##### Document Text Contents

Page 1

433

17th INTERNATIONAL SHIP AND

OFFSHORE STRUCTURES CONGRESS

16-21 AUGUST 2009

SEOUL, KOREA

VOLUME 2

COMMITTEE V.8

SAILING YACHT DESIGN

Mandate

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.

Members

Chairman : A. Shenoi

R. Beck

D. Boote

P. Davies

A. Hage

D. Hudson

K. Kageyama

J. A. Keuning

P. Miller

L. Sutherland

Page 30

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:

h

GZ

FH

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

Page 31

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 et.al., 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.

Page 60

ISSC Committee V.8: Sailing Yacht Design 492

Miller, P. H. (1995), “Design Criteria for Composite Masts”, CSYS Conference,

January 28.

Miller, P. H. (2000), “Durability of Marine Composites”, Doctoral Dissertation,

University of California, Berkeley.

Miller, P. H. (2003), “Design, Verification, and Forensic Correlation of Composite

Yacht Structures”, Advanced Marine Materials Conference, Royal Institution

of Naval Architects, London, UK, October 9-10

Nordisk Bat Standard (1990), ”Fritidsbater under 15 meter”, Det Norske Veritas, Oslo.

Pallu P., Vedrenne J., Devaux H., Balze R. (2008), “An efficient tool for mast design

and tuning”, Proceedings of Madrid Diseno de Yates Symposium, Madrid,

Spain.

Professional Boat Builder (2008), “Closing Molding at Cobalt”, Number 113, July.

Purcell, E., Allen, S. and Walker, R. (1988), “Structural analysis of the U.S. Coastguard

Island class patrol boat”, Transactions of Society of Naval Architects and

Marine Engineers, 96, 221-246.

Registro Italiano Navale (RINa) (1984) “ Regolamento per la Costruzione e la

Classificazione delle Barche a Vela da Regata 12 m S.I. e 6 m S.I.”, Genova,

Italy

Registro Italiano Navale (RINa) (2006), “Infusion as a Composite Construction

Technique for Pleasure Vessels: Guidelines”, RINa S.p.A, Genova

Registro Italiano Navale (RINa) (2007), “ Rules for the Classification of Yachts”,

Genova, Italy.

Riber, H.J. (1993) “Strength analysis of the 470 sailing boat”, Report to Technical

University of Denmark, Lyngby.

Richter, H. and Braun, J. (2003), “Computational Fluid Dynamics for Downwind Sails”,

Proceedings of the 16th Chesapeake Sailing Yacht Symposium, Annapolis,

USA.

Rizzo C., Carrera G. (2007), “Measurement of shrouds deformation a large sailing

ship”, Marstruct Report MAR-W2-7-DINAV 39, Genova.

Robert D., Dijstra G. (2004), “The use of fibre optic strain monitoring systems in the

design, testing and performance monitoring of the novel freestanding

Dynarigs of an 87 m Superyacht by Perini Navi, Design by G. Dijstra”,

Proceedings of the HISWA International Symposium on Yacht Design and

Yacht Construction, Amsterdam, Netherlands.

Shenoi, R A, Conti, P, Turnock, S R and Scarponi, M (2006) “Mini 6.50 Mast

Optimisation using a Design of Experiment Approach and Finite Element

Simulations”, International Journal of Small Craft Technology, The

Transactions of the Royal Institution of Naval Architects, Vol. 148, Part B1,

pp41-49.

Sutherland, L. S. and Guedes Soares C. (2007) “Scaling of impact on low fibre-volume

glass-polyester laminates”, Composites Part A: Applied Science and

Manufacturing, 38, pp 307-317.

Sutherland, L. S. and Guedes Soares C. (2006) “Impact behaviour of typical marine

composite laminates”, Composites Part B: Engineering, 37(2-3), pp 89-100.

Sutherland, L. S. and Guedes Soares C. (2003) “The effects of test parameters on the

Page 61

ISSC Committee V.8: Sailing Yacht Design 493

impact response of glass reinforced plastic using an experimental design

approach”, Composites Science & Technology, 63, pp 1-18.

Trower, G. (1999), “Yacht and small craft construction: Design decisions”, Crowood

Press, Marlborough, Wiltshire.

Uzawa, K. (2001) “America's Cup Yacht”, Encyclopaedia of composite application,

The Japan Society for Composite Materials, Tokyo.

Ward, L.W. (1985), “Sailboat Bow Impact Stresses,” Proceedings, 7th Chesapeake

Sailing Yacht Symposium, Annapolis, MD. pp 13 - 20.

433

17th INTERNATIONAL SHIP AND

OFFSHORE STRUCTURES CONGRESS

16-21 AUGUST 2009

SEOUL, KOREA

VOLUME 2

COMMITTEE V.8

SAILING YACHT DESIGN

Mandate

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.

Members

Chairman : A. Shenoi

R. Beck

D. Boote

P. Davies

A. Hage

D. Hudson

K. Kageyama

J. A. Keuning

P. Miller

L. Sutherland

Page 30

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:

h

GZ

FH

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

Page 31

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 et.al., 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.

Page 60

ISSC Committee V.8: Sailing Yacht Design 492

Miller, P. H. (1995), “Design Criteria for Composite Masts”, CSYS Conference,

January 28.

Miller, P. H. (2000), “Durability of Marine Composites”, Doctoral Dissertation,

University of California, Berkeley.

Miller, P. H. (2003), “Design, Verification, and Forensic Correlation of Composite

Yacht Structures”, Advanced Marine Materials Conference, Royal Institution

of Naval Architects, London, UK, October 9-10

Nordisk Bat Standard (1990), ”Fritidsbater under 15 meter”, Det Norske Veritas, Oslo.

Pallu P., Vedrenne J., Devaux H., Balze R. (2008), “An efficient tool for mast design

and tuning”, Proceedings of Madrid Diseno de Yates Symposium, Madrid,

Spain.

Professional Boat Builder (2008), “Closing Molding at Cobalt”, Number 113, July.

Purcell, E., Allen, S. and Walker, R. (1988), “Structural analysis of the U.S. Coastguard

Island class patrol boat”, Transactions of Society of Naval Architects and

Marine Engineers, 96, 221-246.

Registro Italiano Navale (RINa) (1984) “ Regolamento per la Costruzione e la

Classificazione delle Barche a Vela da Regata 12 m S.I. e 6 m S.I.”, Genova,

Italy

Registro Italiano Navale (RINa) (2006), “Infusion as a Composite Construction

Technique for Pleasure Vessels: Guidelines”, RINa S.p.A, Genova

Registro Italiano Navale (RINa) (2007), “ Rules for the Classification of Yachts”,

Genova, Italy.

Riber, H.J. (1993) “Strength analysis of the 470 sailing boat”, Report to Technical

University of Denmark, Lyngby.

Richter, H. and Braun, J. (2003), “Computational Fluid Dynamics for Downwind Sails”,

Proceedings of the 16th Chesapeake Sailing Yacht Symposium, Annapolis,

USA.

Rizzo C., Carrera G. (2007), “Measurement of shrouds deformation a large sailing

ship”, Marstruct Report MAR-W2-7-DINAV 39, Genova.

Robert D., Dijstra G. (2004), “The use of fibre optic strain monitoring systems in the

design, testing and performance monitoring of the novel freestanding

Dynarigs of an 87 m Superyacht by Perini Navi, Design by G. Dijstra”,

Proceedings of the HISWA International Symposium on Yacht Design and

Yacht Construction, Amsterdam, Netherlands.

Shenoi, R A, Conti, P, Turnock, S R and Scarponi, M (2006) “Mini 6.50 Mast

Optimisation using a Design of Experiment Approach and Finite Element

Simulations”, International Journal of Small Craft Technology, The

Transactions of the Royal Institution of Naval Architects, Vol. 148, Part B1,

pp41-49.

Sutherland, L. S. and Guedes Soares C. (2007) “Scaling of impact on low fibre-volume

glass-polyester laminates”, Composites Part A: Applied Science and

Manufacturing, 38, pp 307-317.

Sutherland, L. S. and Guedes Soares C. (2006) “Impact behaviour of typical marine

composite laminates”, Composites Part B: Engineering, 37(2-3), pp 89-100.

Sutherland, L. S. and Guedes Soares C. (2003) “The effects of test parameters on the

Page 61

ISSC Committee V.8: Sailing Yacht Design 493

impact response of glass reinforced plastic using an experimental design

approach”, Composites Science & Technology, 63, pp 1-18.

Trower, G. (1999), “Yacht and small craft construction: Design decisions”, Crowood

Press, Marlborough, Wiltshire.

Uzawa, K. (2001) “America's Cup Yacht”, Encyclopaedia of composite application,

The Japan Society for Composite Materials, Tokyo.

Ward, L.W. (1985), “Sailboat Bow Impact Stresses,” Proceedings, 7th Chesapeake

Sailing Yacht Symposium, Annapolis, MD. pp 13 - 20.