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**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

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**My Qualifications in Structures and Stability…**

Safety at Sea 2003 Annapolis

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**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! Safety at Sea 2003 Annapolis

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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)! “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” Safety at Sea 2003 Annapolis

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**Static Stability When Really Heeled!**

Limit of Positive Stability “LPS” is when the two vectors are opposite. W B Safety at Sea 2003 Annapolis

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**Static Stability - Beam Effects**

“Ballast Stability” “Form Stability” B W B W Equal Righting Arms! W Different Righting Arms! Safety at Sea 2003 Annapolis

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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! Safety at Sea 2003 Annapolis

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**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 Safety at Sea 2003 Annapolis

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**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! Safety at Sea 2003 Annapolis

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**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! Safety at Sea 2003 Annapolis

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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”. Safety at Sea 2003 Annapolis

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**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 Safety at Sea 2003 Annapolis

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**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) Safety at Sea 2003 Annapolis

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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” < 90o LPS ?) No history Safety at Sea 2003 Annapolis

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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” Safety at Sea 2003 Annapolis

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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! Safety at Sea 2003 Annapolis

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**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 Safety at Sea 2003 Annapolis

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**How Do Naval Architects Design Structures?**

Simple Equations Advanced Computer Programs Rules of Thumb Classification Society Codes (ABS, DnV, ISO) Safety at Sea 2003 Annapolis

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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. Safety at Sea 2003 Annapolis

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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. Safety at Sea 2003 Annapolis

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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? Safety at Sea 2003 Annapolis

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**Why we worry about impact!**

Safety at Sea 2003 Annapolis

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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 Safety at Sea 2003 Annapolis

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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 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! Safety at Sea 2003 Annapolis

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**Final Structural Thought!**

Pedigree is meaningless Safety at Sea 2003 Annapolis

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