Bumps racing is done in Cambridge and Oxford, and also, I believe on the Thames between colleges of London University (though I'm not totally sure about this). I think it originated because the rivers in Cambridge and Oxford are very narrow and preclude side-by-side racing over any reasonable distance.
Bumps racing is processional, it is done in divisions of eighteen boats at a time, each separated by 1 ½ lengths. All the crews are started at the same time, usually by means of a cannon. The object is to catch and touch (yes, touch!) or "bump" the crew in front, without being bumped by the crew behind. If one crew catches the one in front and bumps, the two crews pull into the side of the river whilst the others continue until they have crossed the finish line, or have been bumped/bump themselves. Getting all the way down the course is termed "rowing-over".
The bumps are held over four days and the starting order is inherited from the previous year. When a bump is made, the crews switch places in the following day's starting order, hence a crew can "bump-up" four places in four successive days. If this feat is achieved, crew members are entitled to a painted oar, and the cox a painted rudder, with the names of the crew, college crest, crews bumped etc. This is termed "winning blades". If they reach the head of the first division, they try and remain there until the end of the final day by rowing over, and are termed Head of the River (though, of course only crews within five places of the head have any chance of getting there). Crews at the head of the lower divisions also row over at the bottom of the next division up, and try to "bump-on". Further complications: if two crews bump out ahead during the race, you can proceed to try and catch the crew ahead of them; if achieved, this is termed an over-bump. In the top divisions where crews are evenly matched, it occurs rarely, but in the lower divisions it is more common, as are horrendous pile-ups caused by crews and coxes who are inexperienced and don't get out of the way after bumping sufficiently quickly to avoid the chasing crews.
In Cambridge there are University bumps between the colleges twice a year. The Lent Bumps are usually around the third week in February, and the May Bumps are the second week in June (or so). The town clubs hold bumps in July. Bumps racing is probably the most exciting rowing to watch, with so many crews and the prospect of spectacular crashes. Also, people follow the crews down the river on bikes, signalling by means of whistles, bells and hooters to their crew to tell them how close they are to bumping the crew ahead. Making a bump is very exhilarating, and being bumped is dreadful. Watching that crew sneaking up on you and getting overlap is not at all pleasant. Making the bump is rather different, because, of course, you can't see it (unless you're the cox).
Well, I hope that goes some way to explaining. If you have any other questions on bumps, drop me a line.
Steve [Travis]
Bath Univ BC