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Yacht Design Technology

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Yacht Design Technology

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    1. Yacht Design & Technology Lecture 2 Stability and Safety

    2. Lecture Contents Recap concepts of stability Influence of hull shape on stability Water Ballast & Canting Keels Safety Types of capsize Re-righting

    3. Generation of Righting Moment

    4. Generation of Righting Moment

    5. Generation of Righting Moment

    6. Righting Arm or GZ curve

    7. Righting Arm, GZ Curve As heel angle increases, G & B separation increases to maximum GZ then reduces to zero at limit of positive stability If yacht released at angle less than limit it will return to upright If yacht released at angle greater than limit it will capsize

    8. Righting Arm, GZ Curve If range of stability 180 degrees, it is self-righting GZ curve used to compare designs and assess against regulations

    9. Generation of Righting Moment At small angles GZ (righting arm) equal to GMsinf where f is the angle of heel Therefore GM is a simple measure of initial stability

    10. Influence of Hull Shape Stability characteristics dominated by beam and centre of gravity GM = KB + BM - KG Lower centre of gravity = increase in stability since BM = I/V Increase in beam = reduction in displacement and increase in stability

    11. Different Hull Forms

    12. GZ curves for different Yachts

    13. Traditional Cruiser Heavy displacement, narrow beam & low centre of gravity Relatively low initial stability since beam is small Stability increases until 90 degrees Range of stability is 160 degrees

    14. Modern Cruiser/Racer Lighter displacement, greater beam & higher centre of gravity Greater initial stability since greater beam Stability increases until 60 degrees Lower range of stability, 132 degrees

    15. Modern Catamaran Cruiser Extreme beam. Light displacement with no ballast & high centre of gravity Very high initial stability, but restricted to small angles Windward hull emerges from water at 10 degrees and righting arm reduces rapidly Lower range of stability, 80 degrees Very stable when inverted

    16. Righting moment curves for different Yachts

    17. Righting moment curves Yacht will be in equilibrium in terms of heel angle when: righting moment = heeling moment For identical rigs: traditional yacht heel = 30 degrees modern yacht heel = 25 degrees catamaran heel = 5 degrees

    18. Freeboard Freeboard has significant effect on stability at angles greater than deck edge immersion

    19. Coachroof & Cockpit Volumes A large coachroof increases distance between buoyancy and C of G & hence increases stability at 90 degrees. Cockpit has little effect on stability because remains above waterline at most angles

    20. Coachroof & Cockpit Volumes

    21. Hull Flare Increasing flare has a similar effect to an increase in freeboard

    22. Water Ballast To improve sailing performance by a reduction in heel angle, ballast is frequently moved, or added to the windward side of the yacht. eg. moving crew or water tanks

    23. Water Ballast

    24. Canting Keel Another technique to increase righting moment is to use a canting keel

    25. Canting Keel Current Regulations: Both Yachting Australia & Open 60 class limits max static heel angle to 10 degrees. Plus self-righting test for Open 60 without rig. Volvo 70 specifies minimum capsize angle of 115 deg with keel at max cant which is limited to 40 degrees, plus self-righting test

    26. Canting Keel

    27. Monohull - Casualty Statistics

    28. Multihull - Casualty Statistics

    29. Types of Capsize Overpowered by the wind potential hazard for vessel with low range of stability i.e. less than 90 degrees Knocked down by a gust or squall can happen to any craft, not a hazard if structure remains buoyant and stable at large angles Rolled by a breaking wave small craft more likely to encounter waves large enough to capsize them

    30. Capsize by Breaking Wave Research concentrated on this mechanism since 1979 Fastnet Race (many yachts rolled or capsized, 15 lives lost) Narrow forms less vulnerable than wide forms Conclusion that little to be gained in attempting to reduce vulnerability of monohull yacht to capsize Important consideration is that capsized yacht re-rights

    31. Re-Righting

    32. Re-Righting

    33. Re-Righting Effect of Coach House

    34. Re-Righting Effect of Hull Form

    35. Re-Righting Effect of Mast

    36. Recap/reflect

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