Resistance Ratios

Hydro allows you to store the towing tank model test resistances of a particular model in the .POW file. Once a set of resistances have been stored, the calculate ratios facility can be used. This facility compares the model test resistance with each empirical method's estimation of the model scale resistance and stores the resulting resistance ratios in a .RAT file. These resistance ratios are a measure of how accurate the empirical method is for any particular hullform.

The resistance ratio facility is particularly useful if you have the model test results of, for example, a tanker which can be stored in Hydro and then used in the Calculate Ratios function to determine the resistance ratios. The resulting ratios are then stored in a user selected file with a .RAT extension. These ratios can then be incorporated into any future tanker design analyzed in Hydro to correct the chosen empirical method and 'simulate' the towing tank test.

The choice of calculation for the ratios depends on the published data and methods given. Methods that provide data for the calculation of the residuary resistance (for example, Taylor-Gertler) use the formula:

whereas, methods that provide data to calculate the total resistance directly (for example, BSRA Methodical Series) use the formula:

If any method is inapplicable for the current hullform (which is more than likely since it is difficult to get a hullform that satisfies all the limits for all methods at once), then ratios are not stored in the resistance ratios file for this method.

When you decide to use a set of resistance ratios to correct the predicted resistance, the ratios are used for all conditions analyzed.

Note:
Because the resistance ratios are calculated for all ten resistance methods, the calculated ratios may not be particularly reliable for some of the resistance methods. Therefore, when incorporating a set of ratio resistances into a powering prediction, care should be taken to ensure that the resistance method chosen is applicable to the current hullform and that the resistance ratios in the chosen ratio file are sensible numbers (around 0.8 - 1.5). The ratio resistance file can be viewed as a text file. However, the file is in a fixed format, so editing of the ratio file is strongly discouraged.