1. Seaworthiness and Safety Yacht Design 102
Paul H. Miller
Dept of Naval Architecture, Ocean and Marine Engineering
US Naval Academy
Safety at Sea Seminar

2. Seaworthiness: A Definition
“To be seaworthy, a vessel must be able to defend itself against the perils of the sea…”
A classic example of a “seaworthy design.”
Safety at Sea Seminar

3. The Big Picture: “Design” in Context
12% Design and Construction Issues
1993 Marine Board Casualty (of vessels) Study
88% Owner and Operator Issues!
Safety at Sea Seminar

4. Owner and Operator Issues
Training
Practice!
Education:Safety at Sea Seminars!
Maintenance (Inspect, Maintain, Repair)
Crew Fatigue
Motion? (Bigger is better)
Work load? (Smaller is better!)
Safety at Sea Seminar

5. Fifteen (exciting) Minutes after 79 Boats Started
Owner and Operator Issues Example: 1998 Singlehanded Farallones Race 32 Miles Out into the Pacific and Back
Fifteen (exciting) Minutes after 79 Boats Started
Safety at Sea Seminar

6. 1 Vessel Required USCG Support Swan 47! Flooded
1 Hour Later
25-35 knot winds
12-20 foot waves
1/3 Dropped Out
1 Vessel Required USCG Support
Swan 47!
Flooded
Hatches Improperly Secured!
Safety at Sea Seminar

7. 28’ Herreshoff Rozinante Ketch Ave Speed 7.2 knots
First to Finish
F/27 Trimaran
Ave Speed 9.8 knots
First On Handicap
28’ Herreshoff Rozinante Ketch
Ave Speed 7.2 knots
The Moral of the Story
Both boats were easy to sail, reef, and steer!
Safety at Sea Seminar

8. Design and Construction: Three Basic Rules (Sail or Power)
1. Keep Water Out of the Vessel!
2. Be Able to Direct the Vessel’s Course!
3. Keep the Vessel Upright!
These requirements often conflict with other goals, such as speed or “roominess”!
Safety at Sea Seminar

9. 1. Keeping the Water Out Structure
Wood, Fiberglass, Carbon, Metal, (even Ferro-Cement!) are all acceptable, if sufficient Factors of Safety and quality ($?) construction are used
Reputation of Designer and Builder are a clue, “standards” are another
Thoroughly Survey for Current Condition Prior to Each Passage (“PreFloat”)
Safety at Sea Seminar

10. 1. Keeping the Water Out (2)
Structural “Negative Indicators”
Leaks around through-hulls, hull-deck joint, keel bolts
Noticeable flexing of hull panels that cause joinerwork or furniture to move, doors jammed
Broken joints (tabbing)
Stress cracks
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11. Stress Cracks...
OneAustralia
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12. 1. Keeping the Water Out (3)
Watertight Integrity
Hatches sufficiently strong, bedded and lockable (from inside and out)
Hatchboards sufficiently strong and securable (ditto)
Redundant bailing capacity
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13. Structures References
American Bureau of Shipping Guide for Offshore Racing Yachts
The Elements of Boat Strength by Dave Gerr
Safety at Sea Seminar

14. 2. Directing the Vessel’s Course: Power and Control
Storm Rig & Sails
-Small Sails
-Easy to Rig
-Versatile
-Balanced
-Strong!
2. Directing the Vessel’s Course: Power and Control
Safety at Sea Seminar

15. 2. Directing the Vessel’s Course (2)
Rudder and Keel
Sufficient area
Keep attached!
Weak links
Shaft to tiller/quadrant
Shaft to blade
Cable, sheaves (Think Simple! Tiller?)
Safety at Sea Seminar

16. Suggested Equipment References
US SAILING:
Safety Recommendations for Offshore Sailing or
Safety Recommendations for Cruising Sailboats
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17. 3. Keep the Vessel Upright - Stability
Wind and waves are trying to tip the boat over
Ballast, hull shape and crew weight are trying to keep it upright
Don’t rely on crew weight!
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18. 3. Static Stability
Buoyancy Force acts upward through the center of submerged volume B
Weight Force acts downward through the Center of Gravity W
Sum of the Forces equals Zero!
Safety at Sea Seminar

19. 3. Static Stability When Heeled
Horizontal distance between Center of Gravity and Buoyancy is Righting Arm (RA)
“Give me a lever and I will move the earth!” B W
Righting Moment
= Righting Arm x Boat Weight
= “Stability” W
Safety at Sea Seminar

20. 3. Static Stability When Really Heeled!
Moral: A Low Center of Gravity is Nice!
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21. 3. Static Stability - Beam Effects
“Ballast Stability”
“Form Stability” B B W W W
Righting Arms!
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22. 3. Static Stability Lessons
Beam Provides Stability at Small Angles (<40)
A Low Center of Gravity Always Provides Stability
For the Same Initial Stability a Narrow Boats Needs a Lower CG (More Keel Ballast-More Weight?)
Positive Righting Moment - Boat Will Return Upright RM
Negative RM
Boat Will Capsize
Limit of Positive Stability
Heel Angle 90
135
180
Safety at Sea Seminar

23. 3. Static Stability Curves
Narrow Boat - Ballast Stability
Beamy Boat - Form Stability RM
Heel Angle 90
135
180
With Water Sloshing Inside!
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24. 3. Dynamic Stability
A Vessel’s Response to Gusts and Breakers is a function of:
Static Stability (RA x Boat Weight)
Roll Mass Moment of Inertia
Surface Area
Roll Damping
Luck!
Uh Oh!
Safety at Sea Seminar

25. 3. Dynamic Stability Roll Mass Moment of Inertia Surface Area
= Sum of (weights x “distances from CG”2)!
Separate weights vertically (preferably lower so as to get more RM!)
Surface Area
Reduce Contact Area - dodgers, sails, etc.
Roll Damping
Keel Area Down Low (Bulbs?)
Safety at Sea Seminar

26. 3. Dynamic Stability
“Form Stability vessels tend to follow the water surface!”
“Ballast Stability” vessels tend to follow gravity!
Safety at Sea Seminar

27. 3. Stability Suggestions
Capsize Screening Formula (a rough guide)
LPS >130 for cruisers (may be available from US Sailing for a sistership, or ask a yacht designer)
Safety at Sea Seminar

28. 3. Effect of Size
The Static Stability Curve, Damping and Mass Moment of Inertia terms do not have “length” factors.
Displacement, draft, submerged area, beam, and center of gravity are more important!
Size is relative!
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29. “Desirable and Undesirable Characteristics of Offshore Yachts”
Comprehensive
References
“Desirable and Undesirable Characteristics of Offshore Yachts”
by the Technical Committee of the Cruising Club of America
Safety at Sea Seminar

30. “Seaworthiness: The Forgotten Factor”
C. A. Marchaj
Safety at Sea Seminar

31. Rules To Remember When Selecting, Modifying or Inspecting Your Boat
1. Keep Water Out of the Vessel!
2. Be Able to Direct the Vessel’s Course!
3. Keep the Vessel Upright!
If in doubt, talk to an expert!
Safety at Sea Seminar