Besides the two-passenger, tracked sports models, several other kinds of snowmobiles are in use. Among these are the propeller-driven units which are often called airmobiles, or air-snowmobiles. A few manufactured models are available and are expensive, but most are products of home workshops. The main object of these is speed or racing and they are limited to open-country use. They therefore are not always the best for the average sportsman's needs. Other larger tracked models with standard automobile or truck engines have considerable use in northern United States and Canada. They are multi-passenger vehicles and are used for hunting once snow covers the ground, to supply remote hunting camps and to carry out large quantities of moose or caribou meat. Large snowmobiles are also used in Yellowstone National Park throughout the winter for game viewing and photography.