A charterer may insist that a charterparty contains a clause which causes the shipowner ot take the risks if the vessel is delayed by a "boycott" by labour. For example, waterside woekers (stevedores) who belong to a union which is affiliated with the Internaitonal Transport Workers' Federation, (ITF), may combine to refuse to handle cargo or to refuse to allow a vessel to handle cargo if the shipowner does not have an ITF-apporved collective agreement with the crew and does not possess a "Blue Certificate".