Trans RINA, Vol 152, Part A2, Intl J Maritime Eng, Apr-Jun 2010 degrees of cushion support. This plot shows the
catamaran with no cushion pressure (or associated skirt drag etc.), with 50% and 85% cushion pressure, where it acts as an SES, and with total cushion support, where it may be considered as an ACV. The plot clearly shows the difference in resistance with speed. At low speeds, up to around 25knots, the catamaran is the obvious winner and is not noticeably beaten until about 35knots where the two SES ships become favourable. Above 60knots the level of air support can be seen to be inversely linked to the resistance.
2. AAMV
The basic concept for the AAMV is not new, and aerodynamic lift has already appeared in the literature many times. Perhaps most notably the Ekranocat (a portmanteau from the French word Ekranoplan and catamaran) which refers to the
use of a wing-like
structure to join the hulls of a catamaran. The name was proposed by Doctors [5], Nebylov [25], but the concept has already been suggested elsewhere by Trillo [6]. These vehicles do not use any engine thrust to trap or channel air, only the forward motion of the vessel is used to generate lift. Doctors proposed a catamaran, whilst Walker et al [7] and later Matveev et al [8] suggest using a trimaran. The theory is quite simple. As the boat speed increases the wing generates lift which reduces the hull displacement and hydrodynamic drag at the expense of the far smaller aerodynamic drag. This theory is perfectly sound; however, it has been difficult to propose a vehicle to provide sufficient aerodynamic lift at realistic speeds, especially since the wings are very low aspect ratio. As McKesson points out in [9], if stagnation pressure could be achieved for a vehicle of comparable size and footprint to a 6000tonne SES it would have to be travelling at
around 200knots to achieve total
Figure 1: Optimal resistance curves for a catamaran with varying degrees of aerostatic support [4].
Clearly the ACV is the best for high speeds; however, in reality the presence of waves limits the speed of ACVs severely. This can mean that they are operating at far less efficiency than other vessels, and indeed in particularly rough seas they are often not able to cope at all.
This plot clearly demonstrates the potential for a hybrid vehicle configuration to achieve
greater much better
performance than any of the existing solutions, provided the attributes of each can be successfully combined. It is apparent that a hybrid vehicle capable of performing as a catamaran at low speeds whilst attaining
aerodynamic lift with increasing forward speed could follow the line of least resistance of all of these vessels. Such a vessel should have negligible lift at low speeds of up to 20knots, partial aerodynamic lift at 40 to 60knots and almost total aerodynamic lift above 70knots. This can be achieved by changing the cushion pressure of an SES, however it is the aim of this work to explore the possibility of utilizing aerodynamic lift for this purpose. It is proposed that a vehicle which can maintain adequate lift from aerodynamic forces would have lower profile drag than an SES, require far less engine thrust to achieve the high pressure and would cut
though the
waves unimpeded by a front or rear seal. If feasible, such a vehicle could provide a significant advancement in marine transport. Vehicles which use forward motion for aerodynamic support will be referred to as AAMVs for the purpose of this work.
aerodynamic lift. A lively speed indeed for a 6000tonne ship, but even SES ships do not usually run on full air support and can achieve significant drag reduction at much lower cushion pressure ratios; although around eighty percent air lift seems to be the best for most air cushion type vehicles [4] & [10].
The AAMV design has some distinct advantages,
particularly in that it does not require any front or rear skirt, which eliminates a lot of drag and wave impact problems as well as not needing any extra engines to provide cushion
pressure. This means that any
aerodynamic lift created by fairing the deck area into a wing shape is effectively free, since the profile drag should be much the same, and indeed may even be reduced. Tunnel hull racing boats have effectively been using this principle on a smaller scale for some time [11] & [12]. It is to be noted that they can achieve very high speeds, the world record being 275knots, but even at much lower speeds the aerodynamics can be significant and boats can become fully airborne at speeds closer to 100knots. On a slightly larger scale the KUDU II [13] is an 11m long 4m wide twin hull ram wing planing craft capable of speeds as high as 85knots, with open ocean cruise speeds of around 69knots. This speed is only achievable through considerable amounts of aerodynamic alleviation from the ram wing joining the hulls.
An interesting report by Hockberger [14] describes the performance of a quadrimaran ship with aerodynamic lift capabilities. The vessel's four hulls are almost identical and
form three channels along the Conventional wisdom would most wetted deck. likely lead to a
dismissal of such a design as it appears to have an unnecessarily large surface area. Indeed both Doctors
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©2010: The Royal Institution of Naval Architects
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