Hydrojets

            Underwater jet engines have become a well-practised propulsion technology for marine applications though it does lack some inelegancy in precision manoeuvres, it does offer incredible power. The hydro jet engine is a remarkably uncomplicated device, the simplest of which has no moving parts. The propulsion concept is similar to a ramjet, the main part of the drive is tube, with a wide inlet and narrower outlet, the chamber in between balloons out. In operation water is drawn into the inlet and before it escapes through the outlet, superheated steam is in injected through a collar around the outlet ring. This injection creates a propulsive effect which by expansion, and localised boiling creates shockwaves that forces the water out of the jet engine, and  indirectly draws cooler water in. As long as the steam injection is continuous the propulsive effect is maintained. The power of these engines is phenomenal, especially when compared to other mechanical methods, such as propellers, even modest units, perhaps half a metre long, can produce dozens of kilowatts of thrust.

            These kinds of engines are installed on vessels (usually surface vessels) and enable extremely high power, and high speed. The drawback to these kinds of engines is that speed is require to maintain the efficiency of the engines. At low speeds the engine may falter or cease to work, at any rate the efficiency of the engine is greatly decreased, though similar machines can be used for pumps. Often a rotor assembly (driven by a motor or turbine) is added to the inlet, this not only improves the efficiency at low speeds, but also can be used as a propeller driven propulsion at low speeds.

            The source of power for this engine is the superheated steam, which is preferentially created from desalinated water, to avoid residue problems. The steam can be generated by a number of different means, but all involve heating the source water to high temperature. The only other pre-requisite for the drive steam is a high pressure, generally in excess of 5 bar, the effect is that steam generation favouring ‘closed’ methods (boilers) are preferred as these pressures and higher are easily achieved. Unfortunately this requisite for the engine is also quite dangerous, the superheated steam on the boat is a potential safety risk, though the entire power generation and propulsion systems are automated and shielded, these systems mean that no operator could be at risk whilst the unit is in operation. The outlet stream from the engine is relatively benign, though any living thing passing through the engine is at some risk, for these reasons grill coverings over the inlets are standard. Other aquatic concerns are fairly limited, the engines are not very loud, nor do they cause much splash.

            These kinds of engines are fitted to many types of boats, though generally large monohulls have extremely well developed large engines of this type. The steam generators and associated power generation preclude to a degree this technology on small craft, though sporty high speed motor boats often use this kind of propulsion system. Less conventional boat designs, such as hydrofoils and other planning craft near exclusively use these engines, as they only require a short depth of water, and their thrust is quite directional and efficient, also as they require high speeds to function efficiently the clumsiness of these engines at lower speeds is less a concern.