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Trans RINA, Vol 152, Part A2, Intl J Maritime Eng, Apr-Jun 2010


aerodynamically alleviated hull designs. It is also worth noting that the diffuser hull has four times more lift than the hull shape with very little increase in drag, and that most likely any such shape would already be a significant improvement over the


original aerodynamic drag.


Usually, also for high speed craft, the hull is optimized from a hydrodynamic point of view, therefore from an aerodynamic point of view great improvement can be reached, as demonstrated here.


4.


Figure 7: Relative effect of the various hulls on an infinite Clark Y wing.


In previous papers, such as that by Doctors [5], the hulls are assumed to act as end plates and allow two dimensionality to be applied to the aerodynamics. Figure 7 shows the percentage error of the hull data compared to two dimensional experimental data for a Clark Y wing at infinite height and at a height of 5metres taken from [20]. It can be seen that the effect of the symmetrical Hull shape is extremely detrimental and that assuming the effect is two dimensional and still in ground effect leads to an error of around 55%. By shaping the hulls as Clark Y foils however, the assumption of two dimensionality becomes more reasonable. The actual value lies between that of the ground effect and infinite height, that is to say, the end plate effect of the hulls has not produced as high a value as for an infinite wing in ground effect, but it is better than that of an infinite wing out of ground effect. This is a considerable achievement, since end plates do not usually have such a pronounced effect. They are of course not usually airfoil shaped or in contact with the ground, and this must account for much of the difference. In the case of the diff hull, the hulls have improved the coefficient of lift of the low aspect ratio wing in ground effect beyond that of an infinite wing in ground effect by nearly 18%. This means that the assumption of two dimensionality and dismissal of the effects of the side hulls on a low aspect ratio wing can lead to errors of over 100%. Indeed, since much of the


criticism of


aerodynamically alleviated catamarans is based on the proposed difficulty of providing sufficient lift with a low aspect ratio wing in ground effect, which, it is suggested, will not achieve the two dimensional lift proposed, may in fact be an under estimate if the hulls are designed properly.


It is confirmed that the standard hulls will not allow the assumption of two dimensionality by quite a margin, but the adapted hulls presented here are able to significantly do better than the 2D wing. The final test, using the complete diffuser shaped hull, gives a total CL of 1.151 and a lift-to-drag ratio of 35.9, this can be compared to a low aspect ratio wing of the same dimensions where the CL is only around 0.07 and the L/D as low as 3. This clearly illustrates the importance of hull design in conjunction with wing design when considering


The precise design of such a hull for optimal


performance is beyond the scope of this paper and is not considered further here. The proposed design is a best guess at what may be required and is shown below in Figures 8 & 9.


4.2 RESULTS


The results for coefficient of lift, drag and moment are shown below in Figures 11 to 13. The various coefficients are plotted against the angle of attack (alpha) and are presented for a range of heights (H) between 3 and 7m where H is the height of the origin, as shown in Figure 8, above the mean water surface. The origin is located at the quarter cord point.


COMPLETE AERODYNAMIC HULL FORM


The previous section demonstrated that efficient lift can be generated by suitably shaped ducted hull geometry and that lift values may be much higher than expected. This section aims to provide a complete ship design which accounts for the hydrodynamic constraints as well as the aerodynamic requirements.


To achieve a complete aero-hydrodynamic design for the hybrid vehicle it is necessary to consider the transition states of the vehicle. That is, the at rest requirements, the take-off requirements and the cruise requirements. The static requirements are that the vehicle must float. But beyond this it must float such that the hydrodynamic and aerodynamic surfaces are able to perform once motion has begun. From basic calculations of a prismatic hull using the Archimedean principle of displacement, we can find that a 200tonne planing hull will rest at a maximum draft of about 1.5m. This is based on a 10degree deadrise and


2degree trim angle predicted from the hybrid


stability model used in previous work [22]. It may be beneficial to consider the possibility of greater loading, since the aforementioned model gave reason to believe that a greater load may be supported if correctly designed. As such, a maximum draft


of 3metres is


suggested to accommodate greater static loading. The transitory, or take-off stage, will require the hull to have a good water piercing bow for low speeds as well as a greater trim angle in the bow section to encourage the hull out of the water at lower speeds.


©2010: The Royal Institution of Naval Architects


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