Article: How to use empirical formulas to estimate a ship’s resistance

TheNavalArch is pleased to launch a new article: How to use empirical formulas to estimate a ship’s resistance

The article talks about the popular ‘Holtrop-Mennen’ method of resistance estimation for ship-shaped vessels in detail.

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TheNavalArch – 8 Jun 20

How to use empirical formulas to estimate the resistance of a Ship -...
Read from the Knowledge Hub for Marine Professionals How to use empirical formulas to estimate the resistance of a Ship - TheNavalArch



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I would disagree with classification of methods. What You call 'empirical methods' is actually group of methods:

- Regression methods - obtained by regressions analysis of non-related model test results. Example: Holtrop, Wolfson Unit high-speed and Chine craft methods, Mercier-Savitsky pre-planing, etc.
- Systematic series - based on results of tank testing of hull models with systematical variations of parameters, say Delft series, deGroot, 62, 63, etc.
- Test results for schematic models - say, Savitsky planing (tests on prisms).

Why this is important? Accuracy of each group is different...

 

Perhaps a matter of terminology but the formulas based on systematic test series involving multiple parameters are usually obtained using regression. These formulas include Holtrop-Mennen, the various formulas based on Delft series data, and Savitsky's method. Accuracy of a given method depends on multiple factors. An important factor is how well the hull being evaluated and operating conditions fit within the range of the input data used in developing the method.

 

Yes, exactly - formulas for systematic series can be obtained by regression. Or can be in form of graphs as well, or in form of tables.
However, there is major difference: regression methods are used to obtain formulas for unrelated hulls. Say, towing tank is doing tanks tests for different projects, then they analyses those unrelated hulls and obtain regression formulas. Usually these methods cover wider range of parameters, without relation to particular hull shape. Some regression methods, such as Mercier-Savitsky, include results of testing of few different systematic series.

Systematic series is different - the series is purposely designed to study range of parameters, for given type of hull shape. Before using it, one should make sure the hull shape is similar to shape of series. But accuracy of systematic series is higher.