The term "barging" isn't in the racing rules at all. Nevertheless, the hail "you're barging" is often heard at the starting line, and commonly used in later discussions.

So, what exactly is "barging"? A boat "barges" when she tries to claim room to which she is not entitled at a starting mark. The origin of the term refers to a boat trying to stick her nose in at the "signal barge".

The rules surrounding barging may seem complicated at first, but they're really quite simple. This is one of the instances where RRS 18 "Mark-Room" does not apply. Take a look at the preamble to Part 2, Section C "At marks and obstructions". It says "Section C rules do not apply at a starting mark surrounded by navigable water or at its anchor line from the time boats are approaching them to start until they have passed them". Provided that
  1. the mark is surrounded by navigable water (which is almost always the case), and
  2. boats are approaching the line to start, then
rule 18 (and 19 and 20) does not apply.

Therefore, an outside boat is not obligated to give an inside boat mark-room at the start. When boats overlapped on the same tack approach a signal boat to start, they are governed by RRS 11 "On the same tack, overlapped", not rule 18. As long as she complies with rules 14, 15, 16, and 17, a leeward boat can pinch a windward boat off at the mark, forcing the other boat to drop behind or peel away.

There are a couple of caveats. First, the leeward boat (L) has to "close the door" on the windward boat (W) in time. If L changes course to shut W out, then (in accordance with RRS 16.1) she must give W room to keep clear. L cannot force W into the mark. L must head up early enough that W can turn away and avoid the mark in a seamanlike way. If L waits until W's bow is under the mark (or so close that she can't tack), then L may not luff up.

Secondly, if L has closed the door in time and W "barges in" anyway, L must attempt to avoid contact per RRS 14. L must bear away and let W in. Her only recourse is to protest.

I'll try to put together some illustrative diagrams and post them when I have time.

I hope that helps,
Eric Rasmussen
US Sailing Certified Judge
Chair, SAYRA Appeals Committee