The size of the impact is speed and tow
cable length dependent with slow long tows being the most
sensitive.
Impact of Tow Cable on Buoy
[0042] Assuming a typical tow cable having a diameter of 0.02 m,
mass in air of 0.962 kg/m, CD Axial of 0.015 and CD Normal of 0.4,
the drag forces generated in relation to the tow cable are far
greater than the drag forces of the buoy alone. The performance of
the buoy will be therefore dominated by the necessity to lift the
tow cable and so in almost all cases except very low speeds (0 to
0.5 knots) the buoy stability will be dependent upon three forces,
namely the buoy drag, lift and tow cable tension.
[0043] The ability of the towed buoy system to deliver the payload
into its operating zone is dependent upon the buoy being able to
generate the large lift forces needed to control the tow cable. The
two most common methods of generating significant lift are through
water displacement (buoyancy) and dynamic lift using
hydrofoils.
Buoy Lift
[0044] The lift coefficient of a hydrofoil is dependent upon its
angle of attack with respect to the approaching water. Small
changes in the angle of attack can result in large changes in the
lift coefficient resulting in rapid changes to the buoy's depth. A
buoy that utilizes the lift generated by a hydrofoil needs to be
stable in pitch to prevent rapid changes in depth.
[0045] The buoyancy approach removes the dependency upon through
water speed and angles of attack, ensuring a constant lift force is
available to hold the tow cable high in the water.
[0046] The hydrofoil on the buoy ensures it is capable of
generating the lift required to remain above the submarine. To
maintain a relatively constant lift coefficient the stability of
the buoy in pitch needs to be high. An increase in stability can be
achieved by offsetting the hydrofoil generated forces and the tow
cable connection point on the buoy. For small angles of attack the
variation in drag produced by the hydrofoil are virtually
negligible since drag is less than about 2% of the lift generated
force.
[0047] Instead of pitch generating lift fluctuations, these are
caused by changes in the buoy's through water speed if the pitch is
held approximately constant as the buoy travels in a passing wave.
The orbital particle motion created by the waves interacts with the
hydrofoil (hydroplane). The hydrofoil lift reduces as the buoy
passes through the trailing section of the wave and so the tow
cable tension pulls the buoy down. As the buoy enters the next wave
the hydrofoil lift increases again pulling the buoy and tow cable
up into the wave structure.
[0048] The buoy pitch stability is improved by providing a
significant vertical offset between the buoy CoG and CoB. A mean
nose up pitch orientation of around 8 degrees generates the
required lift. The buoy pitch varies by around +/-8 degrees over a
five minute period, well within the optimum lift range of the
NACA0012 hydrofoil used in the study. This ensures the changes in
the drag coefficient of the hydrofoil remain negligibly small.
[0049] Optimized depth performance for a 3 m/s tow speed at 100 m
depth is achieved with 150 m of tow cable deployed and with 50 kg
of displacement buoyancy assigned to the buoy by reducing its
mass.
[0050] FIG. 11 shows the depth performance in sea states 3, 5 and
6, with all achieving at least 85% of the simulation period at less
than 3 m depth without broaching the surface.
Tow Speed
[0051] At 0 m/s the hydrofoil lift is limited to that generated by
the particle orbital motion and the movement of the buoy as it is
pushed around in the waves. This is unreliable and so additional
buoyancy has to be introduced to support the tow cable. In low sea
states (0 to 4), the buoy attains a compliant depth performance. In
sea states 5 and 6 the performance is less reliable and there is an
increased risk of surface broaches.
[0052] At 6 m/s the buoy is less stable than at lower speeds. By
increasing the vertical and horizontal offset between the hydrofoil
and tow cable termination positions, stability improves
sufficiently. For the 100 m depth case the tow cable length has to
be increased to 230 m. Depth performance with this revised buoy/tow
cable configuration is consistent with that at 3 m/s.
Trim Control
[0053] The dynamic systems for inclusion in the buoy design to trim
the buoy's performance are:
Moving Buoy Tow Point
[0054] At low speeds the metacentric forces act to stabilize the
buoy in roll and pitch but as speed increases the hydrodynamic
forces dominate and any stability provided by the buoyancy and mass
becomes negligible. It is therefore important that the tow point
can be moved to balance hydrodynamic forces at high speeds and
hydrostatic forces at low speeds to improve stability across the
operating range. As tow speed increases, the offset between the tow
point and the CoG increases both vertically and horizontally. This
could be controlled passively by harnessing the changes in the
tension or drag forces, or alternatively, actively by means of an
electrically powered servo motor as part of a closed loop
controlled system.
Variable Buoyancy
[0055] Buoyancy is crucial for very low submarine speeds when the
hydrodynamic lift of the buoy is insignificant. The mass of the tow
cable will have a large effect upon the buoy performance, and this
will change according to the submarine depth. The depth keeping
performance of the buoy is dependent on buoyancy across the
operating speed range, therefore some variation in the buoyancy of
the buoy will be required. The amount of buoyancy variation
required is tow cable mass dependent with the slow deep submarine
case being most reliant.
Active Hydrofoil
[0056] The depth keeping requirements for the buoy can be met with
a fixed hydrofoil which reduces the complexity of the buoy.
However, an active hydrofoil capable of increasing its surface area
and/or varying its pitch angle may be used.
[0057] Having the ability to make these adjustments will not only
widen the operational range of the system but also provide
additional flexibility for operations such as surfacing, deployment
and recovery, ensuring minimal risk of damage to the buoy and
towing platform.
Moving Keel
[0058] The stability of the buoy can be enhanced for high speed
operation by increasing the separation between CoB and CoG by use
of a moveable keel. The moveable keel can be retracted to minimize
the height of the buoy for storage within the vessel bay.
Control
[0059] The buoy contains actuators to tune tow-point location,
ballast/buoyancy, hydrofoil configuration and keel position, but
these are only used to tune the buoy hydrodynamic characteristics
for the specific speed and depth of operation. As the boat speed
and depth do not vary continuously, the actuators on the buoy are
only required to change position occasionally or slowly.
[0060] There are significant advantages to this approach to
controlling the buoy. Firstly, the buoy has a minimum number of
closed-loop control systems and no fast acting actuators meaning it
is much simpler to design, test and prove than a closed-loop
control system. The slow response of the actuators also reduces
their power requirements and allows them to be made smaller. The
position of each actuator can be predefined for the current boat
speed and boat depth, with these values being determined from
modeling and tank trials. This significantly reduces the modeling
and test time required to ensure that a complex closed-loop control
system is stable under all conditions, and replaces it with the
requirement to prove a number of fixed configurations of buoy for
the differing operating conditions.
[0061] The fine control of the buoy depth is performed on a closed
loop basis by adjusting the length of tow-cable deployed.
Buoy Design
Mechanical
[0062] As shown in FIGS. 1 and 2, the buoy 1 has a streamlined body
2 with a hydrofoil 3. Onboard actuators (passive and or active)
control the buoyancy, the hydrofoil length, angle of attack and the
tow-point location. The buoy may be manufactured from composite
materials with static buoyancy provided by sections of high density
foam.
[0063] The payload electronics are contained within pressure
vessels, with the actuators mounted externally from the
electronics.
Actuator Control
[0064] Control of active actuators in the buoy will be commanded
from the towing vessel, with the actuators moving to predefined
positions for the specific speed and depth of the vessel.
[0065] At low speeds, the buoyancy will be increased and the
tow-point moved aft and down. This acts to stop the buoy pitching
up at too great an angle as the natural buoyancy dominates the
hydrodynamic lift of the buoy. At higher speeds, the buoyancy can
be reduced as the hydrofoil provides all the lift required, and a
forward located tow-point provides stability.
[0066] Additionally, as the depth of the boat increases, the lift
required from the buoy will need to be increased to overcome the
increased drag and weight of the longer tow-cable and so the
hydrofoil and buoyancy need to be adjusted accordingly.
Alternative Buoy Designs
[0067] FIG. 12 illustrates a side elevation of a second example of
a buoy of this invention. The buoy 101 has a hydrofoil body 102, a
moveable keel 103, and a moveable tow point 104. The body 102
houses a payload, an adjustable ballast tank 105, a tow point
actuator, and an actuator for moving the keel 103.
[0068] The tow point actuator is operable to move the tow point 104
between a high, forward position for high speed tow, and a low, aft
position for low speed tow. The tow point actuator includes a wheel
104a, which can rotate to move the tow point between these two
positions. The wheel 104a is rotatably mounted on the forward end
of the body 102. This contrasts with the linear movement of the tow
point 4 in the buoy 1 of the first example described previously.
However, the tow points 4 and 104 share the common feature of
enabling translational movement of the tow point with respect to
the remainder of the buoy, under active or passive control. The two
types of tow point are interchangeable on the buoys 1 and 101.
[0069] The keel actuator is operable to adjust the position of the
keel 103. The keel 103 is a heavy, streamlined body of generally
circular cross section rotatably mounted on one end of keel arm
103a, whose other end is rotatably mounted on the underside of the
body 102. Under the action of the keel actuator, the keel 103 can
be moved between a high, retracted position and a low, deployed
position. When in its retracted position, the buoy 101 occupies a
smaller space for storage, e.g. onboard the towing vessel. When in
its deployed position, the CoG of the buoy 101 is moved aft and
downwardly, increasing the metacentric height, and therefore the
stability, of the buoy. The position of the deployed keel can be
adjusted to vary the position of the buoy CoG. However, it is
likely that the keel 103 will be typically fully deployed to
maximise the stability of the buoy 101 in pitch and roll. The keel
103 may be used to house control electronics, if desired.
[0070] The adjustable ballast tank 105 is similar to the ballast
tank 5 of the buoy 1 described previously, and operates
similarly.
[0071] The buoy 101 is connectable to a towing vessel by a tow
cable (not shown) attached to the tow point 104. The buoy may be
operated in a similar manner as for the buoy 1 described
previously.
[0072] FIGS. 13a to 13d illustrate isometric, front, back and side
elevations of a third example of a buoy of this invention. The buoy
201 has an upper hydrofoil body 202, a pair of lower hydrofoils 203
each mounted from the hydrofoil body 202 by a pair of end plates
203a, and a moveable tow point 204. The body 202 houses a payload,
and a tow point actuator. The tow point actuator may be a linear or
rotary actuator similar to that used in the first or second
examples of the buoy described previously for moving the position
of the tow point 204. An adjustable ballast tank may be disposed in
one or more of the upper hydrofoil 202 and the lower hydrofoils
203. The ballast tank(s) may be similar to those used in the first
and second examples of the buoy described previously for adjusting
the buoyancy of the buoy. One or more actuators may be provided for
moving the hydrofoil 202 and/or the hydrofoils 203 for adjusting
the lift generated by the buoy under tow. The buoy 201 is
connectable to a towing vessel by a tow cable attached to the tow
point 204. A tow point guide rail 206 extends in an arc from the
upper hydrofoil 202 and between the two lower hydrofoils 203. The
end of the tow cable, or tether, (not shown) incorporates a slider
block at one end which slidably engages with the rail 206 to follow
the contours of the rail according to the local catenary shape. The
guide rail 206 acts as a manual means of adjusting the tow point
position. A vertical profiled tether will self locate to the lower
central section of the rail 206, and more horizontal tether will
locate on the higher forward rail section. The buoy may be operated
in a similar manner as for the buoys 1 and 101 described
previously.
[0073] 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.
* * * * *