Special Arctic design features included the rounded lines of the ship's body below the water line, an additional belt of ice-floe resistant flush skin-planking (made of oak or larch) along the variable water-line, a false keel for on-ice portage (and for damage prevention from running aground in shallow waters), and the shaft-like upper part and wide lower part (below water-line) of the rudder. Another Arctic feature was the invariable presence aboard any koch of two or more iceboats and of a windlass with anchor rope. Each iceboat had the cargo capacity of 1.5 to 2.0 metric tons (3,300 to 4,400 lb) and was equipped with long runners () for portage on ice. If a koch became trapped in the ice, its rounded bodylines below the water-line would allow for the ship, squeezed by the ice-fields, to be pushed up out of the water and onto the ice with no damage to the body.