1. Sailboat Stability and Structure: The Changing Rules
Prof. Paul H. Miller, D.Eng
Naval Architecture Program
United States Naval Academy
Safety at Sea 2003
Annapolis

2. My Qualifications in Structures and Stability…
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3. Static (not moving) Stability
Buoyancy Force acts upward through the center of underwater volume (Center of Buoyancy) B
Weight Force acts downward through the Center of Gravity W
The sum of these forces equals zero!
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4. Static Stability When Heeled
This lever, the horizontal distance between the Center of Gravity and the Center Buoyancy is called the Righting Arm (RA)!
“Give me a lever and I will move the earth!” (or at least a boat!) B W
Righting Moment
= Righting Arm x Boat Weight
= “Stability”
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5. Static Stability When Really Heeled!
Limit of Positive Stability
“LPS” is when the two vectors are opposite. W B
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6. Static Stability - Beam Effects
“Ballast Stability”
“Form Stability” B W B W
Equal Righting Arms! W
Different Righting Arms!
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7. Dynamic Stability
“Ballast Stability” vessels tend to follow gravity! (They remain more upright!)
“Form Stability” vessels tend to follow the water surface! If the water surface is angled, the boat will be too!
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8. The Righting Moment Curve
Positive area under the curve = work to capsize!
Negative area under the curve = work to re-right!
Narrow Boat - Ballast Stability
Beamy Boat - Form Stability RM
Heel Angle 90
125
180
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9. Dynamic (Moving) Stability
A Vessel’s 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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10. Length is not a big factor
Length is not a big factor! For two boats of the same weight, the smaller will often be more seaworthy!
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11. 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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12. STIX Categories Category Environment Min STIX A
Beaufort 10 (<55 kts)
H1/3<23’ (max <46’!) 32 B
Beaufort 8 (<40 kts)
H1/3<13’ 23 C
Beaufort 6 (<27 kts)
H1/3<6.6’ 14 D
Beaufort 4 (<16 kts)
H1/3<1.6’ 5
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13. STIX FACTORS Starting point is “length” in meters (LBS)
All Factors are near 1.0 ( ) 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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14. 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” < 90o LPS ?)
No history
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15. 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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16. More Suggestions
Look for a boat with an IMS LPS>120 for offshore work.
Question boats with wide beam, high topsides and shoal draft for offshore work.
Don’t add weight up high!
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17. What are “Structures” Design Trade-Offs
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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18. How Do Naval Architects Design Structures?
Simple Equations
Advanced Computer Programs
Rules of Thumb
Classification Society Codes (ABS, DnV, ISO)
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19. 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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20. 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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21. 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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22. Why we worry about impact!
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23. 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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24. Structural Suggestions
A boat designed to ABS should be OK for offshore work.
A boat not designed to ABS may be OK.
As ISO 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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25. Final Structural Thought!
Pedigree is meaningless
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