Next is the J3T back shape. Notice that the cavity does
not separate from the leading edge. This gives the same lift
to drag as the Tulin-Burk section even though it
theoretically is a better back shape. Notice also the cupped
design.
A final design gives better results. Notice how much thinner
the blade is and the sharpness of the leading edge. The J5T
back shape would result in an even thinner section.
However, it’s not that simple. Remember the thrust
versus speed graph from above. Surface piercing propellers
can operate under three different flow conditions illustrated
below. Below also are actual pictures of a test surface
piercing prop. That’s why blade thickness and sharpness
matters. While the exact shape of the forward face of the
blade isn’t as important as the back face, it actually does
matter.
PROPWASH
26
October 2013
Props - The Theory Part One
(Continued from page 25)
NACA series, used to delay cavitation on submerged hydrofoils.
The second shape is an early supercavitating design, the Tulin- Burk
profile, and the third was used on a family of supercavitating props.
Because surface piercing props often operate under conditions
similar to supercavitating hydrofoils, similar blade section designs
can be used. The wedge style in the middle of the picture is
especially useful for its strength. The idea is to fill the cavity formed
by ventilation or cavitation with as much material as possible while
not allowing the forward surface of the section to touch the cavity’s
surface. The bottom surface is formed to provide the best lift to drag
ratio. Below are some computer fluid dynamic studies of flow over
supercavitating sections. The first graph shows the exaggerated
shapes of the bottom of the section – the back surface of the blade.
Notice how they look like the back face of common modified props.
The Tulin-Burke shape (blue line) is similar to a blade with gradually
increasing or progressive pitch. The J3 term shape (red line) looks
like the back face of cupped props.
In the next column is the cavity formed at a five degree angle of
attack on the Tulin-Burk section. Even though the cavity is clear of
the forward face, the simple back shape doesn’t give a really high lift
to drag. That design does allow a thick section, however.