Ralph Mothes and his partner Paloma Werner were sailing off the coast of Cape Town last Sunday when they saw a southern right whale breaching about 120 yards away.
They say it resurfaced it was just 10 yards away and then it breached again onto their ships' deck, breaking the mast in two.
No one was hurt.
The couple says they saw the whale swimming around minutes after the collision and believe it was not injured.
As for the boat, it will cost about $5,000 to fix.