Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014
For the lower angle the flow the outer streamlines follow largely the side of the ship while for the larger angle strong separation occurs in the leeward side leading to a completely different pressure distribution.
Pressure distribution results allow also to have a better insight of the bare hull and superstructure interaction and their contribution to the total pressure force development.
In particular moving from 30° to 55° angle of attack leads to a reduction of pressure level on the yacht frontal areas while the ship side is strongly affected by a pressure increase in the hull and in the superstructures leading to a substantial surge force reduction.
In order to answer the second question it should be thought about the weather conditions that are likely to be when the guests are onboard, and also each external area should be treated in a different way. It is unlikely that the guests will be comfortably sitting on the outside with a wind force greater than 4-5 Bft. With this in mind, a 70% value should probably be alright for most of the external areas, because it means that the wind force will be downgraded to 3-4. It is probably not enough in areas like the swimming pools, where the wind chill can be significant on a wet skin.
Figure 16: streamlines at different wind incidence off the bow (30° left – 50° right)
It would be interesting to set up a unique standard to be applied to measure the wind comfort onboard a yacht. At present, most of the attention to the comfort is drawn on noise & vibration, whilst other aspects of the comfort, which are being neglected, could arise in the near future.
Reported results are simply an example of the results which can be obtained in terms of tracer concentration measurements and the beam wind condition is just an example taken from the available results for every relative incidence between head to the following wind.
For every relative incidence the obtained concentration values obtained are very close to the detection limit of the instrument meaning that no problem with the dilution of the exhaust or their stagnation in critical parts of the yacht should be expected.
Due to the particular exhaust layout it should be actually expected the worse situation on the sun deck at relative incidence between +/- 45° and this is confirmed by the obtained results even if the concentration measurements still remain very close to the detection instrument.
limit of the Figure 17: pressure coefficient distribution on yacht
surface at different wind incidence off the bow (30° left – 50° right)
The model was actually mounted on the wind tunnel floor which simulate the sea surface. The wind tunnel boundary layer was adjusted as well as the turbulence intensity and integral scale length by means of spires and roughness elements placed into the test section aiming a scaled simulation of the natural wind characteristic of the typical open sea condition according with turbulence and boundary layer similitude law.
©2014: The Royal Institution of Naval Architects
B-115
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