Yacht skipper Ralph Mothes, 59, is seen in this amazing photograph dwarfed by the whale as it breaches right next to his yacht Intrepid.
A split-second after the picture was taken the 30ft-long whale slammed into the deck, snapping the steel mast like a twig and crushing the coach roof and side rigging.
Southern rights, an endangered species, can grow up to 50ft long and weigh 60 tons, so this one was probably a youngster.
A tourist from Botswana who was aboard a nearby sightseeing boat captured the before and after shot.
Mr Mothes and his girlfriend and business partner Paloma Werner, 50, took cover as the whale thrashed about on deck before slipping back into the water.
Today the pair were thankful that they were aboard a steel-hulled training yacht, which didn't suffer any structural damage. A fibreglass vessel would have been destroyed.
The incident happened at Hermanus, a popular destination around 80 miles east of Cape Town.
Recalling their close call with the forces of nature, Miss Werner said: 'It really was quite incredible but very scary. The whale was about the same size as the boat.
'We'd spotted it about 100 metres away and thought that was the end of it. Then suddenly it was right up beside us.
'I assumed it would go underneath the boat but instead it sprang out of the sea. We were very lucky to get through it, as the sheer weight of the thing was huge.
'There were bits of skin and blubber left behind, and the mast was wrecked. It brought down the rigging too.
'Thank goodness the hull was made of steel and not fibreglass or we could have been ruined.'
The shaken couple, who are experienced seafarers with the Cape Town Sailing Academy, used their engine to get back to shore in Table Bay.
There were reports that the whale had been harassed by pleasure craft before it leapt onto the Intrepid. An inflatable craft had been seen driving close to the animal moments before the incident.
But experts say the young whale was more likely inexperienced with human contact and didn't know the yacht, which had its engine turned off, was there.
Miss Werner said she first saw the whale when it was about 100m away. It breached once and, before they knew it, the whale was a mere 10m from their yacht.
'There was hardly any wind, so we couldn't get out of the way,' she said. 'We didn't have time to take any evasive action.'
Witnesses said the whale had been seen hitting it's tail on the water, and many thought it was being aggressive.
But Ms Thorton said the whale was probably 'lob-tailing' in order to communicate with other whales.