Page 223 - Practical Ship Design

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Chapter 7

Powering I1

7.1 POWER ESTIMATING USING IN-HOUSE DATA

7.1. I The alternative methods
Most naval architects will have available to them a number of tank test results and
may wish to base power estimates for new designs on these in preference to relying
on methods employing unknown data. The use of in-house data is of course
particularly desirable for a specialist ship if data relating to a similar basis ship is
available.
The main difficulty in using in-house data lies in the fact that such data is likely
to be in a variety of formats depending on the date of the tests and the tank used.
Great care needs to be taken to ensure that the data is used correctly. In principle
there are two ways of working:
I. by using the method used in the tank test report, or
2. by converting the data to an up-to-date method.
The first of these methods is probably the easiest but its use means that the estimate
for the new design can only be as accurate as the method in use at the time of the
tank test and no advantage will be gained from the improvements since that date.
There may also be a need to become familiar with conversion factors for units
that are now rarely used - tons, British horsepower, Imperial knots, etc.
Even the oldest of the methods, which used the Froude friction line, can still
give good results provided an appropriate (1 + X) correlation value is used. And
there is of course a vast amount of data in this format.
If the original tank test results are available, re-analysis of these to the ITTC’78
method is quite easy, but if the results are only available as ship estimates either for
actual ship dimensions or for BSRA standard ship dimensions the conversion
involves a number of steps.