Trans RINA, Vol 152, Part A2, Intl J Maritime Eng, Apr-Jun 2010 AERODYNAMIC LIFT FORCES ON MULTIHULLED MARINE VEHICLES


A G W Williams, M Collu and M H Patel, Cranfield University, UK (DOI No: 10.3940/rina.ijme.2010.a2.169) SUMMARY


The need for high-speed high-payload craft has led to considerable efforts within the marine transport industry towards a vehicle capable of bridging the gap between conventional ships and aircraft. One such concept uses the forward motion of the craft to create aerodynamic lift forces on a wing-like superstructure and hence, reduce the displacement and skin friction. This paper addresses the specific aerodynamic design of multihull for optimal lift production and shows that significant efficiency can be achieved through careful shaping of a ducted hull, with lift-to-drag ratios of nearly 50 for a complete aerodynamic hull configuration. Further analysis is carried out using a hybrid vehicle stability model to determine the effect of such aerodynamic alleviation on a theoretical planing hull. It is found that the resistance can be halved for a fifty metre, three hundred tonne vehicle with aerodynamic alleviation travelling at 70 knots. Results are presented for a candidate vessel.


NOMENCLATURE CL


CD Cm


organizations, where speed often means the difference between life and death.


Coefficient of lift Coefficient of drag


Coefficient of moment


WIG Wing In Ground Effect WISE Wing In Surface Effect ACV


Air Cushion Vehicle


SES Surface Effect Ship AAMV Aerodynamically Alleviated Marine Vehicle B beam (width) of the hull at transom (m) HV Hybrid Vehicle, with aero- and hydrodynamic lift


HV1 First HV configuration, not optimized HV2 Second HV configuration, developed to have the optimum amount of weight sustained by aerodynamic lift


H height of the axis system origin, as shown in Figure 8, above the mean water surface


Fr Froude number, V/√(gL) Fb


g gravitational acceleration (m/s2) L waterline length (m) V vehicle speed (m/s) k ε


1. beam based Froude number V/√(gB) turbulent kinetic energy, k-ε model


rate of dissipation of turbulent kinetic energy k INTRODUCTION


The modern transport market may be thought of as existing in two distinct sections: that of high-speed, low- payload vehicles, such as aircraft, and that of low-speed, high-payload vehicles, such as cargo ships. Constant pressure to carry more load and at greater speed, combined with environmental


concerns over fuel


efficiency has led companies and researchers to look for hybrid technologies capable of closing this gap in the market. Shorter transit time for commercial produce is obviously a valuable asset, as indeed is the case for passenger ferries, where speed comes at a premium. More critical pressure


for high-speed high-payload comes from the military and humanitarian aid


The benefits of a vehicle capable of sustaining high speeds over vast stretches of open ocean whilst carrying significant weight


in cargo are extremely tangible.


Financial, tactical and humanitarian needs warrant a considerable amount of research into the field. A significant


amount of work Russians on WISE craft, has being aircraft aerodynamic advantages of ground effect


been done by the capable


of of


utilizing ground effect whilst flying close to the sea. Many subsequent studies such as Moore et al [1] have shown the


WIG


configurations. Few WIG craft have been produced (most notably in Russia) and design studies have shown that the advantages


are largely


outweighed by the difficulties of flying close to the sea [2]. In particular, the need to perform banked turns and maintain wing strength against potential water collision reduces the practical length of the wings and thus the aspect ratio, which reduces efficiency. Equally, to avoid collisions in even moderately rough seas the WIGE must fly at greater heights, and since ground effect diminishes rapidly with increased height, the combined effect of low aspect ratio wings and greater cruising height often mean that any advantages are lost altogether.


Perhaps the most promising variety of high-speed high- payload vessel are aircushion vehicles, and in particular the SES which have reached speeds of nearly 100knots in calm seas [3]. An SES, having rigid side hulls is better able to maintain cushion pressure in rough seas, however, wave slamming on the front seal results in a significant increase in drag and heavy loading on the vessel, which reduces the speed considerably in even moderate swells. A partial SES described in [26] and [27] explores this concept as well.


Lazauskas [4] presents a detailed comparison of


theoretically optimised vessels with regard to speed. Figure 1 shows the resistance to weight ratio plotted against speed for a catamaran vessel with varying


©2010: The Royal Institution of Naval Architects


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