High aspect foils

15/03/2015 16:27:39
Gareth Griffiths NHRC
Hi guys...!

So what is the recommended foil shape and measurements for the rudder and Centreboards?

I thought JT's boards and rudders were looking very good last season...

I'm keen to make my own, so wondering what foil shape and dimensions are best.

If anyone can help please do.

Looking for the following for both CB and Rudder:

1. Length of Foil

2. Width of Foil

3. How far after from the leading edge is the widest part of the foils shape

4. How far down the blade it starts to taper, when looking at the foil from the leading edge.

18/03/2015 21:28:56
Gareth Griffiths NHRC
No help...?

19/03/2015 07:51:36
Chris Martin
Theres loads of info on DIY foils all over the internet - the Cherub website used to be very good.
I don't think people are going to be too inclined to post the dimensions of their foils on the forum for you to copy. Jon Turner, Dave Winder and one or two others have spent a lot of time and money getting theirs right and making the tooling and I think they would be understandably rather upset. 

19/03/2015 08:07:57
Chris Martin
one decent link.
theres loads of info on NACA sections etc here too

19/03/2015 11:53:02
John Dalby
Hi Gareth,
You may be interested to read a book called "Model Aircraft Aerodynamics" by Martin Simons. Lots of interest and quite readable.
One small point: high aspect ratios usually result in longer boards, the longer the board, the more effort to keep upright!

20/03/2015 06:44:56
Gareth Griffiths NHRC
Chris I don't want to copy. To be honest I am very insulted that you make the rather defamatory suggestion that that is what I am aiming to do.

As you may have noticed from my many posts on this forum I am very interested in the tried and tested history of what people use on their boats from rigging, sails to shapes and designs...!

Regarding making foils I would rather improve on current designs if possible. Jon Turner's are very tidy though. 

Due to the one design nature I am sure that Winders won't fit my boat so copying wouldn't work.

Thanks to all the people who emailed me with their dimensions though.

Very useful.

Cassett rudder as Dave Hayes used last season is a great idea too.

20/03/2015 18:27:16
Gareth Griffiths NHRC
For anyone else interested in making their own foils. Here is a brief outline of the basics in choosing foil sections and why.


All foil shapes come from a series of recognised shapes that were pre-designed and tested by clever bods in lab coats, many moons ago. The most recognised are the NACA four digest shapes.

The four digits relate to the measurement of the width as a percentage of the chord, (the fore/aft length).

As the link above explains, centreboards are generally an NACA0008 and rudders an NACA0012.

The rudder needs a wider foil shape to maintain laminar flow. To prevent stalling, help stop you from broaching on a powered up reach.

28/03/2015 07:36:41
Gareth Griffiths NHRC
For my centreboard i am thinking of using an NACA63010 shape. Max width a little further aft than with NACA0009 or 0010.

Taper to start 260mm from where the board leaves the hull.

The board will be 1370mm long and 240mm chord length.

Straight 90 degree trailing edge with a gentle elliptical leading edge.

Any thoughts guys and girls?

28/03/2015 07:54:05
Gareth Griffiths NHRC
Found this in the archives 


28/03/2015 07:59:38
Gareth Griffiths NHRC
This is an interesting read too...

How to calculate what surface area you need.

First you have to work out the righting moment. Multiply the weight of your crew (in Kilogram) by the distance (in metre) between the boat centre line and the centre of gravity of the person (somewhere at the hip). You get the righting moment in Kilogram * Metre for each person. Add up the righting moment of each person to calculate the total righting moment of the crew. Multiply this righting moment by 9.81 to turn the kilograms into Newtons.

Now you have to work out where your sail force and lift from the centreboard acts on the boat. For the sail force 1/3 of the mast height above deck is probably a good guess. The lift from the centreboard acts at about half the length of the centreboard. Now calculate the vertical distance between the centreboard lift and the sail force.

If you divide your righting moment by this distance you get the required lift on the centreboard in Newton assuming that the boat doesn't heel. In reality the boat will heel but the rudder will also produce some lift. So we just ignore this.

The formula to calculate the required centreboard area is:

A = (L * 2) / (Rho * V^2 * cl)

A = Area in square metres
L = Lift in Newton (as calculated above)
Rho = Density of the water in kilogram per cubicmetre (just use 1000kg/m^3)
V = boat speed in metre per second (2kn = 1 m/s roughly)
cl = lift coefficient

The optimum cl depends on your profile. For laminar profiles it's often quite low to stay within the low drag bucket. For normal profiles 0.3 is probably a good guess. The centreboard starts to stall at a cl of about 1.0 to 1.2. If you manipulate the formula you could work out at which speed your board is going to stall while producing maximum lift. 

The boat speed is squared and therefore it has a big influence on the required board area. If you have a hand held GPS take it sailing to come up with a good number.

28/03/2015 13:46:44
Peter Mason

 Well, there is plenty of fluid mechanics info available on how to design a C/B from scratch and, just to make sure you stay close to reality, there is 60-odd years of empirical experience with Merlins. So, I do not think just making a specially short or long one is going to release a burst of performance.
The principal task of the C/B is to control leeway and the drag of C/B itself is not necessarily the key thing. To do this, it needs firstly adequate area, say 0.3 to 0.4 m2. With this sort of area, a high aspect ratio will also tend to reduce leeway for a given side load but it will be more prone to stall at low speeds.
Turning to the drag, a deeper board will rapidly have lower induced drag but, for a given area, the friction and base drag will go up.(Roughly 50% of the frictional drag occurs in the first 15% of the chord.) So somewhere in here is an optimum depth for a particular set of conditions and sailing style.
A rough calc. for my boat shows that at 2.0 m/s, the viscous drag on the C/B is about 3.6 N and the induced drag is about 2.5 N, i.e. they are nearly equal in value with the induced drag the smaller. I would reckon that this is close to the optimum, bearing in mind that controlling leeway is vital. This C/B is quite small at 0.3 m2 and 1.075 m depth.
If you want to experiment, you could try a sharp trailing edge as any thickness has a huge effect on drag, or you could try crescent shaped tips and straight trailing edges, or best of all, you could try and minimise the dreadful necklace and hairpin vortices which occur where the C/B goes into the hull. These account for a lot of drag.

Some sources which might help are:

- JavaFoil applet which will enable you to derive sectional Cl's and Cd's (my efforts with this gives the best all round performance with a NAC 4-digits series section modified to have the maximium thickness 35% back but with the standard nose radius).
- NACA report 3359 - Tests on 3 semi-elliptical planforms
- NACA report 431 - Round tips versus square tips
- "Minimum Induced Drag of Sail Rigs and Hydrofoils" Tom Speer
- "Fluid Dynamic Drag" Sighart Hoerner
- "Fluid Dynamic Lift"  Sighart Hoerner
- Min. of Aviation Report 3323 "A Review of Research on Two-dimensional Base Flow
- "Parametric Exploration of Wing-Body Junction Flow using Computational Fluid Dynamics" 

28/03/2015 21:30:51
Gareth Griffiths NHRC
Cheers Peter

It's a very interesting subject and I agree that the "necklace" vortex is an area for a great deal of thought...

29/03/2015 14:07:36
Peter Mason
Dear Gareth,
Have just realised that I didn't write anything about rudder design.
Two or three year's ago, I designed a rudder which I had made by Simon Cox at Synergy Marine. The reason for going to him was that he forms the shapes on a CNC cutting machine. I sent him drawings and iges files of coordinate data from Javafoil;  Simon created A CAD model from this information and used this to drive his machine. 
Below is an extract from an email I sent to Simon explaining a little bit of the design basis.
“Dear Simon,
I will have a go at explaining as follows:
1) A Merlin sailing at 2 m/s has a total drag of about 35 Newtons force, i.e about 3.5 Kgs. So quite a small force.
2) Of this drag more than 10% comes from the rudder under steady-state conditions, and much more comes during strong steering actions.
3) The rudder drag has two main components, namely:
a) Form Drag, which is made up of viscous friction drag and drag due to pressure changes front to back. This latter includes surface wave effects and the drag from blunt trailing edges.
b) Induced Drag which comes from carrying a lateral (steering) force and is directly linked to the flow of energy into the tip vortex at the bottom end and extra wave generation at the free surface.
4) Working Area. A good starting point is to have the working area about half that of the centreboard. So about 0.15 to 0.17 sq. m. This fits in with current practice, and with my experience in an experiment I did with a specially small foil (at 0.11 sq. m). This was a disaster in terms of speed round a course, despite lots of calculations which showed that it would be fine.
5) A 10% reduction in rudder drag will give a 1% reduction in total drag, which means a 0.5% speed increase which means a gain of about 25 feet per mile. So about 2 boat lengths per lap in a 4 lap race (at Tamesis).
6) So the idea was to look for a significant reduction in drag over the current run of rudder designs, while maintaining the same level of steering control.
1) Aspect Ratio. I have stuck with the current practice for this (i.e. about 800 mm immersed depth and 200 mm chord). Going deeper brings problems with increased bending in the stock and more likelihood of contact with the bottom. And it is not automatic that drag is reduced by going for a high aspect ratio; true, induced drag will come down as the square of depth but frictional drag will go up as more than half the frictional drag occurs in the first 15% of the chord. Hence, for a given area, a deeper board has more form drag because it has both more leading edge and more trailing edge.  I suspect that total drag round a course is not much affected by aspect ratio with limits.
2) Section.  This is probably the crux of it. As background, NACA 4-digit foils are commonly used for boat foils.  And they work fine as they tend to work well at low Reynolds Numbers, have high resistance to stall and have trailing-edge stall which tends to be “soft” and recoverable.  They were developed by NACA in the 1930’s as good, state-or-the-art foils for general aircraft design of the time, drawing on development work in the US, UK, France and Germany. However, they are not low-drag foils; the shape of the nose section triggers the transition from a laminar to turbulent boundary layer very close to the leading edge. This gives high resistance to stall but high frictional drag. Maybe early aircraft designers were more worried about stall at landing speeds than anything else? Later, in WW2 NACA produced laminar flow foils in the 63, 64 and 65 series to reduce drag on military aircraft flying at 400 to 500 mph. These foils are not generally suitable for boats as they are designed to work at high Reynolds Numbers and are very prone to leading-edge stall.
So, I started to play with JavaFoil to find a low drag section with high lift. I set the Reynolds Number at 400000, which corresponds to a 200 mm chord moving at 2 m/s thru fresh water, and is the lower limit of accuracy for JavaFoil,  anyway. What I found was that a 15% section with the standard NACA nose rad of 2.48%C and the point of max thickness between 35% and 37% back. This section gives
    Max Coefficient of Lift = 1.25
    Coefficient of Drag at Zero Lift = 0.0076
A standard 10% NACA foil would have the following, for comparison:
   Max Coefficient of Lift = 0.95 to 1.00
   Coefficient of Drag at Zero Lift = 0.011.
Hence game set and match. The only potential problem is that the JavaFoil results are calculated values rather than derived from tests and so may be utter bollocks. Who knows? We shall soon see! (This was written before the new design had been used. With use, I cannot tell about the drag reduction but the resistance to stall is very high.)
3) Area. My original rudder has an area of 0.166 sq m. Hence, its maximum steering force at any speed is proportional to the max lift coefficient times the area, i.e 1 x 0.166 = 0.166. This is called the Lift Area. Similarly, its Drag Area is 0.011 x 0.166 = 0.00183.
Now my new foil needs the same Lift Area, as far as I know. To get this with the higher lift coefficient means I only need an area of 0.166/1.25 = 0.133 sq m.
And if I have an area of 0.133 sq m., the new Drag Area becomes 0.0076 x 0.133 = 0.0010 sq m.
Hence, the new frictional drag should be only 55% of the old.
4) Trailing Edge. I can send references on this but any trailing edge thickness has a huge effect on form drag. It really is worth the effort to get this down as small as possible. You never see thick edges on aircraft; 3 mm is a DISASTER for a foil 25 mm thick. It will increase the form drag by 40%.
5) Plan Shape. Two things need to be done here, namely;
- have a shape which minimises the drag induced by load carrying
- have a shape which matches to distribution of local lift coefficient so that all of the area is working as a piece and coming up to stall at the same time.
This subject gets very complex, much more than the equivalent problem for a centreboard. This is because the rudder is a surface-piercing foil and how it behaves is very dependent on the Froude Number of the chord at the surface. In the 1990’s considerable work was done by US research organisations under the PACT scheme for collaboration in the design of one of the America’s Cup boats. I have some of the papers published on this subject, and some of Tom Speer’s work. To cut a long story short, the best rudder profile for normal speeds is full ellipse with the top just touching the surface (this is an elliptical distribution of chord and not necessarily an elliptical shape), i.e. nothing protruding above the surface. Clearly, this is not practical, but the effort should be to get as close as possible to this ideal, and have the centroid of the area about 50% down. The result is a somewhat "pot-bellied" foil with the maximum chord and maximum thickness about half way down the immersed depth."

29/03/2015 16:46:01
Gareth Griffiths NHRC

Thank you so much.

That post is one for this forums archives for sure. Great to read research and theory behind design. Numbers and formula proving theories rather than just speculative meandering.

What you wrote concurs with much of the research that I have done.

To take this forward, chatting with a few close friends who are keen sailors as well as experts in the laminating rudders keels and other pocket knife underwater appendages I have concluded that if I build a CNC mould for a reasonably conservative high aspect centreboard it would be good enough for me to experiment more with blade shapes.

I intend to make a cassett stock so that I can shape HDF carbon rudder blades to trial a few ideas.

Most rudder shapes will be around an 11% chord/width ratio similar to an NACA 0011 shape and therefor will all fit one cassett stock..

If I make a good cassett stock mould then I would be keen to see other Merlin sailors try a few new shapes too. The wider you cast seeds the more precise your results should be.

Thanks again for your musings.

How were the final results?

29/03/2015 18:38:25
Peter Mason

Dear Gareth,

Sounds a good project. If you want to bounce some thoughts off me in the future, my email address is [email protected]

29/03/2015 20:30:35
Gareth Griffiths NHRC
Much appreciated Peter thank you.

30/03/2015 09:39:00
David Greening
the best rudder profile for normal speeds is full 
ellipse with the top just touching the surface (this is an elliptical distribution of chord and not necessarily an elliptical shape), i.e. nothing protruding above
I seem to remember from Aerohydro lectures that there was a guy called Betz who said that it was ok to cut off the tip of an elliptical foil (distribution) with a negligible loss of lift or increase in induced drag (from tip vortices).  
More Hurricane or Mustang than Spitfire.  
This offers some practical advantages because a squared off tip will be easier to maintain and reducced draft may allow you to be inside boat at the Ferry landing!
I did this on a Twelve rudder and banked the gains by reducing the rudder area, I never felt any particular trade off with control.  I have some Autocad drawings for that foil, but had followed much of the same process as Peter.
A good read is Pierre Gutelle's Design of Sailing Yachts which has some good stuff on foils, which is relevant to dinghies.
One thing that Dougal might consider for the 2nd volume is the amount of innovation and experimentation in systems and foils that have gone on in Merlins, and in this case I think of the Haynes' who I think had several small rudders with fences or bulbs on the tips.

30/03/2015 12:01:49
Chris Martin
Well that's my brains blown for the week.....

30/03/2015 16:22:04
Peter Mason

Just to go back to Gareth's model for deriving the area of the centreboard, I think this needs to sit in a wider model of how the total boat behaves going to windward. This should include weight of the crew and how far they are sitting out, apparent wind speed, aero loads on the sails, the corresponding speed thru the water, lift/drag ratios of the sails, all the hydrodynamic drag terms, and so on. All the forces must be in equilibrium and it will take a bit of fiddling with the numbers to get to that position. Then you can investigate, and quantify, the effect on the whole system of changing one component. You could, for example, look at the effect doubling the centreboard area. (The drag goes up X%, speed comes down by X/2% but the leeway is halved.) Or you could look at putting much more lateral load on the rudder. (The leeway would come down but the induced drag would rocket.)

I have such a model for my (lovely) Jack Holt Nellie, circa 1958. This boat is heavily modified from its original form and, for me, is just continuing its racing life as a river special in a development class. 
David talked about cropping the tips off elliptical plan forms. I am sure he is right; the requirement for elliptical distribution of load to minimise induced drag comes from some theoretical mathematical models. Hoerner gives empirical evidence that a simple rectangular plan form foil has lower induced drag than this theoretical minimum. So someone might be wrong!
That said, I think we should not rush to saw the tips off foils but we should be cautious about how a boat's foil tip is formed because such a foil is not always carrying side load, unlike an aircraft wing. Hence having a shape which alleviates local form drag and is not upset by changes in pitch is probably worth thinking about.


30/03/2015 16:34:36
Edward 3340

When I wanted a new shorter rudder I just drew a pretty elliptical shape that the weed would shed from, laminated up some Sitka Spruce  and got the plane out - seems to work fine. With some simple reference lines down the length of the blade you get a pretty fair aerofoil section. If you insert a carbon strip in to the leading and trailing edge when you laminate together it gives you a nice black line to work to - unlike a pencil line it doesn't disappear when you sand the finished blade and allows you to create a rally fine and durable trailing edge.


30/03/2015 18:17:30
Gareth Griffiths NHRC
Cheers Edward

I am going to use foam instead of wood but I agree.

31/03/2015 14:21:33
Andrew Mills
There is some very technical stuff going on here!  We're not Laser sailors, are we!!  However I bet once we all actually start sailing again this off-season foil gazing will drop off the forum

31/03/2015 14:30:38
Gareth - I am planning to use foam for the next one with a sheet of carbon cut to the profile of the blade running down  the middle and with the foam  glued on to either side of it. This way you get the same reference line that I got from the inserts. With a vacuum bagged carbon skin the result should be feather weight by comparison with the glass covered spruce. 

30/01/2017 22:58:53
Gareth Griffiths NHRC

Did you progress with your carbon rudder?


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