An obstruction is

1. object that a boat could not pass without changing course substantially, is she were sailing directly towards it and one of her hull lengths from it.
2. An object that can be safely passed on only one side; or
3. an object, area or line that is so designated in a rule.

However, a boat racing is not an obstruction to other boats unless they are required to keep clear of her or, if rule 22 applies, avoid her.