Foils – Centreboards, Leeboards and Rudders

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My understanding, backed up by some experience, is that there’s a lot of talk about the best foil sections to choose.  Whether Turbulent, Laminar, NACA00xx, Eppler is best.  Also a great deal about tapered, untapered, raked forward or back.

I would argue that so long as any reasonable section is chosen for the foil section that the care in MAKING the foil is the most important aspect.

And for most of us that involves a template.

Making a rudder or centreboard template doesn't add much time but improves the performance of your sailboat dramatically.

Making accurate foils is not difficult at all with a template.  In general people find it is one of the more satisfying parts of building a boat. Accurate foils can easily add 10% to the upwind performance of any boat and add about 4 hours to the building time. The pic below shows the glassed and epoxied foils for an OzRacer before they are sanded and varnished or painted.  You can see the method for making wooden sail boat foils by this method here.

Accurate foils can easily add 10% to the upwind performance of you sailing boat

One of the clues for this was a discussion I had with Aero/Hydrodynamicist Neil Pollock, whose optimised foil design I use on my boat plans working from full size templates.

We were discussing the development of the NS14 class of moving to laminar foil sections well before they were popular in other classes.  Neil had pointed out that the Reynolds number (a hydrodynamic number that takes into account the velocity of the boat and the chord length of the foil as prime inputs) was wrong, the NS14 foils being too narrow and the boats usually at too low velocities.

So in theory, they were unlikely to work to produce significantly less drag in some conditions, but actually were dominated several years of championship level sailing.

My thinking is that it comes down to the care with which the foils were made.  They were so much more accurately shaped and smooth finished that they were able to outperform other foils.  So that is the paramount factor.

So what section?  I use the sections developed by aerodynamicist Neil Pollock in the late ’80s

Oh, but I’m not racing

So let’s have a look at a piece I wrote for my boat plans

Once upon a time when I was starting off in racing sailboats I bought a second hand NS14 – at that time a family style boat that had lots of performance. Over the first season I did OK finishing about midway in my local fleet.

This is a photo of my second NS14 to give you an idea. Photo 2013, boat 1979.

My second NS14 built by Ian Bashford

I had already owned a couple of wooden boats so I was used to doing all my repairs and maintenance myself.

On this NS14 the centreboard and rudder were around the normal length but the boards were a bit wide. So I decided to modify them myself. I sawed the back off both the rudder and centreboard and reshaped them in line with more modern theory – moving the point of maximum width back slightly and in the end producing a pair of really smooth shiny foils.

As a result of this the boat was a little bit faster and my results improved slightly.

A year later I felt that I had got the hang of the boat and was now looking to do everything possible to make it faster. A new mast, new sails the fittings reorganised so the boat would work like clockwork.

It improved a bit more.

Finally I decided to get rid of my homemade foils and buy the best ones available.

The first time I raced the boat was a revelation. Suddenly I was in amongst the top few boats in speed. But more importantly the boat felt completely different.

It would tack and gybe faultlessly coming out of the manouvres with heaps of speed rather than having to get the boat moving again.. When there were big waves and lots of wind it would sail smoothly – before it used to stagger and stall. When sailing in tight conditions with lots of boats around (like milling around before the start) it gave me the confidence to go in close and pick out a good spot without getting into tangles with other boats.

 Qualitatively and quantitatively the boat was much better – safer and more fun.

 The difference between my hand hewn foils and the manufactured ones is not really all that great – almost the same amount of labour – but the manufactured foils were built carefully to accurately reflect the correct airfoil (wing) section.

 They used templates to get that accuracy. With this knowledge I have never needed topay someone for first class foils. About 50 dollars worth of materials and a few hours of light labour (for small boats anyhow) and I can make a set the equal of what I could buy for 600 to 800 dollars.

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