Long cruising keel foil shapes

I am ruminating on the foil design of a fixed cruising three quarter keel.

The design texts are full of advice for fin keel foil shapes but very shy when it comes to longer keels.

Most older vessel designers made no attempt to give any foil shape to a full keel it was just a parallel sided projection down from the garboards faired in as woodwork allowed with the LE shaped to match the stem. It was there to add wetted surface area and damping with no thought to lift/drag relationships.

Three quarter keels take the keel LE back from the bow where it can have a different LE to the stem. Many modern designers with these keel designs don?t adopt a proper hydrodynamic foil but reproduce classic keels and rudders with little thought to the hydrodynamics.

If you want a good lift/drag ratio then the keel is going to be thick in section
For a swept back leading edge with a mid span length of 25 feet (7.62 m) and a 8 % average foil gives a max thickness of two ft (610 mm) at the maximum width (this is for a 45 ft (13.7 m) LWL and a NACA 00 foil) .

If the average goes up to a more desirable 10% then the max thickness is 2 ½ feet or 762mm.

Problem is you end up with a wide and large volume keel this has to be treated as part of the hull for the hydrodynamics of the design, curve of areas etc and the hull design modified to suit.

However it increases frontal area slightly and hence direct downwind form drag. But in a wide keel the ballast and tankage sit lower allowing better sail carrying (round and round we go) Also as soon as there is any leeway it significantly decreases form drag and increases lift ie. when sailing.

Gives a broad base for the vessel to sit on
Easy to access much of the inside of the keel during construction.

So do you take a modern approach or do you bow out to tradition and use a 6% foil so it looks more trad on the lines drawing ?

The engineer and foil specialists say go thick, the artist winces.

Opinions prejudices and facts all welcome.

 

Mike,

I have some experience with the design of a long keel to a large cruising yacht. Keel itself was around 10m long and just under 700mm at its widest making the thickness ratio under 7%.

I too felt that 10% was far too large and ruduced it keeping in mind my desired ballast requirement.

As far as windward performance goes the proof is in the pudding and the boat hasn't been launched yet. Besides, gentlemen sailors don't go to windward...

Brett

 

A boat with a fancy keel will sail on the keel to windward , just dragging the hull along.

GREAT if your racing and want to beat someqone to windward with a small angle

The older boats used the keel as a fence to help the hull work to windward.

This frequently proves faster as the wetted surface will be lower , but you will point a few deg lower.

In the ocean with the need to generate enough HP to climb large waves , most boats wont be pointing as they would in flat water, so the simple wide keel is prefered.

The ability to sit on her own keel , along side a pier or dock , will allow free bottom cleaning as needed , so your transit times will be faster than the "modern" boat that can only haul on rare $$$$$$ occasions .

A porabola on the leading edge of a flat keel is all thats needed for low head resistance , and the flat works well as a foil , just has smaller working high lift angles than curved.

Sail Da Boat Mon! and you can feel it!

FAST FRED

 

A quick look at symetric foils of t/c<~.1 will show that section shape really isn't a player (as long as there is some LE and TE fairing). By the time you reach a t/c =.05 LE and TE fairing doesn't even matter, it could be a square-edged plate for all the flow cares (and square edges may improve lift at low aspect ratios, see the discussion on aspect ratio below). Indeed, having an "airfoil" shape is only really necessary to improve the stall angle and moderate the reduction in the L/D curve at stall (Cavitation effects may also be a concern in some cases).

What does matter is aspect ratio and root drag. Because of the relatively low aspect ratio you do not get as good a lift/drag curve, but you get a higher stall angle (provided you have a somewhat fair section) and may be able to get a greater planform area which offsets the lower lift/drag curve. On the other hand here are other second order effects such a squareing the edges which may improve the situation, see Hoerner, Fluid Dynamic Drag, Section 7.4. Therefor it is important to balance aspect ratio to the lift/drag curve to ensure appropriate angles of attack. Root drag can be extremely high depending on the foil thickness to beam ratio (t/B). For very thin (<.08 t/c) foils attached to very wide hulls (i.e. very small t/B) root drag can be zero or negative (think bilge keels on ships). For plank on edge hulls (t/B effectively 1) the point is moot and it is only wetted surface. For all other appendages you should make sure that t/c < .08 and t/B at the edge of the root radius is < .2 in my opinion. Of course a large root radius will decrease span and therefore aspect ratio so there is a trade off.

 

My Freedom 21 has a "cruising" keel like you describe: long and fat (and it seems to be a pretty NACA foil shapes). It doesn't work very well to windward and is slips sideways easily in light air and rough water. When I heel the boat the rear certerline shifts toward the outside and it effectively lifts the rear of the keel out of the water compared to the front. The more you heel the worse - think of water spilling off the bottom of the keel instead of going to work along it. Now is it strong and it has been pleasantly durable for trailering although it is not always easy to find a sufficiently long ramp.

Since trailering or low draft for that matter is not going to be an issue on a boat the size you are thinking about try something else like better hull shape, twin keels, wings anything really.

Asathor

 

I think hull/keel combinations of this type need to be treated as one entity and optimized to produce an elliptical lift profile. (I don't, however, believe that filletting the keel and hull together in section is helpful). To the extent I understand the answers of Fast Fred & Jehardiman, I agree with them.

At low speed after coming about (i.e. "tacking"), most boats are stalled. To get out of the stall they must first accellerate in the stalled condition. Newer types with a low ratio of lateral area to total wetted surface can have trouble doing this, and need thicker foils (as a percentage). I'm not convinced long keel vessels need such thick foils, and I think they're better off with thinner ones, esp. long keel boats with adequate draft. Airfoil shape remains important, though, as does a good surface near the leading edge.

 

Sounds like there is hardly enough weight in the keel (to EZ trailering) or the boat ,delightfully ,was set up to sail in light air.

Cruising boats will begin to reef at about 15deg of heel , as the speed loss is never worth the discomfort for the resting crew & cook.

Day sailing at 50deg of heal may be MACHO and feel exciting , but putting the boat back on her bottom will work far better to get to windward.

Each design compromise , changes the operating technique to get "best" performance.

FAST FRED

 

Thanks for the inputs, please keep them coming

So all the long keel designers advertising "now with NACA foil" are just indulging in a bit of marketing ?

It seems that the long keel (t/C < 0.1) can just be parrallel sided box with the LE faired to say a parabolic and the last third say brought together as the tail of a NACA foil.

However what do you do when t/C is exactly 0.1 is it worth incorporating a decent foil shape now? If it is then t/B may be worth increasing to get a better foil shape ???

Thats pretty thick, are there any published studies/formula/rules of thumb on the root drag for long keels?

I just measured a few "successful" cruising yachts from the days before the fin keel revolution they have projecting keels that project well below the garboard.

I give here the Keel thickness to Waterline beam ratios: By Vessel name (From the lines given in Hiscocks books)

Mercy Jane 6.3
Alamo 6.6
Wanderer 5.6
Rena 6.6
Omoo 6.3

Kochab 5.3
Restive 5.6

jehardiman's opinion was 5 (a ratio of 0.2) if this is ballpark then the keel can look excessively thick but still be within the desirable root drag figure ???

These last two have a reasonable foil section looks a bit like a Naca 00 and are 3/4 keels the others are sectioned in various ways but the stem shape dictates the LE shape and the balance is faired to the designers whim.
I would like find out if Kochab and Restive were tank tested (not much hope).

I may have to tank test some of these options, I was hoping not to go to that effort for a large wetted SA cruising design.

brett (Aust) I am interested in what you are building.....any more details?

 

All Marketing? Not quite.

As far as sailing performance is concerned, a NACA 00xx section in a long keel will have little impact indeed (as compared to classical straight or rule-of-thumb keels).

For motoring, however, a low drag section such as the a NACA is certainly beneficial in terms of fuel consumption at any given displacement speed.

Long keels in general are all about control and damping, not lift. If you intend to race in nice or moderate conditions, then, and only then, does a fancy fin keel make sense. Under every other circumstance, they don't.

 

I would go for the thin section. If you have a low aspect ratio and the leading edge is swept back (as it usually is with traditional keel shapes) you can get flow separation and a vortex above the leading edge. The leading edge can actually be pointy in this case and the keel will still produce lift at very high angles of attack. The vortex sort of rounds the leading edge and produces the lift. They use this concept on a few military aircraft and get a Cl / Cd of about 5.5 out of it. Not as good as a high aspect ratio foil but military aircraft want to get around the corner and have power to burn.

Cheers,
Karsten

 

It doesn't make any sense to have a large volume of air deep below the waterline. In traditional wood construction, keels often had a lot of buoyancy low down due to building down to a wide structual keel member, or use of a solid deadwood. But in light, modern construction, the added displacement due to a long keel can be substantial and is unnecessary.

 

I'm not building it myself - That's a boatbuilders job.. Large (25m+) cruising yacht. Long keel was selected to give a low draft. If performance was a real issue on this one. there wouldn't be so much teak and polished timber.

 

Just so we are agreed on how to measure t root fillet, t foil, and B I have drawn up a little figure. The real hard part with older style hulls is to define what the "canoe" hull would look like without the foil and the roots. As you pointed out, many hulls have t/B of much less than 0.2, but thats ok as long as the root fillets are well shaped.

I disagree with karsten however on sharp leading edge shape. This I think is one of those things that works in air but not in water. In air, the extreme low pressure on the leading edge suction side vortex quickly stablizes flow due to the high driving pressure to mass ratio. In water however, we have a much lower pressure to mass ratio which means that the flow does not reattach quickly. Additionally, there is the cavitation issue to consider. Now for hydrofoil lifting surfaces where you have 25,000 delivered hp a sharp leading edge and cavitation bubble to develop lift is important; for a cruising sailboat keel a more rounded, fuller, shape is better.

 

Thanks for the posts everyone keep them coming.





Jehardamin
Referring to the diagram posted by you (thanks for your inputs):

In your first post did you mean the outer measurement of the root radius or the inner when calculating t/B. when you said "t/B at the edge of the root radius is < .2 in my opinion."
Also I would like an opinion from you;
If you had a 3/4 keel to design for a single chine steel hull with moderate deadrise and a t/C of 0.1 would you use a box keel with parabolic LE and faired trailing edge, OR would you use a thinner NACA (or other) foil section. ?



Brett
What foil section did you pick for your large yacht? Were you the designer? Can a fellow get hold of you ? send me your email address mjeng1@supernerd.com.au

 

MikeJohns; sorry I was less than clear. I mean that t root fillet is < .2 the waterline beam B.

As for the other, I consider the following foils effectively equal for a moderate speed sailing craft. I would lean towards the DTMB EPH (or modified slightly with a fuller forebody) because of the fuller afterbody. The big choice is where you want CB or CG to end up and how much displacement you need to carry.