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

Safety at Sea 2003Annapolis

Slide 1

Sailboat Stability and Structure:Sailboat Stability and Structure:The Changing RulesThe Changing Rules

Prof. Paul H. Miller, D.EngProf. Paul H. Miller, D.EngNaval Architecture ProgramNaval Architecture Program

United States Naval AcademyUnited States Naval Academy


Slide 2

My Qualifications in Structures My Qualifications in Structures and Stability…and Stability…


Slide 3

Static (not moving) StabilityStatic (not moving) Stability

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

The sum of these forces equals zero!

B

• Weight Force acts Weight Force acts downward through downward through the Center of the Center of GravityGravity

W


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 HeeledStatic Stability When Heeled


Slide 5

Static Stability When Static Stability When ReallyReally Heeled! Heeled!

B

W

Limit of Positive Stability“LPS” is when the two vectors are opposite.


Slide 6

Static Stability - Beam EffectsStatic Stability - Beam Effects

B

W

BW

Different Righting Arms!

““Form Stability”Form Stability”““Ballast Stability”Ballast Stability”

W

Equal Righting Arms!Equal Righting Arms!


Slide 7

Dynamic StabilityDynamic 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!


Slide 8

The Righting Moment CurveThe Righting Moment Curve

RM

Heel Angle0 90 180125

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!


Slide 9

Dynamic (Moving) StabilityDynamic (Moving) Stability

Uh Oh!– Sails SetSails Set

– Static Stability (RA x Static Stability (RA x Boat Weight)Boat Weight)

– Roll Mass Moment of Roll Mass Moment of InertiaInertia

– Surface Area Above and Surface Area Above and Below the SurfaceBelow the Surface

– Roll DampingRoll Damping

– Luck!Luck!

A Vessel’s Response to A Vessel’s Response to Wind and Waves is a Wind and Waves is a function of:function of:


Slide 10

Length is not a big factor!Length is not a big factor!For two boats of the same weight, the For two boats of the same weight, the smaller will often be more seaworthy!smaller will often be more seaworthy!


Slide 11

ISO 12217 – The New Stability RuleISO 12217 – The New Stability Rule

• Created in response to EC RCD 94/25 Created in response to EC RCD 94/25 for selling boats in Europefor selling boats in Europe

• Three parts depending on boat size and Three parts depending on boat size and whether power or sail (part 2).whether power or sail (part 2).

• Gives a “category” rating to boats Gives a “category” rating to boats based on their “seaworthiness factors”.based on their “seaworthiness factors”.

• The factors are combined into a The factors are combined into a number called “STIX”.number called “STIX”.


Slide 12

STIX CategoriesSTIX CategoriesCategoryCategory EnvironmentEnvironment Min Min

STIXSTIX

AA Beaufort 10 (<55 kts)Beaufort 10 (<55 kts)

HH1/31/3<23’ (max <46’!)<23’ (max <46’!)

3232

BB Beaufort 8 (<40 kts)Beaufort 8 (<40 kts)

HH1/31/3<13’<13’

2323

CC Beaufort 6 (<27 kts)Beaufort 6 (<27 kts)

HH1/31/3<6.6’<6.6’

1414

DD Beaufort 4 (<16 kts)Beaufort 4 (<16 kts)

HH1/31/3<1.6’<1.6’

55


Slide 13

STIX FACTORSSTIX FACTORS• Starting point is “length” in meters (LStarting point is “length” in meters (LBSBS))• All Factors are near 1.0 (0.5-1.5) and All Factors are near 1.0 (0.5-1.5) and

modify LBSmodify 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)


Slide 14

STIX FormulaSTIX Formula

• Good PointsGood Points• Empirically based Empirically based

off a “normal” boatoff a “normal” boat• Adjustable/flexibleAdjustable/flexible• Reflects positive Reflects positive

flotation benefitsflotation benefits• Best we have…Best we have…

• Questionable PointsQuestionable Points• Huge overemphasis on lengthHuge overemphasis on length• Based on “experience”, not Based on “experience”, not

basic ocean sciencebasic ocean science• Very political (made to fit Very political (made to fit

current EC designs – beam and current EC designs – beam and hatch problems)hatch problems)

• Possible values not realistic Possible values not realistic (“A” < 90(“A” < 90oo LPS ?) LPS ?)

• No historyNo history

FDFFWMFDSFIRFKRFBDFDLLSTIX BS )25.27(


Slide 15

Stability Suggestions!Stability Suggestions!• Consider Category A Consider Category A

“Oceangoing”, but “Oceangoing”, but possibly only possibly only “Coastal” if STIX “Coastal” if STIX between 32 and 40 and between 32 and 40 and the boat is over 35 ft.the boat is over 35 ft.

• ““B” means “Inshore”B” means “Inshore”• But, small boats with But, small boats with

large displacements large displacements may be a “STIX B” but may be a “STIX B” but should be an “A” should be an “A”


Slide 16

More SuggestionsMore Suggestions

• Look for a boat with an IMS LPS>120 Look for a boat with an IMS LPS>120 for offshore work.for offshore work.

• Question boats with wide beam, high Question boats with wide beam, high topsides and shoal draft for offshore topsides and shoal draft for offshore work.work.

• Don’t add weight up high!Don’t add weight up high!


Slide 17

What are “Structures”What are “Structures”

• Hull and deck Hull and deck “plating” (fiberglass, “plating” (fiberglass, advanced composites, advanced composites, wood, concrete)wood, concrete)

• Connections to other Connections to other components (keel, rig, components (keel, rig, engine, steering, engine, steering, tanks)tanks)

• Rudder, RigRudder, Rig

Design Trade-OffsDesign Trade-Offs• Probability of Probability of

Failure (“Risk”)Failure (“Risk”)• Weight and Center Weight and Center

of Gravityof Gravity• CostCost• DurabilityDurability


Slide 18

How Do Naval Architects Design How Do Naval Architects Design Structures?Structures?

• Simple Simple EquationsEquations

• Advanced Advanced Computer Computer ProgramsPrograms

• Rules of ThumbRules of Thumb• Classification Classification

Society Codes Society Codes (ABS, DnV, ISO)(ABS, DnV, ISO)

2

2

2

2

2)(

)1( y

w

x

wz

Ex


Slide 19

ISO 12215 - StructuresISO 12215 - 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 Based on ABS Guides (sail, power) but have

been watered down a bit for power boats.been watered down a bit for power boats.• Manufacturer-assessed for boats under 39 Manufacturer-assessed for boats under 39

feet.feet.


Slide 20

Structural ApproachStructural Approach

• Code determines a hydrostatic Code determines a hydrostatic pressure based on boat length and pressure based on boat length and depth. (Too conservative?)depth. (Too conservative?)

• Pressure is applied to plating and Pressure is applied to plating and transfers to frames/bulkheads. transfers to frames/bulkheads.

• Minimum structural strength of each Minimum structural strength of each component is checked.component is checked.

• A factor of safety (~2.3) is included.A factor of safety (~2.3) is included.


Slide 21

Important Structural RemindersImportant Structural Reminders

• ““Durability” is not designed into any Durability” is not designed into any of the Classification Society Codes of the Classification Society Codes (impact, abrasion, fatigue)!(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?


Slide 22

Why we worry about impact!Why we worry about impact!


Slide 23

Impact Example – Navy 44 vs…Impact Example – Navy 44 vs…

• Effect of Resin and Effect of Resin and CoreCore

Same amount of glass fiber, different resin

Navy 44

J/24

Test #11


Slide 24

Structural SuggestionsStructural Suggestions

• A boat designed to ABS should be OK A boat designed to ABS should be OK for offshore work.for offshore work.

• A boat not designed to ABS A boat not designed to ABS maymay be OK. be OK.

• As ISO 12215 is based on ABS (at the As ISO 12215 is based on ABS (at the moment)…moment)…

• Add “value” if vinyl ester or epoxy for Add “value” if vinyl ester or epoxy for durability.durability.

• If you can deflect it, it is too weak!If you can deflect it, it is too weak!


Slide 25

Final Structural Thought!Final Structural Thought!

• Pedigree is meaninglessPedigree is meaningless

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

Documents

stability slide

annapolis slide

history slide

center of gravity w

stability suggestions

sailboat stability

form stability vessels

stix b