Trans RINA, Vol 152, Part A2, Intl J Maritime Eng, Apr-Jun 2010
[15] and Tuck [16] argue that fewer hulls are almost always preferable. The quadrimaran, however, produced some interesting results. Along with its natural stability provided by the relative width of the vessel it has additional stability from the time averaged effects of the hulls. This is caused by the hulls sitting atop a variety of wave peaks at any one time without descending into the troughs. Of course, this is largely dependant on the relative size of the vehicle and the waves. Furthermore, the hulls are designed to create dynamic lift and as such, much of the hull area is lifted from the water, significantly reducing the wetted area drag. The most remarkable aspect however, was unanticipated and is due to the aerodynamic effects of the hulls. It was found during trial runs of the 17.5 metre test model Alexander that at full throttle the ship would go noticeably faster into a strong head wind. From this it was realised that the air flow between the hulls was actually creating a ram wing effect and lifting the hulls. Reports from riders who lay down near the bow suggest that a visible depression caused by the air pressure could be seen, and that this actually helped not only to reduce frictional resistance by lifting the vessel out of the water, but also helped to dampen the motion in rough seas. It is also claimed to have reduced the wash height and thus, the wave drag. Unfortunately, the test model Alexander was used as a demonstrator and not as a proper test model, meaning that although reports are generally consistent they must be read with caution. For example, some speeds were calculated by riders who measured approximate distances and timed them on their wrist watches.
Theoretical analysis of wing-ship configurations has
largely been limited to combining a two dimensional wing with an existing hull shape, such as the work by Doctors [5]. The two dimensional approach simplifies the evaluation and is justified on the grounds that the hulls will act as end plates. It should be noted that wing hull configurations are inherently low aspect ratio and as such, any unbound wing would most
likely be very
inefficient. However, end plates do not generally allow a low aspect ratio wing to perform as well as its 2D counterpart [17] & [18] making this seem like an over estimate. Furthermore, experimental analysis such as [7] & [8] shows that low aspect ratio triangular wings in the trimaran configuration are not very efficient. This makes the wing ship look like an untenable option, since the wing will only provide a very small fraction of the vessel support. However, the configuration of a low aspect ratio wing bound by demihulls over a water surface is a reasonably
complex geometry and, if it is to be
simplified, seems more akin to a duct than a 2D wing. For this reason, it was decided that the geometry of a ducted hull configuration should be studied so that a more thorough understanding of the aerodynamic forces on multihulled vessels could be gained, and from this, a reassessment of the viability of aerodynamic alleviation for marine vehicles.
(b) (c)
Figure 3: Profiles of the various hull geometries used, showing the Clark Y (a), ‘Hull shape’ (b) and the ‘Diff hull’ (c).
Figure 2: Solid model combination.
of the Clark Y wing-hull 3. AERODYNAMIC DESIGN 3.1 INITIAL TESTING
The AAMV aerodynamics is a complex and coupled phenomena, being the interaction of the upper deck wing with the side hulls and the free surface. The ducted shape which is created will result
in a pressure difference
through the duct which will affect the free surface shape, which in turn will alter the duct aerodynamics. However, at higher Froude numbers the free surface deformation will become less significant and for cruise conditions it may be fair to assume an infinite Froude number to simplify calculations [19]. It is the aim of this initial study to use a generic hull form to investigate whether there are any significant advantages to using a fully ducted geometry.
The initial model uses a ducted shape shown in Figure 2, where the cross deck is a Clark Y airfoil as shown in Figure 3(a) and the hulls are made of a variety of shapes to provide a comparison. There is extensive data for the Clark Y foil in two and three dimensions in extreme ground effect [20] and it was considered that this could be used to validate the initial CFD model and then provide a comparison when in a ducted form. The Clark Y cross deck was studied with Clark Y hulls, symmetrical foils, and a highly cambered foil.
(a)
©2010: The Royal Institution of Naval Architects
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