Ship Model Propulsion Experiments

Ship model propulsion experiments
analysis and random uncertainty

In this paper two methods of evaluating ship model propulsion experiments using the Load
Varying Method are described. They give model results that are quite close, but the nonlinear
method is preferred and it has the advantage that it can provide the random
uncertainty as well. The linear method, on the other hand, provides criteria or data
consistency checks that the data should satisfy before it is accepted for analysis and
prediction purposes; these checks are not comprehensive, however. The sample size was reasonably large, involving 632 tank runs, covering 13 vessels
of a variety of types, and 29 vessel conditions. Problems encountered in propulsion testing
are discussed and propeller rotational speed is identified as the variable with the least
random scatter present. The scatter is so low in comparison with the scatter in the other
variables, excluding model speed, that the rotational speed can be assumed to be free from
random error. The random error or uncertainty in the measurement of the towing force, on
the jointly propelled and towed model, propagates throughout the analysis, and is central
to the evaluation of the random uncertainties in the other variables and the derived results.
Thus, models that have high scatter in the towing force have high random uncertainty in all
the results. Large heavy models towed with conventional resistance dynamometers are
susceptible to longitudinal model oscillation contaminating the towing force measurement.
This problem is as old as model testing itself, but the development of ships has heightened
it and is the reason for the widespread popularity in using the Load Varying Method of
testing as a means of reducing uncertainty. In choosing the model size for a test, all the
relevant factors should be considered if random uncertainty is to be minimised and not just
the attainment of Reynolds numbers well in excess of threshold values. Large models can
introduce greater uncertainty of measurement, thereby defeating the objective in using
them. It appears that the random uncertainty in the model propulsion results produced at
BMT is low, and probably of much less significance than the random and bias uncertainties
introduced when performing full scale power-speed predictions from model results.

The Load Varying Method of testing has come
to the fore in recent years in attempting to overcome this
difficulty, as well as permitting alternative analyses to be
made at different propeller loadings.
The International Towing Tank Conference (ITTC) has
not made recommendations concerning the two main alternative
testing methods, but has concentrated on producing
a standard analysis and power prediction procedure which
attempts to account for the major scale effects between
model and full scale. However, as the experiment data is
dependent on the test method it is considered that more
attention should be given to this aspect of the work, and this

INTRODUCTION
Since the introduction of ship model propulsion testing
early this century not a great deal of change has taken place
in the basic experimental procedures, apart from improvements
in measuring devices and instrumentation for data
acquisition.
Text books on the subject of naval architecture invariably
still describe the propulsion experiment as though it were a
steady experiment in which the longitudinally applied forces
balance exactly. This is misleading and oversimplifies the
situation. In practice, often, especially with some heavy full
form merchant ship models, the experiment is of a quasisteady
nature, with significant longitudinal inertia forces

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