[0001] The invention consists of a lateral (horizontally) rigid frame sail boom system of
string cable lines, enclosing the major part of a vertically rigid but laterally flexible
airfoil shaped adjustable boom and its sail. A detachable flexible modular skirt,
with inversed T profile, can be attached to the underside of the boom that would increase
the effective sail area and minimise the turbulence cause by the pressure differential
on the two sides of the sail and the boom. The camber is controlled by one or more
laterally expanding rods driven by hydraulic, threaded carrier or similar mechanism.
[0002] In addition, in its complete form, primarily although not exclusively for sizable
vessels, it includes a computerised control system that determines and positions the
rods according to apparent wind parameters so that the appropriate airfoil camber
is attained and maintained by the boom and its sail.
[0003] The aim is to achieve, under given apparent wind parameters, an airfoil shape of
the total surface area of the sail including the boom itself thus maximising propulsive
force.
[0004] Optimum efficiency is attained through the use of modern carbon fiber matrix mix
and impregnation materials in a high profile rectangular cross section design for
the boom with a progressive variation of mass rate of its section in order to achieve
near perfect airfoil shape when deflected by the rod(s). The use of such materials
also aims at minimizing residual elasticity latency and fatigue ensuring long life.
In-mast or boom reefing are compatible. Boom reefing allows for battens that enhance
the sail stability for linear airflow. In-boom reefing is possible without the utilisation
of battens. In-mast reefing provides a simpler and thus more trouble free option.
[0005] With the flexible boom determining its optimum operating form as an airfoil, the
required sail has a practically flat shape.
[0006] As a simple package, the system can easily be installed on existing vessels. In this
form, the invention requires no modifications to the remaining sail propulsion systems
and will result in significant gains in sail power. FBSS requires negligible electrical
power and stored energy and none in case of manual handling.
[0007] When compared to a vessel equipped with a more classical rigging mechanism, and for
the same sail area, the invention provides benefit from:
Boat righting couple reduction due to lower sail center of effort.
Sharper tacking behavior, due to an increase of the effective sail surface.
Smaller ballast and thus lighter and faster boat due to reduction of righting couple
requirement.
Prior Art
[0008] An example illustrates how sails are typically arranged on an average sailing boat
with one mast. The design shown consists of sails attached to the mast, located ahead
of amidships (Figure 1). A triangular head sail ahead of the mast is raised or pre-hoisted
along a forestay which is braced approximately from the bow to near the mast tip.
The clew as the free end of the leech and foot of the sail is hauled tightly by means
of a jib sheet so that the sail can adopt the most favorable possible aerodynamic
profiled shape to take advantage of the apparent wind.
[0009] The draft of this head sail also energetically intensifies the lee flow of the subsequently
positioned mainsail. This ensures the most extensive possible wind deflection without
the lee flow becoming separated from the mainsail, and thus minimises the risk of
creation of complex wake turbulence, which would result in an important loss of sail
driving force.
[0010] For increased efficiency, the mainsail usually has battens attached to it. These
allow for the formation of the most aerodynamically favorable sail profile providing
stability and allowing an increase in the sail surface compared to a more traditional
sail propulsion system utilising a mast of the same length
[0011] The characteristic configuration of the sail profile and its angle of attack is crucial
for the specific propulsion power per square meter of sail area. Utilising current
practices and current sail, mast and boom technology, the maximum production of driving
force is achieved in the upper part of the sail. In the area near the boom, the propulsion-bringing
flow profile is increasingly deflected - and ultimately interrupted - by the turbulence
produced by the boom.
[0012] Neither the angle of attack in the luff zone of a well-positioned main sail, that
is close to the incoming wind direction, nor the outflow angle at the leech following
the forward-driving flow deflection inside the sail are similarly directed to the
angle of attack of the boom. Thus, appreciable power losses occur in the transition
zones from the sail, even when profiled as correctly as possible, in the upper region,
to the straight connected boom.
[0013] Modern sails design has reached its limit in remedying this deficiency. The barrier
of a rigid boom remains insurmountable using existing technology and design.
[0014] This problem is accentuated as the otherwise usable downdraft of the head sail does
not meet an ordered lee flow on the rear side of the main sail.
[0015] In conclusion, when the main sail is attached to a standard profiled rigid boom head,
some wake turbulence is formed along the foot of the sail, caused by the exchange
of air from the luff-side (positive pressure zone) to the lee side (negative pressure
zone). The flow turbulence this produces however is combined with the more important
aforementioned turbulence losses resulting from the straight boom and results in an
important expense of drive energy and should be prevented (figure1).
[0016] Wind tunnel research confirms this situation. An example reference can be found at
http://www.wb-sails.fi/Portals/209338/news/470StreamAnim/index.htm;
[0017] In commercial vessels equipped with successive masts hoisting sails equipped with
booms a similar set of deficiencies occur due to the rigidity of their booms.
[0018] Documents
US 879 986 A,
US 408 902 A and
AT 506 349 A1 propose systems, with different technical packages to attain a flexible boom, that
are more complex, cumbersome, less reliable and more difficult to operate. In addition
none of them proposes clearly a flat sail as part of the system or specifically an
inversed open T modular skirt aiming at increasing effectiveness of sail surface and
minimising local turbulence as proposed by the present invention.
Description of the invention
[0019] The invention, described hereinafter as the
Flexible Boom and its Sail System (FBSS), is designed to address and minimise the effect of the aforementioned disadvantages
(Figure 1). The generic concept is demonstrated in Figure 2.
[0020] The boom is attached to the mast in an articulated manner on all sides by means of
a gooseneck, held in an approximately horizontal position by means of a mast support
and hauled tightly with a sheet. On the upper side of the boom the foot of the sail
is held in a conventional manner, e.g. pulled in a boom groove or bound by means of
marlines in multiple locations. A flexible modular skirt can be attached to the boom
(more details below).
[0021] In its main embodiment the
Flexible Boom and its Sail System (FBSS) consists of a vertically rigid but laterally flexible high profile adjustable airfoil
boom and its flat shaped sail. It is framed by string cables tensioned in a controlled
manner by a mechanism of an expansion rod (Figure 4). In this description a rod is
driven either by a single hydraulic cylinder or a threaded carrier. It rotates inside
an expansion carriage (figure 6) around the boom and locks and expands in opposite
sides as shown in figures 7, 8 and 9. Other appropriate mechanism can be used to drive
the expansion mechanism (EM); their details can be submitted on request. A detachable
flexible modular skirt, with inversed T profile, can be attached to the underside
of the boom (figure10) that would increase the effective sails area and minimise the
turbulence cause by the pressure differential on the two sides of the sail.
[0022] In addition, in its complete form, mainly for sizable boats and commercial vessels,
the invention includes a computerized control system that automatically positions
rods according to apparent wind parameters to attain a near linear wind flow.
[0023] In operation, the luff string cable running along the length of the boom is pushed
away by the rod (Figure 4). This forces the boom to bend thus providing the appropriate
airfoil camber for a linear wind flow around the sail in its entirety including the
boom itself whilst maintaining rigidity laterally at any chosen position.
[0024] In more detail, a hydraulic EM is described first. A threaded carrier expansion mechanism
(EM) is shown as a complement. The concept can easily be extended for larger vessels
to multicarriage packages, depending on the bending force required and distribution
along the boom.
[0025] Amidships the lateral rigidity of the system is ensured by the boom being under no
bending load. The string cables frame attached to it is under no tension (Figure 3).
[0026] In any other position, with the hydraulic cylinder swung on the luff side (Figure
8), the lateral (horizontal) rigidity is ensured by a lateral frame under tension
that includes the boom itself under camber, the rod extended on the luff side and
the luff side string cable under tension. Notice that the lee side cable is at rest.
In simplistic terms the system resembles a drawn bow with the rod as the arrow positioned
off center towards the bows and the hydraulic carriage attached firmly on the boom.
(Figure 4)
[0027] The string cables on each side of the boom are attached ahead as near to the gooseneck
as possible. On the rear they are attached after the point of attachment of the boom
sheet - the exact position depending on the material used for the boom extrusion and
its cross section profile. The aim is to ensure near perfect airfoil shape under load
and thus maximum linearity of airflow with minimum swinging tendencies of the system
on the lateral (horizontal) plane. (Figure 5)
[0028] Technical details of an expansion carriage are shown in Figure 6. The hydraulic EM
is shown in figure 7 (not expanded) and figure 8 (expanded). Similar details of a
threaded carrier EM are shown in figure 9. The length of the expanding rods is determined
by the maximum desired camber.
[0029] The expansion carriage is attached to the boom at approximately a quarter of the
boom length from the gooseneck (drawing 5). In aiming at near perfect airfoil shape
of the sail, its exact position is determined by the material to be used for the boom
extrusion and its cross section profile as well as the arrangements for the kicking
strap.
[0030] The whale shaped base of the carriage, where the boom sits in, has hard elastic airfoil
shaped pads so that with increasing camber the pressure on the boom is distributed
evenly across its surface to ensure minimum contour irregularity and smooth distribution
of load. Due to their elastic properties, the integration of these flanks to the rest
of the carriage and its shape provide for minimum local wind turbulence.
[0031] At each change of course, the crew manages the position of the EM and its driving
rod in order to modify the amplitude of the boom camber ensuring near linear wind
flow of the system. When the boat is facing the wind with the boom amidships the EM
drops amidships and no lateral force is exercised on the boom.
[0032] The sail required is of a simple flat design, as the boom to which it is attached
serves as a guide allowing it, when energised by the wind, to naturally take a smooth
airfoil shape for its entire height including the boom itself. A flexible modular
skirt can be attached to the boom that would minimise pressure differential turbulence
and increase the effective sails area.
[0033] As the boom camber is adjusted according to the apparent wind parameters, the sail
itself, following this camber, will provide maximum possible propulsive power whilst
lowering its center of effort, resulting in to significant increase in performance.
At the chosen camber, stability is attained by absorbing the variation in intensity
and direction of the wind, through the lateral rigidity of the system under the control
of the carriage base pinned on the boom. This arrangement absorbs volatility vibrations
ensuring minimum stress fatigue on the boom and greater airfoil stability for it and
its sail.
[0034] Most of the operational details described above are also applicable to the system
using a threaded carrier (drawing 9) or other expansion mechanism.
[0035] The above description broadly reproduces the more important features of the present
disclosure.
[0036] The proposed system as package attachment, it does not require other modifications
to a vessel s propulsion arrangements
[0037] Given the generic basis of this proposition, the embodiments of the disclosure are
not restricted to this description and the outline of the examples construction which
are arranged in the following description and the figures. Other embodiments can also
be implemented within the scope of the invention as defined by the claims and executed
in different ways; details can be submitted on request. In addition, it is understood
that the phraseology and terminology used is only used for the description and not
as a restriction as it does not demonstrate finer details of the system s design deemed
unnecessary for the purposes of this application.
[0038] The exemplary embodiments of the invention are explained in more detail with reference
to the drawings.
List of Figures
[0039]
- Figure 1
- shows the wake turbulence along the foot of the sail.
- Figure 2
- shows the generic concept of the invention
- Figure 3
- shows the boom amidships
- Figure 4
- shows the boom under camber
- Figure 5
- shows the points of attachment of the tension string cables
- Figure 6
- shows an expansion mechanism (EM) carriage
- Figure 7
- shows states of the expansion mechanism
- Figure. 8
- shows an expansion rod extended.
- Figure 9
- shows a threaded carrier EM
- Figure 10
- shows a flexible skirt and its modular element
Description of the drawings
[0040] Figure 1 shows the distribution of wind flow around a boat in wind tunnel tests.
When the main sail is attached to the boom head, e.g. with a freely tensioned, therefore
profile-able foot of the sail, the formation of some wake turbulence along the foot
of the sail, caused by exchange of air from the luff-side (positive pressure zone)
to the lee side (negative pressure zone) is quite evident. The tow turbulence this
produces coupled with the significant turbulence generated by the straight boom (not
adapted to the direction of flow) are at the expense of the drive energy. The proposed
system greatly reduces this obstacle.
[0041] Figure 2 shows the generic concept of the invention (a bend bow by an eccentric arrow).
It is applicable in all cases of sail booms and their sails as used by various kinds
of recreational and professional sailing boats as well for commercial vessels of all
sizes carrying boom supported sails. The accurate location of the extending rods carriage
and the extension string cables attachment points along the boom length is critical
for the attainment of a near perfect airfoil and maximum effectiveness of the sails
driving capacity. The location of the rear attachment, very close to the rear end
of the boom, moderates the required load for the attainment of certain camber. This
however needs to be balanced with the need to minimise swinging tendencies created
by the position of the boom sheet attachment. The risk of additional turbulence at
the straight part of the boom at its tail after the rear attachment point is considered
negligible.
[0042] Figure 3 shows a boom amidships with its expansion rod system (3). It is attached
to the mast (1) in an articulated manner on all sides in the conventional manner by
means of an appropriately modified gooseneck (2), held in an approximately horizontal
position by means of a mast support and hauled tightly with a sheet. On the upper
side of the boom the foot of the sail is held in a conventional manner. The rod expansion
system (3) is attached to the boom with safety pin, as shown in figure 6 below. Expansion
cable strings (4) are attached to the fore and aft (5), as discussed further.
[0043] Figure 4 shows a boom under camber with a starboard wind and a single rod hydraulic
EM carriage. The boom has swung around the mast (1) via its gooseneck (2) in a favorable
angle to the oncoming apparent wind. The expansion mechanism (EM) carriage (6) is
lifted laterally and the expansion rod has been activated, pushing the luff side string
giving the boom an airfoil shape, in order to achieve optimal shape of the total surface
of the sail and the boom itself. It is thus producing maximum propulsive power under
the prevailing wind conditions. The lee string is not under tension.
[0044] Figure 5 shows the points of attachment of the tension strings on the boom. The front
attachment (3) is located as forward as possible by the mast (1) and its gooseneck
(2). The rear attachment point (5) is located aft near the point of attachment of
the boom sheet. The EM carriage is located around a quarter of the boom length from
the gooseneck. The exact position for both depends on the material used for the boom
extrusion and its cross section profile the aim being to ensure maximum linearity
of airflow with a minimum boom bending and swinging forces. Note that these string
attachments arrangements apply irrespective of the extension mechanism (EM) used.
[0045] Figure 6 shows a typical expansion mechanism (EM) carriage. The boom and its sail
are retained by the expanding pads of the EM carriage via an attachment pin. The carriage
has two slots where the expansion strings rest when not under tension. The EM auxiliary
lifting hydraulic cylinders are positioned on the edges of the carriage to ensure
their free movement when they raise the EM on either side. When the EM is raised,
supporting blades ensure its stability in operation. A turbulence limiting hood minimises
local wind flow disturbance.
[0046] Figure 7 shows the expansion carriage and its hydraulic EM in two phases. At rest
the lifting cylinders are inactive and the EM rests amidships. Both expansion strings
rest in their slots under no tension. When the system is activated, in the second
phase, the EM is raised in an almost horizontal lateral position on the luff side
but the expansion rod is still inactive and the luff expansion string is also at rest.
[0047] Figure 8 shows the expansion carriage and its EM in operation. The EM is secured
laterally on the luff side and the main expansion rod is extended to a position for
attainment of the desired camber, taking the extension string with it. The bow shaped
rigid frame is formed and the boom is bent.
[0048] Figure 9 shows details of a threaded carrier EM that can easily replace the hydraulic
cylinder EM shown in the description above. A small electric motor is located at its
rear side. The threaded rod is guided by a rectangular blade sliding in a slot of
the controlling bloc to be attached to the EM carriage. In such an arrangement alternative
for manual operation can be arranged.
[0049] Finally, Figure 10 show a flexible skirt drawing and one of its modular elements.
The interlocking elements are introduced to a groove fixed to the underside of the
boom, enabling the skirt to assume the camber. As a result the surface of the sail
is augmented whilst, due to the inversed T profile of the skirt, the turbulence created
due to the pressure differential of the two sides of the sail is minimised. The material
used for the skirt is transparent. The actual height of the skirt depends on the space
available on deck and the need for an adequate visibility forward for navigation safety
purposes.