Scheduling a Single Machine to Minimize the Weighted Number of Tardy Jobs

A ski for snowmobiles and the like comprising a ski runner and a keel member resiliently mounted at one end to the runner and pivotally connected at the other end to the runner. In the illustrated construction, parallel longitudinally extending slots are provided in the runner, and the keel member is channel-shaped to provide spaced-apart depending blades which are vertically movable within the slots. The blade edges normally are below the under surface of the runner, and the blades are moved vertically to place the blade edges flush with the runner under surface when an obstruction is encountered.