Full Keel vs Fin Keel
How Lift, Drag, and Leeway are Affected
In Enginering school at the University of Auckland, New Zealand, our aerodynamics professor gave us a practical assignment towards the end of the semester – create a glider using 1 square meter of paper, a stick of balsa wood, and a 100-gram rod of steel. He wanted to see how much we’d absorbed in the class. The winner was to be the one who created the glider that stayed aloft for the longest time when pushed off the second story inside the gymnasium.
Being the renegade disruptor I am, I created a glider much the shape of a hang glider. I even built a little active sling device for the 100-gram weight to simulate a human steering the glider. I was opting for stability. Those who listened in the class built a glider that maximised the aspect ratio of the wings given the 1 sq meter of paper allowed. Aspect ratio is the ratio of length to area. Thus, a long wing has a higher aspect ratio than a short fat wing.
Mine pretty much went straight to the gym floor, while the listeners and followers of pure theory and the mathematical equations we’d learned had quite success.
The lesson was to drive home how aspect ratio has less drag. Less drag means more lift.
How does a higher aspect ratio wing create more lift over a lower aspect ratio wing for the same area of wing?
Since both low and high aspect ratio wings have the same amount of area presented to the air molecules, and lift is proportional to the velocity squared times the area, then the question remains.
The answer lies purely with the energy lost in the vortices coming off the edge of a wing. A fat wing, as above, loses more energy to the vortices than the high aspect ratio glider.
To visualize a vortex of the end of a wind, here is a great photo showing this phenomenon
All of this is energy lost.
Let’s shift to sailboats because a keel is NOT a WING.
A wing is asymmetrical with the curve on top to take advantage of the lift from the airfoil shape. However, a keel on a sailboat must be symmetrical because the water can impinge on either side of a keel, depending on what tack the boat is on.
Notice that the water impinging on the keel creates a force on the keel in the direction opposite to the sideways force created by the wind. Some call this water force “lift”. But it is not lift in an airplane sense because an airplane wing makes the plane go up. In a sailboat sense, the water force balances the sails’ sideways leeway force. And there must be leeway – else if the water came from directly ahead, then there would be no water sideways force on the keel. So forget the notion of lift.
So the job of the keel is to take the water hitting it at a sideways component and use that to push back against the wind, pushing the boat sideways. If not for the keel, the boat would just slip sideways across the water due to the wind pushing it. Whoever invented keels – brilliant!
For now, remember this – you have to have leeway for a keel to work. But how much leeway angle of water pushing on the keel do you need? The stronger the wind, the stronger the keel force is required, and thus, more leeway is required.
What if we could make the keel more efficient to push back on the wind?
Now let’s talk fin keels versus full boat length keels.
Ask a racer, and they will always opt for a fin keel over a full-length keel. The boat goes faster and has less leeway. And they are correct.
A fin keel is simply more efficient at pushing back against the wind. More efficient means it needs less of a leeway angle to push back against the wind. Thus, the racer heads at a better angle to the wind.
So why do fin keels have more push-back force against the wind than full keels? It seems impossible!
Applying a bit of Newtonian physics says that the force on a keel is equal to half the water density x velocity squared x area. But wait – what if the fin keel area was the same as the full keel? All would be the same, right? And even so, usually a full keel would have a larger area than a fin keel (despite the fin keel being a little deeper), so it would seem then that the full keel would do better, not worse.
It is because the Newtonian law assumes a 2-dimensional model. What happens is that there is a difference in the efficiency of the water impinging on the keel for the two different keels.
Thus, the reason for the vortex discussion above.
View the two animations below.
The first is a fin keel showing little vortex spinning of the bottom of the keel.
This is a full keel showing the large amount of vortices and thus the inherent inefficiency.
But this is not the full story – there are other causal effects such as drag. But don’t get down on full keels they do have advantages, however. Standby for more articles.
