Trans RINA, Vol 154, Part C1, Intl J Marine Design, Jan - Jun 2012


In determining the fore and aft rocker I tried to give the boat an easy entry with a soft transition through the mid- section. The run aft is critical to ensuring performance over a range of conditions and points of sail, so I tried to balance a straight run aft for maximum pressure recovery, which promotes planing, with a balanced transition from the mid- to aft sections. By balancing the need for a straight run aft with an even and balanced rocker and diagonals, the boat will sail bow out when fast reaching and running, which has a range of desirable effects including reduced wetted surface and increased control.


Right from my initial concept I had envisaged a boat with high form stability, as this has proven in my previous designs to be effective when combined with a relatively low ballast ratio, average crew weight and light displacement. From observations and experience I have identified that boats with moderate proportions perform the best over a wide range of sailing conditions. There is some evidence that boats with more extreme hull forms can achieve increased performance in some conditions but these gains can be offset by reduced performance in conditions to which they are less suited. It was a high priority for this boat to perform well over a range of wind and sea conditions, due to the changeable conditions experienced around the upper North Island, exaggerated by longer course, passage and overnight races.


Based on these parameters, I developed two candidate designs as computer models, which enabled me to make numerical comparisons


between them. Of particular


interest was the effect of increasing waterline beam on the boat’s small-angle stability and wetted surface area. However, computer modelling is unable


to express


numerically the effect such changes would have on the sailing characteristics of the boat. While it may be clear that one model may, for example, have increased wetted surface or improved righting moment, computational analysis doesn’t provide any feedback on how these differences will actually affect the sailing performance when their effect on the total package is considered, including the effect on balance. These limitations of computer modelling are noted by


several authors,


including Levadou, Prins, & Raven [19], and Roux et al. [20], who state,


One of the major difficulties of such a


computation [using CFD programs] is that the flow over any one of the components — sails and hull — operating in a real sailing boat is a very complex combination of many phenomena, some of which being clearly non-linear. Besides this, a sailing boat is an integrated system in which sails and hull closely interact.


Larsson also notes that “A weak point of most [computer] VPPs is the prediction of the performance in waves . . . Waves create effects in all degrees of freedom . . . therefore a complete model for the wave effects is out of reach at present.” [21]


I then proceeded to hand-draw two draft lines plans for the hull, developing the basic computer sketches and bringing together the characteristics of hull form and weight that I had so far predetermined. This was a major step, as the initial overall lines plan is a major expression of the concept of the boat and has the single largest influence


on the outcome of the performance and appearance.


Developing both of these initial plans as hand drawings as opposed to computer-generated models made for a much more tactile process. The hull lines are built up using a systematic approach,


in which the shape,


characteristics and effect on the design of each line are carefully considered before it is drawn, allowing a clear process to realising the solution to the design concept rather than following a set of computer-generated figures.


First I developed a plan for what I considered to be a boat of moderate dimensions that would fit within the parameters that had been defined for the boat and was consistent with all of the elements of hull form and aesthetics that I had determined. I drew a balanced boat that I considered to be a good all-rounder in terms of performance.


Next, I did a variation on this design which was a more powerful adaptation of the same concept. I kept all of the base parameters the same, including deck line, sheer line and displacement freeboard, but concentrated on giving the hull a higher volume and therefore higher stability.


design, both in


form


I pinned both of these lines plans on my office wall and studied them, evaluating their merits and how each would vary in performance over a range of conditions and scenarios. Each day I would spend time on the process of observation and reflection, considering these two plans and what variations I could make to them and how I could improve the performance potential of the boat’s design.


Through this process I made minor refinements to the design based on improving its balance (see below) and how this would affect the yacht’s performance in a wide range of


sailing conditions. My research and careful


consideration of the design led me to the more powerful of the two models.


3.2 (c) Weight and Balance


As part of the design refinement process and action research spiral, I set up a spreadsheet to develop a detailed weight study for the boat. This went through several iterations as the boat’s design and construction progressed. Every component was weighed as it was built and its longitudinal


centre of gravity (LCG),


transverse centres of gravity (TCG) and vertical centre of gravity (VCG) considered and updated on the spreadsheet to ensure the finished boat would end up


C-32


©2012: The Royal Institution of Naval Architects


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