Long cruising keel foil shapes

Stephen Ditmore
Are there certain foil shapes that you would prefer for long (three quarter) keel on a 56 foot ketch ? Or would you roll your own? If so to e\what sort of formula?

Larsson warns the yacht designer to on no account design your own foil section unless you are an experienced hydrodynamicist ! but probably refers to modern fins.


Jehardiman
Thanks for your posts. I shall have a look into those other foil shapes. Do you have a good text on the subject or is this from personal experience ? My engineering hydrodynamics at university was very mathematical and completely useless.

I may test models of a few of these long keels and report my findings later on.

Anyone else with an opinion please venture it.

 

Jehardiman

JEHARDIMAN

Hi I hope I am not rude asking you directly for advice. I cannot find any local 'experts' who will undertake the deign I need. So in my desperation I am here:

I am a retired school teacher who can weld !

I want a full keel on my home built 60 foot steel sailboat. to replace my shorter one cause the damping is too poor and I want less draught. My last quite similar but smaller boat had a full keel this one has a 10 foot keel on a 50 foot waterline with a rudder skeg and I find (my wife also finds) it a poorer companion to my last full keel.
We also majorly bent the rudder skeg so it needs replacing anyway .

So I too looked up foils and tried to use various NACA series but the keel is too long so gets very wide and cant fit properly into existing hull .

So if your ratio of thickness to cord length is to be somewhere in the 0.6 to 0.7 area what section do you think I should use.


Thanks and sorry to hassle you

John McFarlane
New Zealand

 

Herrishof had a rule of thumb that IF the draft of the boat was at least 1/7 of the LWL the long low AR keel would work.

Your 50 ft LWL would still need 7ft draft to go to windward well.

IF most of your windward work is with diesel assist , you could have less draft .

But the overall stability might suffer, depending on ballast ratio.

FAST FRED

 

John McFarlane, as you may know, changing the keel on a sailboat is not as easy as gas axing one off and welding a new one on. As FAST FRED points out, there are many other considerations. Lets take this off line with a private message/e-mail and we can see if we have all the info for the big questions I may need to ask before we get down in the weeds with something, that in the bigger scheme of things, that is as trivial as section shape.

 

There really isn't a good text because for most applications section shape really isn't that important compared to other things like aspect ratio, pitch monent of inertia, and wave orbitals. Principles of Naval Architecture or Basic Ship Theroy, or Marchaj just about cover what you need to know. Just look at some of the basic research in Hoerner and you soon realize two things. 1) symetric foils are very simple; and 2) airfoils are too thin aft to be good hydrofoils (i.e. NACA 00xx foils have the same lift and less drag at low AOA when towed in a tank backwards ). From there it is just a matter of forcing the foil to fit your needs. The marine environment is much less pure than what aerodynamisists deal with, and water is much more massy and viscous. It is much better to be conserative in your estimates than optimistic. Remember, you are paid by how much people believe your opinions, not what the facts are, and it is better to underestimate performance by 10% than to overestimate it by 1%. The former will have the owners buying you drinks and the latter will land you in court!

As a aside, one of my touchstones for how I rate a hydrodynamisists is to find out how they feel about circulation theroy..... .

 

JEHARDIMAN

I tried a hydrodynamicist first off. He recommended that I try at all costs to widen the foil to around 12% ! To my consternation I found he knew very little of marine architecture. He had his head in the clouds with his aeroplanes. ( No slur on hydrodynamicsts intended ).

I like (construction wise) the simple flat leading edge (LE) found on many older designs. The stem is flat 50mm wide and this flat trails down into the keel LE which widens from 50mm at the top to maybe 200mm at the bottom the keel foil shape. Some of these older keels are quite full aft. I have attached (I hope) a picture of a section of LEXIA's keel (mid span) she was a typical able and successfull performer with such a foil shape (full keel).

I can't help thinking the vortex shedding off the sharp LE corner wouldn't be detrimental to some degree, and that at least a parabolic LE would help. But these flat LE's are very easy to construct in sheet material.

I too am interested in what foil section you would recommend for Johns keel at 6-7%. Would you still use the DTMB EPH ? or the first foil in your picture (This offers a higher volume keel).

Aside: Where do I find the formula to construct a DTMB EPH? I havn't met that one before.

Thanks

JOHN MACFARLANE
contact me if you want (mjeng1@supernerd.com.au) I have various freinds in NZ who could help you engineer the mod if you need that sort of help (once like me you've solved the foil shape issue). Your problem is that many yacht designers are reluctant to design full keels for the reasons apparent in these posts........lack of published guidelines.

 

In reply

J Hardiman, Mike Johns, Fast Fred

Thanks .
I am sorry to cause so much concern.

I should have said I have a local boat engineer who can calculate the required hydrostatics ballast, frame extensions and keel location etc (to survey). He was not happy to design the foil shape ..... I can see why now.

An old wooden boatbuilder here has just said to me that the cross section shape can be widely varied for pretty much the same result with a long keel and you seem to be saying the same thing. So if I go for a thickness of 6 or 7 percent something like the classic flat fronted keel that Mike posted would be really easy to fabricate ( the boatright likes those). Otherwise thwe first of the 3 shapes posted by JH shouldn't be ard to build either, with a bit more information .

I was considering a swept back front edge and a keel amidships of 1 meter depth minimum with a length that fairs nicely into the hull but around 8.5 meters long and 1.2 meter deep so it is a low aspect. This will give around 2.2 meters total draft.

The boat is estimated at 38 tonnes displacement and is ketch rigged with a large mizzen, is a narrowish hull round bilged but the bottom plate is pretty well undeveloped . She is stiff and will cope with the extra wetted friction in low wind situations. The extra bouyancy lower down may well improve her by raising the floatation and reducing the jerkiness due to the low down weight, but the extra keel area will also do this . But as I said I have an adviser, and this is the information I have been told.

So I was just looking for a basic run of the mill shape that was easy to generate and build, that had a bit of volume in the keel to add low bouyancy and give space for extra tanks. I am older now and sail easy courses and use engies to make life easier.

So any suggestions would be welcome and will be better than I would come up with along with my old wooden boatright. I am not looking for the best option just a sensible opinion. Please.


John McFarlane
New Zealand

I access the web through an internet access centre so I am not 'online' properly yet .

 

Part of the problem with designing cross section shapes for low aspect ratio keels is the whole idea lacks validity.

With a high aspect ratio surface, say 6 or greater, one can break the problem of analyzing the 3D flow into two 2D problems: the cross section shape in a plane perpendicular to the long axis of the surface, and a plane that is perpendicular to the flow but located some distance behind the trailing edge (called the Treftz plane). You use the section design to work out the flow in the boundary layer and minimize the skin fricton drag. You use the Treftz plane to calculate the drag due to lift and to optimize the planform shape of the surface. When you combine the drag calculated in these two planes, you get a reasonably accurate result for the whole 3D surface.

But with a low aspect ratio surface, that's not the case. It's really a 3D flow problem that requires you to take the whole shape and flowfield into account at once. Fortunately, the section shape isn't a really big player in this regime, since it's dominated by separated flows and shed vortices. The way you shape the vertical profile is probably going to have as big an effect as the cross section shape.

The square leading edge violates my aesthetic sense, though, even if it has proven to be acceptable in traditional designs. I'd be tempted to shape it into some semblance of a NACA 4-digit or 6-series section's leading edge shape.

 

It's true that the square leading edge is real easy to fabricate, but using a round tube as the leading edge would be just as easy (if it's a steel keel). And it would give a much better section shape.

 

John and all

So from the above posts it would be fair to say for low aspect keels foil that section counts for little. The foil section is more aptly designed with other factors in mind ie. bouyancy, ballast while keeping its max width at < 1/6 of the WL beam.

Importantly there is no benefit from using the foil shapes used for high aspect fins, and the section used on a long keel should be fuller aft than the fin keel foil.
Something like an Eppler foil with max thickness at 50%.
The Performance Measurement Baseline section (PMB ) posted by Jehardiman scaled to around 7% (width to length) would probably be a good contender, easy to generate for the layman who doesn't have access to foil offsets or generating software.

However the side elevation is important the Leading edge (LE) should be swept back on a low aspect keel (Marchaj). The TE is not so important and will be dictated by rudder support and fairing to the Prop.

Foil LE often used to be a square section but is probably better if an elliptical or rounded section is used. But this is not to say that a sqaured LE is actually detrimental ( see ref to Hoerner in prior posts).

There are other factors that need considering; Whether the keel base slopes aft, the required turning rate, Matching rudder size (to say 10% of lateral plane for good control) and probably other things that don't come to mind right now.

 

Mike,

Sorry I haven't replied. Yes I am a designer. The section I used was essentially a 65 series or 00xx naca at about 6-7%(would need to check). I don't hold a lot of importance on the section shape apart from leading edge seperation effects mainly due to the large tip shedding effects on such a long keel. I should note that even though a 6-7% chord thickness is used, the keel still looked "Chunky" from ahead.

Construction wise the keel was a series of plasma cut (cnc) frames and steel plate welded to form a box. A solid round bar was used for the leading ege and a longtl bhd for te trailing edge. It makes sense to use heavy steel plates throughout as steel is quite a bit cheaper than lead. We used a flat bottom plate also for simplicity. If you want some further help give me a yell.

Brett
(Where is Oz are you - I'm qld)

 

MikeJohns; Sorry I didn't get back sooner but life intervened.

1) the section you posted on 10-22 is about a 16:1 foil, and like I posted earlier a foil of that thickness/cord, section shape really dosn't matter. It would be better to have an ellipitical leading edge or to at least roundoff the corners with a 5-10% of face width radius. Keep the trailing edge square.

2) As far as section shape fro a thin keel, here is how I would construct one without parallel midbody (PMB). I would place my maximum thickness at 45% of the cord. I would then place a 3:1 to 5:1 ellipse (to place my CB correctly) for the leading edge. Then strike an arc from the maximum thickness tangent to the surface of the ellipse. That is the forebody, for construction you can cut the LE square, but it should be less than ~2/3 of the total thickness. For the afterbody, cut the trailing edge square at 2 to 10% of thickness. Then strike an arc from the maximum thickness to the trailing edge. See the enclosed figure.

3) Sorry Mike, PMB stands for parallel midbody , not "Performance Measurement Baseline". Sometimes you need additional volume for tankage or ballast. the best way to do this is to add parallel midbody. I wouldn't add over 40% of the cord in parallel midbody. To design this type of section, subtract the parallel midbody length from the cord, this gives you an "effective cord". The foil is designed around the thickness to effective cord length (say we have a 16:1 foil and put in 40% PMB, then we would an effective cord length of 9.6 (16-[16*0.4]) or 9.6:1 foil . After designing this foil we insert the PMB into it at the maximum thickness (see my previous foil post).

 

Everyone thanks for your posts.

Jehardiman
Thanks all is clear.

As for acronyms we could use your PMB as my PMB But yes I now remember I have seen this before.

 

Keel

I don't know how to start a "Post" so I choose you (most senior member)
to find info on keels, starting from scratch/the most basic.

o.What is/are the function(s) of a keel
o.What are the considerations for incorporating a keel
o.Where should the keel be placed (I know it should be aft of the mast base,
and in the center of the beam/width, and .........)
o.To what factors is the placement related and what are the
approximate relations (To mast base, to center of force on sail,
to hight of mast, to crossection of draft/waterline, to ...............)
o.What determines the size, width, length, thickness (related to sail area
and/or beam?)
o.What determines the shape (parallel slab, triangle, ...... ,ease of tacking)


Help, anyone!
Thank you

 

Hello Gudway,

There are books written on the subject and still there are divided opinions on keel design. Ever noticed how the teams hide their keels in the America's Cup?

In a nutshell;

1. The primary function of keels, centreboards and their variants is to limit leeway, and as such they behave as foils. (producing lift). The angles of attack may varies widely in a seaway, sometimes leading to stalling.

Because keels operate at a low angle of attack, a high aspect ratio generally is the most efficient. The leading edge is the most profitable lift producing area of the keel when the angle of attack is small, because under this flow condition the differential in pressure matters most. High aspect ratio implies high lift and a high lift-drag ratio.

2. Thick sections having a parabolic entry with maximum fullness relatively far forward are most resistant to stalling. Laminar flow sections produce low drag under certain conditions

Ideally the leading edge of a foil should be of a section that enables the water to change direction gradually. Parabolic sections do this because the curvature changes progressively. Sharp entries produce less drag under steady conditions with zero angle of attack, but the flow breaks down rapidly in the conditions to which a keel is subjected.
NLF (natural laminar flow) sections have low drag because laminar flow causes substantially less drag than turbulent flow. NACA foils are favorites with many designers. Eg. NACA 63(2) - 015 meaning;
6 - indicates series
2 (subscript) just to complicate things for you and me - actually indicative of the low drag bucket! - useless to use yacht designers
3 - maximum velocity occurs 30% aft
0 - there is no camber (ie.symmetrical section)
15 - the thickness-cord lenght ratio

3. Slight sweepback plus an elliptical planform produce minimal end losses and low induced drag.
The shape of the bottom of the keel/foil affects lift and induced drag.

The mast is actually place forward of the keel. This is called lead and varies with different sail plans and hull designs. Simply put, lead is a percentage
of waterline length, and a combination of factors, CE (center of effort) of sail plan and cetroid of hull lateral plane.
A little weatherhelm is desirible feature.

The keel is placement (and mass) is dictated by weight, LCG, VCG, LCB, CF, CB, etc calculations.

4 - 6% of sail area is a good starting point to determine a fin keel size.

Hope this may help you in a way

Go well

Wynand Nortje