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TitleSailing Yacht Design
TagsYacht Hull (Watercraft) Composite Material Watercraft Water Transport
File Size1.0 MB
Total Pages61
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16-21 AUGUST 2009




Concern for the structural design of sailing yachts and other craft. Consideration shall
be given to the materials selection, fabrication techniques and design procedures for
yacht hull, rig and appendage structures. The role of standards, safety and reliability in
the design and production processes should be addressed. Attention should be given to
fluid-structure interaction effects on hulls, rigs and appendages and their influence on
structural design.


Chairman : A. Shenoi

R. Beck
D. Boote
P. Davies
A. Hage
D. Hudson
K. Kageyama
J. A. Keuning
P. Miller
L. Sutherland

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ISSC Committee V.8: Sailing Yacht Design 462

depends on a number of factors such as: the area and geometry of sails, the apparent
wind velocity and the angle of incidence of sails. The resultant of sails forces FT can be
decomposed into lift L (normal to the apparent wind direction) and drag D (opposite to
the apparent wind direction) and expressed in terms of non-dimensional coefficients
CL and CD. Lift and drag can be measured in the wind tunnel during experimental tests
on scale models and reported in polar diagrams as a function of the angle of incidence


The total force FT can also be decomposed into two other components: the driving
force FR in the direction of the boat's course and the heeling force FH perpendicular to
the boat's course; also in this case non-dimensional coefficients CR and CH are defined.

To compute the load exerted by sails on the mast it is then necessary to know the
coefficients CL and CD or CR and CH; a great deal of experimental data on sails has
been collected by researchers in wind tunnel tests and some of them are available in
literature, such as those published by Marchaj (1962, 1964) or the data collection
gathered on board Bay Bea yacht (Kerwin et. al., 1974).
The sail forces FR and FH can also be determined by considering the hydrostatic
properties of the hull in heeled conditions. The heeling moment MH caused by the
action of the wind on sails is balanced by the righting moment MR rising when the boat
heels. The righting moment for an angle of heel is equal to GZ where is the
displacement of the yacht and GZ the righting arm. The side force FH can be
determined as follows:



where h is the vertical distance between sails’ centre of effort (aerodynamic) and hull
centre of lateral resistance (hydrodynamic).

From the cross curves of the hull it is possible to know exactly the force necessary to
heel the yacht of an angle ; this will be the transverse force developed by the sails in a
quasi-static condition. Assuming proper sail coefficients at the design heel angle , the
apparent wind velocity and the driving force FR can be determined.

The starting point for the designer is then to determine the maximum heel angle to be
assumed for the calculation. For little and medium size sailboats the reference heel
angle for mast and rigging scantling is typically 30 . In the case of big sailing yachts
this could be too large and might lead to excessive mast section dimensions; thus a
maximum heel of 20-25 is often assumed.

Once the driving and heeling forces FR and FH have been calculated and subdivided
between mainsail and foresail, the next problem to solve is how those forces should be
applied on mast and rigging. In a simplified approach it can be assumed that the
mainsail transmits to the mast a distributed load along its length. The simplest way to
apply this load is by a triangular shape as shown in Figure 13a. Taking into account

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ISSC Committee V.8: Sailing Yacht Design 463

that the pressure on the upper part of the sail is greater, owing to the higher wind
velocity, a trapezoidal distribution (Figure 13b) would be more suitable. According to
lifting line theory the pressure follows an elliptical distribution because of the vortex
rising at the upper and lower sail bounds (Figure 13c). The actual pressure distribution
will vary dynamically depending on aspect ratio, twist and sheet tension. For
application to a finite element model, this type of distribution can be well approximated
by a step distribution as shown in Figure 13d. Such a distribution of the load is
conservative towards the bending moment on the mast because the centre of
application of the resulting force is higher than other ones.

Figure 13: Distribution of mainsail load on mast: (a) triangular; (b) trapezoidal; (c)

elliptical; (d) step varied (Claughton, 1998).

As far as the foresail is concerned the total force can be split between the forestay and
the jibsheet. The percentage depends on the tension in the halyard but it is reasonable to
consider that 20% is supported by the sheet and the 80% by the jibstay and,
consequently charged on the masthead. The force on the masthead depends on the
tension in the jibstay and it is a function of the maximum jibstay deflection. The jibstay
tension can be estimated considering the deformed shape of the stay to be a very tight
catenary supporting a distributed load along its length. To know the tension in the
jibstay, it is necessary to impose a minimum, reasonable value for the maximum
deflection. It is sometimes argued that the curvature of the forestay could cause a
stagnation effect on the mainsail and thus consequently decrease the propulsive force
component FR. In order to reduce this effect it is a common practice to pretension the
stay as much as possible increasing the compression and bending stresses on the mast.
In current practice, it can be assumed a maximum stay deflection between 2 and 5% of
the jibstay length.

There are other loads to consider such as those transmitted by boom, the compression at
mast step by an hydraulic jack, the pretensioning of stays and shrouds and the tension
of halyard. In the case of a linear analysis maximum values of considered loads should
be applied to the model. The results of the calculation will be analysed in terms of
stresses and displacements. For what the displacements are concerned it is a common
practice not to allow displacements at the top of the mast higher than 2% of the total
mast height.

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ISSC Committee V.8: Sailing Yacht Design 492

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ISSC Committee V.8: Sailing Yacht Design 493

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