The quick answer to the question is that an air cushioned craft should have as many as it needs to perform safely, but this is not much help when faced with the decision to buy a personal hovercraft. The number of small hovercraft manufacturers has blossomed over the past few years and there are huge variations in performance and material used, depending upon their use in service.
A racing enthusiast lives by the mantra 'the lower the power to weight ratio the better'. To achieve a great ratio, the machine must be made as light as possible, which means that the hull will be streamlined and on-board fitments kept to a minimum. The usual stainless steel accessories would be exchanged for aluminium, which is rigid and light, but strong. A further consideration is that racing ACVs are steered partly by handlebars, and partly by the pilot literally throwing his weight towards the side he wants to travel. A lighter craft would make this easier to do.
Such a craft would have just one two stroke engine, simply because they are lighter - they have no extra oil sump, for example. A duct arrangement is need to separate the airflow form the single fan so that some is directed to the rear, and some underneath the hull for lift and hover. The irony is that extra ducting represents extra weight, so it's a compromise. As might be imagined, safety or noise reduction isn't the primary concern of a racing pilot. He wants to go faster and win against the other competitors. For this reason, racing vehicles always have just one light-weight engine.
In the leisure market, there is more scope for choice and indeed both styles can be found. While it simplifies things enormously if one engine provides forward motion while another pushes air underneath the hull, we have the questions of cost and weight. Weight isn't too much of an issue, as a family man would place the safety of his children at the top of his list of 'must have' features. If two engines are used, then the cost of extra maintenance and fuel should be taken into account, while the designers should have factored in the extra weight when calculating the power needed to 'get over the hump'. This term is applied to the process of lifting a hovercraft off from standing water and requires much more power than just keeping it hovering. More information http://www.leisure-hovercraft.com
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