I have to bring this simple point up since it is clear that at least one good helm doesn't understand and had me rafted up at a recent event.

The basic rule is that at a windward mark no overlap can be claimed.  I was in the Medway Regatta and a Buzz from the Medway YC didn't understand this situation.  The start was from the Wilsonian Club line to a club bouy 200m to windward.  The line was long enough to lay the bouy on port.  However such a short beat meant that many boats would be arriving at the same time, so I elected to start further down the line so that I could tack and approach the windward mark on starboard.  The offending Buzz however arrived on port and expected every starboard tacker to give him room, I narrowly avoided cutting him in half but ended up locked to him as most of the fleet sailed away.

Bad enough but when we had rounded the mark I put up the gennaker and he shouted across that I wouldn't be able to do my turns with a gennaker up.  That was the final staw but as he was sailing from a different club I was not able to meet him afterwards and point out the error of his ways.

Thank you for listening - I feel better now.

"The basic rule is that at a windward mark no overlap can be claimed"

Is that the same for two boats on the same tack with an overlap within the two boats length?

The guy obviously wasn't trying to claim an overlap. He just didn't understand the overiding nature of Fundamental Rule 10:

"When boats are on opposite tacks, a port-tack boat shall keep clear of a starboard-tack boat."

He was probably thinking that he had the right to call for water (a right that he could not possibly have had)

An overlap (for water) can be claimed at any mark - including the windward mark - provided both boats are on the same tack.

Rule 18 governs room at a mark or obstruction, however it does NOT apply when;