Skin boats are the only marine boats to use seal and walrus skins to cover the boat exterior. The skins are soaked in animal oil to keep them waterproof. They are then sewn together with a special double seam that helps keep the water from seeping through the two hides. Blubber or animal fat rubbed along the seams completes the water proofing process, though keeping the skins waterproof requires constant care. On average, a waterproofing treatment lasts from four to seven days, at which point the oils and fats must be reapplied. Additionally, each day the skins must be dried, and on multi-day excursions requires the help of other umiaqs to lift the hull out of the water.