## Yacht Design

Aero-Hydrodynamics of Sailing 3rd edition C A Marchaj ISBN 0 7136 5073 7 Incorporating many years of research, this book takes Marchaj's expositions on the design and handling of sailing boats in Sailing Theory and Practice a stage further. Clear explanations, calculations, diagrams and photographs help to quantify the many factors which determine the performance of a sailing boat. It is an international classic in its field. Sail Performance Theory and Practice C A Marchaj ISBN 0 7136 4123 1...

## High Speed Hydrodynamics

Fig 10.14 Rescue boat in The disadvantage of having the weight this far aft is that the ride may rough seiis (Photo Dan be bumpy in a head sea. Rather than hitting the next wave by the bow Ljungsvik) the hull will land on the afterbody after a jump. A remedy is to trim the hull slightly more by the bow using the trim flaps. Another disadvantage is the non-optimum low speed performance. At lower speeds a larger wetted surface is required, which means that the pressure force is moved forwards and...

## Sail and Rig Design

Fig 7.13 Flow around a m as t sa i I comb in a tion There are two reasons why separation has to be avoided. First, the pressure distribution on the sail is disturbed, essentially in such a way that pressure differences between the two sides of the sail are reduced. This causes a reduction in lift and driving force. Secondly, separation itself causes a drag increase. Experiments at Southampton University with a mast sail combination indicated large effects of mast disturbance. Thus, when a...

## Hydrostatics and Stability

The forces on the stalled surfaces the area is much more important than other geometrical properties, so a long keel yacht will have more damping than a fin-keel one. This is an important conclusion, which speaks in favour of traditional designs and against more modern ones with a small lateral area. It should be pointed out also, that forward speed increases damping considerably, particularly for fin-keel yachts. If the speed is high enough the keel starts working properly and the forces get...

## Propeller and Engine

1 Assume that the velocity at the propeller is equal to the yacht speed, as before. 2 Compute the total resistance and assume that this is equal to the thrust, as before. 3 Compute the propeller loading, KT J2 4 Find the point on the loading curve in Fig 9.14 that corresponds to the computed value, and read the advance ratio and the torque coefficient on the same vertical line. 5 Compute the rate of revolutions from the definition of the advance ratio and the power from the torque and the...

## Hull Design

Fig 5.9 Pressure the pressure in the middle part of the hull is lower. Had there been no distribution with and boundary layer, the pressure forces over the bow would have balanced without separation those over the stern exactly and there would have been no resulting force (neglecting for a moment the effect of the waves, which also have an influence on the pressure). The boundary layer does, however, modify the pressure distribution, and, since the layer is considerably thicker around the stern...

## Principles of Yacht Design

Fig 6.30 Influence of trailing edge geometry on vibration level academic interest. On the contrary, they may cause severe vibrations and noise in the entire hull at speeds where resonance occurs. Using the information in Fig 6.30 these problems can be solved. Advanced section The NACA sections presented above are quite efficient and useful for design most sailing yachts. However, under certain conditions more advanced sections have been used. This is so in connection with the America's Cup and...

## Rig Construction

Required transverse moment of inertia, ix, for the mast 7.25 for wood (Spruce) 70500 E for other materials l(n) actual panel length kj 1.55 for deck stepped masts 1.00 for keel stepped masts When calculating Ix for panel 2 PT is decreased by When calculating ix for panel 3 PT is decreased by D1 is taken from Fig 11.6 for a single spreader rig. D1 and D2 are taken from Fig 11.7 for a double spreader rig. Fig 11.9 Transverse mast dimensioning Transverse mast The tension in the shrouds and stays...

## NRIG

This chapter deals with the dimensioning and construction of the rig. Over the years different methods have evolved, ranging from old rules of thumb for solid wooden spars to sophisticated computer models for exotic composite materials. We will take a middle line, using accepted standard engineering practices as they are used in the Nordic Boat Standard (NBS). The reason for using this NBS standard instead of ABS or Lloyd's Register is the simple fact that NBS is one of the few yacht scantling...

## Balance

Certainly, the underwater body is a wing of a very peculiar shape and thickness distribution, and the aspect ratio is small, but wing theory at least shows that there is no reason to assume that the hydrodynamic CLR should coincide with the geometric one. Various ways to find an approximate position of the hydrodynamic CLR have been proposed. Professor J Gcrritsma suggested a method for fin-keel yachts in which only the keel and rudder are considered. To...

## Keel and Rudder Design

The increase in induced resistance is around 1 for taper ratio deviations of 0.4 from the optimum (as appears from Fig 6.9). Since the induced resistance is only 8 of the total in upwind sailing (and less downwind), according to Fig 5.4, the increase in total resistance due to the too high taper ratio is less than 0.1 . This is certainly compensated for by the stability increase. Thin sections at the root and fat ones at the tip are favourable for several reasons. Most importantly, this lowers...

## Geometry

He hull of a yacht is a complex three-dimensional shape, which cannot be defined by any simple mathematical expression. Gross features of the hull can be described by dimensional quantities such as length, beam and draft, or non-dimensional ones like prismatic coefficient or slenderness (length displacement) ratio. For an accurate definition of the hull the traditional lines drawing is still a common tool, although most professional yacht designers now take advantage of the rapid developments...

## Contents

Preface to the Second Edition - ix 2. Preliminary Considerations 10 Computer aided design of hulls 27 4. Hydrostatics and Stability 30 Transverse and longitudinal stability at small angles 40 Transverse stability at large angles of heel 42 Influence of waves on the righting moment 49 Forces and moments on a sailing yacht 56 Viscous resistance, basic concepts 60 Wave resistance, basic concepts ** 69 Influence of hull shape on wave resistance * 73 6. Keel and Rudder Design 96 Definition of the...

## Introduction

Uring the past 30 years yachting has expanded from being, generally speaking, a minority sport - too expensive for the large majority of people - into a major recreational activity practised by millions all over the world. In the 1960s, many attractive coastal areas were still relatively free from pleasure boats today it can be difficult to find a suitable mooring place for the night. The interest in racing has increased correspondingly at all levels, from dinghy racing to the America's Cup and...

## Grimalkin Yacht

Knowing B', the location of the point where the vertical through B' hits the centre plane M can be found, sec Fig 4.12. BM may then be measured from the figure and the remaining formulae for small angles applied. Curve of static The curve of static stability represents the righting moment at varying stability angles of heel. An example of this is given in Fig 4.13. Since the moment differs from the lever arm only with respect to the constant Ag, the vertical scale could equally well represent...

## Hull Construction

Pmast sin lt i tona2 cos a, - sincej ' P 33247 N 12990 N 42038 N 12990 N y os lt - lt -,2- L22 3 -E- L E Young's Modulus i Moment of Inertia for Hull Girder Maximum Bending Moment Mbhu of the hull occurs in the mast area Mbhu Pmast - Lr L2 L 201375 Nm Fig 12.4 Longitudinal rig With regard to the YD-40, with 7.3 tonnes pressure from the mast forces we can use a simplified model to estimate the required hull girder section modulus SMhull . Considering the yacht to be a beam freely-supported at...

## Methodology

Yacht design is an iterative, 'trial and error procedure where the final result has to satisfy certain requirements, specified beforehand. To achieve this the designer has to start with a number of assumptions and work through the design to see if, at the end, it satisfies the requirements. This will most certainly not be the case in the first iteration, so he will have to change some assumptions and repeat the process, normally several times. The sequence of operations is often referred to as...

## Sailboat Gyradius

And the longitudinal centre of buoyancy should be located 3.5 behind midship. Both these requirements are met. The choice of other shape parameters will be discussed in connection with hull statistics in a later section. Heel resistance When the hull heels due to the side force from the sails, two resistance components develop, as explained in the first section of this chapter. The induced resistance is by far the most important one, but it will not be discussed here, since it is mainly caused...

## List Of Symbols

In general, the symbols used in this book are those recommended by the International Towing Tank Conference ITTC . However, in the chapters on scantling determination hull dimensioning and the Nordic Boat Standard rig dimensioning other symbols have been used. This is to simplify the later use of these standards by readers. distance from neutral axis to centre of area mainsail area, or midship section area below designed aspect ratio and change in aspect ratio, respectively sail area main fore...