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Yacht Design Technology

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Yacht Design Technology

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    1. Yacht Design & Technology Balance & Control

    2. Introduction Important to determine optimum longitudinal position of sail plan relative to underwater body. Sails too far aft = weather helm Sails too far forward = lee helm

    4. Effect of Heel Under equilibrium hydro & aerodynamic forces must act along the same line. If yacht heels: centre of effort of sails moves to leeward centre of effort of hull & keel moves to windward Yacht will tend to luff up. Helmsman applies weather helm to bring hydrodynamic force astern until it is in line with aerodynamic force.

    6. Effect of Heel Due to asymmetry of hull when heeled, the hydrodynamic centre will move slightly forwards. This also increases the imbalance. It is impossible to position sail plan so that yacht is balanced at all angles of heel. Good balance usually sought for small heel angles. Larger weather helm tolerated at large heel angles.

    7. Good Balance Good balance is not zero helm required to steer on a straight course. What? Are you joking? Why? Rudder contributes to hydrodynamic sideforce. Larger the weather helm > larger the rudder sideforce produced. There is therefore an optimum rudder angle for tradeoff between sideforce and resistance. Optimum usually approx. 5 degrees weather helm.

    8. Good Balance Weather helm is also good because: Safety Aspect. When gust hits - yacht will tend to luff up into wind. This will unload sails and reduce risk of heeling. Steering Feel. Advantage to feel effect of gust as increased force on tiller.

    9. Centre of Effort of Underwater Body

    10. Centre of Effort of Underwater Body Geometric CLR is geometric centre of area of underwater profile. Since centre of effort of wing is approx 25% of the chord from leading edge, not at 50% where geometric centre is. The hydrodynamic CLR is quite far from the geometric CLR.

    11. Centre of Effort of Underwater Body Larsson method to estimate hydrodynamic CLR: Assume lies at 25% chord at 45% draft. Draw 25% chord line on keel. Determine 45% draft depth on this line. Assumes effect of rudder and forebody cancel each other out. Only suitable for fin keel yachts.

    12. Centre of Effort of Underwater Body

    13. Centre of Effort of Sails When yacht is running, angle of attack is 90 degrees and centre of effort of sails is at geometric centre. For other courses angle of attack reduces and CE moves forward.

    14. Centre of Effort of Sails

    15. Centre of Effort of Sails Therefore normally there is a considerable distance between geometric CE and the aerodynamic CE. However, in the absence of an alternative method, the Geometric CE is usually used.

    16. Centre of Effort of Sails

    17. Lead Whatever method is used to obtain CLR & CE, their relative positioning is based on experience. The CE is in front of the CLR, with the distance of separation called the lead. The amount of lead will depend on the methods used to find CLR & CE and the type of yacht.

    18. Lead What effect will the following have on the lead? A large beam A long keel Sails with a large aspect ratio A low stability

    19. Lead What effect will the following have on the lead? A large beam A long keel Sails with a large aspect ratio A low stability

    20. Lead For fin-keel yachts: Masthead sloop: 5 - 9% LWL Sloops with fractional rig: 3 - 7% LWL For long keel yachts (use geometric centre for CLR) Masthead sloop: 12 - 16% LWL Sloops with fractional rig: 10 - 14% LWL Ketches: 11 15% LWL

    21. Rudder Balance With the presence of weather helm it is necessary to have the rudder appropriately balanced to avoid excessive effort for the helmsman. Moment on rudder stock = sideforce x moment arm Rudder must not be overbalanced (i.e. centre of pressure forward of rudder stock) since it will become unstable. Suitable location is 50mm behind centre of the stock.

    22. Rudder Balance

    23. Rudder Balance

    24. Recap/Reflect Sum over one complete wave period all local damping waves generated by number of sections along yacht length. Sum over one complete wave period all local damping waves generated by number of sections along yacht length.

    25. Sum over one complete wave period all local damping waves generated by number of sections along yacht length. Sum over one complete wave period all local damping waves generated by number of sections along yacht length.

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