How SailGP grinders power the boat to jaw-dropping speeds

Jonathan Turner
Jonathan Turner
July 11, 2023

It is no secret that the world’s fastest sail-powered boats – SailGP’s high-performance foiling catamarans – feature some of the most sophisticated and technically advanced control systems ever created.

Running along the length and width of the 50-foot-long / 29-foot-wide twin-hull yachts and throughout their towering fully-articulating wingsails are complex electronic and hydraulic networks that enable the six-person racing crew to fly the F50s at speeds approaching 50 knots (58 miles per hour / 93 kilometres per hour).

What is less well-known is the fact that, despite all this on board complexity, the mission-critical control of the F50 wingsail angle of attack to the wind is powered entirely by human energy generated by the crew’s two ultra-fit athletes known as ‘grinders’.

The grinding duo – easily identifiable as the two sailors furthest forward on the boat – face each other (one looking forward and one backwards) over a dual-handled upright winch pedestal connected directly to the F50s mainsail winch.

On-board image of ROCKWOOL Denmark SailGP Team's grinder Hans-Christian Rosendahl who races in the forward-facing the opposite grinder Julius Hallström.

Although overall responsibility for controlling the F50 wingsail lies with the wing trimmer – who, sitting forward of the driver and directly behind the flight controller, uses a set of hydraulic controls to set its 3D profile – that person relies on the grinders to turn the mainsheet winch that moves the rope mainsheet that controls the wing angle.

This mainsheet line held by the wing trimmer is critical to keeping the F50 flying stably and at maximum speed. As such, the rope is in constant motion as the mainsheet trimmer works to regulate the power generated by the wingsail to match the mode the crew are sailing in at any given point on the racecourse. Without the power generated by the two grinders it would be utterly impossible for the wing trimmer to do their job.

The world’s fittest athletes

Given the high power output they are required to generate, F50 grinders typically have highly developed physiques and are amongst the world’s strongest and fittest athletes. Yet despite their advanced physical prowess it is wrong to categorise these sailors purely as the powerhouse section of the crew.

In fact, as well as their gruelling grinding responsibilities – which can see them operating at a heart rate of over 200 beats per minute – the pair are also required at various points on the racecourse to operate the battery powered hydraulic systems that control the trim of the F50’s headsail, as well driving the operation of the boat’s foil daggerboards – and other tasks.       

“I do a lot of grinding, but I think I am more of a jib trimmer than a grinder," comments ROCKWOOL Denmark SailGP Team's grinder Hans-Christian Rosendahl who races in the forward-facing G2 grinder position opposite G1 grinder Julius Hallström.

Hans-Christian Rosendahl, Grinder for ROCKWOOL Denmark SailGP Team

As well as grinding, the G2 grinder also trims the F50 headsail in or out using two foot pedals on the floor of the grinding pod, and – via a set of finger buttons on the side of the pod – can be called on at various times to un-stow the foil daggerboard ready for a gybe or a tack, and/or make changes to the shape of the wingsail. 

“Not all of the teams do it the same way, but on our boat my position is kind of a floating role,” Rosendahl says. “As well as grinding, my position can often involve flying the boat (leaning back into the flight controller’s cockpit to do so) and trimming the jib. During a tack, I would be to leeward, controlling the foil and flying the boat through the turn.”

Switching gears

As Rosendahl explains, there are key differences between the grinding techniques required of the G1 and G2 grinders based on the pedestal winch’s two-speed gearing.

“We have two gears* that are engaged when the G1 grinder grinds backwards or forwards. The G1 grinder mainly grinds with a forward motion that uses the pedestal winch’s ‘big’ gear to deliver maximum power per rotation. Meanwhile, the G2 grinder is standing facing forwards and so is mainly grinding pulling backwards – but, of course, they can also grind forwards in the smaller gear when required.”

*The pedestal winch gearing can be changed between race days to accommodate the F50s different wing sizes and the associated variances in speed at which the grinders are required to move the sheet. 

As an example, when using the larger wing configuration the wing trimmer would often need to depower it by inverting its top section. That action reduces the load on the sheet requiring the winch to be set in a bigger gear to enable the grinders to trim faster.

Conversely, in windier conditions, with the smaller wing setup, the high mainsheet load requires a smaller gear to give more rotations per minute. 

“It’s just the same principle as when you are going uphill or downhill on your bike and you need to downshift the gears so you can generate the right level of power,” says Rosendahl.

The largest 29 meters wing of the newly rebranded F50 catamaran of the ROCKWOOL Denmark SailGP Team foiling around the waters of Chicago

“The G1 grinder’s focus is largely on anticipating the wing trimmer’s demands, so when he thinks the wing needs to come in he will grind on,” says Rosendahl. “Sometimes there is a call from the wing trimmer to grind on, but it works best when there is no need for that communication because the G1 grinder has already anticipated the need to do so.”

Nevertheless, good communication between the grinders and the rest of the crew, as well as the duo’s inherent sailing ability, are critical factors in keeping the F50 constantly flying as fast and evenly as possible on every section of the SailGP racecourse.

When a change to the mode in which the crew are sailing the boat is made – for instance when the wing trimmer applies more twist to the wing – then the grinders are required to simultaneously respond by grinding the wing in closer to the centreline of the boat to compensate for the change.

“You really need to be thinking all the time about what you are doing with the sails and how that impacts the trim of the boat,” says Rosendahl.

“Every time you ease the wing you take a lot of load off the boat so it immediately starts to fly higher. When you trim on it makes the boat fly lower. So when a gust comes it pushes the boat down so you need to ease to bring the boat back up. Likewise, on that first reach, when you ease the jib you off-load the bow so it pitches up and the boat will fly higher.” 

‘Push with everything you have’

Perhaps the busiest point on the racecourse for the grinding team is at mark roundings when the crew switches from sailing upwind to downwind – or vice versa.   

“During a manoeuvre like a bear away or a round up you know you need to ‘give it some’ for the next 10 or 15 seconds, so you just put your head down and push with everything you have,” Rosendahl says.

Achieving the physical profile to fulfil his role as an F50 grinder has seen Rosendahl’s body weight peak at 104 kilograms since joining SailGP – however he has now trimmed down to 92 Kg to help maximise his agility when running from one side of the F50 to the other during tack and gybe manoeuvres. 

His body mass is largely muscle but he confesses to carrying a little fat too: “You can push harder if you are a little bigger in that department”.

Rosendahl has a Technogym machine at home and trains up to eight times per week between SailGP events. If you have one in your nearest gym, give it a go – and imagine that some of those training sessions will conducted at max resistance for up to an hour.

“Not all of them are crazy high heart rate sessions,” he says. “But there would be three to four gym sessions, and I also do plenty of biking, and some open water swimming sometimes.” Around 20 percent of his training is focused on agility.

“I do a lot of leg work and footwork in order to be able to move fast [think American football players dancing in and out of ladders and weaving through cones] plus plenty of step ups and one legged box jumps, and a lot of balance training too.”

ROCKWOOL Denmark SailGP Team's grinders during the teams home event in Copenhagen.

He checks in weekly with the Danish team’s physical coach Craig ‘Oz’ McFarlane who is based in the United States. Each week, ‘Oz’ sets Rosendahl and Hallström individually tailored training programmes, as well as intermittently each month using an upright bike machine to test their power output wattage over 20 minutes and during sessions comprising alternating bursts of 15 seconds on and off.

From the heart

They each have been assessed as having maximum heart rates of a little over 200 beats per minute – matching that of legendary sprinter Usain Bolt in his prime. Both wear heart monitors while racing aboard the Danish F50.

According to Rosendahl, the training days before a regatta – which can see the crews spend up to four hours in intense practice – are much more demanding than the racing itself. Shorter though the race days may be, both he and Hallström push their bodies to the limit over a typical three-race day.  

“Depending on the racecourse, the wind strength, and how hectic the racing is, my heart rate is typically in the range of 180 to 200 for an entire race,” confirms Rosendahl.

Sailing vocabulary

  • Foils - Foils are 'ski-like structures' mounted below the hull of the catamaran. When moved through the water, they generate lift helping the F50 to fly.
  • Upwind - Sailing the boat into the direction of the wind.
  • Downwind - Sailing the boat with the direction of the wind.
  • True wind - This is the wind that you'll feel when stationary and the wind speed you'll see on your local weather forecast.
  • 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.
  • A tack - change course by turning a boat's head into and through the wind.
  • A gybe - change course by swinging the sail across a following wind.
  • Hydrofoil - a lifting surface, or foil, that operates in water. They are similar in appearance and purpose to aerofoils used by aeroplanes.
  • A jib - The jib is the sail at the front of the boat. The jib increases a sailboat's speed simply by adding a significant amount of sail area which catches a lot more wind.
  • Daggerboard - a board that slides vertically through the keel of a sailboat to reduce sideways movement (balancing the boat).
  • Leeward - being in or facing the direction toward which the wind is blowing
  • Trim - simply the running angle of the boat as it makes way in the water
  • Gust - a sudden strong rush of wind.
  • Winch - Winches are drum shaped mechanical devices used to handle halyards, sheets and control lines.

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