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


However, as the hydrodynamic portion is exposed the drag increases rapidly and thus the L/D decreases correspondingly.


Since the aerodynamic drag is likely to be a much less significant factor than the hydrodynamic drag and the aerodynamic lift should affect the hydrodynamic drag, it is reasonable to hope that the higher aerodynamic drag for this configuration will be more than compensated for by the superior aerodynamic lift. However, it should be noted that both the aero and hydrodynamic design of this model are purely demonstrative at this stage and it is anticipated that many improvements could be made to reduce the aerodynamic drag. Significantly, the drag is still quite high at running attitudes where the transom is mostly submerged, suggesting that there is quite a lot of drag arising from the bow design, and that there is much room for improving this. None the less, these initial results show much promise for producing significant levels of aerodynamic support.


5. STABILITY ANALYSIS


The proposed structure has proved that the simple wing in ground effect analysis is not suitable for studying the aerodynamics of multihulls and that significant lift can be provided through careful


geometry. However, the question still remains as to whether this aerodynamic lift will reduce the total drag sufficiently to warrant further


study. The following


section is an analysis of the performance of the proposed configuration combining a basic planning model with the measured aerodynamic properties.


5.1 STABILITY MODEL


The proposed configuration is studied using a Hybrid Vehicle (HV) stability model, developed by Collu [22] specifically for the analysis of AAMV. The model has already


been used to study planing hull [24] previous aerodynamic configurations in conjunction with the author [23].


The model uses a hybrid stability model combining a Savitsky


with the computed


aerodynamic forces which allows it to estimate a running attitude and find the static equilibrium through iterative refinement. This approach allows the vehicle to be studied through a range of speeds, from take off to cruise. The model is only valid for planing speeds and thus is not appropriate at beam based Froude numbers (Fb) lower than 1, which limits the current model to speeds above 20knot. Equally, the model is not able to cope with full air support and returns a null value if the hull leaves the water.


The model is once again 50m long and 25m wide, weighing 300tonnes and having a centre of gravity approximately one third of the ship’s length from the transom, being a distance of 17m from the stern.


Figure 16: Shows a comparison of the various


components of drag for the planing hull and hybrid vehicle.


Figure 16 shows the various contributions to drag for both the Hybrid Vehicle and the planing hull. It can be seen that above speeds of 50knots the hydrodynamic drag of the HV is significantly reduced. This is at the cost of only a very small increase in aerodynamic drag. This model assumes that the planing hull has very little surface above the water, which is why the aerodynamic drag is so small. In reality, if the above water portion of the planing vessel is of comparable size to that of the HV, then the aerodynamic drag of the HV may be smaller than that of a corresponding planing vessel,


©2010: The Royal Institution of Naval Architects A - 47


The model is run in comparison to a planing hull, having exactly the same hydrodynamic properties but without the aerodynamic contribution.


5.2 RESULTS


Results from the static stability model are shown below. From Figure 15 it can be seen that the aerodynamic lift gradually takes over from the hydrodynamic lift and that the cross over point occurs at just after 75knots. It is interesting to note that even at much lower speeds such as 30knots there is still about a 10% aerodynamic contribution.


shaping of ducted hull


Figure 15: Shows the lift fraction for the AAMV as a function of speed.


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