LIFT & THRUST
I have indirectly mentioned lift systems; there are a few basic guidelines to consider. The maximum height at which the craft hovers from the surface is approximately one eighth of its width (any more than this will affect the stability of the craft) and the larger the footprint of the craft relative to its weight the better, a design weight of between 10 and 15 lb per square foot will enable the use of an axial fan.
The axial fan is very good at moving a quantity of air but not so good at generating pressure, therefore the airflow efficiency will drop away very quickly with only a small increase in back pressure. If a craft is undersized or overloaded and the skirt pressure is too high the skirt stiffness will cause excessive drag and wear as well as making it difficult getting the craft over hump [up on plane]. A bigger fan or more a powerful engine will not overcome an inherently bad design.
It should be mentioned that as craft get bigger e.g. 25 feet or more in length, a higher skirt pressure in combination with a skirt geometry giving a more flexible performance e.g. a loop and segment type, will usually use the centrifugal fan, because:
A. The overall craft footprint is bigger and the depression in the water when the craft is at sub hump speeds is proportionately shallower than that of smaller craft.
B. The increase in craft size brings with it its own stability.
C. The increase in skirt pressure will of course be at the cost of increased friction, spray, noise, and fuel consumption, but have the advantage of a greater payload for a given size of craft.
The choice of a ducted axial fan will depend on a combination of factors; the h.p., available flow required etc., but as a basic guideline for any axial fan the smaller it is and the more blades it has the more pressure it can produce - and the more noise! The reverse is true of both lift and thrust fans, the bigger diameter they are with the least number of blades, the more efficient is the air flow and again for a given h.p., that reduces noise.
Of course this is an over simplification, but it helps to explain why as integrated thrust / lift fans get bigger the lift efficiency will drop away, requiring either very fast rotating fans or a lot more blades: both inefficient and noisy solutions.
As with all things, simplicity of design cost etc., will play a part in the designer's options but I would suggest that any designer doing their job properly should start with what they perceive as the most efficient package.
The readily available range of powerful two-stroke engines have meant that even the crudest designs will work, but an efficient design will enable less suitable [from the power to weight of respective] engines to be considered e.g. automotive four-stroke petrol or diesel. The compromise has to suit the use to which a craft is being used; for example operators using two-stroke engines on commercial operations do so because for a given sized hovercraft the payload is increased in comparison with a similar craft using heavier engines. On the other hand for people building craft of their own design or from a kit, the use of a suitable second-hand automotive engine can considerably reduce the building cost.
With regard to thrust systems I have mentioned ducted fans: why ducted?
Primarily for safety reasons when fitted with a 50 mm square metal mesh
guarding the front of the unit and rudders, elevators and possibly vectors
at the rear, the thrust system should be capable of containing any broken
blades, belts etc., as well as preventing any human ingress. It is true
to say that for a given diameter the ducted propulsion system is more
efficient in thrust terms than an open-bladed unit, but it is noisier
than, for example, a broad-bladed, slow running paddle propeller. Noise
is energy, therefore the more power the unit has to absorb for a given
diameter, the more noise it will generate and the amount of thrust per
horse power will reduce. It might be necessary to use more blades to
absorb extra power, as with the lift system the use of more blades will
increase pressure and reduce thrust. But within certain limits some
pressure is not a bad thing as it makes control surfaces [rudders etc]
more effective. In conclusion, the compromise if available, is to fit
the minimum number of efficient profile blades travelling at an adequate
speed to absorb the power available.