Hello Manos,
I conclude the slippage values of your heavy rib are quite normal.
Does that mean your prop efficiency's inevitably bad ?
A converted automatic car with no lockup clutch has similar slippage values
resulting in slip-proportional loss.
Contrararily prop loss is not directly associated with slippage.
Prop efficiency depends on advance coefficient.
See here:
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You must align advance ratio by aligning diameter or pitch so it's under the
efficiency's maximum.
Nevertheless slippage may be 18 % or so.
See the difference to a car with 18 % that would never drive with optimum
efficiency.
Greetings Nordy