The form is mainly used intramurally at the University of Cambridge and the University of Oxford. It is particularly suited where the stretch of water available is long but narrow, precluding side-by-side racing. Bumps racing gives a sharper feel of immediate competition than a head race, where boats are simply timed over a fixed course.
A bump is made when any form of contact is made with the boat in front, or when the stern of the boat behind completely passes the bow of the boat in front. Outright collisions are neither necessary nor encouraged. Due to the fact that bumps racing usually takes place on narrow stretches of water, when contact does occur it is possible for two or more boats to become tangled up. This can lead to the racing line becoming blocked. This can be very dangerous and the possibility of boats getting damaged is high. To avoid this, the cox of the boat being bumped can concede as soon as slight physical contact occurs or even once it is inevitable. Nonetheless, collisions involving several boats are common in bumps racing. An overtaking bump is relatively rare simply because it is easier to make contact with a rival boat than it is to overtake it. A bump of this kind usually occurs when a boat erroneously crashes.
In Cambridge, and at Oxford during Eights Week, once a bump has occurred both crews pull over to the riverbank and take no further part in that race; in Oxford during Torpids a bumping crew pulls over but the bumped crew must continue racing over the entire course and can be bumped by more than one crew per day. Crews in Torpids tend to concede bumps early to avoid being entangled with the crew that caught them: should they be unable to continue, the rest of the division may row past scoring bumps on the bumped crew, sending it to the bottom of the division in minutes. In other competitions there is less incentive to concede before physical contact is made.
The starting order of each day's competition is based on the previous day's results. Any boat that has been bumped starts behind the boat or boats that caught it, whereas a boat which reaches the finish line without either bumping or being bumped is said to have 'rowed over' and stays in the same position. The objective is to gradually progress up the start order day by day by bumping. Typically the first day's starting position is based on the final positions from the previous year, though in lower divisions the boats may be placed according to qualifying races held a few days before the event. (This allows colleges to introduce new crews, hoping that they will move up into the fixed divisions.)
Since all boats row at the same time, it is possible that the boat in front of yours may catch the boat ahead of it before you catch them. Since (except at Oxford during Torpids) these boats both then drop out, you have lost your quarry; but you can still attempt to catch the next boat ahead of you still racing (most often the one which started three places ahead). Success in such a case is called an overbump and, exchanging places with the boat you bumped, you move a full three places up the start order. In rare circumstances boats are able to bump the boat five places ahead — a double overbump — and a very few triple overbumps (of the boat seven places ahead) have been known. Overbumps and variants thereof are most common in the lower divisions.
The ultimate achievement in these competitions is to bump up a place (or more) on each of the four days; a crew achieving this is said to have won its blades. Traditionally members of such a crew purchase a rowing oar in the crew colours and inscribed with the crew names to hang on their wall, though today some purchase a miniature replica for reasons of practicality. 'Blades' are also awarded to the crew that finishes first in the overall order; such a crew is said to be 'Head of the River'. Crews that go down four places are awarded the wooden spoon, and the bottom crew at the end of the competition is known as the 'Tail of the River'.
At Cambridge, the most successful college boat club over the four days of the May Bumps is awarded the Pegasus Cup, sponsored by Milton Brewery.
In both Oxford and Cambridge, there are also separate Town Bumps races in which local clubs compete. Oxford's races, run by the City of Oxford Rowing Club (CORC) are now open to all-comers and are raced in fours, all races taking place on the same day. Cambridge's races are run under the auspices of the Cambridgeshire Rowing Association and are run exclusively in eights, over four nights There are typically four mens divisions with 16 boats (plus a sandwich boat) in each division , and two women's divisions. The bumps are fiercely contested, and the ideal that the bumps should be for local rowers can lead to disputes over whether crews are "legal" Although few rowers worldwide frequent rivers as narrow as the Cam or the Isis, bumps races are sometimes contested elsewhere.
The University of London medical schools' boat clubs United Hospitals Boat Club hold a bumps race over three days each May after exams on the River Thames, racing from upstream of Kew Bridge adjacent to the Royal Botanic Gardens, Kew ending at the UL Boathouse at Chiswick. Traditionally involving only medical school crews, in recent years alumni and non-medical crews have been allowed to enter as invitational crews.
Eton College, where bumping races were first devised and then carried to Oxford and Cambridge by ex-pupils, still has an annual bumping race. It takes place over four evenings, in early May. There are two divisions, with the first division racing after the second division have cleared the water, the boat at the top of the second division becomes a sandwich boat. There is usually one boat entered per house, and one boat entered by college. Because of the dangerous nature of the sport, only fours are used, and only the second and third years (that is, D and E blocks) may row. On the fourth evening there are prizes for the leaders of the chart and also a 'Bumping Cup' for the boat who has made the most bumps over the four nights. The event is marshaled by senior rowers and rowing prefects called The Monarch. The crew training is mainly pupil driven.
A bumps chart is a graphical representation of the week's results; each boat's fortunes can be traced as its line on the chart rises and falls.
Incidentally, in Oxford at least, crews used to race every day until there were no more bumps (i.e. until they were nominally in speed order). This historical set-up could lead to weeks of racing and was therefore abandoned in favour of a four-day version more than a hundred years ago.