Principles of Yacht Design

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G Empty weight of boat [kg] A Full load weight of boat [kg] g Ballast weight [kg] B Maximum beam [m] Loa Length overall [m] As Sail area [m2] RM Dimensioning righting moment [Nm] RMjQ Righting moment at JO degrees heeling with empty weight of the boat [Nm] RM1 Righting moment at 1 degree heeling with empty weight of the boat [Nm] n Number of persons on board Fs Freeboard at mast [m] àrm Additional moment from crew to windward [Nm] HA Heeling arm [m]

~ af if As > RM / (128-HA) the boat is considered « to be a sailing boat, and the rig is to be dimensioned accordingly.

mm 1

Fig 11.1 Definitions and righting moments

Fractional Rig

Fig 11.2 Types of rigs The longitudinal stability of the mast depends on the spread of lower shrouds, runners, inner forestay and location of the mast foot. It is common practice that the transverse and longitudinal stability are studied separately. Compared to a single-spreader, deck-stepped mast, we can increase overall mast stability by increasing the number of spreaders and/or bringing the mast down to the keel. At the same time we get the following pros and cons:

Increased number of spreaders r

+ Thinner mast which gives better mainsail efficiency. + Smaller outer dimensionsAvall thickness give a lighter mast. + Smaller foresail sheet angles are possible.

- More difficult to trim.

_ Often special measures must be made to take care of the longitudinal stability, ie runners, inner forestay, high longitudinal moment of inertia.

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