Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun 2.3 FUTURE CERTIFICATION CODES


The LY2 code is strictly only applicable for vessels with a displacement below 3,000 tonnes, a limit which may be exceeded by modern super yachts. At the time of writing this paper it is understood that the MCA are managing the development, by the Cayman Island Ship Register, of the so-called Passenger Yacht Code (PYC).


PYC will


cover yachts for 12-36 passengers and is intended to follow a similar model to that employed in the LY2 guidance.


Whilst the PYC guidance is awaited with anticipation it is proposed that the current LY2 guidance with respect to the airwake assessment is a solid starting point for super yacht design.


2.4 OTHER IMPLICATIONS OF AIRWAKE


Whilst understandably the CAP437 and LY2 guidance concentrates on ensuring safe conditions within the helicopter landing area, which may indeed be a key aspect of super yacht design, it does not provide advice with regard to other areas


of performance/operation which may be affected by airwake, namely:


• Comfort (buffeting and thermal plumes). • Noise and vibration (turbulence and structural excitation).


• Air quality (exhaust gas dispersion). • Ventilation performance (plume reingestion).


Whilst these factors are perhaps not as high profile as helideck


safety, it is important that they are not


overlooked during the design phase as they may impinge on contractual requirements.


2.5 FUTURE ROTORCRAFT REQUIREMENTS


Whilst the concept of helideck operations usually refers to helicopters, they are applicable to all types of rotorcraft.


A recent article discusses the certification of


helidecks for the operation of a new generation of tilt- rotor aircraft, for example the Augusta Bell BA609 [3]. These aircraft combine the manoeuvrability of standard helicopters with the speed and altitude capabilities of a fixed wing aircraft.


Despite these benefits, aircraft of


this type will present a significant challenge to designers in the future as, under current regulations, the significant wingspan associated with tilt-rotor aircraft correspond to a requirement for ever larger helidecks.


3. 3.1


DIFFERENT APPROACHES EXPERIMENTAL APPROACHES


Historically, airwake assessments have been made using a combination of wind tunnel testing and designer’s experience. Such testing is a valid and useful method for


©2010: The Royal Institution of Naval Architects


flow evaluation and the maturity of the methods involved affords high levels of confidence in the results.


In order to achieve representative conditions within the wind tunnel, it is essential to scale the flow velocities based on the flow Reynolds number (Re), where:


Re = UL ν


(1)


Assuming that testing is conducted in air at nominal atmospheric conditions, the implication of matching Re is that a 1/nth scale models must be tested in the wind tunnel at a speed n times greater than full scale. For even moderate yacht lengths and forward speeds (i.e. 50m and 10kt) care has to be taken to use a model large enough such that the test speed is sufficiently low to avoid introducing high-speed compressibility effects. For very large yachts (upwards of 100m) it may be impractical to make a model sufficiently large to avoid compressibility effects at moderate wind speeds. In this case it may only be possible to achieve very low speed conditions in the wind tunnel.


The larger the model, the larger the wind tunnel required to eliminate the interference of the wall boundaries on the model’s aerodynamics. The hire


of these wind


tunnels can be costly with testing typically costing in the region of £3,000-£5,000 per day excluding set up time. For an extensive experimental programme with complex instrumentation the costs may be considerable.


Once an appropriate wind tunnel has been identified, a skilled model maker is required to construct detailed models of the yacht for testing.


Alternatively,


manufacture of a scale prototype direct from complex Computer Aided Design (CAD) geometry by rapid prototyping is becoming increasingly common.


An alternative to wind tunnel is water tunnel testing. The smaller kinematic viscosity and higher speed of sound of water, compared to air, means that smaller model can be used.


However, whilst it is theoretically feasible to


conduct water tunnel based airwake assessment there are a number of practical considerations which make this approach less common. These include, lesser availability of such facilities compared to wind tunnels, the design of models and instrumentation to avoid degradation in water and withstand higher structural loading


Key to understanding the airwake is the determination of local velocity fluctuations, the thermal field and potentially


gaseous dispersion around the yacht.


Velocity fluctuations can be measured in a number of ways using either traditional


invasive instrumentation


(including pitot probes and hotwires) or using advanced laser based techniques. The capabilities and limitations of these methods are discussed over the following two sections.


B-23


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