clearance between propeller and duct

[Brian.Lin]

Does anyone have paper to talk about the value "C"?

[daiquiri]

Less is better. Zero is ideal, but is unfortunately not practically achievable.

[DMacPherson]

Part of the original NSMB testing of ducted propellers (leading to the Kaplan 19A design) included the effect of tip gap. Standard tip gap is nominally 0.5%D.

Efficiency losses were related not only to tip gap, but to propeller loading (free-run vs bollard/towing) and to pitch loading at the tip. A general summary comes out something like this:

1. Free-run: 2% loss of efficiency for each 1%D increase in tip gap.
2. Towing: 3%+ loss of efficiency for each 1%D increase in tip gap.
3. Losses with wide tip gap can be reduced by as much as half by "unloading" the tip (i.e., reducing pitch into the tip).

So, as already noted, less is better. And you can accumulate efficiency losses pretty quickly with excessive gap.

Regards,

Don MacPherson
HydroComp

[Brian.Lin]

thanks all,