Yacht Design

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Yacht Design Technology

• Lecture 2 Stability and Safety

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Lecture Contents

• Recap concepts of stability
• Influence of hull shape on stability
• Water Ballast Canting Keels
• Safety
• Types of capsize
• Re-righting

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Generation of Righting Moment
Yacht upright in equilibrium Mass of yacht in
air is equal to mass of fluid displaced Buoyancy
force acts upwards Displacement force acts
downwards
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Generation of Righting Moment
Yacht heels B moves to one side G remains in
constant position Intersection of B line of
action and centreline is Metacentre Yacht will
return to upright since GZ, righting moment, is
positive

• M

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Generation of Righting Moment
GZ is negative yacht will continue to roll and
will capsize
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Righting Arm or GZ curve
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Righting Arm, GZ Curve

• As heel angle increases, G B separation
  increases to maximum
• GZ then reduces to zero at limit of positive
  stability
• If yacht released at angle less than limit it
  will return to upright
• If yacht released at angle greater than limit it
  will capsize

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Righting Arm, GZ Curve

• If range of stability 180 degrees, it is
  self-righting
• GZ curve used to compare designs and assess
  against regulations

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Generation of Righting Moment

• At small angles GZ (righting arm) equal to
  GMsinf
• where f is the angle of heel
• Therefore GM is a simple measure of initial
  stability

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Influence of Hull Shape

• Stability characteristics dominated by beam and
  centre of gravity
• GM KB BM - KG
• Lower centre of gravity increase in stability
• since BM I/V
• Increase in beam reduction in displacement and
  increase in stability

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Different Hull Forms
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GZ curves for different Yachts
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Traditional Cruiser

• Heavy displacement, narrow beam low centre of
  gravity
• Relatively low initial stability since beam is
  small
• Stability increases until 90 degrees
• Range of stability is 160 degrees

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Modern Cruiser/Racer

• Lighter displacement, greater beam higher
  centre of gravity
• Greater initial stability since greater beam
• Stability increases until 60 degrees
• Lower range of stability, 132 degrees

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Modern Catamaran Cruiser

• Extreme beam. Light displacement with no ballast
  high centre of gravity
• Very high initial stability, but restricted to
  small angles
• Windward hull emerges from water at 10 degrees
  and righting arm reduces rapidly
• Lower range of stability, 80 degrees
• Very stable when inverted

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Righting moment curves for different Yachts
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Righting moment curves

• Yacht will be in equilibrium in terms of heel
  angle when
• righting moment heeling moment
• For identical rigs
• traditional yacht heel 30 degrees
• modern yacht heel 25 degrees
• catamaran heel 5 degrees

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Freeboard

• Freeboard has significant effect on stability at
  angles greater than deck edge immersion

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Coachroof Cockpit Volumes

• A large coachroof increases distance between
  buoyancy and C of G hence increases stability
  at 90 degrees.
• Cockpit has little effect on stability because
  remains above waterline at most angles

Coachroof
Flush deck
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Coachroof Cockpit Volumes
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Hull Flare

• Increasing flare has a similar effect to an
  increase in freeboard

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Water Ballast

• To improve sailing performance by a reduction in
  heel angle, ballast is frequently moved, or added
  to the windward side of the yacht.
• eg. moving crew or water tanks

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Water Ballast
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Canting Keel

• Another technique to increase righting moment is
  to use a canting keel

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Canting Keel

• Current Regulations
• Both Yachting Australia Open 60 class limits
  max static heel angle to 10 degrees. Plus
  self-righting test for Open 60 without rig.
• Volvo 70 specifies minimum capsize angle of 115
  deg with keel at max cant which is limited to 40
  degrees, plus self-righting test

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Canting Keel
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Monohull - Casualty Statistics
RNLI lifeboat launches (5 year period)
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Multihull - Casualty Statistics
RNLI lifeboat launches (5 year period)
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Types of Capsize

• Overpowered by the wind
• potential hazard for vessel with low range of
  stability i.e. less than 90 degrees
• Knocked down by a gust or squall
• can happen to any craft, not a hazard if
  structure remains buoyant and stable at large
  angles
• Rolled by a breaking wave
• small craft more likely to encounter waves large
  enough to capsize them

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Capsize by Breaking Wave

• Research concentrated on this mechanism since
  1979 Fastnet Race (many yachts rolled or
  capsized, 15 lives lost)
• Narrow forms less vulnerable than wide forms
• Conclusion that little to be gained in
  attempting to reduce vulnerability of monohull
  yacht to capsize
• Important consideration is that capsized yacht
  re-rights

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Re-Righting
Tank tests solitary waves
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Re-Righting
Tank tests irregular waves
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Re-Righting Effect of Coach House
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Re-Righting Effect of Hull Form
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Re-Righting Effect of Mast
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Recap/reflect

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