Example of statistical power prediction


published, students worldwide got in touch. But things did not go quite so quickly in the naval architect world. There were some doubts, they add, as some naval architects still preferred the more traditional methods.


Certainly, one of the factors in its success was that the method emerged at virtually the same time as the computer found its way into the workplace. Computerisation changed things dramatically, Frits says. To make predictions for the resistance of a ship, many large graphs were needed. So it was always necessary to have a huge table! he laughs. “It would take about a day to find the performance of a ship but in the age of the computer just 10 minutes.” The old method was very laborious and also literally you were relying on people’s memory for correlation of comparable cases, he adds.


Available on Internet The Holtrop- Mennen method has been published several times and a PC program (DESP) was intro- duced by the end of the 1980s. DESP is also available on the Internet nowadays. Another important factor was that we decid- ed to focus on a limited set of parameters, stresses Jan. “It is not necessary to have the


complete ship form and all the details to make these calculations. We stuck with this simple approach and I think that is why the method is so widely used. Even now we only use about 15 parameters.” The key to the method’s success has also been its ability to be applied to virtually any ship type.


They both are certain that the method can go even further and could be used to explore extreme combinations of ship dimensions. For instance, the fuller coastal ship could be looked at or flat, wide, shallow-draught vessels. More data could be added to the propeller design part to improve the calcu- lation for all kinds of nozzles, says Jan. “The method is still in development and a new generation can refine and extend our work.”


Replaced by CFD? So as these two pio- neers retire will their method also retire? The answer: a resounding “No”! CFD is substituting some parts of model testing but it is quite unlikely that it will replace this simple power prediction method soon, they stress. It is extremely suitable for the initial design because of the limited number of parameters involved, stresses Jan. “You don’t have to


1986 8


LIFSIM: time domain simulation of coupled 3 body motions


have all the ship’s geometry details available.” The method is also very successful with vessels with lots of appendages such as navy vessels, ferries etc. he adds. “A strong feature is that it can easily be checked against trial results of comparable ships or correlated with model tests on similar cases if you have enough data available. Once you have tested 9,000 models you should be able to predict the performance of number 9,001 with reasonable confidence!”


But of course, it could always be improved, Frits says. Frits has been involved in full- scale trial analysis for the last 35 years, spending many years interpreting and correlating full-scale data in a modern follow-up of earlier correlation studies by Franssen and De Jong. Sometimes he found systematic discrepancies, he says, so there is always room for improvement. The Holtrop-Mennen method was very important for MARIN as well. Interest in the method meant that many new contacts were made all over the world. And as MARIN produces its 9,000th model (see in this Report), it can certainly be proud that the method is one of the most accurate tools the industry has.


1986


SWOPS: first dynamic positioning for floating production


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