Skeg or Spade?

[model 10]

[QUOTE=Juho;2139587]
Aeroplane wings are asymmetric foils. Their design and use are a bit different than with symmetric foils (e.g. best angle of attack is different on different sides).


It was just shown in an above post, that some "aeroplanes" do have symmetrical airfoils. Aerobatic airplanes need to stall pretty much the same way which ever side of the wing is pointing up, so symmetrical airfoils are common. Most all tail surfaces on "aeroplanes" are symmetrical and they're a better example of a sailboat rudder than a wing anyway.

[Juho]

Originally Posted by Stumble

Juho,

I didn't see anything that Dockhead wrote that is wrong, but to vastly simply and point out what I think is the confusion.

Right, nothing wrong. I think I'm just trying to unify the way we see these matters (and terminology), and learn more myself when doing that.

Keel are obviously symetrical side to side. Not because they have to be or that doing so is ideal on a given tack. But because it is equally important that a boat be able to sail upwind on port and starboard tack. So the keel is designed to be equally good on either tack, this the symetric shape.

Yes. Additional daggerboard foils are an interesting potentially asymmetric addition to this.

The way a symetric keel generates lift is due to the direction of the water flowing over it. Water does not flow parallel to the centerline of the boat, it actually flows at a slight angle, called the leeway angle. It is the asymmetric flow over the symetric keel that generates aerodynamic lift.

You are saying "aerodynamic", not just "plywood" lift. That's an interesting topic, related to the profile of the keel.

Theoretically a boat could be designed with an asymmetric keel, and it would significantly out point a boat one one tack, but be disasterous on the other. This is why canting keel boats and fast multihulls have tacking daggerboards. They actually swap one asymmetric board for the other when tacking. But this is far to complicated for a cruising boat to mess with so let's ignore it from here on out.

Ok.

So we are left with a boat that generates lift from the keel by converting leeway into lift. When trying to deign this system the other major factors that need to be taken into account are drag (which always hurts) and speed (more speed = more drag, AND changes the ideal shape of the foil).

So in order to reduce drag there are some simple things that play a part. The first is skin friction, the less surface area of the foil the less friction there is and the better the boat performs. Great, so we want as small a surface area as possible.

Second because there is more drag at edges than along the fin (tip vortexes and slippage) we want the fore and aft length of the keel to be as short as possible.

Third is shape. A hundred years or so ago a series of foil shapes were tested called NACA foils. These basically define the best shape of a symetric keel at any given speed. The faster the boat the narrower the chord, and the further back max chord is. I can't even begin to explain how this works, but there are tables and charts, and it's very authorative with a lot of explanation of you look it up.

I should study the results of those tests.

Given these we now know that we want as small an area as possible, with as short a fore and aft length as possible.

Now deriving how much surface area you need is complicated, because the faster you go the less area you need. But this is far to complicated for me, so let's just assume we need 1% of the sail area as surface area (which is actually pretty reasonable).

So our ideal keel will have 1% of the surface area of our sail area, and be as short (for and aft) as possible, and as deep as possible. In real life there are limits however, first is material properties. We can simply cannot build a keel stuff enough to match what a designer would like to have, so the keel is restricted by the material properties. Second we don't want to go any deeper than X for practical applications. Third, unless there is enough volume in the keel we will be forced to add a bulb which we probably won't want in a cruiser for practical reasons.

Maybe a bulb that has no "nose" forward (grabbing stuff) and that is not too big with respect to the overall strength of the keel would be ok for a cruiser.

So the ideal keel will be 1) as deep as allowed, as short (for and aft) as possible, and 3) thick enough to contain the volume of lead needed for ballast with a shape that minimizes drag and maximizes lift at the expected boat speeds.


Bt the way everything about the keel applies equally to rudders. Except that rudders don't have to worry about ballast volume. And that rudders can change their angle of attack to the water stream. Ideally in a race boat you want to have zero helm, meaning the rudder is pointed strait ahead, which reduces drag to a minimum.

That changing angle is interesting since it may enable some additional tricks / trimming / profile.

BUT there are some practical downsides to such a situation, the less helm you have the easier it is to stall the rudder. Extremely good drivers can control this, bad ones or even good ones that aren't paying attention cannot. Secondly there is a safety factor, ideally if the helm is released you want the boat to point into the wind a little bit of weather healm allows for this, a truely neutral helm will just keep going strait forever.

I guess that safety factor consideration is aimed more at cruisers and less at racers.

On crewed high end race boats it is not unusual to change the rig tune to induce wether helm for deliveries, then tune it back out for racing. The more weather helm (up to a point) the more forgiving a boat is.

Makes sense.

Since everything I wrote about keels is equally true for rudders (except as discussed) it is easy to see why long thin keels matched to long thin rudders have evolved. They reduce drag, increase lift, and allow for higher speeds. On high end maxi racers we are now litterly at the extreme of what material science allows. Boats like Commanche cannot have a longer, thinner, keel because engineers don't have the materials to build one or the knowledge of how to do so yet.

I checked the Commanche profile. Deep keel, much smaller rudder(s), additional daggerboards, canting keel. I guess I should take that as the limits today, and then start reducing some of those measures for cruising use.

So why do skegs suck? Simple, they redirect the water flow around them. Basically the water flowing into the rudder is no longer sliding across the hull, it is now moving parallel to the hull. This means that the rudder cannot derive lift (honestly it just derives far less) without being turned. The skeg effectively permanently stalls part of the rudder. Thus robbing the boat of lift, and adding drag.

Maybe the number one driver behind skegs is safety related concerns (and long keel tradition too). They are not there for racing purposes. But it could be an interesting idea (maybe not usable) that a "two piece rudder" could be used to create and asymmetric foil. That combined skeg+rudder would get the "water sliding across the hull" that you mentioned. But maybe the skeg should in this case be also able to turn to reach the best results, which would make it a non-skeg .

Did you assume that the rudder would point straight forward (making the skeg just a rudder extension that makes the rudder too long), or was there an angle between the skeg and the rudder?

[Juho]

Originally Posted by Dockhead

There's quite a lot of confusion in this post, and making up your own terminology really makes it worse. Confused language leads to confused thought. If you're interested in this stuff, you should read a good book on it.

Sorry if my terminology is unintelligible. I'm not very fluent in English sailing terminology.

And there is no such thing as "asymmetric lift".

My intention was to talk about the lift generated by asymmetric foils.

And to say that "aeroplane wings are asymmetric foils" is false. Many are, but not all.

Yes. I was referring to the glider example case that you took up.

[Juho]

[QUOTE=Guy;2139607]

Originally Posted by Juho

Aeroplane wings are asymmetric foils. Their design and use are a bit different than with symmetric foils (e.g. best angle of attack is different on different sides).

It was just shown in an above post, that some "aeroplanes" do have symmetrical airfoils. Aerobatic airplanes need to stall pretty much the same way which ever side of the wing is pointing up, so symmetrical airfoils are common. Most all tail surfaces on "aeroplanes" are symmetrical and they're a better example of a sailboat rudder than a wing anyway.

Yes. It was just the glider case where we started, that was asymmetric, as opposed to the the keel and rudder that are symmetric.

[TeddyDiver]

Originally Posted by Stumble

So why do skegs suck? Simple, they redirect the water flow around them. Basically the water flowing into the rudder is no longer sliding across the hull, it is now moving parallel to the hull. This means that the rudder cannot derive lift (honestly it just derives far less) without being turned. The skeg effectively permanently stalls part of the rudder. Thus robbing the boat of lift, and adding drag.

Not so simple.. Presuming we have similar foil shapes with a spade and with a skeghun rudders when there's 0 deg of steering the lift and drag are equal. Turning the wheel to lee turns skeghung into assymetrical foil which creates more lift and less drag than a spade rudder. Just think about flaps in an aeroplane.
In reality however it's more complicated to engineer such skeghung rudder than a spade when the aspect ratio grows, but with lower aspect ratios doable. Oversteering will cause both rudders to stall but then a spade with greater steerable surface has better grip and also more drag. That is of course not the case in real life becouse the area of the skeg is in excess off the rudder area but anyways..

BR Teddy

[DumnMad]

The term lift is used for air foils where the air is compressible and density(and hence pressure) is increased on one side and decreased on the other, creating lift. With a deep keel the surrounding water is much the same pressure on each side. The rudder force is produced by the rudder travelling at speed and pushing against the water - not by lift. Sails on the other hand have lift.
The rudder shape is designed to reduce drag, not to create lift.

[valhalla360]

Originally Posted by DumnMad

The term lift is used for air foils where the air is compressible and density(and hence pressure) is increased on one side and decreased on the other, creating lift. With a deep keel the surrounding water is much the same pressure on each side. The rudder force is produced by the rudder travelling at speed and pushing against the water - not by lift. Sails on the other hand have lift.
The rudder shape is designed to reduce drag, not to create lift.

Incorrect.

Due to it's greater density, water is far more effective at creating lift. This is why the sail area is typically several times the keel/rudder area.

You can and do induce lift in a symmetrical rudder by inducing an angle of attack.

Many multihulls with boards use asymmetrical boards so it creates lift exactly like an airplane wing. On cruising boats they typically are not very aggressive so that angle of attack can overcome the asymmetry if the cruiser gets lazy and doesn't swap boards on each tack.

[DumnMad]

Originally Posted by valhalla360

Incorrect.

Due to it's greater density, water is far more effective at creating lift. This is why the sail area is typically several times the keel/rudder area.

You can and do induce lift in a symmetrical rudder by inducing an angle of attack.

Many multihulls with boards use asymmetrical boards so it creates lift exactly like an airplane wing. On cruising boats they typically are not very aggressive so that angle of attack can overcome the asymmetry if the cruiser gets lazy and doesn't swap boards on each tack.

Foils near the water surface do have lift in the pure sense of the word because they suck a hollow behind them but as you note the effect is not apparent on cruising boats.

Keels well below the surface are used to steer by deflecting the water sideways and the force required to do this turns the boat. The droplet or fish-like shape is to do with drag, not lift.

[weavis]

Ive been to Skegness

[TeddyDiver]

Originally Posted by DumnMad

Foils near the water surface do have lift in the pure sense of the word because they suck a hollow behind them but as you note the effect is not apparent on cruising boats.

Keels well below the surface are used to steer by deflecting the water sideways and the force required to do this turns the boat. The droplet or fish-like shape is to do with drag, not lift.

Please explain us also how a propeller works I'd like to hear your explanation

[DumnMad]

Huh. I googled the term lift as used by yacht designers - you are right, it is used this way.

Never-the-less, flow around a foil deep in the water is very different to compressible air flowing around an airfoil.

[Dockhead]

Originally Posted by valhalla360

Incorrect.

Due to it's greater density, water is far more effective at creating lift. This is why the sail area is typically several times the keel/rudder area.

You can and do induce lift in a symmetrical rudder by inducing an angle of attack.

Many multihulls with boards use asymmetrical boards so it creates lift exactly like an airplane wing. On cruising boats they typically are not very aggressive so that angle of attack can overcome the asymmetry if the cruiser gets lazy and doesn't swap boards on each tack.

Exactly. Googling the phrase "hydrodynamic lift" would clear this up.

Lift is generated exactly the same way in water and in air, and foils used in water can be either cambered (assymetrical) or not cambered (symmetrical), just like those used in air. The principles and terminology and even the formulas for hydrodynamic and aerodynamic lift are the same. Most boat keels are designed to a NACA airfoil pattern, precisely so that they will generate lift -- yes LIFT -- in the most efficient possible way.

The compressibility of air becomes a major factor only at supersonic speeds, obviously not a problem on sailboats

[Dockhead]

Originally Posted by DumnMad

. . . Never-the-less, flow around a foil deep in the water is very different to compressible air flowing around an airfoil.

Actually not. The same formulas are used and the same principles apply in all fluids. The forces generated depend on VISCOSITY of the fluid (not density), and so as Valhalla said, the amount of lift generated in water is much greater for a given foil and given speed.

If you put an airplane wing in water, it would work exactly the same way as it does in air, just generating much more lift.

Boat keels are usually designed according to NACA airfoil patterns -- which work equally well in water as in air.


Edit:

Correction -- one of my crew is an aeronautical engineer, and just corrected me on viscosity vs density. Both play a role in the production of lift at different phases.

[Stumble]

Juho,

To correct something I said...

Commanche was a terrible example of foil design as relavent to this discussion, in my head it made sense, but it really doesn't, or to be technical it does but for a whole host of even more complicated reasons. So what was mistake?

Commanche doesn't actually have a keel, at least not the way we have been discussing it. Instead she breaks the two functions of a traditional keel into their respective parts and has seperate solutions to those problems.

The first is to provide RM to the boat. This is done via a canting keel that is pulled up to windward. Because the RM (righting moment) that is created is a function of the angular momentum relative to gravity you get a lot more RM for the same weight by having the level arm parallel to the horizon. So the fin that holds the keel bulb on Comanche is actually designed NOT to generate lift to windward, it's entire design brief is to hold a massive bulb as far to windward as possible while minimizing drag. It has no other function.

Except that technically even this isn't true any more either, modern canters like Commanche actually now have started playing with the tilt of the canting foil. By lifting the forward pivot up a degree or two the fin can generate lift, reduce leeway, and change the load sharing between the keel, daggerboard, and rudder.

Add in the next generation of daggerboard that not only provide lift to windward but also start lifting the hull out of the water (foiling) and we have defended right up to the event horizon of knowledge. The best yacht designers in the world frankly are having a hard time figuring out all this stuff, and it maybe years before any of it becomes accessible to even performance racers, let alone the cruising world.

To generate lift Commanche uses two tacking daggerboards. Basically giant (well giant for a boat) asymmetric foils that don't have to deal with ballast loads at all and can be maximized for lift and minimal drag. She leaves the dock with two of them (port and starboard) and part of the tacking routine is to swap them (there is also some built in redundancy, since the port board can be used on the starboard side by flipping it up side down).

Probably a better example of maximum material capability would be something like a TP-52 without a canting keel. Where no compromise was made for usability or cost.

Keep in mind though that there are real limitations in going to a fin design this extreme for a cruiser. First is simply that ideal shape changes as speed goes down, so if you can't maintain the same speed as a boat like this you need a different shape. A boat capable of maintaining average speeds over 11kn upwind needs a different keel shape than one that is doing 6kn. The 6kn keel is also going to be far more forgiving. Which is why in part keel design on solo boats is actually being driven by the autopilots. As AP's get better at being able to drive to the boats maximum the shapes can be less forgiving than they were. Eventually it is possible to envision a boat that is faster at all times with the AP driving than with a human at the wheel.

See Daggerboard evaluation for an IMOCA 60 yacht : Owen Clarke Design - Yacht Design and Naval Architects for a fascinating description of the issues facing top flight designers with these boats.

[robert sailor]

There is lots of cool stuff happening on the racing front but not much will trickle down to cruisers. Cruisers by and large don't have the skillset to deal with the high performance side of it plus and this is the big item, cruisers load the hell out of boats and weight is the killer of performance and this includes multi hulls as well. We haven't even got to the point where the largest builder of offshore boats can build a rudder properly much less anything else. Race boats need the be very carefully built to have a light strong boat and big builders can build cheap boats but not light and strong. Polux will remind us of the Pogo which is a very cool boat and it is based on a racing hull and it is quick IF it's kept light but every cruiser I ever met is packing more and more stuff on boats these days so forget light. At the speeds most cruisers sail at skeg rudders are fine, probably the partial skegs are a better compromise but don't think skegs are always stronger than spades. These days skegs are probably stronger than spades because the world's biggest builder does a crappy job building spades but if a quality builder built a spade it would be just as strong or stronger than most skegs.