May 7th, 2012
The purpose or use of a rescue hovercraft varies from a single person rescue, for example someone falling thru thin ice, to a mass rescue such as an aircraft crash on normally inaccessible mudflats. The distance to the rescue could be a few meters to hundreds of kilometres from safety.
No one hovercraft configuration could be expected to suit all types of rescue so it is proposed to limit this discussion to hovercraft suitable for rescuing at most a few persons relatively close to conventional transport and help.
Rescue terrain – the terrain over which rescue craft must pass could include mud, salt flat, ice, water (fresh, salt, shallow, deep, fast flowing or strewn with rocks). The landscape could be open with few obstacles or congested with large rocks, trees or undergrowth. There could be a straight line of sight between rescuer and victim or a narrow winding and largely hidden path to the rescue site. Ideally a single rescue hovercraft should be capable of efficient operation over any terrain where any hovercraft is capable of progressing.
Operational requirements for some typical types of terrain:
- Open mudflat or saltpan, shallow water, with or without wind.
- Good directional control needed, especially in side winds.
- Sufficiently high thrust to weight ratio for good hump performance in moderate winds.
- Ability to proceed directly and stop at point of rescue irrespective of conditions.
- Search and rescue down a narrow and winding waterway congested with undergrowth, odd fallen tree limbs and low bridges.
- A high level of control is paramount, including the ability to ‘back-out’ of tight or impassable passages. Pilot errors of judgement should not result in the craft being ‘stuck’.
- Where odd fallen limbs and undergrowth makes a passage ‘too difficult’ it’s a big advantage if the craft is light enough to be manhandled around or over an obstacle or up/down an embankment.
- Ice rescue with or without rough ice areas.
- Whilst some ice fields are completely impassable to any sort of vehicle with low clearance it’s often possible to pick a path between around larger ice chunks or along water leads. The craft must be small enough and manoeuvrable enough to accomplish this.
- Temperatures can fall below minus 40 degrees with significant wind chill factors so engines must readily start and operate at these temperatures and crew will need appropriate protection.
- White-water including operations near dams and waterfalls.
- Rescues of capsized rafters are often required in fast moving water or near dam heads (both upstream and downstream). It does not take much imagination to foresee the results of poor craft control or mechanical failure under such conditions. An ability to move relatively close to the outlet or inlet of a dam and hold position is important if nerve racking.
- Climbing up a ‘white water’ stream requires a high thrust to weight ratio and safely descending such a stream requires a high level of craft control. Operating close to or below hump speed, where the Hovercraft’s air cushion is carried in the direction of the moving water, in these situations reverse and differential thrust is essential if any semblance of control is to be maintained.
Rescue craft are often operated by small town fire services and voluntary rescue groups with very limited budgets so a low purchase price and economical operating costs are essential. This factor usually precludes larger hovercraft and high capital cost vehicles like helicopters.
Operational safety is a concern, especially so because of the volunteer operator with limited training. Craft safety measures such as adequate fan guarding and conservative component loadings are important – fans should not be over sped and engines and transmissions should not be stressed beyond manufacturer’s recommendations.
The ideal rescue hovercraft
From the foregoing remarks it can be seen that not all designs of hovercraft are suitable for all rescue scenarios but the following general points should be considered.
- Craft weight – unlike wheeled and tracked vehicles, hovercraft must lift themselves essentially clear from the surface i.e. they could be regarded as flying machines, thus they must be light. Too often we have seen heavy craft that wont ‘perform’ and the makers response is to add more power – which usually means a heavier engine and transmission thus negating most of the performance gains expected from the extra power. Henry Ford is reputed to have said “weight kills performance” which is as true for hovercraft as it is for racing automobiles.
- Engine selection – Whilst there are many economical and reliable engines on the market few are suitable for small rescue hovercraft. The following table gives some indication of the performance of various types of power plant.
|| Typical HP range
|| Typical installed weight
|| Typical Cost
||70-130 lb (32-59 kg)
|| $5 to 9,000
||120-200 lb (54-91 kg)
||$5 to 7,000
||130-170 lb (59-77 kg)
||300-400 lb (136-181 kg)
||180-220 lb (82-100 kg)
Obviously some engine types will be unsuitable because of high cost or high weight per horsepower but other considerations such as emissions or corrosion problems in salt water may be overriding. Currently the 2-cycle or light 4-cycle engines are the main contenders. Rotary engines look promising but lack large scale production availability. The 4-cycle engine is now in favour because of emission concerns but new direct injection 2-cycle designs may offer a lower weight alternative whilst meeting emission standards. Operation at extreme low temperatures tends to favour the 2-cycle, especially air-cooled designs.
- Control – In the majority of rescue scenarios a high level of craft control is required for useful and safe operation. Often operating conditions are far from ideal with moderate to high winds being most troublesome.
The older designs of hovercraft with simple thrust devices, especially if the lift and thrust are integrated, can be most difficult control in strong wind conditions. Rescue craft must be designed to handle, as well as possible, these conditions by employing thrust vectoring means, such as reverse thrust, puff ports or whatever else can be devised to maintain adequate control. An ability to statically hover and creep forward, under full control, overcomes the danger of pushing slabs of broken ice into the victim.
Picking up individuals from water or ice demands precision craft control – there’s no point in running down or drowning a victim in spray whilst attempting to manoeuvre close enough to pick them up.
- Propulsion – Unfortunately in fully amphibious hovercraft no highly efficient means of propulsion has yet been devised. Air propellers and ducted fans typically only produce 3-6 lb of thrust per horsepower.
If very large diameter thrust rotors could be used ratios in excess of 10 lb per horsepower could be achieved but this is impractical – even moderately sized propulsors can make craft passage under obstacles such as low bridges and tree branches impossible. Also the windage effects on large rotors and thrust ducts can be a severe embarrassment in other than calm conditions. The best that can be achieved currently is to keep the thrust devices as small as possible, consistent with maintaining adequate performance without excessive power input or over-speeding rotors.
Reflecting on other fields of fan and propeller technology, such as jet engine development, it can be judged that light hovercraft propulsion has to travel a long developmental path before anything like ‘most efficient design’ can be claimed.