Water jets are no longer used just for thrill-seeking jet-skiers. Now larger boats in a variety of applications are using the technology for propulsion – so what’s powering the trend?
The first thing that may come to mind when you think of water-jet propulsion is the jet skis that are popular with holiday-goers and adrenaline junkies. Traditionally the system has been used in small vessels such as these and the jetboats used by coastguards, the navy and others. Since 2000, however, this technology has increasingly been used by larger vessels such as military ships and ferries. But what makes water jets (also called pump jets) appeal more to operators in these fields than traditional propellers?
Marine propellers, acting in combination with rudders, have been the main way of propelling and positioning a vessel. But in the 1950s, New Zealander Sir William Hamilton developed a practical water jet, initially for use in shallow, fast-flowing rivers where propellers could hit the bottom. The technology is now widely used in a range of marine craft including patrol boats, surface effect ships, hydrofoils and motor yachts.
What is a water jet
A water jet creates a propulsive thrust from the reaction created when water is forced in a backwards direction – the effect described in Newton’s Third Law of Motion: ‘every action has an equal and opposite reaction’. In a jetboat, the discharge of a high-velocity jet stream generates a reaction force in the opposite direction, which is transferred through the body of the jet unit to the craft’s hull, thus propelling it forward.