What price water speed improvements of amphibians

Abstract

INTRODUCTION THE DESIRABILITY to have military vehicles move unimpeded through any type of terrain including bodies of water has long been established. The problem of floating and propelling such vehicles, being primarily land vehicles, has always been approached with a particular mission in mind. Once designed though, the vehicles often then are used for a wide variety of applications. The basic vehicle mobility specifications have never been categorized in a quantitative manner, therefore no really satisfactory compromise between land and water performance has ever been found. This paper is designed to present a brief review of the problems encountered in the design of amphibious craft. It is not intended as a design guide, but more a presentation of observation and conclusions gained from my experience of over 10 years' testing of full scale and model amphibious vehicles. It discusses the more important basic hydrodynamic fundamentals applicable to conventionally designed amphibious vehicles, the reasoning underlying these fundamentals, and the basic advantages and disadvantages of various approaches. Any vehicle design is a grouping of many compromises. Most land vehicles are relatively short and wide, with a length-width ratio of about 1.5-2. This ratio has developed from a reasonable compromise between vehicle control and ability to negotiate obstacles. Vehicle height is limited for sideslope stability, but good ground clearance dictates a high body floor. Wheels and/or tracks are located such as to give good break angles and angles of approach and departure. Their size and construction has to be sufficient for good mobility but not so large as to make the vehicle unwieldy or, conversely to consume excessive space at the expense of useful load. Any shape which becomes excessively long must be divided into several sections, such as articulated or trained vehicles. Compromises are no less present in amphibious vehicles, for all conflicting problems inherent in the design of land vehicles are present, plus the additional requirements required to float and propel itself adequately in water. Any shape, immersed in water, will float only after it has displaced its own weight in water. Most land vehicles are too dense to float. Special measures, therefore, must be taken to decrease their density (weight/volume) or increase their water displacement. In order to provide floatation, usually hulls or external displacement bodies are added to a basic land vehicle. More often than not these floatation devices, even an integral hull, are an impediment to land travel. To reduce this impediment some vehicles use detachable floatation devices. These, however, are usually bulky and awkward to stow on the vehicle. The user is then faced with the dilemma of carrying the floatation kit the life of the vehicle for the small percentage of the time that it is