John Trudgill leapt into action when his younger brother Pete was knocked into the water by a wave at Stonehaven Harbour.
But he found that the rope attached to the life-saving device was knotted.
John spent five minutes desperately trying to untie the knot in the hope that he could save his brother.
Paramedic Pete Trudgill, 43, was in the stormy sea for more than 40 minutes before he was pulled from the water.
Paramedics and doctors at Aberdeen Royal Infirmary tried to revive him but the father-of-two died the next day.
Now John is campaigning for better safety measures to be put in place at Stonehaven Harbour to a repeat of the tragedy.
John, of Stonehaven, said: "The location where he was washed in has the most dangerous piece of water in the area, and it is the only section without a barrier or railing.
"If there had been a railing along that wall he would not have been washed into the sea.
"I got the life-ring out of the container only to find the rope was tied in a big knot.
"When Pete was washed out to sea a large wave came down and we think knocked him on to rocks. That happened within the first 10 minutes - and you are wasting five minutes trying to get a big knot undone."
John has started a petition to improve safety at Stonehaven Harbour which has already attracted around 300 signatures.