What foil to reduce drag at circular motor shaft?

Hallo,
I'm building a solar electric boat 21' (6,3 m) for slow cruising (max. 10 kt ?) in sheltered small inland waterways in Germany. She is a Kat and driven only by two trolling OBs with circular shafts (diam 28 mm, 1.1 inch). To reduce the shafts drag in water I will cover them with a suitable airfoil profile.

With only very basic knowledge in hydrodynamics I think a NACA 0021 could do it (Chord 180 mm, 7"). Comments appreciated, thank you.

 

I have added a foil and there was a significant improvement. I also added an antiventilation plate because the propeller was sucking air.

 

If all you want is to reduce drag and vibration, no lift; then I recommend something like the drawing below. This is based on the TMB EPH prop shaft strut shape, which is the specific application you need. The NACA 0000 shapes are too thin aft for good performance in certain hydrodynamic, vice aerodynamic, situations.

 

@jehardiman
Thank you!

 

Any photos Gonzo please? Interested In the AV plate mostly.

 

I never took any photos. It was a tear shaped flat plate. I made it that it could slide up and down to find the best spot.

 

You might want to have a look at

Airfoil database search http://airfoiltools.com/search/index?MAirfoilSearchForm%5BtextSearch%5D=strut&MAirfoilSearchForm%5BmaxThickness%5D=&MAirfoilSearchForm%5BminThickness%5D=&MAirfoilSearchForm%5BmaxCamber%5D=&MAirfoilSearchForm%5BminCamber%5D=&MAirfoilSearchForm%5Bgrp%5D=&MAirfoilSearchForm%5Bsort%5D=1&yt0=Search

These are used for wings struts on some general aviation planes and ultralights / LSA's.
I haven't tried them in real water but in my CFD sims they tend to do very well for their enormous thickness (which is just the point of this design).

Unsurprisingly, the shapes are very similar to the strut Jehardiman posted, but they come in three thicknesses, complete with point coordinates and pics detailed enough to be used as patterns. (There are even Lift/Drag curves in the "airfoil details" - filter it down to RE 500 000 - 1 000 000 and only Ncrit 5 for, uhm, the "least fishy available approximation" for your case, but keep in mind that these characteristics are for an infinite wing span: real-life characteristics will be several times worse for your strut fairing. You can also use the site to compare to other symmetric shapes (maximum camber = 0 in the search function).

 

Post #252 and follow ups. Switching out from the standard weedless prop also seems to help a lot.

Efficient electric boat https://www.boatdesign.net/threads/efficient-electric-boat.27996/page-17

 

@CocoonCruisers
@portacruise
Thank you both!
When I tried to store the graph (linked #252, "test results") my computer told me it was already stored. Many months ago I think. The photos reminded me, that I was doing a slightly similar wooden
profile (Göttingen type) years ago which was never in the water.

 

Why not just throw the whole shaft away and build an FRP leg with the wiring molded in. Probably be about 8mm at widest part if you do it that way.

3 Conductor Tunable White Flat Power Wire - 10mm - Per Foot | Super Bright LEDs https://www.superbrightleds.com/flat-power-wire-3-conductor-10mm-nfls10-fpw-3?utm_campaign=Child+-+Organic+Shopping&utm_source=More+LED+Lighting+%3E+Power+Wires+%26+Cables&utm_medium=NFLS10-FPW-3&gad_source=1&gclid=Cj0KCQjwpZWzBhC0ARIsACvjWRPmNNnftel8apLd0ngoRL3ICqMmBfzKgdLh7K4Y2Lqqh_TFfAP6y9EaAjuFEALw_wcB

 

Thank you.
No way to do this in the near future. (OTOH something similar to this is already planned for the next generation AB.) The linked conductor would carry only about a 10th of the needed current.

 

Rather than throw the whole shaft away, maybe consider a trailing shaft electric motor Drive, which is much more efficient? This link gives you an idea about how that is done on a very large boat.



Notice how the water flow to the propeller is not blocked off or disturbed compared to what occurs with a submerged Motorhead trolling motor. The propeller for some craft can be adjusted to run in submerged or Surface piercing mode.

 

@jehardiman Okay, I give up. I figured out, after a couple of hours, that TMB-EHP referred to the the Taylor Model Basin's Extended Hydrofoil Project. But after reading everything I could find about it, I had bupkis on the strut fairing you pictured. I did find the strut profiles on the PCH-1 demonstrator. But nothing on a blunt streamlined strut with high transverse modulus or nonstructural low-drag fairing applications. There is apparently a shape like you pictured studied for blimps and other streamlined bodies of revolution, but I can't find one like it for nonstructural strut fairings. Where does your geometry come from?

 

TMB does stand for Taylor Model Basin, but EPH stands for elliptic-parabolic-hyperbolic which is a description of the 3 parts that makeup the forebody, mid-section, and trailing edge of the shape. Originally developed for deployable ASDIC/SONAR head struts in early WWII to meet the low-drag low-vibration needs for higher speed and more accurate attack runs. This modification, along with MOUSETRAP/HEADGEHOG and millimeter RADAR, won the Battle of the Atlantic. The shape went on to be used in many other applications where a low-drag low-vibration shape was needed. It is in the public literature, but has a very low profile (pun intended).

Edit: Actually I should have said centimeter RADAR...millimeter was post war.

 

So the TLA horizon was earlier than I realized.

Four-letter acronyms are often humorously called XTLAs - extended three-letter acronyms.