Transcript Sailboat Stability and Structure

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
Slide 1
My Qualifications in Structures
and Stability…
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Slide 2
Static (not moving) Stability
•
Buoyancy Force
acts upward
through the center
of underwater
volume (Center of
Buoyancy)
B
• Weight Force acts
W
downward through
the Center of
The sum of these forces equals zero!
Gravity
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Slide 3
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
B
least a boat!)
W
Righting Moment
= Righting Arm x Boat Weight
= “Stability”
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Slide 4
Static Stability When Really Heeled!
W
Limit of
Positive
Stability
“LPS” is when
the two
vectors are
opposite.
B
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Slide 5
Static Stability - Beam Effects
“Ballast Stability”
“Form Stability”
B
B
W
W
W
Different Righting Arms!
Equal Righting Arms!
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Slide 6
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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Slide 7
The Righting Moment Curve
Positive area under the curve = work to capsize!
Narrow Boat - Ballast Stability Beamy Boat Form Stability
RM
0
Heel Angle
90
125
180
Negative area under the curve = work to re-right!
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Slide 8
Dynamic (Moving) Stability
A Vessel’s Response to
Wind and Waves is a
function of:
–
–
–
–
–
–
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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Uh Oh!
Slide 9
Length is not a big factor!
For two boats of the same weight, the
smaller will often be more seaworthy!
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Slide 10
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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Slide 11
STIX Categories
Category
Environment
A
Beaufort 10 (<55 kts)
H1/3<23’ (max <46’!)
Beaufort 8 (<40 kts)
H1/3<13’
Beaufort 6 (<27 kts)
H1/3<6.6’
Beaufort 4 (<16 kts)
H1/3<1.6’
B
C
D
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Min
STIX
32
23
14
5
Slide 12
STIX FACTORS
• Starting point is “length” in meters (LBS)
• All Factors are near 1.0 (0.5-1.5) 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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Slide 13
STIX Formula
STIX  (7  2.25 LBS )  FDL FBD FKR FIR FDS  FWM  FDF  
• 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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Slide 14
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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Slide 15
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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Slide 16
What are “Structures”
• 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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Slide 17
How Do Naval Architects Design
Structures?
• Simple
Equations
• Advanced
x
Computer
Programs
• Rules of Thumb
• Classification
Society Codes
(ABS, DnV, ISO)
  w  w
E

( z ) 2  2 
2
(1  )
y 
 x
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2
2
Slide 18
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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Slide 19
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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Slide 20
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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Slide 21
Why we worry about impact!
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Slide 22
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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Slide 23
Structural Suggestions
• A boat designed to ABS should be OK
for offshore work.
• A boat not designed to ABS may be OK.
• As ISO 12215 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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Slide 24
Final Structural Thought!
• Pedigree is meaningless
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Slide 25