Yacht Design

1
Yacht Design Technology

• Aerodynamics

2
(No Transcript)
3
(No Transcript)
4
(No Transcript)
5
(No Transcript)
6
Introduction

• Sails are examples of lifting foils, similar to
  aircraft wings, propellers, keels.
• Usual design requirement for foil is to maximise
  lift force and minimise drag.
• However for sails the requirement is to generate
  maximum driving force without incurring excessive
  side force or heeling moment

7
Introduction

• Therefore sails differ from most foils since
• they need to operate at high levels of lift i.e.
  closer to point of maximum lift.
• they have a flexible geometry.
• they have virtually no thickness.
• they have a large camber.
• usually there are two or more sails operating in
  close proximity.

8
Airflow Around Aerofoil

• Two regions
• Outer region where fluid frictional effects
  negligible
• Region over foil downstream in which frictional
  effects are important, boundary layer and wake.

9
Airflow Around Aerofoil

• Boundary layer is thin At trailing edge approx 3
  5 of chord.
• Smooth, orderly, streamlined laminar flow will
  undergo transition to turbulent flow.
• This transition is delayed with a favourable
  pressure gradient (falling pressure). In
  contrast, an adverse gradient will cause almost
  instantaneous transition.

10
Airflow Around Aerofoil

• Separation If pressure gradient is sufficiently
  adverse, boundary layer is unable to maintain
  contact with foil surface reversed flow will
  occur between boundary layer and foil surface.

11
Airflow Around Aerofoil

• Near the leading edge a much stronger adverse
  gradient is required to cause separation.
• However a return to a favourable gradient can
  cause reattachment.
• Separation bubbles will exist on each side of a
  sail set behind a mast.

12
Airflow Around Aerofoil

• As lift increases the pressure gradients on
  suction surface becomes more severe.
• Separation point for trailing edge moves forward.
  Also leading edge bubble lengthens.
• Two zones merge and entire upper surface flow
  lifts off foil.

13
Airflow Around Aerofoil

• As this happens the lift will rise to a maximum
  and then reduce.
• Simultaneously the drag will increase sharply.
• This is called stalling the foil.

UPWIND Sails operate below stall CLOSE
REACHING Near point of maximum lift OFF WIND
Stalled state
14
Aerofoil Design

• Choose pressure distribution that will lead to
  proper boundary layer growth along foil.
• Optimise laminar flow over suction surface
  gentle favourable pressure gradient.
• Limit adverse gradient over rear part of foil to
  delay separation point.
• Then derive foil shape that will give required
  pressure distribution.

15
Pressure Distribution

• Pressure distribution around foil dependent on
• Shape of camber line
• Distribution of thickness across chord
• Angle of attack

16
Angle of Attack

• Ideal angle of attack fluid flow divides
  smoothly either side of leading edge of foil.
• High angle of attack
• point of attachment moves to pressure face,
  giving sharp suction peak.
• pressure rises again immediately causing adverse
  pressure gradient.
• This adverse pressure gradient responsible for
  creating leading edge separation bubble, causing
  early transition to turbulent flow, leading to
  stalling.
• Low angle of attack
• point of attachment moves to suction face,
  pressure peak is on pressure surface, with
  corresponding separation effects over surface of
  foil.

17
Angle of Attack

• Foil drag usually at minimum when operating close
  to ideal angle of attack.
• Ideal angle of attack increases as foil camber
  increases, as does lift generated at ideal angle.
• The range of angle of attack for satisfactory
  performance is dependent on radius of leading
  edge thicker foils are more forgiving.

18
Sails

• Ultimate thin aerofoil, therefore need to be
  operated close to ideal angle of attack.
• Sail camber needs adjusting as sail lift
  requirement changes. Therefore sail flattening
  devices are utilised.
• Flexibility of cloth allows camber line to change
  shape, e.g. as suction peak develops at low
  angles of attack luff lifts to windward.
• Wool tufts etc may be used on sails to identify
  flow states ensure correct sail settings

19
3-D Effects

• Foil with finite span has 3-D effects associated
  with pressure equalisation around the foil tips.
• Pressure face pressure reduces towards tips
  flow is deflected towards tips.
• Suction face pressure lowest at mid-span flow
  drawn in towards mid-span.

Flow around tips from pressure to suction face
results in formation of tip vortices which extend
into downstream wake.
20
3-D Effects

• The downwash flow is perpendicular to the foil
  and wake, directed from suction side to pressure
  side.
• The downwash has the effect of rotating fluid
  flow onto the foil and rotating lift and drag
  axes.
• This rotation reduces the effective angle of
  attack reduces lift.

21
Maximum Drive Force
Plot of total aerodynamic drag versus total lift
envelope of performance for different sheeting
angles.

• Corresponds to maximum forward drive force
• In practice best boat speed obtained using
  slightly less lift. Best light wind sail setting.
• In heavy weather sail force reduced to limit
  heeling

22
Optimum Planform

• Usually design foil to generate Lift whilst
  minimising Drag
• Minimise induced drag
• Since high downwash high induced drag
• Design foil with spanwise distribution of twist
• Transfer lift from region of high downwash to
  region of low downwash
• Reduce downwash
• Elliptical planform, e.g. Spitfire wing, is ideal
  since produces uniform downwash.
• However max lift/drag is not our aim for sails

23
Optimum Planform

• Then what is our aim for sails?
• Maximise boat speed on given course in given wind
  strength.
• Light weather max. driving force
• Heavy weather max. driving force within max.
  heeling moment
• For heavy weather therefore a sail which
  generates lift to windward at masthead can
  generate much larger driving force lower down the
  sail.
• To achieve this sail requires considerable twist
  near masthead.

24
Optimum Spanwise Sail Loading
25
Sail Twist
26
Sail Twist
27
Sail Interactions

• Sails in close proximity will interact in two
  ways
• Individual sails will operate in modified
  airflow due to presence of other sails.
• If gap between sails is sufficiently small the
  leading sail guides flow onto leading edge of
  trailing sail.
• Ahead of sail is a region of upwash.
• Upwash from trailing sail increases apparent wind
  angle of leading sail.
• Rotation of lift vector on leading sail increases
  forward drive improves performance.
• Trailing sails performance is reduced due to
  reverse effect. Therefore foresail is more
  effective than the mainsail behind it. Foresail
  carries a heavier load per unit area than the
  mainsail.

28
Sail Interactions
29
(No Transcript)
30
5 min Presentation/Activity
Tacking Gybing Boom vang Cunningham Spinaker
gybe Sail Battens Outhaul Jib sheet Main
sheet Traveller Deck winch
Backstay Man overboard! Basic sail trim Start
procedure (racing) Typical racing courses Strong
wind strategies Sailing in the Olympics Spinaker
trim Knots Crewing basics
31
(No Transcript)