Andrew Buchanan pulled Ian Buchan from his capsized trimaran after his young son spotted the sailor clinging to the top of his boat in rough seas.
Mr Buchanan, 45, had been on a fishing trip off the Aberdeenshire coast with father-in-law Brian Topp, 64, and sons Matthew, 12, and Rory, 8, when the drama unfolded.
Their boat - The Shindig - and Mr Buchan’s small trimaran were the only vessels in Crawton Bay, south of Stonehaven when the weather deteriorated.
But as the Buchanan’s started to head for shore, young Rory spotted Mr Buchan was in trouble - half a mile from the coast on his capsized vessel.
Mr Buchanan, a financial consultant, said: “It was quite a rough day.
“We lifted our anchor and were thinking about leaving because it was getting really, really windy.
“As we turned to go back, my eight-year-old son said, ‘That man’s boat is upside down’.
“It was getting really windy and rough and he just had shorts and a top on, so I went over to him. He was just out for a sail.
“It wasn’t his fault his boat had gone over. It was very gusty.”
The Buchanan’s found Mr Buchan half in the water and clinging for safety from the top of the boat.
And as the wind increased he was getting pushed further out from the coast.
Sensing the danger Mr Buchanan tried to pull the boat back onto its keel but with no luck.
He said: “It was too rough and dangerous and the guy was getting cold and I took him aboard my boat.
“It was really quite rough and, where he was, he was getting blown farther and farther out.”
Mr Buchanan then tied Mr Buchan’s capsized craft to his own 20ft sports fishing boat and started to tow it back to shore.
He said: “My boat’s got a big engine and I thought I was capable of towing him.
“We got him in as far as we could, but his mast was bigger than we thought.
“We didn’t want to damage the boat.
“We decided to just stop there.
“It took us about 40 minutes to tow him in and that’s when I phoned one of the helmsmen at Stonehaven RNLI.”
The Stonehaven inshore lifeboat was launched at 6.20pm on Saturday to aid the stricken sailors.
The lifeboat volunteers succeeded in righting the trimaran.
A spokesman for Aberdeen Coastguard said yesterday: “We had a call from someone who reported that a trimaran had capsized about four miles south of Stonehaven.
“The guy had been in the water but was now aboard another vessel.
“They had been towing the trimaran to shore but had run into some trouble with the tow.
“The Stonehaven lifeboat was dispatched and everyone was safely returned to shore.”
Mr Buchanan’s father-in-law Mr Topp had just returned home from offshore and had fancied a fishing trip.
But the weather, which had been closing in for most of the day, almost convinced Mr Buchanan - who lives in Stonehaven with his wife Rachael and his two other sons, Sam and Ross - to cancel the trip at the last minute.
He added that if the family had not been fishing nearby than Mr Buchan could have been in the water a lot longer.