Note: Descriptions are shown in the official language in which they were submitted.
lZ~9529
SWING SAIL BOAT
FIELD OF THE INVEWTION
This invention relates to sailboats and more
particularly to a rigging configuration for sailboats.
BRIEF DESCRIPTION OF THE PRIOR ART
Numerous configurations for the rigging of sailboats
have been developed over the years. A type of rigging
prevalent today includes a mainsail roughly in the shape
of a right triangle affixed along the entire length of a
vertical edge (thle "luff" edge) to a stationary vertical
mast. The entire length of a horizontal edge (the
"foot") is affixed to a boom. The boom is pivotably
mounted to the mast at the right angle (the "tack") of
the sail such that the opposite end of the boom (at the
"clew" of the sail) can swing transversely across the
stern of the boat. A mainsheet extends between the boom
and a point on the deck at the approximate horizontal
center thereof to prevent the boom from swinging outward
~0 beyond a desired angle.
Conventionally rigged sailboats can also include a jib
which is a foresail, also roughly triangular in shape,
attached along its entire luff edge to a forestay
extending between the bow and a high point on the mast.
12~9529
--2--
A jib usually does not include a foot boom; instead a
jibsheet is attached to the clew of the sail and is
fastened adjustably to the deck near the leeward side of
the cockpit.
A sailboat may also include other sails, such as a
mizzen or a staysail. The present invention, while
described primarily with respect to boats rigged as
described above, is equally applicable to many boats
with other types of rigging.
When a sailboat is running before the wind, its
operation is fairly simple. That is, the more sail area
that can be presented to the wind, normal to the wind
direction, the greater the force propelling the boat
forward. Since the propelling force is effectively
applied at a point high above the deck, however, this
force also creates a pitching moment which tends to
upset the boat's longitudinal trim by forcing the bow
downward into the water. The effect of the pitching
moment counters the propelling force to some degree and
limits the ultimate forward velocity of the boat. In
serious cases, the pitching moment can force the bow of
the boat so far into the water that a high wave can
cause the boat to capsize. See Yachtina, March 1985,
pp. 68-71 for a graphic illustration of this.
When a sailboat is moving across the wind or into the
wind, additional complications arise which seriously
affect its performance. These factors, which are
covered in detail in two books by C.A. Marchaj, Sailina
Theorv and Practice (1964) and Aero-DYnamics of Sailinq
(1979), both published by Dodd, Mead and Company of New
York City, are briefly explained in the following
discussion as they pertain to the invention.
Heelina Forces. When sailing close to the wind, the
1279S;29
--3--
mainsail takes on an airfoil shape due to the difference
in pressure that develops on the outside (forward or
leeward) surface and the inside (sternward or windward)
surface. This difference in pressure creates a force
effectively applied normal to a point on the sail known
as the center of effort. This force ~FT) has components
in the longitudinal and transverse directions relative
to the hull of the boat. The longitudinal component
tends to propel the boat in a forward direction, whereas
the transverse force tends to cause the boat to drift
sideways. The transverse orce also acts as a couple
with the center of lateral resistance of the boat to
cause the boat to roll about a longitudinal axis away
from the wind, a motion referred to as heeling. Since
the hull of a boat generally is not perfectly
symmetrical below the deck line, heeling causes changes
in the drag of the hull moving forward through the water
as the angle of heel is changed. Thus, a hull designed
to have minimum drag at one angle of heel will not be
operating at maximum efficiency at another angle.
Heeling can also be uncomfortable to some passengers
and, in the case of wind-driven or wind-assisted cargo
ships (see Fishman, "A Fresh Wind for Sail Power," New
York Times, Sect. 6, Page 166 (12/6/81)), can cause
cargo to break loose from its security and result in
damage.
Hull Immersion. The transverse force on the sail that
causes heeling is part of a torque acting about a
longitudinal axis of the boat. At zero transverse
force, the boat is in the upright position and, as the
force is increased, the boat heels over. As the mast
moves out of the vertical position, the transverse force
increasingly develops a component directed downward
relative to the water, which has the effect of forcing
the hull deeper into the water. This increases the area
of the hull wetted by the water, thereby increasing the
~2~95;~
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drag of the hull.
Lonaitudinal Trim. The total force on the sail is
directed normal to a plane of the sail at its center of
effort, which is approximately at its geometric center.
The longitudinal portion of this force propels the boat
forward, but also creates a torque which pushes the bow
of the boat into the water with respect to the stern.
This is the same disturbance to longitudinal trim which
was described above with respect to a boat travelling
downwind. It causes the hull to move forward in the
water in a disposition other than that designed for
maximum efficiency.
Shape of Sail. When sailing very close to the wind, the
airfoil of a sail which provides the propelling force to
a boat produces a larger total force on a tall, narrow
sail than on a short, broad one of the same area. Thus,
to get the greatest propelling force from a given sail
area, the sail must be as tall as possible. This effect
is measured by the aspect ratio defined as follows:
L2
A.R. = --
SA
where L = height of sail
SA = area of sail.
However, a sail with a large aspect ratio also has a
high center of effort and therefore a greater heeling
and pitching torque. These torques therefore impose a
limit on the highest aspect ratios that can be usefully
attained. Aspect ratios between 1.0 and 2.0 are on the
low side, while 4.0 to 4.5 are considered to be the
highest that can usefully be obtained under normal
circumstances.
Directional Balance. The center of resistance of the
hull to the motion of the hull in the water is located
~279~2~
close to the fore-aft middle of the boat and
approximately at the middle of the keel or centerboard.
The lateral dragging ~orce of the water on the boat is
effectively applied at this point. This force, when
projected onto a horizontal (relative to the water)
plane together with the force-on-the-sail vector,
defines a torque or couple which determines the
directional balance of the boat. When the force-on-
the-sail vector, extended in both directions and
projected onto the horizontal plane, lies before a
similar projection of the resistance force vector, there
is a tendency for the boat to turn off the wind, and the
helmsman must compensate by holding the boat into the
wind. On the other hand, when the projected and
extended sail force vector lies astern of the resistance
vector, there is a tendency for the boat to turn into
the wind, and the helmsman must compensate by holding
the boat in the downwind direction. The magnitudes of
the torques determine the ease with which the helmsman
can steer the boat, with small torques providing good
directional balance or a "light helm." A boat trimmed
with poor directional balance has the added disadvantage
of increased rudder drag.
Mast-Sail Interactions. In one conventional
arrangement, the sail is fastened to the mast by means
of slides that ride in a track on the mast, and so the
sail always bears the same geometric relationship to the
mast regardless of wind direction. The flow of wind
around the mast creates eddies and other flow
disturbances that reduce the effectiveness of the
airfoil of the sail in generating aerodynamic force.
The effect of the mast on sail efficiency can be quite
substantial. It can be reduced for a single wind
direction by giving the mast an appropriately
streamlined shape, such as that of a teardrop, but this
would exacerbate the problem for another wind direction.
79S29
One solution to this problem is to provide a mast of
streamlined cross section, and allow the mast to rotate
about a vertical axis depending on the wind direction.
See U.S. Patent No. 4,230,060 to McCoy. This
arrangement places a heavy strain on the pivot point at
the base of the mast and requires special rotation
devices aloft where the stays and spreaders are attached
to the mast.
In the past, attempts were made to improve the operation
of sailboats by allowing the mast to slant under wind
pressure. See, for example, U.S. Patent No. 3,487,800
to Schweitzer, which discloses a sailboard having such a
pivotable mast. For larger boats, the pivotable mast
was frequently accompanied by a keel which would pivot
in response to the slant of the mast so as to counteract
the heel of the boat. This type of arrangement is shown
in U.S. Patent No. 3,903,827 to Marcil and U.S. Patent
No. 4,117,797 to Kelly.
Other arrangements that have been proposed for reducing
the deleterious forces described above include a kite or
wing sail such as that shown in U.S. Patent No.
4,068,607 to Harmon.
U.S. Patent No. 4,345,535 to Ross shows an apparatus for
countering the heeling torque. In this apparatus, a
mast is mounted on a hull so as to permit the mast to
pivot, at its base, either to port or starboard in
response to the wind. By mechanical means the pivoting
of the mast is used to apply a vertically downward force
on the windward side of the hull, thereby tending to
right the boat. The mainsail is mounted on an auxiliary
mast suspended as a pendulum from the top of the primary
mast. The lower end of the auxiliary mast rides in a
track which permits the auxiliary mast to swing to port
or starboard only, by an amount determined by the pivot
~2795~9
angle of the primary mast. Ross also shows a foresail
mounted on a jib foot boom which can pivot at the end
thereof attached to the primary mast.
U.S. Patent No. 4,005,669 to Klemm also discloses a mast
which is tiltable as a pendulum. Like Ross, however,
the movement of the base of the mast is limited by a
track to the port-starboard direction. See also the
patents listed as prior art in Klemm.
U~S. Patent No. 4,044,702 to Jamieson discloses an
arrangement comprising a vertical mast with a short,
fixed-length boomlet rotatably attached to the base
thereof. (Figs. 8-11). The tack end of a longer foot
boom is attached to the free end of the boomlet such
that the foot boom can rotate with respect to the
boomlet about an axis parallel to the mast. The sail is
then attached at its three corners respectively to the
top of the mast, the tack of the foot boom, and the clew
of the foot boom. An alternative embodiment is shown in
Fig. 12 of Jamieson. This arrangement is not intended
to reduce the deleterious forces described above, but
rather to take advantage of a slot effect created when
the sail is positioned aweather of the mast.
SUMMARY OF THE INVENTION
An objective of this invention is to provide a sailboat
which reduces or eliminates some or all of the above-
described disadvantages.
Another objective of this invention is to reduce or
control the heeling of a sailboat when sailing cloce to
the wind.
Another objective of this invention is to reduce the
sidewise, downwind motion of a sailboat when sailing
~279~29
--8--
close to the wind.
Still another objective of this invention is to reduce
or partially reverse the immersion of the hull when
sailing close to the wind.
Still another objective of this invention is to permit
the use of a sail of higher aspect ratio and total
aerodynamic force than is possible with conventional
designs.
Still another objective of this invention is to reduce
the disturbing effect of the mast on the flow of air on
the lee side of the sail.
Still another objective of this invention is to improve
the longitudinal trim of the hull of a sailboat.
Still another objective of this invention is to improve
the directional balance of a sailboat.
Still another objective of this invention is to permit
the sail to be raised higher above the water.
Still another objective of this invention is to provide
a luff boom of cross-sectional shape that will produce
minimum disturbance on the sail.
Still another objective of this invention is to provide
increased control of steering when running before the
wind in a following sea.
Still another objective of this invention is to provide
increased forward thrust on the sail by reducing the
tendency to stall as a result of increasing the angle
between wind and the leading edge or luff of the sail.
~L2795~:9
Still another objective of this invention is to allow
the design of a more efficient jib.
Still another objective of this invention is to allow
the design of a jib with a curved luff.
The aforementioned objectives and others can be achieved
by mounting the mainsail between two booms: one (the
"luff boom") generally vertical running along the luff
of the sail from tack to head, and the other (the "foot
boom") generally horizontal running along the foot of
the sail from tack to clew. The luff boom may have a
streamlined cross section with the sail fastened either
inside or close to the trailing edge thereof. The luff
boom may also be inserted in a pocket in the luff edge
of the sail. The luff boom is attached to the mast by a
fitting that allows rotation about (1) a vertical axis
parallel to the mast, (2) a horizontal axis normal to
the mast and passing through the fitting, and, if
desired, (3) the pivotal point on the luff boom, which
is to say that it can rotate in its own plane. The
fitting i5 joined to the luff boom at a pivot point
above the center of effort of the sail. Since the
fitting can be at the top of a shortened mast, no part
of the mast need be on the lee side of the sail when the
luff boom is tilted out of the vertical position.
~o fully position the sail, two sheets are used--one
fastened near each end of the foot boom. Since the mast
in this invention may be free-standing without stays or
shrouds, provision can be made to raise or lower it
along its vertical axis by having it composed of
telescoping sections. See U.S. Patent No. 4,016,823 to
Davis. The mast could then be extended or retracted by
having external screw threads on the internal member and
matching internal screw threads on the external member
so that as one member is rotated the mast extends or
~7g52~
--10--
retracts.
If the boat includes a jib, it too can be swingably or
tiltably mounted. In this situation, the luff edge of
the jib is affixed to a luff boom which is pivotably
mounted at its head end to a high point on the mast.
The luff boom may have a streamlined cross section. A
foot boom may be desirable under some circumstances, but
is not required. A jib tack sheet is secured near the
tack end of the luff boom and runs through a swivel
block on the centerline of the deck near the bow. A jib
clew sheet is fastened to the clew of the jib and runs
through a swivel block mounted on the centerline of the
deck immediately before the mast. The swivel block may
also be mounted next to the cockpit. Thus, like the
mainsail, a tiltably mounted jib can be operated either
in the conventional manner or with the luff boom tilted
toward the leeward side of the boat. Other sails, too,
can be tiltably mounted.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more clearly understood by
reference to the accompanying drawings, in which:
Fig. 1 is a side view of a prior art sailboat rigged in
a conventional manner;
Fig. 2 is a side view of a sailboat according to the
present invention in operation running before the wind;
Fig. 3 is a bow view of a sailboat according to the
present invention in operation sailing close to the
wind;
Fig. 4 is a sectional view of the prior art mast shown
on the sailboat depicted in Fig. l;
~279~;2~3
Fig. 5 ;s a sectional view of the mast, luff boom and
pivot joint shown on the sailboat depicted in Fig. 3;
Fig. 6 is a side view, partly cut away, of a portion of
a telescoping mast which may be used with the present
invention; and
Fig. 7 is a side view of another sailboat according to
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the invention in comparison with prior
art arrangements will now be described with reference to
the accompanying drawings, throughout which like parts
are ~iven like designations. Though many of the
advantages of the invention are presented by reference
to the physical forces involved, it should be recognized
that such references are greatly simplified to more
clearly illustrate the appropriate phenomPna.
In Fig. 1 is shown a side view of a prior art sailboat
10 with a mainsail rigged in a conventional manner. It
comprises a hull 12 having a deck 14 on the top surface
thereof, and a keel 16 and a rudder 18 depending from
the bottom of the hull 12. Internal to the hull 12 is a
cockpit 22, and a cabin (not shown) with a roof 20. A
mast 24 is affixed to the hull 12 and extends through
the cabin roof 20 substantially vertically with respect
to the hull 12. A forestay 26 extends from the top of
the mast 24 to a point at the bow of the boat 10, and an
backstay 28 extends from the top of the mast 24 to a
point at the stern of the boat on the centerline
thereof. The forestay 26 and the backstay 28 are
tightly secured so as to oppose sternward and forward,
respectively, bending of the mast. Additional stays
(not shown) oppose sideways bending of the mast.
~L2795~
-12-
Pivotably mounted to the rear of the mast 24, at a point
above the cabin roof 2~, is a boom 30. For reasons
which will become apparent later, the boom 30 will
sometimes be referred to herein as a foot boom. The
S mounting of the boom 30 to the mast 24 is such as to
permit the boom 30 to deviate horizontally from the
centerline of the boat by approximately 90~ in each
direction, and to deviate upward by approximately 20-.
A mainsail 32, having a luff edge 34, a leach edge 36, a
foot 38, a tack 40 between the luff 34 and the foot 38,
and a clew 42 between the foot 38 and the leach 36, and
a head 43 at the top of the sail 32, is mounted between
the mast 24 and the boom 30. The luff edge 34 may be
secured inside a slot (not shown) in the top of the boom
30 and extending most of the length thereof. A
mainsheet 48 is secured to a shackle 46 near the rear of
the cockpit 22 and extends upward to a block 44 mounted
near the clew end of the boom 30. The mainsheet-48
passes through the block 44 and through a second block
45 located near the middle of the boom 30, and down to a
locking block 47 located near the front of the cockpit
22. The block 47 is designed to permit the length of
the mainsheet 48 to be adjusted and locked at a desired
length. A boom vang 49 is shown extending between a
point on the boom 30 forward of the block 45, and the
base of the mast 24. Also shown in Fig. 1 is the center
of effort CE of the wind on the sail 32 and the center
of lateral resistance CLR of the boat 10 in the water.
This sailboat 10 is subject to the disadvantages
previously described.
In Fig. 2 there is shown a sailboat 100 according to the
present invention. The mast 24 is replaced by a
shorter, sturdier mast 50, and the stays and shrouds are
removed. The luff edge 34 of the mainsail 32 is affixed
along its entire length to a luff boom 52 which is
attached at a pivot point to the top of the mast 50 via
lZ79;;~9
-13-
a fitting 54. ~he fitting 54 permits the mainsail 32 to
tilt about an axis normal to mast S0 and lying
substantially in the plane of the sail, and also to
rotate about the vertical axis of the mast 50. Although
not necessary to the invention, the fitting 54 of the
present embodiment also permits rotation in the plane of
the sail. The pivot point on the luff boom 52 at which
the fitting 54 is attached is located above the center
of effort CE as projected onto the luff boom 52. The
mainsail 32 extends from the trailing edge of the luff
boom 52 whereas the fitting 54 is attached to the
leading edge.
The foot boom 30, which was attached to the mast 24 in
the prior art arrangement of Fig. 1, is now attached
near the lower (tack) end of the luff boom 52 in the
same manner. The luff boom 52 extends down below the
point where the foot boom 30 is attached. The boom vang
49 is attached between the lower end of the luff boom 52
and a fairlead 51 located on the foot boom 30 between
the block 45 and the luff boom 52. The fairlead 51 is
needed in the present embodiment to prevent the boom
vang 49 from interfering with a tack sheet, to be
described below. As in the prior art arrangement, the
mainsheet 48 is attached to the shackle 46 near the rear
of the cockpit 22, passes through the two blocks 44 and
45 on the foot boom 30, and i5 held taut by the locking
block 47 near the front of the cockpit 22. To avoid
confusion, the mainsheet 48 will sometimes be referred
to hereinafter as the clew sheet or main clew sheet 48.
A new tack sheet 56 is attached to a car 60 which rides
on an arcuate track 57. The track 57 is attached to the
cabin roof 20 and orien~ed so that the centerpoint of
the arc is disposed on the roof 20 just behind the mast
50, and the two ends of the arc are disposed on the roof
20 aft of the centerpoint of the arc and on either side
of the centerline of the boat. The tack sheet 56
lZ79~
-14-
extends from the car 60 up to and in through a hole in
the lower end of the luff boom 52, out through another
hole in the luff boom 52 just below the point where the
foot boom 30 is attached, through the fairlead 51,
through a block 58 mounted on the foot boom 30 next to
the block 45, and down to a locking block 59 mounted
near the front of the cockpit 22 next to the locking
block 47.
In its simplest mode of operation, the tack sheet 56 is
close hauled so as to prevent the luff boom 52 from
tilting away from the mast 50. In this configuration,
the operation of the sailboat reduces to that of the
prior art arrangement. When the tack sheet is let out
to some extent, the advantages of the present invention
begin to become important. Since the center of effort
CE of the wind on the sail 32 is below the axis of tilt,
the force F~ which the wind exerts on the sail urges the
sail to tilt, the tack end of the luff boom 52 moving
away from the mast. This is illustrated in Fig. 2 for a
boat sailing downwind and in Fig. 3 for a boat sailing
close to the wind. The luff boom 52 tilts in this
manner until the tack sheet 56 becomes taut and
restrains further tilting. Since the total force FT
produced by the wind is applied in a direction normal to
the plane of the sail 32, any component of FT in the
vertically downward direction will be reduced or
cancelled, and could potentially result in a net upward
force. The plane of the sail, as that term is used
above, varies depending on many factors. See the
Marchaj references. However, it is approximately that
plane defined by the tack, clew and peak of the sail.
In operation, when running before the wind (see Fig. 2),
the tack sheet 56 is let out and the luff boom 52
allowed to tilt forward, so that the mainsail 32 is
positioned generally in a plane parallel to the beam of
1 ~7~
-15-
the boat 100, but tilted so as to pass through the top
of the mast 50 and a point on the deck 14 forward of the
mast 50. When the wind fills the sail from behind, some
air will spill out under the foot, but the remainder
will generate a force FT directed generally
perpendicular to the plane of the sail 32. The force
therefore has an upward vertical component, or more of
an upward vertical component than it has on a
conventionally rigged sailboat. As long as the center
of effort CE is before the pitching axis, this upward
component will reduce the pitching torque. The designer
can ensure that the center of effort CE is forward of
the pitching axis in a known manner by weighting the
stern of the hull 12 more than the bow, and/or by
placing the mast 50 sufficiently forward consistent with
good directional balance.
When sailing close to the wind (see Fig. 3), the tack
sheet 56 is again let out, permitting the luff boom 52
to tilt leeward, away from the mast 50. The clew sheet
48 is also let out until a desired trim is obtained.
Since the luff boom 52 is both tiltably and rotatably
attached to the top of mast 50, the fitting 54 will
rotate such that the tilt axis remains approximately
parallel to the foot boom 30. The tack (lower) end of
the luff boom 52 is now both forward and alee of the
mast 50. When the wind comes around the lee side of the
sail 32, a pressure differential is created which
generates a force FT directed generally normal to a
plane of the sail. This force has components in the
forward, lateral and vertical directions. If the boat
is not heeling, the vertical component is directed
upward. If the boat is heeling, which is entirely
possible depending on the trim chosen, the downward
vertical component present on a conventional boat will
be reduced or cancelled.
7~529
-16-
This has a number of positive effects. First, when the
boat is running before the wind, the pitching torque
will be reduced or eliminated if the center of effort CE
is forward of the pitching axis. Second, hull immersion
caused by the downward component of FT will be reduced.
Third, the heeling torque will be reduced because the
portion of FT directed normal to the torque arm (i.e.,
normal to a plane passing through the center of effort
CE and the heeling axis) is reduced. Additionally, if
the mast 50 is retractable as described hereinafter, it
can be lowered to reduce the length of the moment arm.
This is possible because as the boom 52 tilts, the tack
end thereof moves upward and increases the clearance
between it and the deck 14 or cabin roof 20.
Fourth, since the magnitude of the portion of FT in a
plane parallel to the water is reduced, the magnitude of
the force vectors tending to steer the boat off course
is reduced, thereby improving the boat's directional
balance. This effect depends in part on the angle of
heel. Fifth, since a more upright sail will spill less
air when the boat is heeling, it should be possible to
sail closer to the wind than is possible on a
conventional sailboat. Moreover, on small boats, if the
boat is held flat in the water, a hiking strap in the
cockpit and a hiking stick on the tiller become
unnecessary. The tiller could even be replaced by a
wheel for further convenience. On large racing yachts
it is no longer necessary to have two wheels, port and
starboard.
In Fig. 4 there is shown a sectional view of the mast 24
on the conventionally rigged sailboat 10 of Fig. 1. The
mast 24 has an oblong cross section, the major axis of
which is parallel to the centerline of the boat. The
mainsail 32 is shown secured to the trailing edge of the
mast 24 and is extending somewhat to port, which is
127~S~'~
-17-
appropriate for the wind direction depicted as arrow 62.
As can be readily seen, the mast 24 blocks a portion of
the wind and causes eddies and other disturbances on the
lee side of the ~ainsail 32.
S On a sailboat according to the present invention, it is
possible to take advantage of known streamlining
technology. In Fig. 5, there is shown a sectional view
of the mast 50, the fitting 54, the luff boom 52, and
the mainsail 32, all of the sailboat 100 depicted in
Figs. 2 and 3, and all positioned appropriately for the
wind direction shown by arrow 62. The luff boom 52 has
a teardrop shaped cross section, the leading (rounded)
edge 66 of which is directed windward and is tiltably
attached to the mast 50 through the fitting 54 to be
described in more detail below. The luff boom 52 is
disposed inside a pocket 67 formed in the luff edge of
the mainsail 32, so that the mainsail 32 extends from
the trailing (pointed) edge 68 of the luff boom 52.
Since the fitting 54 is mounted rotatably about the
vertical axis of the mast 50, it can be seen that the
luff boom 52 will rotate about that axis until its
leading edge 66 is directed substantially windward,
thereby causing minimum disturbance to the wind flow for
virtually any wind direction. Moreover, as is better
seen in Fig. 3, the mast 50 causes no disturbance to the
wind flow on the leeward side of the mainsail 32 because
the tilt axis is at the top of the mast 50. Any
disturbance caused by the mast 50 occurs on the windward
side only, where a smooth flow is less important.
In the embodiment so far described, the mast 50 is
freestanding, without stays or shrouds (except possibly
a forestay). It must therefore be made of high strength
material. For large boats a carbon fiber composite may
be required, but for smaller boats an aluminum alloy
will suffice. In another embodiment it might be possible
~27gs~
-18-
to use a weaker material and add shrouds, such as by
continuing the mast above the fitting 5~ to a point
higher than the highest point reachable by the top of
the luff boom 52. This would sacrifice some of the
S advantages of the invention, but is nevertheless within
its scope.
A specific embodiment of the fitting 54 will now be
described with reference to Fig. 5. Mounted inside the
mast 50 at the top thereof is a vertical support bearing
(not shown) comprising two bearing rings, one disposed
just inside the mast 50 and one disposed several inches
down. A retaining ring 170 is visible in Fig. 5 at the
top of the mast 50. A shaft 172 passes through the two
bearing rings and the retaining ring 170 and is welded
to the side of a cylindrical container 174. Inside the
container 174 is a horizontal support bearing (not
shown), comprising two additional bearing rings mounted
just inside the two opposite ends of the container 174.
A second shaft 176 passes through these two additional
bearing rings and protrudes out one end of the container
174. The protruding end of the second shaft 176 is
welded to the side of a cylinder 178. A clamp 180
comprises two side plates 182 and 184 which are
rotatably attached to the two opposite ends of the
cylinder 178, so as to permit the clamp to rotate about
the axis of such cylinder. The two plates 1~2 and 184
extend radially out to one side of the cylinder 178 and
clamp onto either side of the luff boom 52. Three bolts
(one shown as 186) traverse the space between the two
side plates 182 and 184, at a point between the cylinder
178 and the luff boom 52, and may be tightened to secure
the luff boom 52 between the two plates. In operation,
the vertical support bearing inside the mast 50 permits
the luff boom 52 to rotate about the vertical mast axis;
the horizontal support bearing inside the container 17g
permits rotation about a tilt axis normal to the mast;
~.27~2~
--19--
and the mounting of the clamp 180 to the cylinder 178
permits rotation of the luff boom 52 in the plane of the
sail.
As mentioned previously, when the luff boom 52 is in a
tilted position, the tack end thereof and the foot boom
30 are raised higher above the deck 14 and cabin roof 20
than when the luff boom 52 is in an untilted position.
A sailboat rigged according to the present invention can
take advantage of this additional clearance to lower the
center of effort CE of the mainsail 32. This can be
accomplished by building the mast 50 in two or more
round telescoping sections 70 and 72 as shown in Fig. 6.
The outer section 70 makes up the lower portion of the
mast 50 and includes screw threads 74 on its inside
surface. The inside section 72 makes up the upper
portion and includes screw threads 76 on its outside
surface, the screw threads 76 being adapted to mate with
the screw threads 74. The outer section 70 is mounted
rotatably about its central vertical axis on bearings
(not shown), and a large wheel 77 is rigidly attached
to, and concentrically with, the outer section 70. The
inner section 72 is prevented from rotating by means not
shown. A mast so built can be extended or retracted as
desired~by rotating the wheel 77 in one direction or the
other, as appropriate, thereby keeping the center of
effort CE as close as possible to the center of mass of
the boat. Alternatively, the mast can be extended so as
to move the sail higher, into a region of higher wind
velocity. A motor may also be used in place of the
wheel 77.
The general principles of the tilt sail rig can be
applied also or instead to a jib or foresail with
advantages similar to those for a mainsail. A unique
advantage that the tilt sail principle confers on a jib
is the ability to use a curved luff.
~;~79S2~
-20-
The conventional jib is disposed with the luff fastened
along the forestay (which runs from the peak of the mast
to some point at the bow on the centerline) and held
straight by tightly securing it at the tack and peak.
Rotation of the jib around the line of the luff is
possible and the sail is trimmed by the sheet at the
clew. However, the constraint of holding the luff of
the sail means that no rotation of the jib other than
around the forestay is possible.
An important function of the jib on a conventionally
rigged sailboat is to modify and control the wind going
past the mainsail. This is done by extending the foot
of the (Genoa) jib creating a slot between the main and
the iib so that the wind speed on the lee side of the
mainsail is increased. While this modification improves
the effectiveness of the mainsail, it does so at the
expense of a large amount of sail area and results in a
jib of low aspect ratio and, therefore, low aerodynamic
force. Furthermore, a jib with a long foot is difficult
to manipulate during tacking.
An embodiment which applies the principles of the
present invention to a jib will now be described. In
Fig. 7, there is shown a sailboat 100 having a tilt-
mounted mainsail 32 as described with respect to Figs. 2
and 3. In addition, there is shown a jib 110 having a
head 112, tack 114 and a clew 116. Fitted inside a
pocket (not shown) formed in the luff edge of the jib
110 is a luff boom 118 with a teardrop-shaped cross
section. The head end of the luff boom 118 is attached
to the mast 50 near the top thereof by means of a joint
120. The joint 120 permits the jib 110 to tilt about an
axis normal to the mast 50 and lying substantially in
the plane of the sail, and also to rotate about an axis
substantially coincident with the luff boom 118. This
is accomplished by means of, for example, a ball and
79529
-21-
socket joint (not shown), or by suspending the top of
the boom 118 by a snap hook and eye 123 from an arm 121
clamped near the top of the mast 50. Although not
necessary to the invention, the luff boom 118 is also
pivotable about an axis normal to the axes of tilt and
rotation.
Attached to a shackle 122 at the tack end of the luff
boom 118 is a tack sheet 124. Tack sheet 124 is
threaded through a block 126 on the deck 14 near the bow
and adjustably secured to a winch and cleat (not shown)
near the cockpit 22. A clew sheet 128 is attached to
the clew 116 of the jib 110 and is also adjustably
secured near the cockpit 22 to a winch and cleat (not
shown). Although not part of the present embodiment,
the foresail rigging may also include a foot boom.
As with the mainsail 32, the jib 110 may be operated in
a conventional mode by close hauling the tack sheet 124
and thereby preventing any tilt or pivoting of the luff
boom 118. The advantages of the invention appear,
however, when the tack sheet 124 is let out and the luff
boom 118 is allowed to tilt outward or pivot forward.
When running before the wind, the tack and clew sheets
124 and 128 for the jib 110 may be let out to position
the jib similarly to the mainsail 32 as shown in Fig. 2.
That is, the clew 116 is out to one side of the boat 100
and the tack end of the luff boom 118 tilts forward and
upward (because the tilt axis lies above the center of
effort CEJ on the jib 110) to rest somewhat above and
forward of the hull 12. If desired, the mainsail 32 can
be set out to port (as shown in Fig. 2) and the jib 110
can be set out to starboard, or vice versa.
As with the mainsail 32, the total force of the wind
acting on the jib 110 can be considered to act on a
center of effort CEJ and in a direction substantially
~Z7~S~
-22-
perpendicular to an average plane of the sail. The
force on the jib 110 in this position therefore has a
component in the upward vertical direction relative to
the boat 100. The vector sum of this force acting at
CEJ, and that produced by the wind acting on the
mainsail 32 at its center of effort CEM, comprises the
total force FT produced by the wind acting on the two
sails 32 and 110. The total force FT acts effectively
from a combined center of effort CE located on a line
extending between the center of effort CEM on the
mainsail 32 and the center of effort CEJ on the jib 110.
Since each force individually has a component directed
vertically upward relative to the boat 100, so does the
combined force FT. Similarly to the one-sail embodiment
previously described, this upward component tends to
improve the longitudinal trim of the hull 12 by reducing
the pitching torque, and also to reduce hull immersion
by tending to lift the hull 12 out of the water.
When sailing close to the wind, again, the same
advantages over conventionally rigged boats can be
obtained as are obtained in the one-sail embodiment.
This is accomplished by letting out the jib clew sheet
128 only enough to permit the clew 116 to swing a small
distance out to port (assuming as before that the wind
is coming from the starboard bow), and by letting out
the jib tack sheet only enough to permit the luff boom
118 to tilt to port by a small angle. If the same is
done with the mainsail 32, the total force FT acting at
the center of effort CE will have an upward component
relative to the water, or less of a downward component
than in a conventional boat. The configuration improves
longitudinal trim and directional balance, reduces hull
immersion and heeling, and permits the boat 100 to sail
closer to the wind than does a conventional design.
Additionally, since the luff boom 118 can rotate about
its longitudinal axis, it can always be rotated so that
~2795~9
its leading edge is directed windward, thereby
minimizing the disturbance of the wind flow on the lee
side of the jib 110. Moreover, since all the above-
described advantages reduce the drag of the hull 12 in
the water, the present invention permits the use of a
jib with a shorter foot and a higher aspect ratio than
conventional designs. Alternatively, if the sailor so
desires, a jib with a long foot may still be used to
create a slot for passage of wind between it and the
mainsail 32.
The principles of the present invention are also
applicable to other sails, such as mizzens and
staysails.
The invention has been described with respect to
particular embodiments thereof, and one skilled in the
art can now easily ascertain its essential
characteristics. Various changes and modifications are
possible to adapt it to various usages and conditions,
all within the scope of the invention.
