Safety at Sea 2003

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Sailboat Stability and StructureThe Changing
Rules

• Prof. Paul H. Miller, D.Eng
• Naval Architecture Program
• United States Naval Academy

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My Qualifications in Structures and Stability
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Static (not moving) Stability

• Buoyancy Force acts upward through the center of
  underwater volume (Center of Buoyancy)

The sum of these forces equals zero!
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Static Stability When Heeled
This lever, the horizontal distance between the
Center of Gravity and the Center Buoyancy is
called the Righting Arm (RA)!
Righting Moment Righting Arm x Boat Weight
Stability
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Static Stability When Really Heeled!
Limit of Positive Stability LPS is when the two
vectors are opposite.
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Static Stability - Beam Effects
Form Stability
Ballast Stability
B
W
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Dynamic Stability
Ballast Stability vessels tend to follow
gravity! (They remain more upright!)
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The Righting Moment Curve
RM
0
Heel Angle
90
180
125
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Dynamic (Moving) Stability
A Vessels Response to Wind and Waves is a
function of
Uh Oh!

• Sails Set
• Static Stability (RA x Boat Weight)
• Roll Mass Moment of Inertia
• Surface Area Above and Below the Surface
• Roll Damping
• Luck!

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Length is not a big factor!For two boats of the
same weight, the smaller will often be more
seaworthy!
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ISO 12217 The New Stability Rule

• Created in response to EC RCD 94/25 for selling
  boats in Europe
• Three parts depending on boat size and whether
  power or sail (part 2).
• Gives a category rating to boats based on their
  seaworthiness factors.
• The factors are combined into a number called
  STIX.

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STIX Categories
Category Environment Min STIX
A Beaufort 10 (lt55 kts) H1/3lt23 (max lt46!) 32
B Beaufort 8 (lt40 kts) H1/3lt13 23
C Beaufort 6 (lt27 kts) H1/3lt6.6 14
D Beaufort 4 (lt16 kts) H1/3lt1.6 5
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STIX FACTORS

• Starting point is length in meters (LBS)
• All Factors are near 1.0 (0.5-1.5) and modify LBS
• Displacement Length Factor (FDL)
• Beam Displacement Factor (FBD)
• Knockdown Recovery Factor (FKR)
• Inversion Recovery Factor (FIR)
• Dynamic Stability Factor (FDS)
• Wind Moment Factor (FWM)
• Downflooding Factor (FDF)

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STIX Formula

• Good Points
• Empirically based off a normal boat
• Adjustable/flexible
• Reflects positive flotation benefits
• Best we have

• Questionable Points
• Huge overemphasis on length
• Based on experience, not basic ocean science
• Very political (made to fit current EC designs
  beam and hatch problems)
• Possible values not realistic (A lt 90o LPS ?)
• No history

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Stability Suggestions!

• Consider Category A Oceangoing, but possibly
  only Coastal if STIX between 32 and 40 and the
  boat is over 35 ft.
• B means Inshore
• But, small boats with large displacements may be
  a STIX B but should be an A

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More Suggestions

• Look for a boat with an IMS LPSgt120 for offshore
  work.
• Question boats with wide beam, high topsides and
  shoal draft for offshore work.
• Dont add weight up high!

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What are Structures

• Hull and deck plating (fiberglass, advanced
  composites, wood, concrete)
• Connections to other components (keel, rig,
  engine, steering, tanks)
• Rudder, Rig

• Design Trade-Offs
• Probability of Failure (Risk)
• Weight and Center of Gravity
• Cost
• Durability

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How Do Naval Architects Design Structures?

• Simple Equations
• Advanced Computer Programs
• Rules of Thumb
• Classification Society Codes (ABS, DnV, ISO)

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ISO 12215 - Structures

• Also created in response to EC RCD 94/25
• In development (behind 12217)
• Same environmental A, B, C, D categories
• Based on ABS Guides (sail, power) but have been
  watered down a bit for power boats.
• Manufacturer-assessed for boats under 39 feet.

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Structural Approach

• Code determines a hydrostatic pressure based on
  boat length and depth. (Too conservative?)
• Pressure is applied to plating and transfers to
  frames/bulkheads.
• Minimum structural strength of each component is
  checked.
• A factor of safety (2.3) is included.

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Important Structural Reminders

• Durability is not designed into any of the
  Classification Society Codes (impact, abrasion,
  fatigue)!
• The design may not be what is built!
• Small data set
• Fatigue!
• Damage History?
• Other pieces?

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Why we worry about impact!
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Impact Example Navy 44 vs

• Effect of Resin and Core

Same amount of glass fiber, different resin
Navy 44
J/24
Test 11
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Structural Suggestions

• A boat designed to ABS should be OK for offshore
  work.
• A boat not designed to ABS may be OK.
• As ISO 12215 is based on ABS (at the moment)
• Add value if vinyl ester or epoxy for
  durability.
• If you can deflect it, it is too weak!

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Final Structural Thought!

• Pedigree is meaningless