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, give her room or mark-room or, if rule 22 applies, avoid her. A vessel under way, including a boat racing, is never a continuing obstruction.
A continuing obstruction is one that takes a considerable amount of time for boats to pass, either because of its size or because of adverse current. Since the above ISAF definition clearly states that a vessel underway is never a continuing obstruction, a cruise ship or tanker which VSK skippers occasionally are faced with qualifies as an obstruction but cannot qualify as a continuing obstructionunless it is aground or at anchor.