Prepare for take-off: How the world’s fastest catamaran takes flight

If SailGP was a movie, perhaps Top Gun would be it. Some of the world’s most elite and high performance operators, pulling incredible levels of G-forces, in state-of-the-art, flying machines.

After all, they’re called Flight Controllers for a reason – they have one job, and that’s to get the boat flying… and keep it there. And though there might be a few thousand feet difference between taking off to the skies or on the seas, the concept is similar.

Prepare for take-off

In many ways, the process of taking a SailGP F50 foiling catamaran from a standing start to flying above the water at speeds up to 100 kilometres per hour (km/hr) is not dissimilar to an aeroplane setting off down an airport runway. 

In the case of the F50, however, there is no jet engine providing the forward motion to drive the air over the wing. Instead, the sailors have to use two types of wind power to do the job: first the ‘true’ wind (the wind that makes flags fly); and second the ‘apparent’ wind (the wind that is created by an object moving through the air – like what you feel if you stick your hand out of a car window on the motorway).

^{F50 foiling (flying) over calm waters}

The true wind is what the crew use to get the boat moving forwards in the first place and, as it does so, the highly efficient wing quickly begins to generate its own apparent wind. This in turn makes the boat accelerate more and generates even more apparent wind.

09 January 2023

How SailGP’s foiling F50 catamarans sail so much faster than the wind

However, it is not as simple as just sheeting on the sails and leaping up on the foils. To make the transition from ‘displacement mode’ (hulls in the water) to flying above the water is all about minimising ‘drag’ – the backwards force you feel on your body when you ride your bike, or when you trail your hand through the water on a moving boat.

Popping it on the foil

To get the boat moving, the crew drops the foil on the hull ‘to leeward’ (that’s the opposite side to where the wind is coming from) fully down to avoid sliding sideways rather than moving forwards.

The crew’s flight controller makes sure the cant of this foil and the two rudders are set for minimum drag. Likewise the wing trimmer (controller) will configure the wing to be as aerodynamically efficient as possible as the boat starts to move.

14 February 2023

SailGP’s Flight Controllers

“The process of getting the F50 airborne starts with sailing the boat with minimum drag in the water, which means you don’t have any lift on the foils and the shape of the wing is also very minimised in terms of drag as you try to accelerate,” Denmark SailGP Team driver Nicolai Sehested explains.

^{ROCKWOOL Denmark SailGP Team foiling (flying) over the shallow water around Navi Pier in Chicago.}

“You have a foil down to stop the leeway slip because otherwise the energy going into the boat would be a sideways force rather than driving the boat forwards. But there is no lift applied to the foil or to the rudders which are at a neutral setting to reduce drag to a minimum.”

As the boat accelerates and reaches takeoff speed – somewhere around 30 km/h – the Driver, Wing Trimmer, and the Flight Controller have to work in perfect unison to ‘pop’ the boat out of the water and onto its foils.

Here is how Sehested describes the process:

“Once you get to 30 km/h you can start to add lift to the foils and try to get the boat out of the water. At that point it is all about how light you can make the platform. 

“The takeoff process is a three, two, one countdown from me on the wheel. As I reach ‘one’ the flight controller will add maximum rake on the foils – somewhere around seven degrees of positive angle of attack – and to the rudders to lift the boat out of the water. 

“At the same time the Wing Trimmer will reduce the power in the wing, because that is generating a down pressure on the platform that’s trying to push it back down into the water. 

10 January 2023

How ‘cavitation’ and ‘dirty air’ affects the performance of SailGP’s F50 catamarans

“That’s done by easing out the wing to a wider angle and twisting off the leech (trailing section) so that there is no load in it just for that instant when the boat is taking off.

“As soon as the boat is out of the water it accelerates pretty quickly so you can quickly start taking on power in the wing again to help that acceleration. But not too soon that it pushes the boat back down into the water.”

In SailGP, even for the best teams, achieving  this complex set of actions requires split-second timing. The ‘back three’ positions must work in harmony, hitting the sequence with the collective dynamism of a symphony orchestra.

Sehested estimates the F50s need about 13 km/h of true wind to achieve take off on flat water. As soon as the boat is airborne, it is freed of the drag created by the hulls in passing through the water, and with just the minimal drag of the narrow chord foils and rudder blades in play the boat speed rockets up quickly. 

“It takes more lift on the foils to get the platform out of the water than it does to keep it in the air,” Sehested says.

^{Top view of the foining F50 of the ROCKWOOL Denmark SailGP Team. The image shows what is happening under the water and how the catamaran is 'flying'}

“In 12–13 km/h of breeze the boats can be doing 40 km/h of boatspeed and we can actually achieve boat speeds of close to four times the wind strength at certain angles – like on the first leg of the course where we are ‘reaching’ (sailing with the wind blowing from the side of the boat).”

09 January 2023

How SailGP’s foiling F50 catamarans sail so much faster than the wind

Flight mode 

Once the F50 is up and flying, the crew quickly switches to a new mode as it configures the boat for both stable flight and maximum speed. 

“Once you are foiling the goal is to have the platform level both fore (front) and aft (back), and side to side,” Sehested comments. 

Achieving this careful equilibrium requires the sailors to work in unison to finely balance out the forces generated by the wing and the opposing ‘righting moment’ force delivered by the foil and rudders.

Put simply, in sailing terms, ‘righting moment’ is the force that prevents any sailboat from being blown over by the wind. 

In conventional yachting, the more a boat heels the less efficient its keel becomes at driving it forwards – and the more it slips sideways. Likewise, on the F50s, generating the right amount of righting moment to keep the platform flat side to side translates directly into increased power and, ultimately, maximum speed.

Over three seasons of SailGP racing, the crews have developed sophisticated methods for optimally configuring the F50’s foils and rudders for this purpose. 

^{A close-up of the F50s hydrofoils cutting through the water.}

A key technique is the application of ‘rudder differential’. This is where the two horizontal blades (called ‘elevators’) that endplate the rudder fins can independently create either lift or downforce in accordance with the individual cant angle of the rudders. 

By setting the leeward (furthest away from the wind) elevator at a positive angle of attack to produce lift, and the windward (closest to the wind) elevator at a negative angle that produces a huge (many hundreds of kilos) downforce, the crew can hugely increase the F50’s ‘righting moment’ (the force that opposes the heeling effect induced by the wind on the sails) – and that translates directly into more power and speed.

The maximum possible differential between the rudders is around seven and a half degrees and is normally set by the driver using a single button control built into the steering wheel.

“From the wheel, I can decide how much lift and how much differential I want to put in the rudders,” says Sehested. 

“In perfect conditions you would be at maximum differential on the rudders to create maximum righting moment and power. Even though this creates more drag it is still faster to go for max righting moment.”

Although in winds above 19 km/h the Danish crew generally leave the rudders set at maximum differential, Sehested says it is important to be constantly adjusting their rake to maintain the optimum ‘pitch’ (its angle from stern to bow) of the platform around the various legs of the racecourse.

^{Denmark SailGP team balancing F50 while catching upwind (truewind from ahead)}

“Whatever the conditions, we are constantly adjusting rudder lift to maintain the optimum pitch. If you tried to sail downwind (true wind from behind) with the same amount of rudder lift as you had upwind (truewind from ahead) then one or both of the rudders can come out of the water causing a nosedive crash.”

Keeping the boat on that knife edge between flying at maximum speed and wiping out because you pushed too hard becomes more and more difficult as the wind increases and the boat speeds ratchet up to close to 100 km/h.  

At that point, just sailing the boat in a straight line is a high speed balancing act that requires all six sailors on the boat to be thinking and acting as one coherent unit. Then throw in eight other F50s, all dogfighting for the best position on the racecourse, and the parallel with the Top Gun movies becomes even more appropriate.