Sec. 76.28 



m'.SK.N OF SPECIAL-PURPOSE CRAFT 



807 



lllllll 



in nil 



fin 



Fig. 76.T One Type of Tracked Amphibian 



Official U. S. Navy photograph. Note the M-shaped 



paddles or cleats on the two moving tracks and the 



relatively close spacing of these paddles along 



the tracks. 



employing one of the several possible methods of 

 water propulsion: 



(1) The paddle track has the advantage that the 

 same mechanical installation is utihzed for run- 

 ning on both the water and the land, including 

 propulsion as well as maneuvering. If it is suffi- 

 ciently strong and durable it is satisfactory for 

 extended land travel but not for high speeds 

 ashore unless it can somehow be rubberized. 

 Balancing this present shortcoming is the great 

 advantage that the track furnishes the only 

 known adequate and acceptable propulsion in 

 the complete range of media from clear water to 

 hard ground, comprising silt, sand, vegetable 

 growths, and mud of all possible consistencies. 



(2) The screw propeller is adapted only for use 

 in media having the consistency of water. Un- 

 doubtedly, it is the most efficient of the propulsion 

 devices, and capable of producing the highest 

 speeds at which craft of this kind can travel in 

 water. The propeller can be housed and at the 

 same time protected in a tunnel under the stern 

 of the craft, similar to that on a shallow-draft 

 vessel. An adaptation of the outboard or swinging 

 propeller is possible. It is even practicable to 

 provide, as John Ericsson did for the American 

 auxihary sailing ship Massachusetts in 1845, a 

 propeller carried by an arm swingmg in a trans- 

 verse plane [Isherwood, B. F., "Engineering 

 Precedents for Steam Machinery," Vol. II, pp. 

 213-220]. When working it is swung down so that 

 the propeller is below or at least abaft the hull. 

 When not working it is swimg up, well clear of the 

 ground and inside the frontal or transverse 

 projected area of the vehicle. The screw propeller 



will give good performance provided water can 

 flow easily to it, either from the side or under the 

 bottom. 



(3) The central paddlewheel is specially adapted 

 to divided-hull craft of the catamaran type. It is 

 simple mechanically, is easily constructed, main- 

 tained, and repaired, and is reasonably efficient 

 for shallow-water craft. It is admittedly bulky, 

 lumbersome, and heavy for the power delivered. 

 I 1) The sternwheel is indicated only for raft-like 

 craft designed for traveling in extremely shallow 

 water 



(5) A water jet to propel a heavy, resistful am- 

 phibian is, with its ducts, likely to occupy more 

 space than can be devoted to it. Such a device 

 might serve for a lighter craft. 



(6) The amphibian driven by a paddlewheel of 

 some kind or by one or more screw propellers 

 needs a good rudder. Probably it needs more than 

 one to approach the maneuverability of the 

 tracked vehicle, which can change its track 

 speeds or even go astern on one track while 

 gomg ahead on the other. Rudders in amphibians 

 propelled by paddlewheels or screw propellers 

 are almost of necessity placed in the outflow jets 

 from those propulsion devices. 



(7) Despite its clumsy form, maneuverability of 

 an amphibian may not be too difficult to achieve 

 because the length-beam ratio of such a contri- 

 vance is usually about 3 or 4 and rarely exceeds 5. 



Despite the fact that an amphibian is of no 

 practical use unless it can travel in water, it is 

 possible that whatever water-propulsion device 

 is fitted to it may have to play a secondary role 

 to the land-propulsion gear. Because of the variety 

 of possible configurations, not much can be said 

 as to design for water propulsion except to em- 

 phasize that the water must have a reasonably 

 good path to flow to whatever propdev is fitted. 

 Because of the interference effects described in 

 Sec. 32.2, the cleats on moving paddletracks 

 always give the best performance when they are 

 spaced as far apart as the considerations of 

 ground or land travel permit. 



Although the water-excludmg portion of an 

 amphibian may look more hke a covered wagon 

 than a boat or ship, the law of Archimedes still 

 apphes. The craft sinks in the water untU the 

 weight of water displaced equals its scale weight. 



Because of the relatively high bow-wave crest 

 and deep folloAving trough created by the am- 

 phibian body when running at moderate speed. 



