PTP:
Quote

Proper Course A course a boat would sail to finish as soon as possible in the absence of the other boats referred to in the rule using the term. A boat has no proper course before her starting signal.


You will most often not need it, but if sailing in a mixed fleet proper course might differ wildly. Especially so downwind.
If you go trough the case book, you will find lots of examples of proper course and how to handle it. If you are in doubt, the other boat is sailing proper course <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />
http://www.sailing.org/rrs2005/casebook2005.pdf


Thanks for the replies on the original question. Lets go hair splitting..

The "leeward did not give us time to avoid him" defense, would that hold up as there is time to avoid him, but you will have to hit the mark?
I dont see how that defense can hold if windward have time to tack off, so lets disregard that.

What if the windward marker was an iron pole, or an inflatable with some sort of metal on top and bottm (there are some sort of fishermans marker with a huge iron ring on top that is often used here. I dont go near the thing!). Obviously, you would get some serious damage from hitting an iron pole, and light damage from hitting the "fishermans marker"?
I that case, believe that leeward would have to give both boats to windward room to avoid damage to their boats, but must/should protest. We are still asuming that boats to windward dont have time to tack off or shoot over the mark.