Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014


In response to Prof Chris Baker: An increasing number of yacht owners are requesting the Dynamic Positioning System (DPS) which is a computer controlled system to automatically maintain the ship position and heading by using her own propellers and thrusters.


For maxi and mega-yacht applications DPS is used to get and maintain a fixed point or to maintain a defined course at very low speed, mostly in good weather conditions or in any case in low Beaufort sea states. Typical applications are maintaining ship position and orientation in sheltered waters without anchoring, or course keeping in narrow passages. Position reference sensors, combined with wind sensors, motion sensors and gyro compasses, provide information to the system about the ship position and the magnitude and direction of the acting external forces.


The system software contains a mathematical model of the ship that


knowledge, combined with the sensor Aiming to have a reliable model of


hydrodynamic model of the yacht and the location and propulsive


includes an aerodynamic model, an characteristics of the thrusters. This


information,


allows the computer to assess the steering angle and thruster output for each thruster.


the yacht


aerodynamics, which is essential for a good performance of the DPS system in order to maintain ship position and orientation in


sheltered waters without anchoring, aerodynamic forces knowledge is predominant.


So the experimental assessment of the ship aerodynamic behaviour through wind tunnel tests on a scale model is very significant for the design of the installed DPS system and its performances which heavily depend on the aerodynamics loads on the vessel.


The aerodynamic coefficients for merchant ships are available, as extensive wind tunnel tests have been done in the past on the various ship types, but there is a lack of information for yachts, as not many tests have been performed, and also


because of the variety superstructure shapes which heavily affect the results.


The results of the simulation are usually given in the form of capability polar plot.


Figure 18 shows an example of capability polar plot which,


for every relative incidence, provides the


maximum wind speed (in Knots) allowed by the DPS system in order to guarantee the capability of maintaining the desired ship position and orientation.


The aerodynamic coefficients are useful also if ship motion simulation softwares are employed, again the effect of the wind so far has been taken into account by applying other ship types coefficients.


As previously mentioned it would be interesting to set up a unique standard to be adopted with reference to the wind comfort onboard a yacht.


In reference to the last point raised, the pollutant


concentrations in this instance were so low that there’s no issue about the acceptable values. However, an average threshold value is 1000ppm of CO2.


of


Figure 18: Capability polar plot It


design of a yacht will be driven by the requirements of sea keeping


is however unlikely that a superstructure exterior or


dynamic positioning performances,


therefore it is hard to tell which coefficients are good or bad. The propulsion and motion control systems must be designed to meet the required performances having the superstructure shape as a design constraint.


In principle it should be possible to adopt one of the comfort criteria usually used for urban pedestrian comfort evaluation (e.g. DMI Wind Comfort criteria) where the evaluation of discomfort phenomena is analysed in probabilistic terms using the speed up measurements and evaluating the probability of exceeding of a threshold level suitable for different activities.


The main problem in the present case is that the anchorage site is in principle never the same so it’s difficult to identify a reference area for the wind climate definition in terms of wind statistic distribution.


B-116


©2014: The Royal Institution of Naval Architects


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