Quick question...

So you're on port.. windward of you is another boat (also on port). You want to tack onto starboard but the other boat is just taking you way out to the layline... How much of a gap would you need to be able to tack onto starboard and call him to tack also? Just enough space to complete the tack or is there some ettiquette/rule involved here?

Rule 13: while you are tacking you must keep clear of any boat that is not tacking. So if he has to change course to avoid you before you have completed your tack you have broken rule 13.

But then rule 15 comes into play: at the moment you complete your tack (and assuming that he hasn't also tacked) you become right of way boat by being on starboard tack when he is on port tack, BUT rule 15 says that you still have to initially give him room to keep clear - you have to give him enough time and space to keep clear of you without him having to make an unseamanlike maneuver or start that maneuver before you have completed your tack. If he is going to have to tack to keep clear of you then you need to give him enough time and space to make a seamanlike tack (and he doesn't need to start that maneuver until you have completed your tack). Likewise, if he is going to have to duck to keep clear of you, you need to be far enough in front of him that he can safely go behind you without having to make an unseamanlike bear away starting at the point when you completed your tack (and not before).

NB At no point do you have the right to hail him to tack (unless you are approaching an obstruction).

(For the experts in the room, yes I know I have ignored some of the finer details of the rules.)

Sorry Sam, but the other boat does NOT have to take action when it becomes clear that you are tacking (unless that is necessary to avoid contact, in which case the tacking boat has already broken rule 13). It is only once the tacking boat has COMPLETED their tack and become right of way boat that the other boat has to start to take action to keep clear.

Case 27 doesn't actually say that a boat is entitled to 1 to 2 seconds of thinking time, it merely notes that in the specific circumstances of that case the boat concerned "had perhaps one to two seconds to decide what to do and then do it", implying that that wasn't sufficient time for them to avoid contact, and hence they didn't break rule 14.

How much room (time and space) a boat needs to keep clear when another boat acquires right of way will depend upon the specific facts of each case, including e.g. the type of boat and the conditions at the time.

Yes, you are right, the tacking boat does not have RoW until she reaches close hauled but if it is obvious she is tacking the keep clear boat cannot claim extra time to 'have a think about it', she must be prepared to take action the moment RoW hits close hauled. I chose my words badly but, in the real world, it would be a foolish skipper who claimed he thought RoW was bluffing when she tacked.

Originally posted by Rupert The difference between rules and tactics.

Yup...

If the leeward boat is too close to the windward boat, so they cannot complete the tack to starboard, then this gives the windward boat freedom to hold the leeward boat beyond the layline and make the tack to starboard when they have a comfortable lay.

The leeward boat has two tactical options to combat this.
1) Luff aggressively (assuming overlap established from behind) so that the windward boat tacks off before the lay. This only works from a long way out, or if there is significant starboard still to do and not a lot of boats on the lay.  
2) Foot off to create enough gap so that a tack can completed (sometimes called 'gaping off'. The leeward boat is then free to tack on a layline which suits them. The windward boat will then have decision to tack under and tight on the lay, or duck, concede position, but sail a safer layline. 

You can see this play out from 5:50 onward in this video
[TUBE]79X_-cScgm0[/TUBE]
5:55 we tack on a port boats leebow, for a long port approach to the windward mark. I was hoping to leebow them and pinch up, then tack clear ahead on to starboard at the layline. 
up to 7:12 I try to squeeze them out, but it doesn't work. 
7:12 onwards they are controlling in that I can't complete a tack to starboard, they hold us out past the layline. Perhaps I should have footed off to create a tacking gap, but in this instance it would have meant going out in to more current. So I just accept them holding us out past the layline. 

Lesson really was to have crossed them a 5:40 and been the inside boat in less current.