How does a jet boat work? Inside New Zealand's river machine

Hamilton did not start from nothing. In the early 1950s he followed the American Hanley hydrojet, which used a pump to draw water in and push it out through a steerable nozzle.¹³ His first version used a round centrifugal pump that fired water out under the boat and reached a modest 11 miles per hour.¹⁴ It moved, but slowly, and the nozzle still sat beneath the hull where rocks could reach it.

The breakthrough came in 1954, when the jet stream was moved so that it left the boat above the waterline.⁹ Speed rose to 17 mph and every underwater appendage disappeared.⁹ The first unit of that kind was named Quinnat, a vertical-shaft centrifugal pump driven through a right-angle gearbox.¹⁵ Its first trials took place on the dam and water race at Irishman Creek; then, early in 1954, it made a slow but successful run up the Waitaki River.¹⁶

Hamilton was modest about all this. He often said he had not invented the waterjet at all and handed the honour to Archimedes.¹⁷ That is fair in a way: pumping water to push a boat is an old idea. What he did was make it rugged, simple and shallow enough to work on real New Zealand rivers, and that is the version every Queenstown boat still uses. Next time a driver says the boat was invented here, that is the story behind it.

The single most asked question after a ride is how shallow can a jet boat go, and the honest answer depends on the boat. The principle is the same everywhere: HamiltonJet's intake sits flush with the hull bottom, so the boat can reach shallow water and even land on a beach without damaging its drive.³ With no propeller, rudder or shaft below, a jet boat on the plane rides on a thin film of water.

Operators quote their own figures. Shotover Jet says its boats can manoeuvre in water as shallow as 10 cm in the canyons.²⁶ KJet goes further for the braids of the lower Shotover, saying its boats run in water less than 5 cm deep because the jet unit is flush with the underside.²³ Both are talking about a boat already skimming at speed; at a standstill any hull sits deeper, so drivers keep moving when the water gets thin.

There is a safety side to the design too. HamiltonJet notes that having no underwater appendages lowers drag and makes the boat safer around people and wildlife in the water.²⁷ Shotover Jet puts the contrast simply: a propeller boat works best on calm, flat water.²² On a lake that is fine; on a river where the bed changes after every flood, it is the jet that keeps going while a propeller boat would be stuck on the first shingle bar.

The design kept improving quickly. The first Rainbow units appeared in 1956, and about 100 were made and sold in New Zealand.^30,31 In 1957 came the Chinook, a twin-impeller axial-flow unit that pushed water straight through and was far more efficient.¹⁰ In 1963 the Colorado series simplified the design so much that it halved the cost of a unit.¹² Each step made the boats cheaper, lighter and better on rough water.

The machine soon proved itself far from home. In 1960 his son Jon played a key part in the Colorado River expedition that became the first to journey upstream through the Grand Canyon.¹¹ In the 20 years that followed, jet boats went up the Sun Kosi, the Sepik, the Zaire, the Ganges and the Amazon.³² A farm workshop idea had become the way to reach places no other boat could, from Himalayan rivers to tropical floodplains.

Bill Hamilton was knighted in 1974 for services to manufacturing and died on March 30, 1978.^33,34 The company he started says it has now installed over 60,000 waterjets worldwide, in ferries, rescue boats and patrol craft as well as river runners.³⁵ The boats you see in Queenstown are the most visible part of that legacy, but far from the largest; the same idea now moves ferries carrying commuters.