Practical Wings

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While on one doubts the performance benefits of
wings, the obvious drawback of practicality and cost has, so far, kept them
away from your local sailing fleet.  AdvancedWing Systems is planning to change this. Greg Johnston has taken the time to
explain and introduce us to their semi ridged wing sail system:

Practical Wings for Sailing Boats

The America’s Cup is creating a lot of
interest in wing sails.  The AC 45 catamarans are a great showcase for
solid wing technology.  Wing sails have been proven to give significant
performance gains.  We are seeing rigid wing sail creeping into more and
more classes of boats.  However, this type of technology is really not
practical for most yachts.  The infrastructure required to rig, unrig and
store wings for other than off the beach craft or those with very deep pockets,
is simply not practical.

Does this mean that we are destined to
keep refining the single surface sail to try to extract that last bit of
efficiency?  As rigs get taller and masts get slimmer the drag associated
with rigging to support the sail become self-defeating.  I think we have
reached the top of the technology curve for single surface sails.  The
cost of minor performance improvements is spiralling upwards.

There is an alternative.  Double
surface, semi rigid wing sails can offer great performance and still keep costs
to reasonable levels.  Presented here is one such design.  It is a
simple technology that can be applied to every day sailing craft.
Figure 1: A 25m2 Wing Sail being used on a 7.2m Sports Boat

The system is based on a counter
rotating mast which supports two fully battened sails.  The mast can be
much larger in section than a conventional mast.  This means that thinner
walled sections can be used and less supporting rigging is required.  The
mast forms the leading edge of the sail and parasitic drag from the mast is
almost entirely eliminated.  This translates into a stiffer, lighter, and
lower drag rig.  Figure 2 shows a typical wing section developed by this
system.  The mast is free to rotate with the boom.  Figure 4 shows
what this looks like in practice – there is good flow attachment on both sides
of the sail – all from a sail that can be reefed and stowed.

Figure 2: Mast and Sails form a cambered, asymmetrical wing section
Figure 3: View Between the Sails
Figure 4: Leeward and Windward Mast to Sail Transitions

This mast and sail configuration is
capable of producing very good aerodynamic characteristics.  Computational
Fluid Dynamics (CFD) modelling of the wing sections achieved show maximum
coefficients of lift (CL) of around 1.8 for a sail operating in around 10 knots
of apparent wind.  While not as good as the high CL’s achieved with the
multi-element hard wings it is still much better than can be expected from a
single surface sail.  However, it is not all about max CL.  When
sailing upwind a high lift to drag ratio (L/D) is required.  CFD analysis
shows L/D of 80 for the section shown in Figure 2.  Furthermore, the twist
through the sail can be controlled to ensure good span wise flow distributions
and also allow depowering of the head.

The mast shape used is not
aerodynamically optimal, however, when considered in the context of an
operating yacht and the compromises that result from the structural and rigging
requirements, the above results represent a good outcome.  As a
comparison, results for a NACA6409 wing section are shown at Max CL (1.623) and
Max L/D (84.9).  See Figure 8 to Figure 11.

A significant advantage of a thick wing
sail with small or no mast drag is that the drive from the sail is
substantially better than for a conventional sail.  On an aerofoil, the
greatest pressure differentials are closer to the leading edge.  Figure 5
shows the pressure gradients of the section in Figure 2. Figure 6 shows the
area behind the mast of a conventional sail that is affected by the mast. 
The wings forward half generates most of the force, while the sail is only
effective once the flow has re-established further down the sail.  The
upshot of this is that the wing’s force is in a more forward direction,
resulting in more drive and less heeling force (ignoring that the forces are
greater anyway).
Figure 5: Pressure Distribution
Figure 6: Separation Behind a Mast
Figure 7: Forward Driving Force
Figure 8: CFD Analysis at Max CL
 Figure 9: CFD Analysis at Max L/D
Figure 10: NACA6409 at Max CL
Figure 11: NACA6409 at Max L/D

Rotating masts present problems with
stayed rigs, particularly on narrow hulls which require spreaders to maintain
stay angles.  We developed a self-supporting spreader system to overcome
this problem.  The spreaders are locked into place by the wires or rod and
only exert perpendicular load onto the mast panel they support.  This
means the mast can rotate freely behind the spreaders.  Figure 12 shows
the spreaders are attached to the mast by a ring running on a plastic bearing
surface.

Figure 12: Mast and Spreader Attachment
Figure 13: Mast Step

The mast is stepped onto a stainless
steel ball.  In Figure 13 the original mast post has been replaced with a
tube with the ball mounted onto it.  In this instance there is adequate
sideways support for the mast step.  When docked, the rigging does not
stand out as being notably different than a conventional rig – the twin sail
tracks are what give it away.

Figure 14: The Wing at Dock

These staying arrangements mean that the
wing sail can be retrofitted easily to most boats.  We recommend aft swept
spreaders, so ideally the target boat has this staying arrangement
already.  Multi-spreader rigs are also possible, as are mast head and
fractional rigs.  Our own rig is fractional and supports a fractional
asymmetrical spinnaker.  However, we are about to fit a mast head
spinnaker for lighter winds.

The rig controls are quite simple. 
Our boat uses a full width main traveller at 80% of the boom.  This
replaces the vang and allows us to get full sheet angles when the wing is set
with normal operating thickness.  Wider sheet angles are possible if the
wing is flattened. In these circumstances depowering is usually the objective
so releasing main sheet allows the boom to travel further and also depowers the
top of the sail.

Mast rotation is always set relative to
the boom.  So when you sheet, the mast moves with the boom.  The
rotation can be controlled by ropes and pulleys or electronically using a
linear actuator – as in Figure 15 – mounted in the front of the boom.  An
advantage of the actuator method is that the wing section can be computer
controlled using wind speed and direction, boat speed and direction, and sheet
angle as inputs.  Precise feedback can also be given to the crew on angle
of incidence to aid trimming.



Figure 15: Electronic Linear Actuator Controls Mast Rotation

The whole package is quite light. 
Our 25m2 wing, including all rigging, halyards, controls, sails and battens
weighs in at less than 90kg – or about 3.6kg/m2.  This weight is achieved
with aluminium structural spars and spreaders and stainless steel
rigging.  Compare this to an AC45 wing which is about 4.5 kg/m2.

The exciting thing is that we are at the
beginning of a new technology curve. Performances and weights are all open for
significant improvements.  We are designing multi-element semi-rigid
wings, with flaps and slots, while maintaining the ability to stow and
reef.  We are interested in building relationships with manufacturing and
distribution partners.  Visit us at www.advancedwingsystems.com

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  Comments: 1

  1. Anders Nilsson


    What do you want to show with figure 7. Isn´t obvious that when you release the sheet you get the force to be more pushing forward?