A midnight mercy dash to help a stricken solo yachtsman off the southern coast of New Zealand has helped end a harrowing and dangerous ordeal.

Frenchman Thomas Ruyant was racing in the Vendee Globe when he thinks he hit a shipping container, causing his boat's hull to open and the frame to start coming apart.

Help arrived just before midnight in the form of Bluff coastguard Stuart McLachlan as the yacht threatened to break in two in Foveaux Strait.

Mr McLachlan told 1 NEWS Mr Ruyant's experience would have been terrifying.

"If you stand in the cockpit going over the waves, you can see the front of the boat going up and down about a foot and a half just flexing and the rigging all loose," he said.

"I can't imagine what he was going through, we only saw it in relatively calm conditions, how much the boat was moving."

The solo sailor was escorted by the coastguard for seven hours, reaching Bluff this morning, giving Mr Ruyant time to reflect on the ordeal.

"It's scary, it's violent," he said.