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The most important question BEFORE you buy a hovercraft

   

   Almost all manufacturers of small hovercrafts describe their machines using various parameters like engine power, fan diameter, maximum payload and so on. All these informations are very useful but say nothing about one of the most important ability of hovercraft - the ability to start from water with full load. This ability is described by two parameters: TOH-1 (time over hump with pilot only) and TOH-MAX (time over hump with full load). During normal water operation a hovercraft rides on top of the water surface rather than in the water. When starting up from a stop on water, a hovercraft transitions from displacement mode (the air cushion generates a “soft” depression in the water – rather than the damaging “hard” depression caused by a displacement boat) to full hover mode (flying above the surface) at a speed known as "hump speed" (usually around 18-20 km/h for most craft)[1].  See videos and graph for H37:  TOH-1     TOH-2   
 



   This effect does not occur when traveling from land onto water. Hovercrafts are machines for activities on water or in mixed mode-land and water. All another technical parameters can be wonderful but if your hovercraft can't start from water with full load nothing can help you!- design, engine, fan diameter, all these informations are in this case useless.
   So if you have decided to buy a hovercraft ask the dealer about video with full start sequence from velocity 0 km/h (after engine was stopped for minimum 5s!) to the velocity of minimum 30 km/h. Why we should to examine time for increasing the velocity to 30 km/h?. It's sometimes difficult to recognize exactly the time point of transition from displacement mode to full hover mode. If the hovercraft can achieve 30 km/h we can be sure that it is “over hump”.
   The second, but not so critical parameter is the ability achieving a maximum declared speed.

See also: A buyers guide to Cruising Hovercraft of the HOVERCRAFTCLUB of Great Britain

[1]-Theory and design of Aircushion Craft, Liang Yun, John Wiley & Sons Inc, NY 2000.

 

Authors:  Dipl. ing. Krzysztof Kruciński and  mgr inż. Jerzy Woźniak
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