Adding jet propulsion to yacht requires weighing pros and cons

Nov 8, 2016 by Rich Merhige

Water-jet propulsion can best be described as the propulsive force provided by the jet-reactive thrust of high velocity water ejected through a nozzle. This is similar to the propulsive drive of a jet aircraft, however, rather than using air as the working substance, water is fed into a high-powered pump impeller, which adds high velocity energy and pushes it aft through a nozzle.

This type of propulsion system works in relation to Newton’s Third Law of Motion: Every action has an equal and opposite reaction. Propulsive thrust is generated within the system from the reaction created when water is forced in the aft direction.

Waterjet propulsion goes back to 1661 when Toogood and Hays described a ship with a channel down the center of a vessel with an Archimedes screw. Water was sucked in at the bow opening, accelerated by the screw pump and ejected at the stern, the higher velocity providing thrust.

Benjamin Franklin proposed a waterjet boat in 1775 and only a few years later in 1787 James Rumsey demonstrated a waterjet vessel to George Washington on the Potomac River. James Hamilton was the pioneer of the modern waterjet for small craft when, in 1954 in New Zealand, he developed a small high-speed waterjet-propelled boat.

How this happens is simple. A high velocity jet stream of seawater is discharged through the waterjet back into the sea. This high-speed water generates a reaction force in the opposite direction, transferred through the body of the jet unit to the craft’s hull, which propels it forward. For speeds above 30 knots, waterjets are more efficient than conventional propellers.

A typical waterjet system includes a flush-mounted inlet channel on the hull bottom guiding the water to a rotating pump impeller, a stationary guide vane package, an outlet nozzle and a steering/reversing mechanism. The water from under the vessel is fed through an inlet duct to a precision inboard pump, usually mounted at the transom, adding head to the water. This head is then applied to increase the velocity when the water passes through an outlet nozzle into the ambient atmospheric pressure. The steering and reversing forces are generated by deflecting the jet stream by a flat-bucket gear, which is normally hydraulically operated.

Why would a boat operator choose a waterjet system over a standard propulsion system? There’s many advantages that are noteworthy, but these systems also have disadvantages. Here’s a brief look at both sides. Advantages include:

* High efficiency. At medium to high planing speeds, there is a higher propulsive coefficient than even the best propeller systems achievable.

* High speed. Many waterjet systems are optimized to reach speeds up to 50 knots.

* Excellent maneuverability. Precise steering control is available at all speeds.

* Low drag and shallow draft. Absence of an exposed propeller reduces hull resistance. The waterjet is also flush with the hull bottom, making it possible to access shallow water areas.

* Smooth and quiet. No exposed propeller means no hull vibration, no torque effect, and no high-speed cavitation.

* Maximum engine life. There is no possibility of engine overload under any conditions.

* Marine Safety. No propellers to harm people in the water or marine life.

For some, though, the disadvantages far outweigh the benefits:

* High cost. Most waterjet propulsion systems cost significantly higher than most standard propulsion systems.

* Low efficiency at low speeds. The waterjet propulsion system has a lower propulsive coefficient than standard propulsion systems at low speeds. The thrust force is dependent on the change in momentum, and the water-jet develops less thrust at low and intermediate speeds because of the inherent difficulty it has in handling large mass flow rates at low speeds, whereas the opposite is true for conventional propellers.

* Clogging. The seawater intake grill can become easily clogged with sea debris.

The success of a waterjet installation requires a systemic analysis of the jet’s performance across the vessel’s entire speed range. The hull form, hull bottom loading, hull resistance, number of propulsors, jet size selection, and power requirements (engine size) are all considerations in a waterjet vessel.

Rich Merhige is owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Ft. Lauderdale ( Comments below.