Could a sailboat outrun a balloon, between two points, floating downwind? It’s an age-old sailing conundrum, often debated in yacht clubs around the world.
The concept is quite simple: but where a balloon could float directly from point A to B, making a sailboat travel fast is all about angles.
Despite seeming counterintuitive, the concept of sailing faster than the wind isn’t new – it’s actually a feat that sailboats have always been able to achieve if sailed at the right angle to the breeze.
This is called ‘tacking’ and ‘gybing’, and it’s why you see the SailGP boats zig-zagging each other on the course so often.
Back to our pop quiz: surely this extra ground covered, finding angles of attack to the wind, would put the boat at a disadvantage to the balloon, right?
Well, in the case of a normal sailboat, yes – but when we’re talking about a state-of-the-art SailGP F50, the boat would still have the edge.
Beating the balloon
You see, SailGP boats have an incredible ability to travel up to four times faster than the wind that propels them.
They do this by capitalising on what sailors call ‘apparent wind’ – this is the force generated when an object moves through the air. You might recognise the feeling when you ride your bike, or when you put your hand out of the window of a moving car.
‘True wind’, meanwhile, is what makes flags fly, flutters the leaves on trees, and is what sailors use to get their boat moving forward when they hoist their sails.Then, as they move through the air, the sails start to generate apparent wind too, and it is this combined velocity of true and apparent wind that results in conventional boats being able to sail up to one and a half times faster than the wind.
Bending the laws of physics
So how do the F50s seemingly defy the rules of physics to fly at speeds many times higher than the wind is blowing?
The answer to that question comes down to one concept: the elimination of ‘drag’. Drag is the slowing effect that water has as it clings to the hull of a boat moving through it. The more area of the hull that touches the water – yacht designers call this ‘wetted surface’ – the more the boat’s speed is restricted.
Conventional racing yachts are designed to minimise wetted surface as much as possible, but the introduction of foiling boats that fly above the water on hydrofoils means hull drag can be eliminated entirely. With just the minimal wetted surface of the hydrofoil in the water, foilers are freed from the limitations of drag allowing their boat speeds to soar.
In SailGP, a team that completes a whole leg of the course without touching the hull into the water calls this a ‘dry lap’ – and this used to be the holy grail of foiling just five years ago. But today, with the fly time of many of the SailGP teams close to 100% in the right conditions, a new performance bar has been set.
Before we discuss how the carefully crafted pair of hydrofoils lift the boats clear of the water, it is worth noting that F50s are long, wide, and (for their size) comparatively light catamarans. In sailing terms that’s the perfect ‘platform’ for efficiently converting the power of the wind into forward motion.
As Nicolai Sehested – driver of the Denmark SailGP Team presented by ROCKWOOL – explains, the F50’s dimensions –15.24 metres (m) long and 8.8 m wide – play a significant part in developing the power and speed required for the boat’s hydrofoils to work.
The Denmark SailGP Team's F50 in Copenhagen sailing next to a cruise ship. The wing shown is the medium-sized wing with a height of 24m.
“These catamarans are light and they are very wide which means they have the power needed to accelerate to one and a half times the wind speed – fast enough to climb out of the water when you add lift to the foils.
“If you put foils on a normal monohull you probably wouldn’t get to the speed required to lift it out of the water.”
As efficient as the F50 platform is, however, it still requires significant power to get it moving in the first place and to accelerate it to a speed where the foils can do their job. The delivery mechanism for this power comes not from a conventional ‘soft’ sail, but from a highly complex three-dimensional wing sail.
SailGP Wingsail being mounted ahead of the race. Every morning before sailing, the catamarans are assembled and craned into the water. At the end of the day, the F50s are lifted back out of the water and disassembled for maintenance.
Working in the same way as an aircraft wing – but mounted vertically – as the wind blows over the leading edge of the wing, it is forced to travel further over one surface than the other. This extra distance causes a pressure delta between one side of the wing and the other, and, just as on an aeroplane, the result of that delta is lift – or in the case of the F50, forward motion.
So sophisticated are the control systems built into the modular F50 wings that they can be articulated perfectly by the sailors to change shape as required to deliver precisely the right amount of power for any given situation. These settings are called ‘twist’ and ‘camber’ and finding the correct combination can cause significant speed benefit on the race course.
The final pieces of the F50 design jigsaw are the boats’ underwater appendages: two straight blade rudders that terminate in horizontal t-sections, known as ‘elevators’; and a pair of L-shaped carbon daggerboards.
A close-up of the F50s hydrofoils cutting through the water.
The daggerboards are constructed using super-strong high modulus carbon fibre and can be individually raised and lowered, and canted laterally and back and forth (aft and fore, in sailing lingo).
As well as stopping the boat sliding sideways like the keel of a conventional yacht, the horizontal sections at the ends of the foils create hydrodynamic lift as the water flows over it and as the boat propels forward
Canting the foil backwards increases its angle of attack and creates more lift, while canting it forwards reduces lift and can even create a downforce.
Meanwhile, on the back of the F50 the vertical blades of the rudders enable the driver to steer the boat from the twin ‘steering wheels’ (or helms, in SailGP terms) on deck, but each rudder can also be individually canted so that the horizontal elevators generate lift or downforce. (More on how the crews use this setup later).
Ultimately, achieving stable flight aboard an F50 comes down to how well the crew can balance out the forces generated by the wing sail, the foils, and the rudders.
The complexity of the onboard systems required to coax more than 2,000 kilograms (kg) of boat and 483 kg of crew bodyweight into the air and keep it there is simply mind boggling.
However, it takes more than advanced technology to get an F50 around the confines of a SailGP racecourse, and the sailors who race these next generation boats have had to dramatically upskill their techniques, reaction times, and overall racing strategies, to do just that.
In reality, the chasm that exists between conventional racing yachts that sail on the water and the F50 catamarans that fly above it is so vast that many believe SailGP should really be regarded as a separate sport in its own right.
It’s sailing – yes – but not as we know it.
Faster and faster
As breathtaking as the current speeds of the F50 catamarans may be – the top speed in the 2022 season of SailGP was just under 100 kilometres per hour (km/hr), highway driving speed on most European motorways – it seems they are set to go even quicker.
The only thing that is stopping the SailGP boats from breaking through that 100km/hr barrier even faster is a hydrodynamic concept known as ‘cavitation’, which occurs at very high speeds as the water around a hydrofoil begins to vaporise. This stops the foil from working properly, resulting in instability and sometimes causes the boat to crash out of the air.
However, planned enhancements to the SailGP fleet during 2023 are designed to significantly delay the onset of cavitation. As well as upgraded control systems and new steering wheels, the F50’s foils will be upgraded to a new T-shaped design terminated in titanium wings.
These evolutions should not only help to delay cavitation, but the new foils are also predicted to reduce the need to apply rudder differential to generate righting moment – thereby reducing drag and increasing overall speed.
Experts believe that those innovations could see the SailGP fleet break the 110-120 km/hr barrier before the end of Season 4.
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- Tacking & gybing - Turning the boat to a point where the wind comes from the opposite side of the boat. It's called tack when the bow moves through the eye of the wind, and gybe (or jibe) if it's the stern.
- True wind - This is the wind that you'll feel when stationary and the wind speed you'll see on your local weather forecast.
- Induced wind - The 'wind' you'll feel when moving. This the air resistance or 'drag' created by any object moving through the atmosphere. The induced wind is equal to the speed of travel but in the opposite direction.
- Apparent wind - Apparent wind is a combination of the induced wind and the true wind. By travelling at an angle to the true wind the combined apparent wind becomes bigger than the true wind, enabling boats to travel faster than the true wind speed.
- Drag - The resistance of an object moving through air or water. As the hull or foils move through the water, drag is created. Because water is much denser than air, it creates more drag, hence lifting the boat out of the water allows it to move faster.
- Pressure Delta - Difference in or change in pressure between two points.