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Abstract

The International Moth is a single-handed development class of racing dinghy. The intention of the class rules is to give the designer and builder the fullest liberty in design and construction to develop and produce faster boats.

The rules can be summarised as follows :

Maximum length 3355 mm
Maximum beam 2250 mm
Maximum sail area 8 m2
Maximum luff length 5185 mm
One person only
One sail only
Multihulls, trapezes, moveable seats and sailboards are prohibited

The lack of restriction within these rules has led to some interesting craft which have tested their designers' ideas. Because any successful design has to be competitive on all points of sailing and in most wind strengths and sea conditions, this has inhibited a lot of the more radical craft from widespread success.

Over the sixty year history of the class, it has always been a handful of boat builders, sail makers and enthusiastic sailors who have developed their boats. The majority of moth sailors merely copied or bought the boats that were apparently fast at that time. In the last few years the fastest boats have almost become "off the shelf" products. The modern sailor does not have sufficient time, equipment, knowledge or enthusiasm to create his or her own competitive boat. This is as much a reflection on society as it is upon the sailors within the class.

Being one of the last enthusiastic sailors, I have built my own International Moth incorporating existing ideas in its design. The construction methods and materials were governed by what was available and affordable. The structural design was based on a "that looks about right" philosophy. Although building boats on experience , empirical data and an intuitive feel may sound primitive it can produce impressive and competitive results.

To progress in my quest to build faster boats I must step up a league and incorporate engineering techniques and scientific theory in a new design. This report is a technical investigation into aspects of Moth design, with a view to developing a new competitive design.

 

List of Tables

 

1.1 Comparison of speeds from different scales.
1.2 Aussie Axeman hydrostatic data.
1.3 WSH hydrostatic data.

2.1 GZ data for Axeman.
2.2 GZ data for WSH.
2.3 Calculated quasi-static stability for WSH design.
2.4 Calculated quasi-static stability for both designs.
2.5 Calculated quasi-static stability for WSH design with the mast heeled to leeward.

3.1 WSH design LCB position with -20 cm of trim.
3.2 Axeman LCB position with -20 cm of trim.

4.1 Calculated values of CL and CD for a foil with sweep where induced drag and profile drag are the same in magnitude.
4.2 Calculated values of CL and CD for a foil with sweep where induced drag is 50 % larger than profile drag.

6.1 Standard rudder design lift and drag characteristics.
6.2 Design 1 rudder lift and drag characteristics.
6.3 Design 2 rudder lift and drag characteristics.
6.4 Design 3 rudder lift and drag characteristics.
6.5 Correction factors for profile shape on rudder lift and drag coefficients.
6.6 Actual values of lift and drag for a standard rudder at 5 knots.
6.7 Actual values of lift and drag for a design 2 rudder without foils at 5 knots.
6.8 Actual values of lift and drag for a design 2 rudder with foils at 5 knots.

7.1 Mast section radii and weight.

 

List of Figures

 

Appendix 3

1.1 Aussie Axeman measured sections.

2.1 Graph of the GZ curves for the two hull forms.

2.2 Graph of total quasi-static stability for WSH design

2.3 Graph comparing the two designs total stability

2.4 Graph of quasi-static stability with the mast heeled to leeward.

3.1 Foil characteristics of the NACA 65.012 section. Reprinted from "Theory of wing sections". by Abbott and Von Doenhoff.

4.1 Forces developed by a single Bermudan sail in beam reaching condition. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.2 The effect of the gap under the boom on L / D ratio of a Finn-type sail. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.3 Graphs investigating spanwise lift distributions. Reprinted from Transactions RINA 1992 paper titled "The optimisation of aerodynamic lift distribution for a heeled yacht in a wind gradient". by A. H. Day.

4.4 Approximate suitable spanwise lift distributions for a Moth sail.

4.5 Effect of camber on L / D ratio. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.6 Variation of lift with incidence for circular arc section of different camber ratio. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.7 Section characteristics CL and CD of 2 sail sections with masts. Camber ratio is about 12 %. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.8 Air flow over mast and sail. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.9 Mast and sail combinations.

4.10 Moth sail profile.

4.11 Effect of angle of sweep on induced drag of foils of larger aspect ratio and elliptic or nearly elliptic profile. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.12 Graph of sail characteristics with rake.

4.13 Components of total drag CD plotted against CL2. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.14 Graph of sail characteristics with rake.

4.15 Lift and drag coefficients of a Finn-type rig at different kicking strap tension. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.16 Polar diagram of sail coefficients of Finn-type rig at different kicking strap tension. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.17 L / D ratio for a Finn-type rig at different kicking strap tension. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

4.18 Effect of changes in luff and foot tensions on L / D ratio. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

5.1 Aussie Axeman sail profile showing the centroid of area.

5.2 WSH sail profile showing the centroid of area.

6.1 Effective aspect ratio factor for a number of foil forms with various tip modifications. Reprinted from "Aero-hydrodynamics of sailing". by C. A. Marchaj.

6.2 Foil characteristics for the NACA 0012 section. Reprinted from "Theory of wing sections". by Abbott and Von Doenhoff.

6.3 Foil characteristic for the NACA 64.012 section. Reprinted from "Theory of wing sections". by Abbott and Von Doenhoff.

6.4 Rudder L / D ratio plotted against angle of attack.

6.5 Rudder drag plotted against lift.

7.1 Graph of mast section radius and weight.