Saving time with quasi-steady technique


The quasi-steady measurement technique, the latest development in propulsion testing, comes under the spotlight.


Michiel Verhulst, Jan Holtrop & Hans van der Kam m.verhulst@marin.nl


In the quasi-steady technique a gra dual variation of the rotative speed of the pro- peller is imposed, while the forward speed of the model is kept constant. Thus, the load of the propellers continuously changes during the measurement run. An essential assumption of quasi-steady experiments is that each instantaneous condition encountered during the quasi- steady test is representative for the corre- sponding steady condition. In a conventional propulsion measurement each run is represented by a single set of average values of the towing force, the pro- peller thrust and torque for one particular combination of model speed and propeller rotation rate. In the quasi-steady technique a range of loadings is covered, not just one, as seen in the figure. Now each measure- ment run is represented by three sets of measured values, corresponding to the minimum, average and maximum loading covered.


Accurate analysis A clear advantage of the quasi-steady method over the conven- tional one is that separate load-variation runs can be dispensed with. This means that a complete and accurate analysis can already be made while the carriage is being returned to its starting position and a full- scale prediction for the tested speed is also included. The analysis may help to select the next test condition. For complex/hybrid propulsor configurations,


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an independent variation of the load of the two different sets of propulsors is essential to capture the possible interaction between these sets and to account for separate scale-effect corrections. By the quasi-steady technique these loadings can be varied simultaneously during a single run, instead of having to test a large matrix of relative propulsor loadings.


Of course, the quasi-steady method offers time savings because of the smaller number of runs needed. Hence, quasi- steady propulsion testing is ideally suited for a large series of propulsion tests, e.g. a program with several trim/draught combi- nations. In addition, experiments with only one single test speed can be done very cost effectively by MARIN’s new measure- ment procedure.


QSM Conventional


Towing force F Thrust-Towing-force samples of one run by the Quasi-steady method versus the Conventional method


Total thrust T


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