Does anyone have details of ideal merlin rudder sections please?

I'd be interested in the ideal NACA number, aspect ratio and chord length.

Thanks in advance.

Jon

have you used the forum search facility (top left, under logo) - there has been much discussion before on this topic (though perhaps not all useful!).

I don't have info on the correct sections but there is some airfoil plotting packages such as compufoil or profoil which have libraries full of symmetrical sections

Plotting is not a problem,  I'm just after the best section code to use for a Merlin.

Unfortunately the terms "NAC", "NACA" and "Foil Section" don't bring up anything useful.

I suspect that this information may well be (quite reasonably so) regarded as a bit of a trade secret.

try these 2 links. i did clear out all discussions over 2 years old, so perhaps the ones i am thinking of have gone now.

http://www.merlinrocket.co.uk/forum/main/topic.asp?topic=1368

http://www.merlinrocket.co.uk/forum/main/topic.asp?topic=1485

And of course...

C J Marchaj of Southampton University (Probably long retired.) wrote a definative book on the subject, precis: leading edge 2mm dia, trailing edge no more than 2mm wide and razor sharp edges (same applies to the transom.) thickest but 1/3-1/2 way back very smooth. not surprisingly he's more detailed! Should be able to get book from a good library search under author.

Thanks Mr Geek!

I take it you mean "Aero-hydrodynamics of sailing" which I have found in my university library.  I will be grabbing that and Mr Bethwaite's Tome at the first opportunity.

Jon, 
There are two books by Marchaj, and you want the later one-published about '79 I think. It's seriously heavy going unless applied maths is your strong suit, but worth it. If more people read and understood it, there'd be a helluva lot less time and money wasted in pointless "revolutionary" developments which will never fly, and a lot less b----x spouted on forums like this - not my words, but those of John Flewitt, Marchaj's then boss and head of the Woolfson Labs at the time!
Rog

Bob Fishers otherwise simplistic book "Crewing Dinghies and Small Keelboats" has some stuff, quoting NACA numbers etc., since he also claims to be able to work wood to 3,000ths of an inch.....I leave you to draw conclusions (The late and much missed Rod'McAlpine-Downie who was the thinking mans yachtsman of 1957-1974(ish.) always said that even in a C Class Cat' they's never noticed any difference whatever they did as long as it was smooth rounded and sharp at the front and tapered towards the back!)Bob's book in its third chapter has the stuff, there are also some pictures of Merlin Sailors of the period (Late 60's.).

Just to rub it in, just back from a sail in 30degrees sun and force 4! Using the net and browsing the excellent but somewhat dusty shelves of the Royal Bombay Yacht Club Library (There is even a Merlin Rocket Book; I think I left it 15 years ago!) and this might help further especially for you fellow mathematicians who understand duplexed quadric planes!
Manfred Currey (Original designer of the Porshe/VW Beetle cat shape writer of good yotty tomes.) Circa 1936 writes that he recommends Goetingen 444.
The Bob Fisher NAC no's are: 0006 & 0009.

Studying this at uni at moment, but in terms of big Yachts, racing yachts are mainly using the 66 series sections for keels because they have a more promenade drag bucket (see Theory of Airfoil Sections by Abbott and von Doenhoff).  But a more comment section is the 63 series as it has a wide drag bucked, compared to the 66 which is thin.  Drag bucket - see plot of Cd/Cl.  Also average thickness/ chord ratio for a racing yacht is 8-9%.  All this might be completely irrelevant to dinghy, but i don�t see any reason why it doesn�t apply.

Sorry that was .. a more common section is 63 series....

Ancient Geek, you should have grabbed that MR book back, and got it on ebay!

Jon,
Claire L is right, but as with most things, read the small print. The low drag bucket is only "available" IF, and only IF, the surface finish on the foil is immaculate, i.e. polished to a mirror finish, and this means just what it says, not "quite smooth", not "OKish",not even "matt but smooth" but as in "see your reflection in it" smooth (yes, really!). Bethwaite has something to say on this point as well, if I recall right.

Oh no Sahib, goodness gracious me no, the LIbrary is magnificent, the esteemed Royal (now a Republic!) Bombay (now Mumbai!) Yacht Club (Still!) is truly a thing to be encouraged and preserved for instance a post sail bath is in a cubicle with tabs you hang over the door more hot water  or more cold water and a bucket is poured over the partition within seconds, fresh lime soda or cold beer appears through a wee hatch at your elbow and bizarre this overhead Sky TV!!! Or BBC W
oprld Service TV. (Indian Mastermind ("What is your name"? - "Pass"!) a cult viewing.

Four digit NACA sections are considered to be pretty good for rudders, i.e. 0006, 0010 and so on.  The last two digits are the thickness to chord ratio %'age.  The thinner the foil the more prone to stalling, the thicker the more drag.  You could measure some successfull foils to get the thickness that seems to work in Merlins, usually the quoted value is the average t/c ratio, so take measurements at a number of heights (or depths!).

If you can't find the offsets Larsson and Eliasson's Principles of Yacht Design has offsets for a 0010 section, you can adjust the offsets by a scale factor.

One advantage of 0000 sections over bucket sections is that the are less critical to surface finish and can be realistically made by hand using templates.  The bucket sections really need to be machined to acheive acceptable tollerances.

Regards all,

David
N3461

P.S A website with loads of goog stuff on this is www.boatdesign.net.

NACA 0009 rings a bell from somewhere....

www.philsfoils.com gives interesting info on foil construction and design.

Try a 64A010 section for your centre board and a 0010 for your rudder section. Abbot and Von Doenhoff 'Theory of Wing Sections' will give you all the foil characteristics. The NACA catergorise foil shapes with letters and numbers;
e.g 64A010 translates as 6~series number, 4~location of minimum pressure aft of the leading edge, A~ indicates that from 80 % aft of the leading edge the curve becomes a straight line, 0~indicates a symetrical foil, 10~ thickness in percentage of chord. 
For a 64 foil the maximum thickness is 40 % of chord.

The 0010 foil is explained as 0~a symetrical foil, 0~location of asymmetry {zero for a symetric foil}, 10~ thickness in % chord.

If you read David Winders article in the library you can understand why foils cost so much.

Thickness is 40% of the chord?! So for a chord length of 15 inches,it's 6 inches thick. Shome mishtake, shurely?

That or boasting again.

And presumably no need for any extension!!

I think he means the thickest part (DOH) id 40% of the way back!!! Or am I assuming something here?

Hmmm….
64A010 where the last two digits give the thickness ratio in % of the chord. In the case of the 64 series, as Ancient Geek says, the maximum thickness occurs at 40% distance along the chord from the leading edge.

The important bit of information is the minimum pressure and where this is located aft of the leading edge. This is why the four digit series is better suited to being a rudder as the location of minimum pressure is close to the leading edge, which allows it a greater lift coefficient at high angles of attack. The down side is you get increased drag.

So when your agony aunt tells you size and shape don’t matter, she’s probably a board sailor. Believe me if you want to satisfy the love of your life you need to make sure you pick the right foil series to suit the application.