## Principles of Yacht Design

that is acting on the cross-section itself. This shear force also gives a shear stress along the beam or panel.

Global loads One case of global loadings primarily concerning ships and bigger yachts, are the bending-moment conditions of hogging and sagging: hogging when the wave crest is amidships and sagging when the wave trough is amidships with the crests at bow and stQrn. Normally hogging/sagging calculations are not performed on pleasure yachts below approximately 100 ft (30 m) (Fig 12.1).

Fig 12.1 Hull girder requirements (ABS)

The ABS (American Bureau of Shipping) guide for building and classing offshore racing yachts and the ISO 12215 Standard cover vessels up to 80 ft (24 m) and does not require the calculations of bending moments and hull girder strength, but the ABS rule for motor pleasure yachts stipulates a minimum hull girder section modulus SM at amidships varying with length, breadth and block coefficient. (See Fig 12.5 for how this is calculated for the YD-40.) This ABS formula is valid for yachts shorter than 45 m and made of fibre reinforced plastic with speeds below 25 knots. The beam (B) of the vessel is not to be greater than 2 times the depth of the canoe-body (Dj.

The minimum ultimate strength of the hull material, tensile or compressive, is gu, whichever is less in N/mm2. L and B are length and beam in metres, and Cb is the block coefficient of the vessel. Typical values for the block coefficient are 0.35 to 0.42.

The other big villain which inflicts deformations is loading from the rig in sailboats. The loads come from the shroud tension to windward, and the tension in the fore-and-aft stays. The former is directly coupled

Wavo Prof Ha

Compression In .Bottom-

Sagging Condition

Section Modulus requirement for the hull girder: SM , =■ 0.01 L2- B> (l2.7-.14L)'(Cb+.7)-80 • 400/<Ju [cm^J

L = (Loa + )/2. B = Bmax , Cb = 0.3 to O.4. <JU =115 to 150

Hogging Condition

-Long. Mast Pressure [7.3 Tonnes) = Hu/i-Bending Force

Typical Rig Forces on a 40' Sailing Yacht

Tot. Mast Pressure inci. Long—, Transv— & Pressure from Halyards:

17 Tonnes

Hull Section wants

Fig 12.2 Forces from the to the boat's righting moment and the latter to the need for a straight rig jibstay to get the best performance from the sails.

On a 'normal" ballasted sailing yacht the accumulated pressure on the mast foot, coming from stays, shrouds and halyard tension can reach a value of double the boat's displacement. Loadings from the shrouds are of the same magnitude as the displacement, and halyard tensions are approximately 15% of displacement. The tension in the fore-and-aft stay inducing a longitudinal bending moment in the hull girder, results in a