Rendimiento académico

Having formulated a dynamic equation and demonstrated the computer simulation of a surface ship under tow situation, and investigated characteristics of the responses, there was now a need to investigate whether a real physical model behaved in the manner predicted by the computer simulation.

The small Circulating Water Channel (CWC) at DRA Haslar was made available for this purpose. This facility limited the size of model to approximately 1 metre length which was considered rather too large a scale factor to accurately predict full scale hydrodynamic forces appropriate to a whole ship. However the size of the facility allowed the tests to be conducted with just two people and changes to the test parameters could easily be made. In addition the continuous running channel with stationary tow post allowed each run to complete its full motion leading to a final steady state, i.e. either docile tow or sheered off to the side, for the CWC does not require waiting times between the runs or return of the carriage as is the case in a ship towing tank.

The model used was 1:100 scale frigate hull already available from previous resistance tests. The instrumentation to obtain records of the angular motion and tow forces were arranged using a camera as a back up recording. The transducers were based on two rotary potentiometers and strain gauged post which had been made in University College at very little cost. For steady state tests direct measurement of the position of the model in the tank was possible and that provided a basis for the determination of yaw, sway and rudder derivatives. One difficulty encountered was the very low value of towing force on the model such that was insufficient for the existing standard tank dynamometer to measure. For the dynamic tests a video camera was used to record the

perspective analysis generated data on the motions. The resulting plots were very similar in character to those generated on the computer simulation.

A considerable number of runs were accomplished indicating the potential of the method to provide a simple and low cost means of evaluating the ship manoeuvring characteristics. Variations in tow length and tow points showed the ability to arrange suitable tests to achieve different responses. The tests also showed the possibility of determining the rudder derivatives. In general the model behaviour was qualitatively similar to the previous linear simulation studies.

Subsequent analysis of the results shows that from the final steady state tow it is possible to obtain consistent values of N^, Y^ and plus any initial bias derivatives

Y* and N* of the model. In particular it is possible to obtain a good location of the sway force i.e. which is termed the Neutral point in submarine analogy. This is obtained without the need for accurate force measurements. The rotary and inertial derivatives can be obtained from the damped oscillatory motion of the model before reaching the steady state tow. Quite good results were obtained from the linear equations of motions by choosing moments when one of the motion variables is zero. The first such state of the transient motion appears to give the best results as subsequent conditions occur when all the motions are small.

These tests revealed that the angle of the tow line to the central line of the tank can be quite large and therefore in the analysis the component of the force across the model should be taken as the sine of the angles i.e. (Tsin(t% + j9)) and not as a linear representation {T{a + P)). Despite this the heading angle of the model remained small (i.e. sin P - p, cos P ~ 7). However the actual motions of the model in sway and yaw are within small perturbation theory and if the lateral displacement variable rather than the tow line angle a , is chosen to be used in the analysis the tow input force can be treated as linear:

sin a = and P = small values

s ' m ( a + p ) = s m a c o s p + c o s a - s i n p

Chapter 4 Model Tests At Haslar Circulating Water Channel________________________ Secondly rather than running the model with the rudder amidships it was found to be advantageous to run the model with a small angle on the rudder. In this test the model was started from the opposite side of the tank and veered across the tank with a damped oscillatory motion taking up a steady state with the tow line at an angle to the tow post and the model at a small angle to the flow direction. From this the steady state equilibrium between the tow force, the rudder force and the hydrodynamic force on the inclined hull can be obtained. Subsequent analysis showed that from the steady state readings it was possible, by varying the towing point, to obtain sufficient data to yield the sway force and moment derivatives, the rudder force moment derivatives plus any bias derivatives on the model. This was extremely useful as it reduced the amount of information that had to be analysed from the transient motion of the model prior to reaching the steady state.

Another phenomena noticed was that if the trim of the model was varied e.g. by altering the ballasting so that there was a greater stem trim, the final steady state of the model shifted from its original condition and therefore the experiment was sensitive to this change of loading of the model and the change to marginal stability with forward trim.