[hpv-boats] Propeller optimization

[Peter Raymond]

I have the book "Fluid-Dynamic Drag" by S. F. Hoerner that has a ton of
interesting information in it.  One of the charts shows induced drag for
trapezoidal wings with different ratios of root to tip cord.  I used this to
look at minimizing induced drag for a given amount of sail area and
overturning moment for a sail boat.  As I recall, for a given aspect ratio,
minimum induced drag comes with the tip cord at 1/3 of the base cord.
However, on a sailboat you are limited by stability, so induced drag vs
overturning moment is important, not induced drag vs aspect ratio.

What I found was that by increasing the sail cord at the base and reducing
it at the tip I could increase the aspect ratio of the sail and for the same
overturning moment the net induced drag dropped.

I would expect some similar effect for propellers, although significantly
more complicated.  In a propeller the aspect ratio is limited by material
properties.  If you increase the taper of the propeller the base of the
blade gets wider and thicker and so stronger and stiffer.  This might not be
the most efficient shape for a given aspect ratio, but it might let you run
significantly higher aspect ratio and I would expect the higher aspect ratio
to give a net benefit.

Unfortunately, you could make it more complicated still by including the
thickness ratio in the optimization.  For a given propeller design there is
an optimum thickness ratio, but using one thicker than optimum would give
more strength and stiffness and support a higher aspect ratio.  Even load
distribution could be optimized this way.  Increasing the loading at the
roots reduces the stress on the blade and allows a higher aspect ratio.

I wouldn't want to try to write the code to optimize all of the variables,
but it does seem some improvement could be made by changes that would
improve the  mechanical/strength efficiency of the design.

Peter Raymond