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Introduction

In this report various aspects of an International Moth racing dinghy design are investigated with a view to developing a competitive Moth design. The Author's existing boat, which is a copy of the 1995 World Championship winning Aussie Axeman boat, has been used to provide data which can be compared with any new design.

 

Hull Design and Stability

Hull design is investigated leading to the presentation of a new International Moth hull, the WSH design. An attempt to model dynamic stability is carried out using quasi-static models. The transverse and longitudinal stability of the new WSH design is compared against the Aussie Axeman hull form which is considered to have acceptable stability characteristics. A suitable foil on the tip of the rudder is seen to give the relatively unstable WSH design similar longitudinal stability to the Aussie Axeman.

 

Sail Design and Balance

The sail and rig are investigated using data from "The Aero-hydrodynamics of Sailing" to illustrate the factors involved and how the handling characteristics can be controlled and modified. Methods of depowering with sail twist and mast rake are discussed. A sail profile and its area measurement, in accordance with the International Moth class rules, are also presented.

Vector analysis of a complex mast raking control system is shown in an attempt to provide an easier and more accurate iterative method for determining the shroud and forestay positions. Until now a lengthy experimental approach has had to be used when fitting out this particular mast raking system.

The factors involved in balancing the yawing moments of a sailing craft are discussed. The basis for a rule of thumb to determine the correct position of the dagger board and the mast step position for an International Moth design is suggested.

 

Rudder Design

The rudder design is investigated by presenting equations that can be used to evaluate various rudder designs. Four rudders were numerically evaluated and a suitable improved rudder design (complete with longitudinal stability foil) is compared to a standard rudder design.

 

Mast Design

A crude analysis of the working loads on the rig is attempted in order to produce a structural specification for the mast. This analysis is shown to be unreliable because it predicts that the working loads on the mast are in excess of the Euler critical buckling load and that would break existing International Moth masts.

A method for calculating the modulous of a composite mast section is presented. A worked example shows how the bend characteristics of a competitive parent mast can be mimicked.