You  cannot use a continuig obstruction to disadvantage a leading boat.

Buoy Room - Continusing Obstruction

RULE 

Part 2 - When Boats Meet

Section C - At Marks and Obstructions

Rule 18.1 When Rule 18 Applies

...it does not apply

(d) if the mark is a continuing obstruction, in which case rule 19 applies.

Buoy room 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... 

PURPOSE

Protect the rights of the leading boat.