Resistance & Powering Coefficients





Resistance & Powering Coefficients
In order to evaluate the success of the extrapolation process from model to full size, it is necessary to measure the resistance of the ship with sufficient accuracy for comparison between the measured ship resistance & that predicted from model tests. Present methods of measuring ship resistance are inaccurate & costly. The ship has to be taken out of service & special instrumentation & personnel have to be on board. Inaccuracies in the resistance measurement result from:

1. Noisy thrust measurements if a thrustmeter is installed or even worse - if thrust is not measured, from thrust estimates derived from propeller torque or engine power;
2. Use of scaled model measured wake fraction, thrust deduction factor & thrust coefficient, which we know suffer from scale effect;
3. Inaccuracies in speed determination from differentiation of distance with respect to time; &
4. Correction for current, wind & waves.

Delays in trials to await better sea conditions are very costly, & conducting trials in dubious weather is also costly by throwing good money after bad in trying to get something useful out of the analysis of bad data.

Knowing the wake fraction of the ship is an essential element in propeller design & the wake fraction suffers severely from "scale effects", as can be demonstrated from hydrodynamic theory. Improved methods of correcting for scale effect in wake fraction will be of great benefit to the propeller designer.

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