Monday, February 5, 2018

More Propeller Thoughts

Some time back I did a mind dump of some thinking about boat propellers.  And one of the comments on that post hit a nerve - one that I have been thinking about for years.

Tip vortices.  What are these?  They are the spiraling water that slips off the ends of the prop blades when it is turning.  They come from the fact that water on one side of the prop is at a higher pressure than water on the other side.  This arrangement holds just fine until you get to the end of the blade, and then the high pressure water just spills off the blade and joins the low pressure on the other side, making a vortex.  For visualization, the same thing happens at the ends of an airplane wing, causing sometimes beautiful effects.  And drag.

Wing Tip Vortices

Making vortices uses energy - energy that could have been used to propel water astern giving thrust.  So, how to stop this waste?  On an airplane wing (or a keel...), one way is to put up a fence to stop the spill-over, thus the development of winglets and winged keels.

So what would a fence on a propeller blade look like?
  • Start with a conventional propeller.  
  • Add a ring that goes all the way around the ends of the blades.  
  • Extend the blades profile to meet the ring.  

This is an interesting example - the ring here is being touted as a guard, which of course it is.  But it almost meets the purpose of a fence.  It falls short only in that the ring is not wide enough to fully cover the ends of the prop blades.

Stationary ring bolted to engine
Why doesn't this prop guard achieve the purpose?  In fact, this is probably worse than no ring at all.  The tips will still be forming vortices, which will then immediately impact the (stationary) ring, creating additional turbulence and drag.    It is important that the tips extend to and attach to the ring, and that the ring rotates with the propeller.

Ducted fans have been using (stationary, however) rings forever.  And the cross section of the rings is designed to minimize flow turbulence as the fluid enters the duct (look at the leading edge of a jet engine cowling for an example).  If the rotating ring had such a cross section, drag could be reduced even further.

Now, if only I had a bronze foundry and some propeller tooling to play with...

If someone out there wants  to do the experiment, I need a RH 20x14 prop to fit a 1.25" shaft...

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