The US. Navy is in the process of reexamining the ship model powering correlation policy to reflect the ship-model correlation allowance experience obtained on 17 ships since 1980. A correct estimate of the correlation allowance is crucial to the accurate model test based prediction of ship power and propeller RPM Relative to the old correlation data, the new post-1980 correlation data tends to confirm the correlation allowance for mid-size ships. Relative to the Naval Sea Systems Command (NAVSEA) correlation curve, the correlation allowances for long ships over 600 ft and for short 200. ft ships tends to be lower.
Both the predicted power and the predicted RPM is compared to ship trial data for all the new data. The model test data was reanalyzed with the correct correlation allowance determined from ship powering trials in order to determine the accuracy of the propeller RPM prediction methodology. Propeller pitch was known for certain on fixed pitch propellers and on controllable pitch propellers operated at pitch values limited by mechanical stops. For these cases. the propeller RPM calculated with the post-trial correlation allowance was within +0.9% and -2.4% of the trials measurement. These data tend to validate the model test extrapolation procedures in use at the Naval Surface Warfare Center, Carderock Division (NSWCCD) for the typical combatant I auxiliary Naval ship.
On ships with controllable pitch propellers operating at the nominal design pitch there was much greater scatter in the predicted RPM, indicating that the pitch of the operating propeller was not really known accurately. The propellers may have been operating at slightly off design pitch conditions. The development of in-hub pitch sensors is highly recommended.
The correlation allowance for ships launched after 1982 is on the average 0.0002 lower than the correlation allowance for ships launched prior to l 982. Ships with ablative type paint tended to show lower correlation allowance than ships without ablative paint. Explanations for the decreased correlation allowance for the post-1982 launched ships are examined in the paper.
