Dramatic footage shows four men desperately clinging to a boat on the edge of 40m dam in South Africa

Terrifying footage shows four men desperately clinging onto a stranded boat on the edge of a 40m dam spillway as heavy flowing water rushes beneath them.

The heart-stopping moment took place at the Grootdraai Dam in Mpumalanga, South Africa on December 29 after the boat experienced mechanical failure and drifted towards the damn wall, getting trapped at the spillway.

The four men gripped onto the boat, using their weight to stop it from being swept over the spillway edge and down the staggering drop into the Vaal River.

The National Sea Rescue Institute (NSRI) launched a multi-agency operation at 7.15pm to try rescue the men.

In a statement, the NSRI said there was many risks involved and “decisions made in seconds could determine success or failure.”

Rescue boats that were launched several kilometres upstream tentatively approached the boat, calculating the best way to go about the unfolding situation.

The strength of the white water initially made it difficult for the men to be reached.

After several hours, officials decided to divert the water flow by opening three sluice gates at the spillway which automatically created a vacuum under the boat, preventing the boat from being swept over the edge.

At around 9.40pm, the boat was stable long enough for rescuers to go in and attend.

The four men, all of which wearing life jackets, were rescued using a throw line with a buoy attached and pulled to safety.

The sheer force of the water meant rescue crews only had one chance, and thankfully they were successful.

Once the men were far enough from the edge, they were pulled onto the rescue craft and assessed by paramedics.

Thankfully all the men came out of the dramatic ordeal unscathed.

However, their boat was not quite as lucky - it still remains trapped on the edge of the weir.

The NSRI is being commended for saving the four men’s lives.