Trans RINA, Vol 152, Part B2, Intl J Small Craft Tech, 2010 Jul-Dec
It should be noted that goals of minimising resistance and maximising velocity made good are differentiated by rig dimensions, ballast ratio, foil configuration etc.
4. TECHNICAL ANALYSIS
The main program calls the subroutines prepared for the design of the yacht. These subroutines are combined to create a template. It consists of some
structured
subroutines for input, output, definition of constraints and subroutines prepared for calculation of design parameters and constraints (design analysis).
The design analysis subroutine starts with a set of owner’s requirements (Table 1) and a set of input data for the twenty-nine system variables (Table 2). It then estimates the necessary
parameters to define the
geometry of hull, keel, rudder and sail particulars. The masses of rig and sails, deck gear, machinery, interior fittings and respective KGs (height of centre of gravity above the bottom of keel) are estimated by the method as suggested in [4]. The ballast material is chosen so that the entire volume of keel is filled in and its specific gravity is not to be greater than that of lead. The keel mass and its KG are estimated as suggested in [21]. The masses of deck and equipment, interior fittings and machinery are assumed as the mean of lower and upper limits as suggested in [4]. An inboard engine is assumed for the 2 cabin and 3 cabin yachts. For the 1 cabin yacht it is assumed that an outboard motor weighing 40 kg is fitted and the mass of fuel is 20 kg.
As the cost data are difficult to obtain, the approximate cost (Euro) of the yacht is estimated using the formula suggested in [4]. As these cost data are for 2003, the cost was adjusted using an escalation factor of 1.05 per year. The resulting estimated costs shown are for 2009 values.
The program next estimates the velocity made good (VMG)
for each of seven Froude numbers selected
within the range of data available for the estimation of resistance and sail forces using a subroutine for the prediction of velocity as suggested in [4]. The maximum of the seven VMGs is estimated by a routine that uses a search technique by the Golden Section Method [22]. The other particulars at the maximum VMG are estimated by an interpolation subroutine.
There is an option for the STability IndeX (STIX) – (part of the ISO Stability and Buoyancy standard) to be estimated. For the European market, it is mandatory to include the calculation for STIX. The particulars required for the STIX can only be calculated accurately if the lines plan of the vessel is available. As the lines plan is not
developed at this preliminary design stage, the
righting levers are estimated as suggested in [4], and the approximate method as suggested in [23] is adopted to calculate a value of STIX.
The initial point for starting the optimisation is varied within the range of principal design parameters and after a few runs the final solution obtained is the global solution i.e. it is arrived at consistently, irrespective of the initial point.
5. RESULTS AND DISCUSSION
Two important considerations must be borne in mind when interpreting the results. Firstly, the model settings used are intended only to be an example of what can be achieved. Secondly, a designer may, and most probably will select different goals, weightings and constraints to those used here, based on their own experience and using their own databases. For example, the model selected here fixes the main hull shape and stability once the length has been determined, which in turn is governed largely by internal layout. This results in a model that optimises as much for comfort and cost as it does performance. The effect can be seen when true wind velocity is varied, revealing little change in displacement, GM or sail area.
The model is applied to the three sets of owner’s requirements shown in Table 1.
The resulting principal design parameters are shown in Figure 1. It shows that the main dimensions increase as the number of required cabins increases, as expected. The dimensionless ratios also change, as shown in Figure 2. This illustrates the effect of using such an optimisation technique. Figure 3 shows mass, cost, velocity made good and length overall, for yachts of all three configurations.
A sensitivity analysis is carried out varying true wind speed between 10 and 20 knots. Performance and cost as a function of wind speed are plotted in Figure 4, Figure 5 and
Figure 6 As the general arrangement is not defined at the
preliminary design stage, the down flooding angle is assumed to be one degree greater than the angle of the vanishing stability.
for 3 cabin, 2 cabin and 1 cabin
configurations respectively. The cost of the yacht is effectively independent of wind speed because it is determined largely by vessel length, which in turn is governed more by
interior volume than sailing performance for the way the model has been set up here.
A sensitivity analysis to loading condition is conducted for the 2 cabin configuration by varying the number of cruising days from 2 to 16. The results are shown in Figure 7 and Table 3. The results are again as expected; the longer passage time requiring more stores and a heavier boat to accommodate them, with a consequent increase in construction costs.
©2010: The Royal Institution of Naval Architects
B-77
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66