## Propeller and Engine

Propeller Propeller blades act as wings when the propeller rotates and advances characteristics through the water. A section of a blade at a certain radius is shown in Fig 9.5. It can be seen that the resulting velocity, to which the blade is exposed, is composed of the axial component (due to the forward motion) and the tangential component (due to the rotation). The former is normally not exactly equal to the yacht speed, but somewhat lower, since the propeller operates in the wake behind the...

## Principles of Yacht Design

Total wetted surface less than on a boat without steps, generating the same lift discharge the exhaust gases through the step. In this way the gases will be suckcd out, improving the efficiency of the engine. Since the lift is now spread to several surfaces along the hull (see Fig 10.12) the longitudinal stability becomes very large. It is difficult to change the trim. This is no problem in smooth water, but in a seaway the hull may tend to follow the contour of the waves. Larger hulls may...

## Keel And Rudder Design

In the design of keels and rudders well established principles from aircraft aerodynamics may be employed. Although most aircraft today fly at speeds at which the compressibility of the air is important (more than 100 m s), much information may be gleaned also for the incompressible water flow, partly due to the early aerodynamic research carried out more than 50 years ago. In this chapter we will first give a short introduction to the basic principles of the flow around a wing (keel or rudder)...

## Hull Design

To be any particular variation with length, at least not below 12 m (Lwl), which is the upper limit for the data used for evaluating these numbers. Our YD-40 represents the average with a ballast ratio of 0.45. The keel mass is 3250 kg and the light displacement 7250 kg. It should be mentioned, finally, that the official displacements used in the statistics may be slightly low, due to optimistic weight calculations even for the light condition. In reality, the length displacement ratio and the...

## Hydrostatics and Stability

Fig 4.20 Influence of wave on the stability curve - Grimalkin reductions may occur, as shown in Fig 4.20. Grimalkin's stability curve is shown for a wave height of 12 metres and a wave period of 9 seconds. These extreme conditions were actually measured in the Fastnet disaster in 1979. It is seen that on a wave crest the stability is almost halved, and this is at a position when the yacht is most exposed to the wind.

## 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...

## Rig Construction

2) Single Lowers with Inner Fore stay Aft stay is optional on fractional boats below 1 tonne of dispiacem ent. 4) Runners with Checkstay (Only on Fractional Rig) 5) Single Lowers with swept Spreaders (Only on Fractional Rig) 6) Simple Rig with no or short Spreaders The foremost sail carrying forestay shall have a breaking strength (Pfo) of at least, The inner forestay shall have a breaking strength (Pf.) of at least The aft stay shall have a breaking strength (P ) of at least PQ sin af y sina c...

## 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...

## Balance

Fig 8.6 Position of centre of pressure for plane wings of varying aspect ratio Xcp Distance from leading edge to centre of pressure thai for a rudder hung below the bottom of the hull the effective aspect ratio is twice the geometric one (as explained in Chapter 6). Tt is seen in the figure that the centre of pressure moves towards the leading edge when the aspect ratio goes to zero. It is of the utmost importance that the rudder is not over-balanced (ie has its centre of pressure forward of...

## 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...

## High Speed Hydrodynamics

Dynamic stability There are two important dynamic stability phenomena for high speed hulls. One is caused by the large centrifugal forces generated when a hull at high speed changes its direction. The other occurs due to the suction forces which may be generated near the chines due to convexity of the hull buttocks. We will deal with both in the following. When the rudder is given an angle of attack a force is generated sidewards. This causes the hull to start moving in this direction and,...

## 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...

## 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...