A large sound alerted four fishermen to a 150 horsepower motor falling off the back of their boat about 10 kilometres south of Killarney.

Koroit police Senior Constable Chris Kelly said the two men and two 11-year-old boys were heading out to chase tuna about 10am when the incident happened.

"They were going out fishing with another boat about 10am and were about 10 kilometres offshore, almost directly south of Killarney, when the motor fell off," he said.

"They heard a large donk sound, turned around and the motor fell off the back of the boat.

"The 5.75 metre Cruise Craft started taking on water. They radioed for help and their friends in the other boat were only 50 metres away."

Senior Constable Kelly said the two men and two boys were able to transfer to the other boat and went back to Port Fairy.

The fibreglass boat was bought 18 months ago for $50,000.

"They were very lucky indeed. They would have been in real strife if they were by themselves," Senior Constable Kelly said.

"They went back to the Port Fairy boat ramp and no one was hurt or even wet.

"The two boys were checked out by ambulance paramedics and they were OK."

The boat is believed to be between 15 and 20 years old and the 150hp Suzuki four stroke motor is understood to have ripped off the whole transom.

The Port Fairy Marine Rescue Service launched a search for the boat in the hope of locating and retrieving it.

Duty officer Tony Goode said rescuers spent four hours looking for the vessel but were unable to find it.

He said they had been told by Water Police to go out straight away to ensure it wasn't posing a danger to other vessels.

He said the vessel would have sunk within 20-minutes of the fishermen getting off the boat with Marine Rescue arriving at the GPS coordinates soon after.

"They were very, very lucky," he said. "We were very quick to get out there. There was no sign of it."

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