The drag of a pressure distribution on a water surface was an early focus for engineers working on Air Cushion Research. The earliest concepts were based on air jets forming a curtain to contain the air cushion, so this is a logical approach. Two general questions needed solutions, first what was the effect of the plan form geometry, and second, what was the influence of a channel boundary, as is the case when a model is towed in a towing tank. Extended from this is what is the effect of a yawed pressure distribution. Finally, what is the effect of water depth on the waves generated by the cushion.
We present here three papers that addressed these topics in the early days of development. The data presented is in graphical form, hand plotted as this was before computers became generally available for such work. There are some important general learnings from these papers, in addition to some guidance that may still be useful to designers.
The Wave Drag of Hovercraft, by M J Barratt, HDL/63/127
This Technical Note has been transcribed as the text part is not available. The figures are all in original form. Mike looked at rectangular and elliptical plan forms. He also looked at the effect of limiting wave height before breaking. At the low end of the speed range this effectively cuts off the peak of the secondary hump. Shallow water also affects this, moving primary hump to lower speed.
Overall, a rectangular pressure distribution seemed to give lower overall drag performance compared to an elliptical distribution from this work.
Wave Resistance to the motion of a uniform planform pressure distribution of rectangular planform in a canal of finite width and depth, by D R Lavis, TA/DRL/Am/6046
This Technical Note presents calculations made by computer using the theoretical work of Newman and Pool (given as ref 1 in the document) to give resistance coefficients for the pressure distributions where the Wave Resistance coefficient is CD = ρ g R / (pc2 B). Calculations were carried out for L/B of 4, 2, 1.6 and 1.33, water depth ration H/L of 0.25, 0.5, 0.75, 1.0 and 10.0, and channel width W/L of 0.75, 1.0, 1.25, 1.5, 2.0 and 10.0.
The author discusses the effects of water depth and channel width on primary and secondary hump magnitude and makes some comparison with earlier work carried out on an elliptical planform.
The consequences of wave steepness limits on secondary (lower speed) hump are also mentioned.
This data is simply the analytical results for a pressure distribution. It can for a useful start for expectations when testing a ‘real’ ACV, whether an SES where the addition of side hulls has to be superimposed and friction drag from bow and stern seals, or for an amphibious ACV where the drag of the wetted skirt has to be added. The graphical form makes quick reference possible.
When testing in a towing tank it is possible to use an equivalent approach as for fast boats, where in this case the cushion pressure induced wave drag is calculated, and this is removed from total drag measured in that tank so as to gain insight on the friction drag from the skirts, particularly if cushion pressure loading is varied as this will change the skirt wetting. Care is needed of course with scaling.
A theoretical and Experimental study of the wavemaking of hovercraft at arbitrary planform and angle of yaw, by J T Everest and N Hogben, Ship Division, National Physical Laboratory, RINA paper W10 1968.
This paper presented to the Royal Institution of Naval Architects in 1968 was based on work carried out at NPL Ship Division. The authors built on the work of Newman and Poole, and Barratt, developing a computer programme to generate the wave profile under a pressure distribution of arbitrary shape, and also angles of yaw to direction of travel; and from this infer the drag force due to wavemaking.
The analytical work was followed by model tests carried out at No3 tank of NPL at Feltham (the largest and fastest facility in UK at that time) where a rectangular annular jet model, and a scale model of the HDL HD-2 research ACV were carried out. Carpet plots are presented summarising the results.
Correlation between analytical predictions and the model tests was carried out. The approach of assessing the energy of generated wave forms to calculate wave induced drag was validated. For the HD-2 model initial tests were at high cushion loading and the consequent skirt wetting drag meant correlation was not direct. Tests carried out at lighter cushion loading where the skirt wetting was much reduced gave closer correlation.
A key finding of this work was that at large angles of yaw the side force could be as high as 50% of the wave drag along the craft centreline, thus generating a significant overturning moment.
The Prediction of Resistance of Surface Effect Ships, by H Oehlmann, MTG Marinetechnik GmbH, J C Lewthwaite, Consultant Naval Architect
MTG Marinetechnik in Hamburg has designed a large SES called the SES-700 Fast Test Craft. A 5m long model of this design has been extensively tank tested at the David Taylor Research Centre (DTRC) in Washington D.C., USA. In addition seakeeping data for a number of UK SES craft have been purchased from Hovermarine International and analysed in order to extend the database with respect to differing hull forms and size variations.
In this paper presented at the ASNE Advanced Vehicle Conference, Arlington VA, USA in June 1989, the results of a study into the resistance of various SES in both calm water and head seas, are presented. Generalised methods of prediction of resistance have been developed which can be applied to new designs. Several theories of the individual resistance components have been incorporated into a computer program named SESDRAG, and examples of the output from this program are given for a new SES design
NOTE: Members may find it useful to also read Professor Lawrie Doctors paper given to the Fifth International ACV Conference in 2003 which has some useful thoughts on the drag of SES compared with Catamarans, and the sidewall breadth/displacement. This paper can be found here.
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