Foil design - basic design for home building

Hello everybody,

our homebuilding project have now reached the point where foils are becoming a real concern. Decks are being installed and the other small bits are being made or sourced.

I have researched costs of purchasing foils, but these are so expensive that I have to evaluate homebuilding before comitting to purchasing. I am sure commercially available foils are superior in quality and handling but I need to evaluate homebuilding.

Before deciding on materials and methods, I need a basic foil design for rudders and daggerboards.

The boat will be sailed in 2-up mode mostly and dominantly in 1 to 8 m/s windspeed. We assume normal crew weight.

As a rule of thumb the area of the daggerboards should be 2% of jib+mainsail according to some sources on the internet. Aspect ratio of 4 to 4.5 is considered "normal".

For daggerboards NACA0007 is a conservative choice. For rudders NACA0012 is a conservative choice.

Planform would be parallel leading and trailing egdes with a small diameter round-off at the leading egde bottom.


Have I got this mostly correct? Any comments and recommendations?

Rolf,

Excellent question. Not much information out there. I would like to know what the area on an existing F16, F18 and N20 board is, as well as the basic chord length.

I need to pull up my numbers, but if I recall correctly a few things came to light in my analysis:

1) The NACA 6 series laminar foils offered very very little performance over the NACA 0012 or 0009. All is lost if the foils gets gummed up in any way.
2) Need to check the Eppler foils again, they were better in higher lift conditions than the NACA series but the NACA foils have lower drag in low to modest lift coefficient regime (sub Cl=0.5).

Here is a white paper from Tom Speers on his H105 foil, but I don't think the .dat file is available: http://www.foils.org/hysecdes.pdf

More info from Tom: http://www.boatdesign.net/forums/sailboats/best-foil-shape-rn-250-000-750-000-a-1577.html

I would use a NACA0009 on the daggerboards and NACA0012 on the rudders if I were to build tomorrow.

Rolf
A little different direction... but I had a thread going on over at Sailing Anarchy that may overlap a little on the home building aspect.
http://forums.sailinganarchy.com/index.php?showtopic=129899
I was doing a brain storm session with the guys over there about home building curved dagger boards, there might be some golden nuggets that you could use on straight boards as well...or at least construction techniques....I put the same question here and only got one response...but I am going on 87 posts over there. Maybe something to comb thru if you have the time....

Best Regards,
Robert

Rolf,

Aspect ratio of 4 is pretty low for a daggerboard. Some dimensions from F18 high AR daggerboards: http://www.reverie.ltd.uk/product_list.php?group=M%20FOIL

I'd be skeptical about going lower than AR=10.

AR=10!? That sounds like the AR for a glider from the 1930s

Laminar foils are not worth it as the theoretical "drag bucket" is so hard to utilize. Your analysis is the same as mine there

I'll not consider construction methods before I know the design

You should be sure what you understand as area and aspect ratio. There is a exposed area (which stick out of the hull) and the complete area. E.g. 0.2 sqm area and 1 m span or 0.1 sqm and 0.5m span.
Once you fixed your average chord, I know the Reynoldsnumber. I can than look for a good foil for this Reynoldsnumber.

Naca 0007 is very thin, which means less maximum lift. In other words with a thicker section you can have a smaller chord, hence less drag while still having the same lift and hence manouvreability.

In general airfoils are not to bad, but there are
a) much more advanced sections the NACA four digits and
b) dedicated hydrofoils offer more potenial

However you should be sure to achieve accurate surface and shape quality with your building process. If not you can take any tear drop shaped something.

Personaly I would take a hard wood plank, let it CNC machine and cover it with epoxy, then glass fibre+expoxy.

Cheers,

Klaus

just checked: the boards of my Bimare are 1.2m long, the wetted length or span is 0.7m and the chord is 0.25m, hence the in water area is 0.175 sqm per board (it feels ok for 2 up sailing, but a bit too much for single handing, however I have only 14sqm sail area upwind)
Rudders have a submerged span of 0.35m and an average chord of about 0.18m.

Richard Roake (Retired Geek) suggested Eppler E836 scaled to 12% thickness ratio to me a while ago.

Target a chord around 150mm (anything smaller is to hard to build.

Span wise what did Phil recommend? Don't go any less exposed area than his original design.

0.175 sq.m (Klaus's Number) sounds about right for area so if you were 150mm that would give you an exposed length of 1166mm and a board length around 1700.

Hunt around and see if you can find someone upgrading their F18 foils.

Hi Klaus,

I always think about wetted surface when the subject is daggerboards and rudders, so AR should be discussed as wetted AR. Thanks for pointing out the difference in how the term can be used

So first decide on the AR, then the chord and everything falls into the "best" slot afterwards by analysing the numbers?
I suppose there are physical limitations and non-achievable theorethical gains that also go into the analysis.


Sooo.. How do one decide on what AR to go for and the chord?

Thanks Chris,

was the Eppler E863 suggested both for the daggers and rudders?

150mm is noted! Great to have some numbers!

I am in catamaran no-mans land but are looking at websites to see what is in the market.

Not for a rudder for the light wind conditions you stated above, as it has low cLmax. It is optimised for laminar flow at low cL and maybe cavitation resistance.
The effect of lift and cavitation is very noticeable, as the boards have to provide all of the lateral resistance. On the other side the profile drag of board and rudder are only a minor part of the total drag build-up. Saving 10% of a little bit isn't worth to loose 10..20% of all your lateral force.
It is probably better to maximise the section for lift and reduce the total board area, if you dare to.
Considering the aspect ratio or better span, you should consider the maximum depth you want to have. Long boards prevent the boom from swinging free when they are up and you have to manouvre at the beach, confined water, etc. From a pure performance point you want long narrow boards, but this is limited to the minimum chord and the ever increasing bending moment (longer board, which is thiner because of low chord,if you kee the area constant) which finally leads to breakage. Selecting a thicker section with slightly higher drag helps to go for higher span as it is more resistant to the bending moment. I think a NACA 0012 is overall better than the Eppler section for a cat. It could be modified with a more forward maximum thickness position and a pointier nose for your light wind specification.

Cheers,

Klaus

Here is a comparison of most of the sections mentioned so far on a board shape that is similar to the curved DNA boards, but rolled flat. The AR is around 12.5, chord is 150mm for the first 788mm and then tapers to a 33mm chord at 895mm of span. This is all done in the program XFLR5 (you can find it on SourceForge, its free) and the model is the underwater portion of the boards joined at the root end. The drag numbers are not real in that they don't account for the hull etc, but it does give a reasonable approximation of the likely differences between the various sections against each other.
As Chris noted, the 12% version of the E836 is a good choice and the Naca 0012 is less than average at low leeway angles, and not bad at high angles, but you shouldn't be sailing with that much leeway.
As for areas, the DNA boards are about 1.836% of sail area (btw, this is not the right way to size boards unless you have no other options) which I consider large.
Boards are trending to higher AR's, 11 to 12.5 are relatively common now although mine range from 14.4 to 15.2
In the pictures with the boards themselves in it, the solid purple line is the transition line on the weather side of the board, the dashed purple line is the same on the leeward side, the magenta area behind the board is viscous drag, the orange is induced drag, the green is lift distribution and the green arrow is the center of lift. The rest should be self evident.
Cheers
Richard

more pics

more pics

more pics

oops...I forgot to mention that you can use the E836 for both boards and rudders with no problems at all. On the LR cats our rudders are just chopped down board sections and the section we use has a lot of similar characteristics to the E836

And this means that LR rudders are too large for its job. A NACA 0012 has a cLmax of about 1.2 at low Reynolds numbers and the Eppler section below 1. Hence the rudders could be 17% smaller and still give the same manouvreability. Apart from the reduced friction and profile drag of the reduced rudders you save some weight at the stern of the boat.
Just hijecked the plot from Tom Speer from his posting on boatdesign.net to show this important effect.



Looking at your diagrams I would think, that something with your calculation is wrong too. You should check, if you have selected the right parameters (Reynoldsnumber, Ncrit, etc.) for the calculation. Again Tom Speer's plot may give you some hints.

Cheers,

Klaus

Thanks everybody! I now heave a lot of information to ponder and decide on

Some of the graphs here go above my head I am afraid. I want to look at this some more and think about all the arguments.

From a practical point of view I like the simplicity of using the same mould for both rudders and daggerboards.
"Traditional wisdom" or knowlegde says that rudders and foils are very different as the AoA on rudders will be more variable and greater. Thus a different foil is "optimal" for each application. This discussion probably needs some tank testing in a lab to determine the differences?

PS: Last night we built chainplates in carbon for the boat. Will be interesting to see how they come out after cooking.