Have you been crashing delicate rowing boats at top speed, catching crabs and showing off your race starts for spectators at the Plough?

If you have, then you've been at the Cambridge Town Bumps this week participating in a rowing tradition that is more than 160 years old and takes over the banks of the River Cam for four hopefully balmy July nights every year.

Both Oxford and Cambridge universities hold their own student versions of these races twice a year, but only Cambridge boasts its own for townies, where anyone who can put a crew of eight together has a chance to wear the willow that signifies race victory on a night.

"The glory of the Bumps is that everyone can have a go," former Cambridge University chief coach and ex-New Zealand international Duncan Holland told Reuters.

About one percent of the 100,000 or so residents in Cambridge actually row in the bumps, while thousands more line the banks and pubs like the Plough along the river to cheer on normally mild-mannered townsfolk thrashing down the river.

The bumping race concept is fairly simple.

Boats containing eight crew are lined up along the river about a boat length and a half behind each other. A crew must catch the boat in front to get a "bump."

Getting a bump earns the crew the right to adorn themselves with strips of willow from trees along the bank. But it also means the crew advances one starting position for the next night and must chase - usually - a faster crew.

A bump each of the four nights wins a crew their "blades," old-fashioned oars decorated in honour of the victories.

Elite crews - often seeded with top university or international rowers - compete for the honour of becoming Head of the River, the crew who starts first in the first division and has no one to chase, but everyone chasing them.

It sounds all rather genteel, with evening BBQs outside boathouses by the river after the races and dinners on the last night at some of the lovely dining halls in Cambridge colleges.

But don't be fooled, it's carnage on some nights.

Race marshals and bank party supporters for each boat ride along the tow path on bicycles at breakneck speeds shouting encouragement and using complicated whistle signals to tell their crews how close they are to a bump.

Boats career into the river bank, hit each other and sink. Sometimes crew members are ejected at speed from the boats or lose a tooth when they lose control of the oar, known as "catching a crab."

The two most dangerous spots in the boat are the coxswain (the person who sits in the stern and steers the boat) and the crewmember rowing in the bow.

Coxswain Fiona Knights said sometimes crews coming up behind will try to hit the coxswain to force them to concede.

"I've had a blade hit me in the lower back, scrape all the way up to my ear and rip out my earring," she said.

Andrew Watson, who comes from a celebrated Cambridge rowing family and at 42 years old is still competing at elite events like the Henley Royal Regatta against top rowers half his age, said most things about the bumps have remained the same since he first rowed it 27 years ago.

"When we used to row then, if you could taste the blood in your mouth, then you knew you were doing it," he said. "Now they're wearing heart monitors and using sports science."

Local rowing legend Paul Knights, who has been head of the river five times and boatman for Queens' and Magdalene College since 1984, said the only difference between the Town Bumps and the university version was the full day's work first.

The 51-year-old former British indoor rowing champion and another veteran of Henley said the unique beauty of bumps racing instead of the usual side by side racing at regattas has brought him back every year since 1972.

"It's the thrill of the chase," he said.

"You're the hunter being hunted."