Drag Limits

Hydrofoils and more…

In theory a sailing boat can travel infinitely fast.  How?  The wind generated by forward movement (we know as being a component of apparent wind) continues to increase with speed.  With the increase in apparent wind everything changes, and more speed is theoretically possible.  We should have video games based on this theory!  Apparent wind has been better understood over the past few decades than at any previous time.

If we look at what prevents the sailing boat from achieving this huge speed, we see two distinctly separate forms of drag.  These are hydrodynamic drag, created by the hull going through the water, and aerodynamic drag, created by the sail traveling through the air, and the imperfections of airflow around the sail.  Indeed the airflow going over the surface of the sail is one of the “improved” components that wings offer on the current America’s cup boats, allied to the foiling effect, which simply reduces (but far from eliminates) hydrodynamic drag.

We may first look at hull drag to begin to understand the limitations that water brings to sailing boat speed.  If we look at ice sailing, they travel at many times the speed of conventional sail-craft, yet share similar rigs.  This is because the friction of the blades is considerably less than the friction (drag) of a hull through water – absolutely no other reason.  The same can be said for sail buggies.  There have been many attempts to minimize hull drag.  The biggest breakthrough in conventional hull drag minimization is probably the better understanding of planing.  Planing to windward was initially introduced in the Flying Dutchman around 50 years ago.  We can loosely define planning as when the boat rides on, or overtakes, its own bow wake so that it is riding over the water, rather than going through it.  The turbulence caused by this can be seen in pictures of any planing boat – plenty of spray and wake, so the drag caused by wake (turbulence / drag) is evident for all to see.  Maximum hydrodynamic efficiency is when the wake and spray are minimized.

Hydrofoils provide another attempted answer to minimizing hull drag.  The challenge with hydrofoils is that they come with an inbuilt problem – gravity.  In order to minimize drag, the hull has to be lifted clear of the water.  This means a hydrofoil will always have to be angled to lift, which in itself produces massive drag, and the heavier the boat the more drag it produces. This drag, albeit of a much smaller object (the hydrfoil) being driven through the water, is still substantial.  Again we may see the benefits and effects of this on the America’s cup 72, Foiling Moth and other sail boats being adapted for foiling.  However great the reduction in drag, the hydrofoils produce their own intense turbulence caused by supporting the weight of the boat.

In short, we have not come anywhere close to minimal hydrodynamic drag, but the strides we have taken are so large that sail boat performance has increased massively since the boffins have been working in this direction.  Ironically the humble windsurfer probably leads the way in this field, yet the IMOCA 60s have made many leaps that few would credit as being possible.

In terms of aerodynamic drag, there are simple rules, and many less simple.  The deeper the sail, the more “powerful” it becomes, meaning that it can accelerate quicker than a flatter sail, yet its top speed is not as good due to the air being bent as the sail uses it to produce drive.  The greater the draft (depth) of the sail, the more likelihood of turbulence, which is drag, which is slow!  A “powerful” sail, by definition, is therefore slow unless it can be flattened as the speed increases.

If we take the aerodynamic drag factor alone, there are complex problems to accelerating well and going fast.  If we take the hydrodynamic drag factors also, they inhibit our potential speed by the hardware – the boat design and the class restrictions.  The entire field of drag is essential to understand for success in sail racing.  This is an area where we can never gain a big return, but the details are truly important when striving for “race pace” at the front of the fleet.  At Toplevel Sailing (formerly WSA) we seek to deliver understanding of change of drag with speed, and the requirements of adjustment that this brings.

The America’s Cup boats have dedicated trimmers, one and two person Olympic sailing classes do not have this luxury.  We therefore acknowledge that there is a trade-off between speed and the attention to other tasks, which is always a compromise, but one that will improve with focus and experience.

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