On efficiency


Efficiency is generally defined as the ratio between the forward component of the wind force on the sail/wing divided by the lateral component of the wind force. If the efficiency is high, the boat is fast, all else being equal.  That is why the AC72 boats and other multihulls have wings. The wings project vertically upwards, resulting in a very high efficiency and very fast boats.

Yet, the current world record holder Sailrocket has a wing that is tilted to windward. Clearly, a tilted wing can not have a very high efficiency or at least not as high as the AC72 wings. Moreover, the earlier record holders, i.e. windsurfers, have a sail that is titled backwards over a considerable angle and is tilted to windward over a smaller angle. Clearly, windsurfers also have a lower efficiency than boats having a vertical wing. Yet windsurfers are very fast. In fact, windsurfers are considerably faster than any sailboat of the same size. How can these obviously incorrect angles of the wing or sail achieve such high speeds? It doesn't make any sense.

The answer is: efficiency. But not the kind that designers generally use. 

Imagine a wing that projects vertically upward, like the AC72 wing. Now tilt the wing in your mind over 1 degree about a virtual axis defined by the apparent wind. What happens with the forces? The force on the wing now has an upward component. The upward component is small, but noticeable. The efficiency has decreased somewhat, because the tilting results in a greater lateral component for a given forward component.

And here comes the point.

The gain in the upward component is larger than the decrease in efficiency. Or in other words, of the resultant vertical and lateral forces acting on the boat by the water, the vertical force has decreased by let's say one Newton, while the lateral force has increased by considerably less than one Newton. Or in other words, you just saved a Newton of upward water force on the boat, while spending less than a Newton of lateral water force. Or, in yet other words, you have improved the power to weight ratio to a greater extent than the efficiency has decreased. 

Another way of explaining this is that efficiency is a 2D parameter, which does not take into account vertical effects. The positive effect of the tilted wing is real, but is not expressed in the efficiency. This is why at Wiebel, we calculate with 3D efficiency instead of 2D efficiency. 3D efficiency is defined as the forward component of the wind force acting on the wing/sail divided by the total lateral and vertical forces acting on the boat by the water. If you use the 3D parameter of efficiency, you can calculate the beneficial effect of tilting the wing (or sail). The beneficial effect is actually quite high. 

That is why Sailrocket is so fast. Sailrocket is the most efficient sailboat in history. Not in 2D, because in 2D Sailrocket sucks. But in terms of 3D efficiency, Sailrocket is great. Forget about the zero-heel concept, it does not add to the speed. It is the 3D efficiency that does the trick. Or in other words, the forward component of the wind force divided by the total lateral water forces and the total vertical water forces acting on the boat is very small. Or in other words, the 2D efficiency of Sailrocket is lower, but all else is not equal.

The same applies to windsurfers, but to a lesser extent. The tilting of the sail to aft over a considerable angle in combination with the tilting to windward over a smaller angle and the outward rotation of the boom results in an upward component. Some designers don't believe this is true, but it is. If you ever get a chance to look at a windsurfer in top view (from a bridge looking down), take care to notice that in top view you are looking at the convex side of the sail. That means there is an upward component. The upward component results in a high 3D efficiency and makes wndsurfers go fast. Put a vertical yet scaled down AC72 wing on a windsurfer and it won't go nearly as fast. 

That is also why Wiebel will be very fast. It is optimised in 3D. In 2D Wiebel sucks, just like Sailrocket and windsurfers do. But in 3D Wiebel is just right, and getting better all the time.

So next time you look at an AC72, consider that the wing/boat combination is optimised in 2D, but not in 3D. That is why AC72's, great as they are, will never break any world record. The sailing world is nowhere near the optimal sailboat yet, at least not a sailboat that can actually change tack. What we have to do is learn the lessons from Sailrocket and windsurfers and apply these lessons to sailboats as good as we can. That is exactly what we are doing at Wiebel. It will take some years before 3D optimised sailboats will participate in races like the America's cup. But that is not a problem. The wine that is in the barrel does not become sour, as we say in the Netherlands. It stays good. Once we drink that wine, we will enjoy it to the max. At Wiebel, we hope to drink that wine, preferably with you. We know it's good, because we have the first test results,and because we can see from Sailrocket and windsurfers that it is good.

The question remains what the best tilting angle is, and if the wing should be tilted more to leeward as Sailrocket does, or more to aft as windsurfers do. But that will be discussed in a next blog. Hope to welcome you on our website next time. 


Walter Hart








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