International Journal of Small Craft Technology
PREDICTING THE HYDRODYNAMIC PERFORMANCE OF VERY HIGH SPEED CRAFT – A NOTE ON SOME OF THE PROBLEMS M Renilson, QinetiQ Sea Division, UK. SUMMARY
Predicting the hydrodynamic performance of very high speed craft is a particularly challenging task. As high speed craft tend to be smaller than conventional craft, with consequently lower design budgets, the budget available to predict their performance is often smaller than that available to the designers of larger conventional craft.
The paper discusses the issues associated with predicting the hydrodynamic performance of high speed craft. It is shown that numerical techniques are generally not considered sufficiently accurate for many aspects of high speed craft performance, and that many of the assumptions made when conducting model experiments on conventional vessels, and extrapolating the results to full scale, are not valid for high speed craft.
1. INTRODUCTION
Predicting the hydrodynamic performance of very high speed craft is a particularly challenging task. Often these craft are smaller than conventional vessels, with smaller design budgets and hence smaller development budgets.
Many of the assumptions used when predicting the performance of conventional
vessels, either with
numerical or experimental techniques, do not apply to high speed craft, and if care is not taken the performance predictions will be inaccurate.
2. RESISTANCE AND POWERING
The relative importance of the various components of resistance are
different for very high speed craft
compared to those for conventional craft. In particular, spray drag, which is generally ignored for conventional craft, can become important, as can wind and appendage resistance.
Computational Fluid Dynamics, (CFD) is not currently capable of being used to accurately predict the resistance of very high speed craft, largely because of the difficulty in predicting the running sinkage and trim, which are influenced by small changes in pressure around the hull. Many attempts have been made to solve this problem, using both panel methods and RANS approaches.
In addition, it is currently very difficult to predict the flow around a fully unwetted transom, and often it is not even possible to obtain numerical convergence.
It is
difficult to determine the transition from the wetted to the unwetted cases.
Thus, it is necessary to resort to physical models. Unfortunately the models used tend to be smaller for high speed craft than those used for conventional craft.
© 2007: Royal Institution of Naval Architects
This is partly due to the need to obtain high Froude numbers, and partly because the budget for small high speed craft tends
conventional craft. This means that smaller tanks are often used, which with limitations on the top speed possible, results in much smaller models than typically used for larger craft.
With high speed craft the model Reynolds numbers will be higher than for conventional vessels, and the shape of the stern sections make them less likely to suffer from laminar separation, so this may not be such a serious problem for unpropelled models.
A major difficulty with using physical models to predict the resistance of a high speed craft is that, for most hull forms, the underwater shape varies significantly with speed. This means that the wetted surface, and the form factor, are both functions of speed, and need to be determined for each speed before the classical Froude scaling procedure can be applied. This can have a major influence on the results. If the static wetted surface is used the resistance of the full scale vessel will be severely underpredicted.
Hull forms which rely on a cushion of air unwetting a central region, such as the Stolkraft, are particularly prone to difficulties with scaling from model to full scale. (Lund, 1988).
A further difficulty with using scale models to predict the performance of high speed craft is that the running trim can have a major influence on performance, and must be modelled correctly.
to be smaller than that for larger
The propulsion system and the
effects of appendages, such as spray rails, will both have a significant influence on the running trim and must be properly taken into account.
Chines and spray rails need to have very sharp corners on the model scale, as otherwise their effectiveness is not
B-13
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54