http://www.keelsandsloops.org.uk/humber-keel-comrade/humber-keel-origins.html Humber Keel and Sloop Preservation Society - Humber Keel Origins] , accessed 6 February 2011 Early keels were designed to work in waterways which were only 3 feet deep, but could still carry 40 to 50 tonnes of cargo. ''