Michael Schmiechen, Berlin:
Towards a rational evaluation of ship speed trials

Abstract

The present paper is a summary of recent work undertaken by the author to promote a more rational evaluation of ship speed trials. In a series of sample evaluations it has been shown that the consistent application of systems identification techniques provides trustworthy results with a minimum of plausible and acceptable conventions and without reference to model test results, as it should be.

The paper contains direct links to all relevant material being published on the website of the author and to be found as well in his Bibliography on Propulsion and under his Recent papers in the section 'On the evaluation of ship speed trials'. An earlier version of this summary, already including the reduction to the no wind condition was the note on Further evaluation. The data and the evaluation of example 21010, the latter including the the first attempt at a reduction to the no wind condition, have been removed as obsolete from the website at a rather early stage of the development, but have of course been archived for purposes of research.

A new example has been added 1999.06.24: 05010_data_orig, 05010_eval_rat.
The examples may still contain errors due to misinterpretations of traditional sign conventions!

Introduction

The traditional way of conducting and evaluating ship speed trials is very costly and involved and at the same time the results are not very trustworthy. The reason for this situation is that the logics behind the whole procedure is not very clear to say the least.

Consequently an attempt has been made to promote the necessary clarification and perform a systematic reanalysis of trials data made available to the author up to date. The ideas developed although in principle simple and conventional, but not traditional are certainly not readily acceptable for the community concerned, but maybe they can form the nucleus of a discussion, and the economic advantages will finally lead to their adoption.

In a proposal for a clear-cut procedure, prepared as a contribution to the current ISO/WD 15016, it as been shown how the power functions of ship propellers in the behind conditions can be identified together with the current velocities with systems identification techniques from a minimum of test runs, and with a minimum of conventions and without reference to model test results, as it should be.

The approach is based on the theory of conflict resolution, on simple models of hull-propeller interaction and current velocity as well as the basic facts of systems identification in noisy feed back loops. This is the approach systems engineers, knowing little or nothing about ship theory, would take. It is not only more adequate, but much more convincing and trustworthy than the traditional approach, avoiding unnecessary and maybe even irrelevant diversions. The sketchy style in the format of the layout of the standard serves to stress the point and needs of course to be changed, if the ideas are followed up.

The work is related to experience gained in applications of the rational theory of ship hull-propeller interaction during full scale tests with the German research vessel METEOR and the Blohm + Voss experimental SES CORSAIR. Material concerning these tests and their evaluation has been published on the website of the author as well and is to be found in his Bibliography on Propulsion and under the Recent papers in the sections 'On the propulsion tests with METEOR, Proceedings 2nd INTERACTION Berlin '91' and 'On the propulsion tests with CORSAIR'.

Motivation

The stimulus for the recent activities of the author has been the Japanese ISO Committee Draft (CD) 15016. The first reaction of the author was a different Draft proposal together with Cover letter and two examples, Data of example 1, Evaluation of example 1, Data of example 2, Evaluation of example 2.

The reservations concerning ISO/CD 15016 were, and are even stronger now, that an ISO standard should not just continue to refine past practice, but should meet the highest 'standards' and take advantage of the latest state of the art and technology of systems identification, not only in view of the legal implications, but the requirements of ISO 9001 as well.

And the standard must be the result of a joint effort of the whole community concerned. In the interest of the profession, science and technology, and the costumers, yards and owners, a serious discussion not only of the details, but of the fundamentals in the first place, is strongly suggested. Naval architects need to take the discomfort of the industry, they are serving, very serious and come themselves up with adequate solutions before outsiders or industry tell them what they better should do or should do better.

Principles

The principles proposed are to keep the models as simple and the method as transparent as possible in order to make the results as truthworthy as possible. Consequently the rule adhered to is to keep separate problems separate as far as possible and keep the exposition as simple as possible.

The resulting procedure is:
and, for the purposes of the study started to convince the community concerned,
whichever has been used by the yard or institution supplying the data.

The first task has been solved in terms of a powering model and a current model as shown already in the draft proposal and in the accompanying examples and in the many new examples, last update and additions 1999.02.09: under the Recent papers in the section 'On the evaluation of ship speed trials'.
As mentioned in other places the data and the evaluation of example 21010, the latter including the the first attempt at a reduction to the no wind condition, have been removed as obsolete from the website at a rather early stage of the development, but have of course been archived for purposes of research.

The usual, rather involved iterative solution of a problem with at least five unknows is replaced by the straightforward solution of a system of linear equations. The second and third tasks could not be performed in the examples due to lack of data. Task 4 has been performed as far as data have been made available.

One general observation concerns the choice of the power to be used in the evaluation. In proving the conformance with contract conditions, the shaft power measured, the brake power, appears to be the most adequate reference, as it does not require further assumptions on top of those necessary anyway, i. e. for the strain gauge measurements in the usual absence of calibrations proper.

Evaluation

The advantage of the procedure advocated is that a minimum of assumptions, i. e. conventions to be agreed upon, are necessary. No references to model or resistance data are necessary. Further, even with moderate deviations from the contract conditions the powering performance does not change, provided the submergence of the propeller does not change. Changes of salinity can be accounted for computationally.

If the submergence of the propeller changes with load conditions, especially from fully to partially submerged, trials have to be conducted at all relevant conditions anyway. Subsequently the powering performance can to identified separately at all these conditions.

The objection to this procedure was and is that the traditional procedure goes further and establishes powering performance at certain service conditions, e. g. at the no wind condition. By doing so, a Pandora's box of problems is being opened. But if one wants to do this, for whatever reason, one should look for a solution following the principles stated.

After having reached this point it may be rightly suspected that any educated guess, i. e. systems identification of the wind effects and reduction to the no wind condition, might be more convincing and consistent than the traditional procedures followed so far. Consequently, as a first attempt, a very simple procedure has been developed, the first results published in the example 21010, later considered as obsolete for various reasons and consequently removed from the website, perfectly agreeing with those obtained by the yard following the traditional procedure!

The model used in the program is a linear to cubic interpolation of the power required due to water resistance and a cubic interpolation of the power required due to wind resistance. This model is of course open for discussion as are the extensions to account for other deviations from contract conditions. In order to identify these effects correctly the conduct of the trials will have to be changed as has been proposed earlier by the present author since 1980; see his Bibliography on Propulsion.

The idea can be extended to phenomena as e. g. changes of trim etc, as soon as the corresponding changes are being performed during the trials and the parameters are available for purposes of correlation. And in due course the effects of waves, shallow water etc will have to be considered consistently with the procedure developed sofar.

Of course this will lead in many cases to the use of data, which have been used up to now as well. But the use may be different. In that sense the whole exercise is to be considered as a necessary rationalisation of the traditional procedure. The former appeal is repeated: naval architects should do this better themselves before other people tell them what to do better.

Request for data

In the meantime, on occasion of the ONR Symposium in Washington 1998, the fundamentals and the first two examples published earlier have been explained to a group of colleagues. In a paper On the Logics with presentation, prepared for that purpose, a more detailed exposition is to be found.

One problem in carrying out the systematic reanalysis of trials data, which aims at the comparison of the results of traditional evaluations with those of the rational evaluation proposed, is that many of the data sets provided are incomplete.

Consequently more and complete data sets in a format discussed below have been asked for and as well as the permit to publish the results on the website of the author in the format of the examples; see the request for data to colleagues and institutions.

The data made available so far are very different not only in extent, but in layout, including the proper identification of the data. Evidently there is a very strong demand for a standard concerning this format, long before any standards concerning the execution and evaluation of full scale trials and model tests. Such rigorous standards, i. e. conventions, will be necessary as well due to the fact that in the presence of noise, the results strongly depend on the procedure.

The differences in the presentation of model test results has in fact been the reason for the formation of the former, famous ITTC Presentation Committee, the name of which has been changing through Information Committee to, presently, Symbols and Terminology Group.

'Some Fundamental Considerations Concerning the History and Recent Development of the ITTC SaT List, the International Towing Tank Conference Symbols and Terminology List' can be found in a paper, which has been presented at the First International Conference on Maritime Terminology. The current ITTC SaT List can be downloaded from the ITTC Website.

The uniformity of presentation is required more than ever before for the purposes of quality assurance according to ISO 9001 and for the purposes of product data model technology (PDT) aiming at the exchange of data in neutral formats, e. g. the ISO/STEP format. Evidently this format is not concerned with neither the generation nor the use of the data.

A a later stage the format will have to be linked with the ISO/STEP Shipbuilding activities and the emerging Application Protocol Ship Hydromechanics. But before this the fundamental questions associated with the change in paradigm promoted have to be solved with a minimum of overhead and as convincingly as possible. The environment of Mathcad is considered to provide a particularly intuitive access to problems and solutions.

Proposed format

From the exercises in conjunction with the creation of the rational theory of ship hull-propeller interaction and for the purposes of the present study the following appear to be the minimum data necessary:
Very desirable data are:
In the present study predicted values have not been made available and contracted values only on a very limited scale. One observation in analysing the data is that very often the quality of the data is very 'poor', i. e. that they exhibit systematic errors. This would not matter if this would happen randomly, i. e. if the samples would be large enough for a statistical treatment proper. In case of the small samples available other methods have to be relied upon. In fact some yards and institutions appear to use normalized data in the way proposed to scrutinize the data.

Conclusions

According to the comparisons available so far the current velocities and the powering characteristics in the behind condition can be identified without any reference to resistance and model data. The results are in all cases very close to the values obtained by traditional methods. And the technique proposed permits to establish transparently the conformance with conditions predicted and contracted on the basis of model tests or determined by other procedures as e. g. the traditional method and detect inconsistencies in the latter.

Looking forward to the serious consideration of these remarks as well as the elaborations and the numerous examples provided, and maybe some response the author remains with best regards,

Michael Schmiechen.

© 1999 Michael Schmiechen, Berlin
Last update and additions: 1999.06.28