Third-placed Sydney to Hobart supermaxi SHK Scallywag 100 was forced to sail without electronics and instruments after rough seas wreaked havoc early in the race.

The 100-footer crossed the finish line at 4:30am (AEDT) on Wednesday, almost three hours behind line honours winner Black Jack.

It took the lead out of Sydney Heads on Sunday before tough conditions battered the 88-strong fleet, eventually forcing 36 boats to retire.

Skipper David Witt, who ticked off his 25th bluewater classic, said he was proud of his crew for keeping the boat in the race up until Tasman Light.

He said SHK Scallywag 100 lost 40-50 miles dealing with breakages and crew were left to hand wind some winches.

"We lost all electronics and we sailed three-quarters of the race with no instruments at all," Witt told reporters in Hobart.

"On a 100-footer that's a pretty difficult thing to do.

"We had a lot of problems. The crew did an amazing job. We probably don't deserve to have a 100-footer here (considering) the condition the boat ended up in after the first night."

Christian Beck, owner and skipper of second-placed LawConnect, which held the lead at times off Tasmania's east coast, said it was by far his toughest Sydney to Hobart.

"Big waves, a lot of wind. Boats crashing onto it, people vomiting everywhere. It was very, very tough," Beck, who has competed in four events, said.

"I personally stayed in bed during that whole thing. I was not able to really do a lot anyway. It was just survival mode really."

Beck said water damage put the yacht's positional tracker offline for several hours, meaning the crew was sailing "semi-blind" until an onboard IT-guru fixed the equipment.

"We didn't have updated weather reporting and didn't know where the other boats were on the tracker. It's not easy to fix computers, let alone if you're bashing through a swell," he said.

LawConnect crew member Ty Oxley said it was his slowest finish in the 10 Sydney to Hobart races he's sailed on supermaxis.

"I don't think I've ever done a race where we haven't had the spinnaker on. It was upwind all race," he said.

"This is what is used to be like, I think we've been spoiled the past few years."