Stability, Safety and Stability, Safety and Seaworthiness in Offshore Seaworthiness in Offshore

BoatsBoats

28 Feb 201028 Feb 2010

Paul H. Miller, D.Eng. P.E.Paul H. Miller, D.Eng. P.E.Professor of Naval ArchitectureProfessor of Naval Architecture

United States Naval AcademyUnited States Naval Academy

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SeaworthinessSeaworthiness““To be To be seaworthy, seaworthy, the vessel the vessel

must be able must be able to defend to defend

itself against itself against the incursion the incursion and perils of and perils of the sea…”the sea…”

A classic example of a seaworthy design.Low center of gravity, versatile rig, narrow beam.

From “Seaworthiness: The Forgotten Factor” by C.A. Marchaj

But perhaps a bit impractical and it has a horrible rating!

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Typical Typical Boating Boating

StatisticsStatistics

11% Design, 11% Design, Construction Construction and and Equipment Equipment IssuesIssues

A reality check:Question: What percentage of recreational marine accidents are related to design, construction or equipment?

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Human Error is the biggest Human Error is the biggest problem!problem!

89%89%Owner and Owner and Operator Operator MistakesMistakes(Training,(Training,Attitude, Attitude, Preparation, Preparation, Weather, Weather, Fatigue, etc)Fatigue, etc)

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““To finish first, first you have to finish.”To finish first, first you have to finish.”

The keel The keel is still is still there, but there, but the mast the mast is not!is not!

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What is your acceptable risk?What is your acceptable risk?

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How Much Risk Will You Take To How Much Risk Will You Take To Cross an Ocean?Cross an Ocean?

““How Lucky Do You Feel?!”How Lucky Do You Feel?!”

Is this your acceptable level?

Or, is this?Risk

Preparation + Training + Cost + Attitude + Routing

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For your level of acceptable risk…For your level of acceptable risk…If your level of training and experience is minimal, then:•Get more training/practice (go to sailing camp!) •Adopt a more conservative attitude to routing•Increase your boat preparation/equipment•Choose a more conservative boat design!

Some great voyages Some great voyages have been made have been made in in spite ofspite of the boat the boat design and design and equipment!equipment!

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Design Attributes:Design Attributes:The Basic Rules to Reduce RiskThe Basic Rules to Reduce Risk

(Sail or Power)(Sail or Power)

1.1. Watertight integrity: Keep Water Out! Watertight integrity: Keep Water Out!

2.2. Stability: Keep the Vessel Upright!Stability: Keep the Vessel Upright!

3.3. Maintain Steering and WeatherlinessMaintain Steering and Weatherliness

4.4. SimplicitySimplicity

These requirements often conflict with other goals, such as speed vs. comfort vs. cost vs. draft vs. …

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Watertight Integrity (In Brief)Watertight Integrity (In Brief)1.1. Hatches/boardsHatches/boards

2.2. Port lights (storm Port lights (storm shutters)shutters)

3.3. Vents/DoradesVents/Dorades

4.4. Through hullsThrough hulls

5.5. MastMast

1.1. Rudder shaftRudder shaft

2.2. Control linesControl lines

3.3. Prop shaftProp shaft

4.4. Locker coversLocker covers

5.5. Charlie NoblesCharlie Nobles

6.6. Construction Construction (ABS Rules)(ABS Rules)

Hint: Give your boat a good shower!Hint: Give your boat a good shower!Watertight is good, airtight is not!Watertight is good, airtight is not!

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Lessons Learned and RelearnedLessons Learned and Relearned

1.1. The 1979 Fastnet The 1979 Fastnet pointed out the pointed out the dangers of wide dangers of wide beam and light beam and light displacement.displacement.

2.2. The Rating Rules The Rating Rules were changed to were changed to encourage encourage stability. (’98 stability. (’98 Sydney-Hobart)Sydney-Hobart)

3.3. Boats became more Boats became more stable at large heel stable at large heel angles.angles.

4.4. Today, many Today, many “cruising boats” “cruising boats” have wider beams, have wider beams, lighter lighter displacements and displacements and higher CG’s than higher CG’s than pre-1979 boats…pre-1979 boats…

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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)

• Weight Force Weight Force acts downward acts downward through the through the Center of GravityCenter of Gravity Sum of the Forces equals Zero!

B

W

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Static Stability When HeeledStatic Stability When Heeled

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 right a boat!)

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Static Stability When Static Stability When ReallyReally Heeled! Heeled!

B

WW

Moral of this story: A Low Center of Gravity is Nice!

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

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Static Stability - Beam EffectsStatic Stability - Beam Effects

B

W

BW

Righting Arms!

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

W

Equal Righting Arms!Equal Righting Arms!

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Static Stability LessonsStatic Stability Lessons1.1. Beam Provides Stability at Small Angles (<40Beam Provides Stability at Small Angles (<40oo))2.2. A Low Center of Gravity A Low Center of Gravity AlwaysAlways Provides Stability Provides Stability3.3. For the For the SameSame Initial Stability a Narrow Boat Needs a Initial Stability a Narrow Boat Needs a

Lower CG (More Keel Ballast-More Weight?). Lower CG (More Keel Ballast-More Weight?).

RM

Heel Angle0 90 180125

Positive Righting Moment - Boat Will Return Upright

Negative RMBoat Will CapsizeLimit of Positive Stability

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Righting Moment CurvesRighting Moment Curves

RM

Heel Angle0 90 180125

Narrow Boat - Ballast StabilityBeamy Boat - Form Stability

Beamy Boat With Water Sloshing Inside!

Positive area under the curve = work to capsize!

Negative area under the curve = work to re-right!

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Dynamic (Moving) StabilityDynamic (Moving) Stability

Uh Oh!

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

• 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!

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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!

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Best Approach to Evaluating Best Approach to Evaluating Capsize ResistanceCapsize Resistance

• A large area under the Positive A large area under the Positive Righting Moment Curve (and small Righting Moment Curve (and small negative area!)negative area!)

• High Limit of Positive StabilityHigh Limit of Positive Stability• Large DisplacementLarge Displacement• Low Center of GravityLow Center of Gravity• Possibly available from the designerPossibly available from the designer

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Less Rigorous But Easier EvaluatorsLess Rigorous But Easier Evaluators

3.3. Capsize Screening Formula (a Capsize Screening Formula (a roughrough guide of form vs. ballast stability)guide of form vs. ballast stability)

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64)( 3

1

lbsBoatWeight

Beam

1.1. LPS >125 for cruisers (may be available from US Sailing LPS >125 for cruisers (may be available from US Sailing for a sistership, or ask a yacht designer)for a sistership, or ask a yacht designer)

2.2. Ballast/Disp>40% and no shoal keels!Ballast/Disp>40% and no shoal keels!

What What happened happened to CG and to CG and length?length?

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Screening Factor ExamplesScreening Factor Examples

LOA = 28’LOA = 28’BEAM = 10.5’BEAM = 10.5’DISP = 7400 LBDISP = 7400 LBCSF = 2.15CSF = 2.15

Hunter 28

LOA = 28’3”LOA = 28’3”

BEAM = 9.8’BEAM = 9.8’

DISP = 7450 LBDISP = 7450 LB

CSF = 2.0CSF = 2.0

Tartan 28 Hallberg Rassy 29

LOA = 29’3”LOA = 29’3”

BEAM = 9.33’BEAM = 9.33’

DISP = 8360 LBDISP = 8360 LB

CSF = 1.8CSF = 1.8

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More Screening Factor ExamplesMore Screening Factor Examples

LOA = 28’2”LOA = 28’2”BEAM = 8.9’BEAM = 8.9’DISP = 9000 LBDISP = 9000 LBCSF = 1.7CSF = 1.7

Cape Dory 28 Remember Remember that the that the CSF CSF equation equation does not does not include include some some important important terms!terms!(CG, (CG, damping, damping, MMI, etc.)MMI, etc.)

Rozinante

LOA = 28’LOA = 28’

BEAM = 6.25’BEAM = 6.25’

DISP = 7100 LBDISP = 7100 LB

CSF = 1.3 !CSF = 1.3 !

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Is Bigger Always Better?Is Bigger Always Better?1.1. The Static Stability Curve, Damping and Mass Moment The Static Stability Curve, Damping and Mass Moment

of Inertia terms do not have “length” factors.of Inertia terms do not have “length” factors.

2.2. Center of Gravity, weight, draft, surface area, and Center of Gravity, weight, draft, surface area, and beam are more important than length!beam are more important than length!

Bottom LineAn increase in length leads to greater comfort, possibly higher stability, higher loads and lower maneuverability.

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Is “Fast” Unsafe?Is “Fast” Unsafe?Can a performance boat be safe?Can a performance boat be safe?

Things to consider:Things to consider:1.1. Outrun weatherOutrun weather2.2. Crew weight! (wide Crew weight! (wide

beam)beam)3.3. Your ApproachYour Approach4.4. Weatherliness Weatherliness

(upwind ability)(upwind ability)

So the answer is “maybe”!

Note the insufficient rudder area!

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Rudder and Keel AreaRudder and Keel AreaCan You Climb Off a Lee Shore?Can You Climb Off a Lee Shore?

1.1. Stability and Stability and appendage area are appendage area are relatedrelated

2.2. At lower speeds you At lower speeds you need more areaneed more area

3.3. Rough guide:Rough guide:• Keel Area Keel Area >> 3.5% of 3.5% of

Sail AreaSail Area• Rudder Area Rudder Area >> 1.5% of 1.5% of

Sail AreaSail Area

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Three Quotes To RememberThree Quotes To Remember

“A Modern Sailboat…“A Modern Sailboat…1.1. cannot be too stable.”cannot be too stable.”

2.2. can be either too weak or too strong.”can be either too weak or too strong.”

3.3. can have rudders and keels that are can have rudders and keels that are too small and are too shoal.”too small and are too shoal.”

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Some “Lower Risk” Offshore DesignsSome “Lower Risk” Offshore Designs(for a couple)(for a couple)

Flicka 20CSF=1.8

Great for newlyweds!

Frances 26CSF=1.7

Enough for 2?

Pearson 30CSF=1.9

Value Cruiser?

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More “Lower Risk” Offshore DesignsMore “Lower Risk” Offshore Designs(for a couple)(for a couple)

Crealock 34CSF=1.7

Enough for 2?

Cal 40CSF=1.8

A great all-round design

Alerion Express 38CSF=1.9

My dream boat?

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A boat, although a A boat, although a good design, is good design, is only as seaworthy only as seaworthy as the condition it as the condition it is in and the skill is in and the skill of the crew that of the crew that sails it!sails it!

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Attributes that increase riskAttributes that increase risk

1.1. Shoal draft keels Shoal draft keels (too high a CG, (too high a CG, reduced reduced weatherliness)weatherliness)

2.2. High windage High windage rigging (steps, rigging (steps, main furlers)main furlers)

3.3. Boats that rely on Boats that rely on crew weight for crew weight for stabilitystability

1.1. Small keels and Small keels and rudders (lack of rudders (lack of control at low control at low speed)speed)

2.2. Racing rigsRacing rigs

3.3. Flat bottomsFlat bottoms

4.4. Large windowsLarge windows

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““Learn from the mistakes of others, Learn from the mistakes of others, because you won’t live long enough because you won’t live long enough

to make them all yourself”to make them all yourself”

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“Seaworthiness: The Forgotten Factor”

C. A. Marchaj

“Principles of Yacht Design”

Larsson and Eliasson

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From US SAILING: Equipment GuidesSafety Recommendations for Offshore Sailing

or

Safety Recommendations for Cruising Sailboats

And, “Safety From Capsizing; Final Report”

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“The Seaworthy Offshore Sailboat”

John Vigor

“Desirable and Undesirable Characteristics of Offshore

Yachts”by the CCA

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American Bureau of Shipping Guide for Building and Classing Offshore Racing Yachts

The Elements of Boat Strength by Dave Gerr

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One final thought:

Buy the smallest boat you can afford…

And go have fun!