Figure 4.2 Examples of speed/trim curves 5.1 HEAD/BOW QUARTERING/BEAM SEAS
It is normal practice when predicting the motions of a vessel in head and bow quartering seas to assume that it is advancing at a steady speed. Unfortunately, for a small high speed vessel this assumption may not be valid, as the surge force can influence the longitudinal motion significantly in some cases. In addition, for very small high speed craft the operator will be continually varying the throttle setting, and possibly altering course, to enable the vessel to perform at its best in the waves.
It
is not always possible to assume that predictions based on steady velocity will bear any resemblance to full scale performance.
Although, as noted above, existing theories are based on assumptions which are not valid for small high speed craft, they can often be applicable with care beyond the range where they are strictly valid as shown for example by Macfarlane and Renilson, 1995.
Variation in towing method
0 1 2 3 4 5
SS3 Motion response at 30 knots
Tow post Tow line
This method has the advantage of being able to simulate the upthrust from the propulsion system in a manner that realistically represents the change in direction of this force as the model is pitching in the waves. In addition, it has the added advantage that it reduces to zero when the model becomes airborne as it comes off a crest.
If SS4 Sea state
Figure 5.1 The effect of towing configuration on motions in a head sea
The conduct of model experiments to predict the motions of very high speed craft is not altogether straightforward. Often the models used, and the speeds required, mean that it is not possible to use a self-propelled model, and it is necessary to make use of a towed model. In this case, the method of towing the model is crucial, and has to be decided with care, as the towing method itself can
B-18 SS5
the method of simulating this upward force did not do this then the model behaviour under these circumstances would be very unrealistic, since this component of force dominates as
the other hydrodynamic forces temporarily been reduced to zero.
Finally, it is worth noting that the use of the results from these tests is likely to be to estimate dynamic loads, and to determine the effects of the motions on human performance.
have
When towing a small model in waves it will not be possible to continually vary the angle of the towline to take into account the effect of trim on the thrust line, as recommended for tests in calm water. In addition, if the full scale propulsion system generates an upward thrust this can not be simulated by removing mass, as could be done in calm water, since this will affect the mass, LCG and radius of gyration, which will influence the results. One alternative approach, adopted by QinetiQ when conducting such tests, is to make use of a trim tab to provide the upward force. This is illustrated in figure 5.2.
influence the results. Figure 5.1 shows the results from a small high speed model using two different tow methods.
Further, as noted above, a small high speed vessel may experience significant
variations in speed due to the
waves, and consideration ought to be given to towing the model with a constant force rather than at a constant speed. Even this will not simulate reality exactly, and unless a soft spring is incorporated in the towing system is very difficult to achieve.
© 2007: Royal Institution of Naval Architects
RMS pitch (degs)
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