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

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Safety at Sea 2003 Annapolis Slide 2 My Qualifications in Structures and Stability…

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Safety at Sea 2003 Annapolis Slide 3 Static (not moving) Stability Buoyancy Force acts upward through the center of underwater volume (Center of Buoyancy)Buoyancy Force acts upward through the center of underwater volume (Center of Buoyancy) The sum of these forces equals zero! B Weight Force acts downward through the Center of GravityWeight Force acts downward through the Center of Gravity W

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Safety at Sea 2003 Annapolis Slide 4 B W 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 Give me a lever and I will move the earth! (or at least a boat!) Static Stability When Heeled

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Safety at Sea 2003 Annapolis Slide 5 Static Stability When Really Heeled! B W Limit of Positive Stability LPS is when the two vectors are opposite.

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Safety at Sea 2003 Annapolis Slide 6 Static Stability - Beam Effects B W B W Different Righting Arms! Form Stability Ballast Stability W Equal Righting Arms!

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Safety at Sea 2003 Annapolis Slide 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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Safety at Sea 2003 Annapolis Slide 8 The Righting Moment Curve RM Heel Angle Narrow Boat - Ballast Stability Beamy Boat - Form Stability Positive area under the curve = work to capsize! Negative area under the curve = work to re-right!

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Safety at Sea 2003 Annapolis Slide 9 Dynamic (Moving) Stability 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! A Vessels Response to Wind and Waves is a function of:

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Safety at Sea 2003 Annapolis Slide 10 Length is not a big factor! For two boats of the same weight, the smaller will often be more seaworthy!

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Safety at Sea 2003 Annapolis Slide 11 ISO – The New Stability Rule Created in response to EC RCD 94/25 for selling boats in EuropeCreated 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).Three parts depending on boat size and whether power or sail (part 2). Gives a category rating to boats based on their seaworthiness factors.Gives a category rating to boats based on their seaworthiness factors. The factors are combined into a number called STIX.The factors are combined into a number called STIX.

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Safety at Sea 2003 Annapolis Slide 12 STIX Categories CategoryEnvironment Min STIX A Beaufort 10 (<55 kts) H 1/3 <23 (max <46!) 32 B Beaufort 8 (<40 kts) H 1/3 <13 23 C Beaufort 6 (<27 kts) H 1/3 < D Beaufort 4 (<16 kts) H 1/3 <1.6 5

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Safety at Sea 2003 Annapolis Slide 13 STIX FACTORS Starting point is length in meters (L BS )Starting point is length in meters (L BS ) All Factors are near 1.0 ( ) and modify LBSAll Factors are near 1.0 ( ) and modify LBS Displacement Length Factor (FDL)Displacement Length Factor (FDL) Beam Displacement Factor (FBD)Beam Displacement Factor (FBD) Knockdown Recovery Factor (FKR)Knockdown Recovery Factor (FKR) Inversion Recovery Factor (FIR)Inversion Recovery Factor (FIR) Dynamic Stability Factor (FDS)Dynamic Stability Factor (FDS) Wind Moment Factor (FWM)Wind Moment Factor (FWM) Downflooding Factor (FDF)Downflooding Factor (FDF)

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Safety at Sea 2003 Annapolis Slide 14 STIX Formula Good PointsGood Points Empirically based off a normal boatEmpirically based off a normal boat Adjustable/flexibleAdjustable/flexible Reflects positive flotation benefitsReflects positive flotation benefits Best we have…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 < 90 o LPS ?) No history

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Safety at Sea 2003 Annapolis Slide 15 Stability Suggestions! Consider Category A Oceangoing, but possibly only Coastal if STIX between 32 and 40 and the boat is over 35 ft.Consider Category A Oceangoing, but possibly only Coastal if STIX between 32 and 40 and the boat is over 35 ft. B means InshoreB means Inshore But, small boats with large displacements may be a STIX B but should be an ABut, small boats with large displacements may be a STIX B but should be an A

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Safety at Sea 2003 Annapolis Slide 16 More Suggestions Look for a boat with an IMS LPS>120 for offshore work.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.Question boats with wide beam, high topsides and shoal draft for offshore work. Dont add weight up high!Dont add weight up high!

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Safety at Sea 2003 Annapolis Slide 17 What are Structures Hull and deck plating (fiberglass, advanced composites, wood, concrete)Hull and deck plating (fiberglass, advanced composites, wood, concrete) Connections to other components (keel, rig, engine, steering, tanks)Connections to other components (keel, rig, engine, steering, tanks) Rudder, RigRudder, Rig Design Trade-Offs Probability of Failure (Risk) Weight and Center of Gravity Cost Durability

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Safety at Sea 2003 Annapolis Slide 18 How Do Naval Architects Design Structures? Simple EquationsSimple Equations Advanced Computer ProgramsAdvanced Computer Programs Rules of ThumbRules of Thumb Classification Society Codes (ABS, DnV, ISO)Classification Society Codes (ABS, DnV, ISO)

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Safety at Sea 2003 Annapolis Slide 19 ISO Structures Also created in response to EC RCD 94/25Also created in response to EC RCD 94/25 In development (behind 12217)In development (behind 12217) Same environmental A, B, C, D categoriesSame environmental A, B, C, D categories Based on ABS Guides (sail, power) but have been watered down a bit for power boats.Based on ABS Guides (sail, power) but have been watered down a bit for power boats. Manufacturer-assessed for boats under 39 feet.Manufacturer-assessed for boats under 39 feet.

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Safety at Sea 2003 Annapolis Slide 20 Structural Approach Code determines a hydrostatic pressure based on boat length and depth. (Too conservative?)Code determines a hydrostatic pressure based on boat length and depth. (Too conservative?) Pressure is applied to plating and transfers to frames/bulkheads.Pressure is applied to plating and transfers to frames/bulkheads. Minimum structural strength of each component is checked.Minimum structural strength of each component is checked. A factor of safety (~2.3) is included.A factor of safety (~2.3) is included.

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Safety at Sea 2003 Annapolis Slide 21 Important Structural Reminders Durability is not designed into any of the Classification Society Codes (impact, abrasion, fatigue)!Durability is not designed into any of the Classification Society Codes (impact, abrasion, fatigue)! The design may not be what is built!The design may not be what is built! Small data setSmall data set Fatigue…!Fatigue…! Damage History?Damage History? Other pieces?Other pieces?

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Safety at Sea 2003 Annapolis Slide 22 Why we worry about impact!

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Safety at Sea 2003 Annapolis Slide 23 Impact Example – Navy 44 vs… Effect of Resin and CoreEffect of Resin and Core Same amount of glass fiber, different resin Navy 44 J/24 Test #11

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Safety at Sea 2003 Annapolis Slide 24 Structural Suggestions A boat designed to ABS should be OK for offshore work.A boat designed to ABS should be OK for offshore work. A boat not designed to ABS may be OK.A boat not designed to ABS may be OK. As ISO is based on ABS (at the moment)…As ISO is based on ABS (at the moment)… Add value if vinyl ester or epoxy for durability.Add value if vinyl ester or epoxy for durability. If you can deflect it, it is too weak!If you can deflect it, it is too weak!

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Safety at Sea 2003 Annapolis Slide 25 Final Structural Thought! Pedigree is meaninglessPedigree is meaningless

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