for lack of a more precise definition. (This apprehension may be partial or substantial or complete yet undefined)


The problem is that while this may be valid, it is essential to avoid preconceptions; these freeze the design and make conceptual movement impossible, thereby limiting the creative possibilities. This takes substantial experience and judgement.


When the himself/herself about


designer enters the


into a dialogue design, the with development


process using sketches, is essentially an iterative process, the movement within the design space is not unidirectional as the designer explores and moves closer to an appropriate “fit”. In the case of marine design there is, and must be, a constant reference back to the physical performance requirements of the


design, be they


structural, constructional, or emotional, psychological, symbolic, ergonomic etc.


It is believed that this referential iterative process is familiar and common in all design activity.


3. YACHT DESIGN METHODOLOGY


There are also various methodologies used by yacht designers. Again these are all similar. The usual analogy used to describe the methodology is the design spiral this is a iterative “trial and error”, procedure where staring with a set of assumptions the designer works through empirically stages spiralling in towards as close a “fit” as possible to satisfy the design requirements, this is repeated through many iterations until the desired result is produced. [7]


At the initial stage the designer will define a set of geometric dimensions and select coefficients and ratios. In yacht


design these relate to linear dimensions,


volumes, mass, hydrostatic and hydrodynamic properties, sail area to displacement ratio. etc. Probably the most elementary are overall dimensions and some reasonable estimate of weight (displacement)


These various values are then applied to the development of an initial set of lines. Most yacht designers will build an elementary mathematical model initially, then begin to develop a lines plan from the mid-ship section, waterplane, profile, plan etc. [7]


Nowadays yacht designers will probably begin a new design with an existing design on file in their computer, or set up a basic grid and sketch in the various views and sections needed to work through developing a set of lines.


Typical dimensions relating to Geometric shape and


proportion, (Linear dimensions and Areas) are:-  LOA (Length overall of hull)  DWL (Design waterline)  BOA (Beam overall)


Even at a very early stage, if minimum performance requirements such as sailing performance are critical then the design will be put through some form of sailing performance analysis or comparative analysis. (e.g. Velocity Prediction Programme (VPP) Fluid Dynamics analysis (CFD) etc)


or Computer


If a minimum condition is the primary driving force as in the case of an America’s Cup yacht, then increasingly use is being made of hullform optimisation programmes and substantial tank testing. This is hardly surprising as the costs of not achieving the required performances are substantial.


It is anticipated that the automation of design in this way will continue to increase; one envisages quite soon various virtual design options being sailed off against one another, around a course using a specific set of sailing conditions, with 3D virtual models and performance data projected onto the monitor in real time.


The implications of such automation of engineering and naval architectural techniques are interesting in relation to marine design, as it indicates (to the author at least) that in the future the naval architect will be freed to undertake more creative development in technical design (for example new types of hullforms) while the less demanding


minimum condition designs will allow


substantially more creative input and design control by the marine designer.


By way of illustrating just how widely variations for a set of design parameters might be interpreted,


two very


rough concept sketches for an 18ft trailer sailer, are shown below. They serve to show how widely variations for a set of design parameters might be interpreted, we hope they serve to illustrate, at the initial conceptual stage at least, how qualitative judgements are an essential part of the design process.


Larssen and Eliasson [7] produced what is now considered to be the most popular (and useful) general


 BWL (Beam waterline)  BWLmax (Maximum beam on waterline) 


B/L ratio (beam to length) etc.


 S/A Sail Area  Midship Area coefficient  Waterplane Area coefficient


While examples of ratios and coefficients related to Topological shape (or Volumes) are:-   


Froude number


S/L (speed to length) Prismatic coefficient


 Block coefficient 


 Length/Displacement ratio 


Displacement/Length ratio (D/L) Sail Area/Displacement ratio.


© 2008: Royal Institution of Naval Architects


B-25


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