Making NACA and highly accurate centreboard and rudder foils at home

Several of the Group were building NACA foils.  But the OzRacer and OzGoose Plans have templates for quick and accurate shaping.

Making without the templates … several were feeling that they couldn’t make them to a greater accuracy than 1/16th of an inch.

So some were suggesting using a CNC machine.

The discussion goes on to cover our option where we include an accurate template for a Aerodynamicist created foil which is easier to build and allows easy tapering.

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.

My old NS14 from the late 1970s

Every type of boat, from dinghy, to cat to keelboat right through to hydrofoil boats perform better with accurate foils.

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.

A plane can cut to a tenth of a millimetre and sandpaper on a block can be even more precise.

This is what we do.  With some computer assistance to print some templates.

We go a different way from NACA with Storerboats. We noticed that while there were arguments about what type of foil section to use … laminar flow or turbulent flow … it always seemed that the fastest boats had the most accurate foils whatever the section was.

So we use an easy to build section developed by aerodynamicist Neil Pollock some years back. And the plans include accurate templates to get consistent shape.

Foils - accurate centreboard and rudder blades are essential to the good performance of Storer Boat Plans

One of the big flaws of NACA foils for home builders is that if any taper is required (it reduces drag) it is very difficult to homebuild because of multitude of templates and the guesswork about what happens between them.

The Pollock sections allow tapering without affecting accuracy.

And they save on wood and labour.

So that is why they are an integral part of sailboat designs by storer boat plans.

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