Williams showed off their 2018 Formula One car on Thursday, with technical head Paddy Lowe signalling a change of direction as the former world champions seek to get back among the frontrunners.

Images of the Mercedes-powered FW41 were shown on a big screen at a launch in London, without the car being physically present. Those who had hoped to see it in the flesh will have to wait until testing in Barcelona on Feb. 26.

The car is the first under Lowe's guidance, with the Briton joining from dominant Mercedes last March with plenty of expectation.

"No pressure, Paddy," joked deputy principal Claire Williams.

The British-based team have also recruited former Ferrari chief aerodynamicist Dirk de Beer, who started at much the same time as Lowe.

"The car has many new features, most of which are not all that obvious, but externally the team has pursued a very different aerodynamic concept which has allowed us some significant progress in aerodynamic performance," said Lowe.

"All Formula One cars are an evolution of what has gone before to some extent, but the FW41 does involve a number of departures from the directions that have been pursued in the past."

Lowe said there had also been some "radical changes" to the car's packaging to incorporate developments from Mercedes.

Williams, fifth for the past two seasons after finishing third in 2015, could have a tough year ahead with the youngest lineup on the starting grid in Canadian teenager Lance Stroll and Russian Sergey Sirotkin.

Sirotkin, 22, replaces retired Brazilian Felipe Massa.

The gulf between Williams and fourth-placed Force India, who have the same Mercedes engines and operate on a similar budget, ballooned to 104 points least year from 35 a season earlier.

McLaren, now with Renault engines after ditching Honda following three dismal seasons together, and Renault's own works team are also expected to be breathing down their necks.

Williams were, however, the only team outside Mercedes, Ferrari and Red Bull to finish on the podium last season thanks to Stroll's third place in Azerbaijan.

The Canadian, now 19, was the youngest rookie podium finisher in the sport's history and also the youngest front row starter.

"I am a different person to where I was last year," said Stroll. "I learned from the tough times... and I enjoyed the good times."

Poland's Robert Kubica, who had been hoping to return to the starting grid for the first time since a 2011 rally accident partly severed his right arm, will be the team's reserve and development driver.

The season starts in Australia on March 25.

Haas set Formula One's launch season rolling on Wednesday as the first of the 10 teams to offer a glimpse of their new car complete with the new 'halo' head protection device.

The Ferrari-powered U.S.-owned team, who finished eighth overall last season for the second year running, released digital images of the VF-18 car on their website and social media.

"The biggest part of the car’s evolution was the addition of the halo," said team principal Guenther Steiner in a statement.

"It took quite a bit of study by the aerodynamicists, but the designers had to work hard to modify the chassis so the halo could survive the mandated loads."

The halo, fixed at three points with its central upright in front of the driver and an overhead loop, is designed to protect the otherwise exposed helmet from bouncing wheels and flying debris.

The Mercedes team's technical head James Allison said last week that the device can withstand the weight of a London double-decker bus.

With few other rule changes for 2018, Steiner said the car was "less about reinvention and more about refinement.

"You see elements we had from last year on the car this year," he added.

"Our 2017 car was actually pretty good, but we didn’t always get the best out of it, and that’s what we aimed to change in 2018. We got the car as light as possible to carry more ballast. We were able to do a better job of putting the weight where we wanted it."

Testing starts in Barcelona on Feb. 26, with Frenchman Romain Grosjean and Denmark's Kevin Magnussen returning as Haas's line-up for 2018. The season-opening race is in Australia on March 25.

Williams are due to unveil their new car on Thursday with the remaining teams holding launches, virtual and real, during the course of next week.

Champions Mercedes, with Britain's Lewis Hamilton going for his fifth title this season, will present their car at Silverstone on Feb. 22.