modelled correctly. Great care has to be taken with this, and depending on the method of model construction this can be quite difficult to achieve.


For a towed model, being able to simulate the correct influence of the propulsion system is not straightforward. If the vessel is to be propelled by propellers it is very likely that the shafts will be inclined. The tow force will need to be applied at this angle with respect to the model, rather than with respect to the carriage, which can present problems when the model trims.


Manually


varying the tow height on the carriage to ensure that the tow angle is correct with respect to the model is possible, however sufficient run length is required.


Also, if the shafts are inclined, the propeller will not be operating in axial flow, and hence will also generate a force at right angles to the shaft line, which may need to be taken into account. This can be done by removing weight at the position of the propeller, if the effect is known.


This is often neglected when testing towed models of high speed craft.


If the full scale vessel is to be propelled by waterjets then the additional mass of water in the waterjet inlets will need to be included in the mass of the model. This will also affect the trim.


Options available for propulsion of high speed craft are as follows:


1. In addition, the suction of the


waterjet inlets, and the reduction in pressure over the aft portion of the vessel may need to be considered. This is sometimes neglected when testing towed models of high speed craft.


If a propelled model is used these problems are not so severe, however care needs to be taken with the model to full scale thrust scaling.


As noted above, a major difficulty with predicting the resistance of a high speed craft using a small scale model is the determination of the form factor which is a function of speed. For conventional craft the classical assumption is that this doesn’t vary with speed, and hence it can be obtained by extrapolating the results to zero speed, and making the so called Prohaska plot, Harvald, 1983.


With high speed craft which change their running sinkage and trim dramatically with speed, such as planing craft, it is obvious that the assumption that the form


factor remains constant with speed is not


applicable. In fact, for any high speed craft which runs with an immersed transom it will not be possible to make this assumption.


One accepted practice is therefore to neglect form factor and use the 1957 ITTC correlation procedure. However, work carried out by Armstrong (1999a, & 1999b) has shown that substantial form factors do exist, even for very slender catamaran hulls with transoms, and that this method can yield substantially incorrect results.


2.


3. 4.


Conventional propellers (usually multiple) on angled shafts;


Supercavitating propellers (again, usually multiple and on angled shafts);


Surface piercing propellers; and Waterjets.


Each of these approaches to the propulsion of high speed craft has its own issues and problems, and the designers and manufacturers have developed their own technical solutions and specialist expertise.


3.1 CONVENTIONAL PROPELLERS


As noted above, there are considerable difficulties using conventional propellers above about 35 knots, and cavitation concerns are paramount. At these high speeds it is difficult to avoid uncontrolled cavitation, and the associated erosion. Although it is possible to predict the thrust and torque


of a conventional non-cavitating


propeller reasonably well using even fairly simple panel methods, it is impossible to predict the performance, or the extent of cavitation, on a cavitating blade.


It is therefore necessary to resort to model experiments which need to be conducted in a high speed cavitation tunnel capable of simulating the appropriate cavitation number.


3.


There are a lot conventional


PROPULSION of


propellers for


difficulties associated with high speed craft.


It


generally accepted that conventional propellers suffer from excessive cavitation above about 35 knots, and the hull form


combined with the level of power required, means that if propellers are used they are usually attached to angled shafts, resulting in them operating in oblique flow.


is associated with high speed operation,


It is well known that shallow water effects can have a substantial influence on the resistance of high speed craft.


This effect is exacerbated if the vessel is in


laterally confined waters as well. Unfortunately, most model tests are conducted in tanks where the Froude depth number is large, and by the very nature of the situation there is also a severe lateral width restriction. Care must be taken to ensure that this does not influence the results (Doctors & Renilson, 1992).


Ideally such a tunnel ought to be able to be


fitted with a foreshortened hull model to give the correct wake, however this requires a larger tunnel, and is a very expensive process. In many cases adequate information about the performance of the propeller, and its cavitation


B-14


© 2007: Royal Institution of Naval Architects


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