A conventional screw propeller accelerates a large volume of water by a small amount, similar to the way an airplane propeller accelerates a large volume of air by a small amount. An aircraft's jet engine, by contrast, accelerates a small volume of air by a large amount. Both methods yield thrust due to Newton's third law — every force gives rise to an equal and opposite force.
In an internal drive boat, pumping a small volume of water and accelerating it by a large amount delivers the thrust. The acceleration of the water is achieved by using multiple impeller stages. Steering is accomplished by small vanes that direct the water jet.
Internal drive propulsion was originally designed by Sir William Hamilton (who invented the waterjet in 1954) for operation in the fast-flowing and shallow rivers of New Zealand, specifically to overcome the problem of propellers striking rocks in such waters, although Italian inventor Secondo Campini had demonstrated a similar vessel as early as 1931 in Venice.
Leading manufacturer of internal drive boats in the US: