The Physics of Sailing

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The Physics of Sailing
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Outline

• Hulls
• Keels
• Sails

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Hulls

• Hull Speed
• Resistance
• Shape
• Stability

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Hull Speed

• Hull speed is determined by the length of the
  boat.
• Water waves are dispersive, i.e., their speeds
  depend on the wavelength of the wave long
  wavelengths are faster.
• Boats generate a wave at the bow. The speed of
  this wave must equal the speed of the boat. (This
  is the speed with which the crest is being forced
  to advance.)

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Hull Speed

• At first, the boat moves slowly and the bow waves
  generated have short length several waves are
  seen along the side of the boat.
• As the boat moves faster, the wavelength
  increases, until it equals the length of the
  boat.
• When the wavelength becomes longer than the boat,
  the stern begins to fall into the trough of the
  wave and the boat is ploughing uphill on the
  bow wave.
• The resistance increases dramatically.

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HULL RESISTANCE

• Surface Resistance
• Shearing
• Turbulence
• Reynolds No.
• Eddies
• Separation
• Shape

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Friction Intermolecular forces
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Roughness

• Hull should be smooth.
• Bumps will introduce turbulence sooner and/or
  will produce larger turbulence.
• Polishing does not help very much.
• Shearing must take place!

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Hull Shape (Form Resistance)

• Hull shape determines how fast a boat can
  accelerate and how fast it can go in light
  winds.
• Generally speaking, narrower, shallower hulls are
  faster, but less stable and hold less cargo.
• Exact shape for fastest hull is still a subject
  of debate.

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Modern Racing Hull Design

• Narrow, sleek bow
• Shallow, flat bottom toward stern
• Square stern, normally above water line
• Able to plane under certain conditions

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Keels

• Keels are necessary to provide resistance against
  side-slipping, and to provide counter balance
  for sideways force of wind on sails.
• A large keel adds a lot of surface resistance.
• Want a balance between positive keel action and
  negative keel resistance.

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Wing theory

• Keels and sails act like airplane wings i.e.,
  they can provide lift.
• Proper design helps a lot!

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Lift (Bernoullis Principle)?
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Sail and Keel Lift
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Fluid flow around wing
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Typical Cruising Keel
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Racing Keel
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Shallow draft keel with wing
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Keels and Stability
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Sails

• Sails provide the power.
• Sails act like wings and provide lift and
  generate vortices.
• Ideal sail shape is different for downwind and
  upwind
  Downwind sails should be square-shaped
  (low aspect ratio).
  Upwind sails should be tall
  (high aspect ratio) to minimize vortex generation.

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