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Aviation History
1964
1964 - 1185.PDF
Air-Cuihion Vehicle* Hovercraft Ferry ... has been taken of waves arising from distant storms, increased height of waves due to wind against tide or from refraction of waves from estuarial banks or from steepening in shallow water, nor of waves generated by passing ships. Nevertheless, it is considered that the techniques now available can be very useful in the initial assessment of wave characteristics. Perhaps more than on any other sea- going craft, the hovercraft is likely to be affected by topographical influences; the amphibious hovercraft inevitably passes through regions of very shallow water and possibly surf. Both these circumstances produce steeper and often higher waves than winds generate. Therefore it soon becomes most desirable that the actual conditions on a proposed route are sampled over a reasonable period of years. A wave shape envelope based upon visual observations at sea differs con- siderably in form from the envelope now proposed, Fig 5, and confirms sus- picions regarding the tendency for the human observer to exaggerate wave heights in the seas normally encoun- tered. The weather ships and some light vessels now regularly measure sea con- ditions. In terms of wave height and length some data is shown in Figs 3 and 4. Before decisions can be taken about the necessary height of the "hard" structure above the calm water to avoid impact in the wave heights assessed or measured, the response of the craft to waves requires some attention, in much the same way that the freeboard of a displacement ship may be related to wave height and pitch and roll ampli- tude. Resonance and build-up of amplitude occurs in a certain band width about an encountered wave period, where the encountered wave period is derived from the wave length in the direction of motion divided by craft speed. For the short sea routes Figs 5(a) and 5(6) show the frequency of occurrence of waves of given encountered period; by the nature of the definition the faster the craft the shorter the time between wave crests in head seas but beam seas show no apparent shortening. Natural oscillation data for the present-day family of hovercraft with cushion pressures (or base loading, a measure of structural density) of up to 601b/sq ft are illustrated by Fig 6. The ordinate scale of stiffness has units "per- centage centre of (lift) pressure shift per degree." Stiffness is calculated from the NORTH SEA FLIGHT International supplement, 23 April Fig 3: Wave height probability V so100 Ui <J 8 0—i Z a. cc u O o o40_ UJ o ce _, 20~ z z< n NORTH ION S U < QF FEET SEA O < I 5 0 FEET < CQ Mm FEET 100 60- 4 0- 20- - n ENGLISH CHANNEL ON S i- O Q j "" < <* UJ < Q I 50 FE ET 5 0 FEET < 5 0 FEET INCREMENTS OF 50 FEET OVER 350 FEET applied moment required to roll or pitch the craft, i.e. Mpercent C.P. shift/degree = — X 100 YV is where M is moment to tilt craft through 1 degree in roll or pitch; W is craft displacement weight; L is cushion dimensions either beam or lengthwise. Unless special arrangements are made a stiffness of 2£ per cent in pitch and li per cent in roll may be expected. A period of 3 to 4sec about both axles will be exhibited by a craft of 150 tons dis- placement weight. Assuming a band width of excessive response to waves ±£sec about the critical region, a craft maintaining 6Okt cruise speed should meet beam seas Fig 4: Wave length probability which for 6 per cent of the time intro- duce roll amplitudes greater than the wave amplitude; in head sea conditions there is less likelihood of meeting reso- nant conditions—about 3 per cent of occasions in the North Sea area and 1 per cent on the Dover - Calais route. Naturally, the well-known ship tech- niques of changing course or speed to avoid excessive pitch or roll oscillations are available to the hovercraft captain- Quite marked changes of course are required and speed reduction is likely to make matters worse for these routes; the driver may be reluctant or unable to increase speed to move away from this uncomfortable condition. The alternative course of action to changing the encountered frequency *s 58
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