Michael Storer Boat Design

PD RACER FAQ

Are NACA foils worthwhile?

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Several 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.

Working 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.

This 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 boats.

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 in '66.

:-)  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.

Our smallest 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.

When 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

Best Regards
Michael




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