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. An object that can be safely passed on only one side and an area so designated by the sailing instructions are also
obstructions. However, a boat
racing is not an
obstruction to other boats unless they are required to
keep clear of her or, if rule
23 applies, avoid her. A vessel underway, including a boat
racing, is never a continuing
obstruction.