An obstruction is

1. an object that a boat could not pass without changing course substantially, if 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.