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Slide 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
Slide 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
Slide 3: The Big Picture: “Design” in Context 1993 Marine Board Casualty (of vessels) Study 12% Design and Construction Issues 88% Owner and Operator Issues! Safety at Sea Seminar
Slide 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
Slide 5: 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
Slide 6: • • • • • 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
Slide 7: • 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
Slide 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
Slide 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
Slide 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 Safety at Sea Seminar
Slide 11: Stress Cracks... OneAustralia Safety at Sea Seminar
Slide 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 Safety at Sea Seminar
Slide 13: Structures References American Bureau of The Elements of Boat Shipping Guide for Strength by Dave Gerr Offshore Racing Yachts Safety at Sea Seminar
Slide 14: 2. Directing the Vessel’s Course: Power and Control Safety at Sea Seminar
Slide 15: • Rudder and Keel • Sufficient area • Keep attached! 2. Directing the Vessel’s Course (2) • Weak links • Shaft to tiller/quadrant • Shaft to blade • Cable, sheaves (Think Simple! Tiller?) Safety at Sea Seminar
Slide 16: Suggested Equipment References US SAILING: Safety Recommendations for Offshore Sailing or Safety Recommendations for Cruising Sailboats Safety at Sea Seminar
Slide 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! Safety at Sea Seminar
Slide 18: 3. Static Stability • Buoyancy Force acts upward through the center of submerged volume B W Sum of the Forces equals Zero! • Weight Force acts downward through the Center of Gravity Safety at Sea Seminar
Slide 19: 3. Static Stability When Heeled “Give me a lever and I will move the earth!” B W W Righting Moment = Righting Arm x Boat Weight = “Stability” Horizontal distance between Center of Gravity and Buoyancy is Righting Arm (RA) Safety at Sea Seminar
Slide 20: 3. Static Stability When Really Heeled! W W B Moral: A Low Center of Gravity is Nice! Safety at Sea Seminar
Slide 21: 3. Static Stability - Beam Effects “Ballast Stability” “Form Stability” B W W Righting Arms! Safety at Sea Seminar B W
Slide 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 Limit of Positive Stability 0 Heel Angle 90 Negative RM Boat Will Capsize 135 180 Safety at Sea Seminar
Slide 23: 3. Static Stability Curves Narrow Boat - Ballast Stability Beamy Boat Form Stability RM 0 Heel Angle 90 135 180 With Water Sloshing Inside! Safety at Sea Seminar
Slide 24: • 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! 3. Dynamic Stability Uh Oh! Safety at Sea Seminar
Slide 25: 3. Dynamic Stability • Roll Mass Moment of Inertia • = 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
Slide 26: 3. Dynamic Stability “Form Stability vessels tend to follow the water surface!” “Ballast Stability” vessels tend to follow gravity! Safety at Sea Seminar
Slide 27: 3. Stability Suggestions • Capsize Screening Formula (a rough guide) Beam 2 1  BoatWeight (lbs )  3   64   • LPS >130 for cruisers (may be available from US Sailing for a sistership, or ask a yacht designer) Safety at Sea Seminar
Slide 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! Safety at Sea Seminar
Slide 29: Comprehensive References “Desirable and Undesirable Characteristics of Offshore Yachts” by the Technical Committee of the Cruising Club of America Safety at Sea Seminar
Slide 30: “Seaworthiness: The Forgotten Factor” C. A. Marchaj Safety at Sea Seminar
Slide 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