When using NavCad's reference propeller (for" Aligned series" or "KTKQ data"), should I be using one of the Rn basis options or setting it to "Unknown"?

Let's start by considering why we need propeller scale correction in the first place: to match the scale of the propeller calculation to the scale of the analysis. It will be most typical that the basis for the propeller KT-KQ is at model scale and the analysis is at a full ship scale. The propeller series polynomials (e.g., B Series) reflect model scale KT-KQ test data, and therefore a scale correction is used (via the "Propeller options") to expand the model scale values (for a typical 250 mm or 10 inch model propeller) to the full scale propeller diameter. Most individual model test reports that you might enter as KT-KQ data also are reported at model scale (but not always).

All scale correction (whether that be for a propeller or hull) requires that the correct scale be defined for the data as entered. If you are entering the the model tested KT-KQ data (without any expansion or correction to full scale) then NavCad benefits from knowing the original Reynolds number (RN) for the test data. However, not all tests are conducted in the same way nor presented with complete information. If each test point (J) includes the specific RN figure for the test, then you should select "Defined" as the "Rn basis", and explicitly enter each RN value. If this is not given, but there is reference to testing by "Constant RPM" or "Constant speed"), then use these and enter toe appropriate constant value. NavCad can then derive the properly scaled RN.

Remember, this RN information is not about how the KT-KQ values are to be scaled, but the status of the test data as reported.

Sometimes, however, you might have KT-KQ data that has already been expanded to full scale figures. In this case, enter the data in the same way as for model scale, but use the full scale RN values that were defined in the report. If you have no knowledge of the scale of the test data that you are entering, then as a last resort you can set the "Rn basis" to "Unknown". This assumes a nominal model scale RN figure for the data.

Further comments about scale correction for reference ship data: It would be typical that a reference ship file is prepared in NavCad using model scale hull values. Reference ship propeller data, however, is not relevant to aligned resistance prediction (nor the aligned prediction of hull-propulsor interaction coefficients). It would be useful, however, in running a calculation to check the complete self-propulsion model test.

If you are interested in setting up a full resistance and propulsion review for the model test, you should use model scale propeller data for propeller KT-KQ and enter the corresponding "Scale basis" data. For the propulsion analysis, you should not use any "Scale correction" in the "Propeller options" group. Propeller scale correction methods are only for model-to-full scale.

« Go back

Powered by Help Desk Software HESK, brought to you by SysAid