Are not all starts gates???

'Gate' starts, otherwise called 'rabbit' starts are described in the 505 Class Standard SI

SI 12.1 and 12.3 prohibit any boat from interfering with the pathfinder (who is on port tack).  This does not rely on the definition of 'obstruction'.

SI 10.6 designates the Gate Boat (sometimes referred to as 'guard boat') as a starting mark, so boats starting are required not to touch it by rule 31, and rule 19 is disapplied by Preamble to Section C, so, again there is no reliance on the definition of 'obstruction'.

We use similar text as referred to above but, whilst it prohibits any boat interfering with the gate boat, if a boat was being pushed into a position that will interfere with the gate boat by a third right of way boat (eg a leeward boat pushing a windward boat up into the gate boat), it is unclear what rights it would have in attempting to avoid that position.  As in the original question, if one chose to designate the gate boat as an obstruction, would that clarify the situation? 

Are you talking about a rabbit start with, or without a Guard Boat?  In a club race without a Guard Boat, you would need different language.

I wouldn't be very comfortable about the 'obstruct' language in SI 12.1:  why did they not say 'keep clear of the pathfinder'?

I think the concept is that the Guard Boat, which is the starting mark will be between other competitors and the  pathfinder, except for a starboard tacker crossing in front of the pathfinder, when the 'obstruct' language will work.

For the SI to designate the pathfinder, a port tack boat, as an obstruction (with respect to starboard tack or port tack leeward boats)  would contradict the Definition  Obstruction third sentence, and would be impermissible under rule 86.1.  ISTM that a SI requiring boats to keep clear of the pathfinder during the starting sequence would make the pathfinder an obstruction without the need to say so.