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...



Captain Curmudgeon said...

Dear Bob: Prop vortices and their elimination. It also becomes a financial issue. Developing a way to rescue lost energy on a propeller or ducted fan will only save 2 or at best 3 percent of the thrust. Looking at it over time makes it a better idea. An aircraft with a life expectancy of say 50,000 hours saving 3 % equated into fuel burn is a fair amount of 💰 to the owner. This becomes cumulative when you add wing fences, commposit fan blades shaped to improve flow and not to forget the new age jet engines that now aren't making thrust like a jet. They are now spinning a highly engineered fan. A ducted fan. We may now assume the entire new design to be as much as 12 % better and over the same 50,000 hours of flight that save a ton of fuel. To accomplish this on a sailboat would defeat the reason for owning one. Sailboats are in my mind designed to be enjoyed and used as a way of relaxing from life's stress. It's seems to me there is a saying that fits here. It's not about speed or getting somewhere it's about the journey. Take up crosswords and fine beer. Relax and enjoy life. Retire from such thoughts. Go for a walk with the wife, smell the roses.

Darin Christen said...

Kort nozzles use a stationary ring. I wonder if rotational drag from a rotating ring would be a drawback.

Darin Christen said...

I hope retirement gives me more time for "such thoughts".

Robert Salnick said...

Darin -

I believe that the skin drag from the rotating ring would be very significantly less than the drag caused by the tip vortices.

For large propellers, a new problem arises - centrifugal force. It may be that for ship-sized propellers the centrifugal forces that the ring would experience are too great for bronze of an appropriate cross section. But certainly not on my 20" prop...


Robert Salnick said...

Captain Curmudgeon -
Hello from a fellow curmudgeon!

This is the kind of thinking that happens with a glass of wine down below, tied to the dock on a rainy windswept day...


The Cynical Sailor said...

Imagine all the interesting things you could create if you had a bronze foundry :-)

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