Definition
A jet drive is a propulsion system that draws water through an inlet in the hull bottom, accelerates it through an internal impeller driven by a marine diesel or petrol engine, and expels it through a steerable nozzle at the transom. Thrust comes from the velocity change of the water mass; steering comes from vectoring the nozzle. There is no propeller behind the boat.
Background and use
Jet propulsion is the default choice in two distinct duties on the superyacht tender deck. The first is shallow-water capability: with no propeller hanging below the hull, jet boats can operate in water depths down to the impeller intake (often 30-40cm) without grounding the running gear. The second is guest safety around the boat: a propellerless transom is a meaningful difference when guests are swimming, snorkelling, or boarding from the water.
Williams Jet Tenders is the name most owners encounter; their 285 to 565 range covers the small-to-medium garage-fit market. Castoldi (Italy) builds larger jet drives used in 8-12m custom limousines and beachlanders. HamiltonJet and Doen produce the heavy-duty units used on superyacht chase tenders and military-derived support boats. On larger superyachts and on commercial ferries, twin and triple jet installations using HamiltonJet HM or HJ series are common.
The trade-offs are well understood. Jet drives consume more fuel than propellers at low and cruise speeds, particularly below planing threshold; the velocity-change mechanism is inefficient at low water-mass throughput. They lose efficiency further when the impeller takes in air or weed. And bollard pull is poor compared to a propeller of equivalent power, which means jet boats are not the right choice if towing capacity matters. Where the duty cycle is high-speed planing in clean water with propellerless safety required, jets are the natural answer.
Related considerations
- Impeller wear is the headline maintenance item; expect inspection annually and replacement every two to four seasons depending on use.
- Weed and debris ingestion is a real-world performance killer; specify a clearable inlet design where possible.
- Jet boats are difficult to reverse or hold station in current and wind compared to propeller-driven equivalents; train operators specifically.
- Cavitation damage on the impeller and tunnel is common in shallow operation; budget for refurbishment.
- Bollard pull is roughly half that of an equivalent propeller installation; do not specify a jet-driven boat for serious towing duty.