of the PD Racers were building NACA foils. The discussion was
that several were feeling that they couldn't make them to a greater
accuracy than 1/16th of an inch.
A CNC machine would take the fun out!!! :-)
The efficiency of NACA sections rely on getting very close to the required shape.
to a sixteenth will lose a lot of the advantages. It will be
better than just grabbing a plane and going for it - but trying to get
closer will have significant benefits.
There were a series of
tests done here years ago on NS14s which were the predecessors of the
TASER dinghy. Basically a foil shaped by eye and painted
perfectly was 4 minutes in an hour slower than one shaped carefully to
the correct NACA section and given a similar perfect paint job.
may not matter to the cruisers (bless their cotton socks!) but if you
want you boat to sail really nicely it does make a big
difference. There is probably little point in expending the
extra labour if you don't care much about making a show against other
4 minutes actually expands out into a much bigger margin
because the advantage is upwind only - reaching and running it won;t
make any difference - so the end result is that the NACA section saved
you 4 minutes in every 30 or something over 10%.
You need to
mark the foil out as accurately as possible - below might give you some
idea of how we go about it. Basically tools - particularly planes
have a much tighter tolerance than 1/16 of an inch. Sandpaper on
a block is finer still.
There is a long history in these
colonies (Land of OZ) of hand shaping accurate foils. Luckily we
aren't hampered by the imperial measurement system which we got rid of
:-) That was a sorta joke (as my Chinese friends
would say a "cold" joke ie nobody laughed except me) - the following is
our method of getting finer points - which will work with any
measurement system - but you might need to do a bit of spreadsheet math
to work out if applying it to the imperial system.
usual measure is the millimetre (three of them to 1/8") and it is easy
enough with a fine pencil to mark half or a quarter of one or a third
by interpolating (guessing) the points.
If I could draw 2/5ths
by eye I could get closer approximations but I haven't a clue what
2/5ths looks like whereas I am pretty clear on quarters and thirds!
So we can mark 0.25, 0.33, 0.5, 0.66, 0.75 pretty easily.
really keen you can divide those marks in half. eg mark 10.75 and 11
and then split the difference for something close to 10.9.
This is what we do. With some computer assistance to print some templates