The advantages of jet drives have been obvious for years,
but recently they have picked up in popularity. This has happened in large part
because of their success in PWCs built by several companies, and in jet-powered
runabouts built by Yamaha and Sea-Doo. In fact, the 23-foot Yamaha runabout is the largest
selling boat that size in the U.S. Maine boat builder Hinckley has been installing
them for over 10 years in its picnic boat series, Huckins has installed them in
its new 56-foot boats, and last year they were even put in a new U.S. warship complete
with joystick steering. They are adaptable to virtually all types of engines from
outboards to large diesels.
Drawing of a Hamilton jet unit. Hamilton has
been making jet drives since 1954.
Water jet propulsion has many advantages over other forms
of marine propulsion, such as stern drives, outboard motors, shafted propellers
and surface drives. These advantages include...
• Precise steering control at all boat speeds
• "Zero Speed" steering effect provides 360° thrusting ability for docking and holding
• Sideways movement possible with multiple jet installations.
• High efficiency astern thrust with "power-braking" ability at speed
• Propulsive coefficients as good or higher than the
best propeller systems achievable
at medium to high planing speeds
• Hole shots faster than prop-driven boats
• Better fuel efficiency than most other propulsion options
Low Drag and Shallow Draught
• Absence of underwater appendages reduces hull resistance
• Shallow draught - the water jet intake is flush with hull bottom to allow access
to shallow water areas and beach landings with no risk of damage to the drive
• Ideal for skinny, rocky rivers
• No protruding propulsion gear eliminates impact damage or snags
• Minimum downtime and simple maintenance routines
• No worries about bent shafts or struts
• No cutlass bearings to be replaced
• No stuffing box or rudder shaft leaks
Smooth and Quiet
• No hull vibration, no torque effect and no high speed cavitation
• Low underwater acoustic signature
• No exposed propeller means safer environment for swimmers and animals
• Warning: Jet thrust can be dangerous
Maximum Engine Life
• Jet unit impeller is finely matched to engine power
• Power absorption is the same regardless of boat speed
• No possibility of engine overload under any conditions
• Single packaged module
• No heavy and expensive gearbox required for many installations. Simple driveline
from engine to jet coupling
How a Water Jet Works
A water jet generates propulsive thrust from the reaction created when water is
forced in a rearward direction. It works in relation to Newton's Third Law of Motion
- "every action has an equal and opposite reaction". A good example of this is the
recoil felt on the shoulder when firing a rifle or the thrust felt when holding
a powerful fire hose.
Put simply, 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, propelling it forward.
In a boat hull the jet unit is mounted inboard in the aft section. Water enters
the jet unit intake on the bottom of the boat, at boat speed, and is accelerated
through the jet unit and discharged through the transom at a high velocity.
The picture shows where water enters the jet unit via the Intake (A). The pumping
unit, which includes the Impeller (B) and Stator (C), increases the pressure, or
"head", of the flow. This high pressure flow is discharged at the nozzle (D) as
a high velocity jet stream. The driveshaft attaches at the coupling (F) to turn
Steering is achieved by changing the direction
of the stream of water as it leaves the jet unit. Pointing the jet stream one way
forces the stern of the boat in the opposite direction which puts the vessel into
Reverse is achieved by lowering an astern deflector (E) into the jet stream after
it leaves the nozzle. This reverses the direction of the force generated by the
jet stream, forward and down, to keep the boat stationary or propel it in the astern
Installation is Easy
This is a cut-away drawing of the stern sections
of a conventional boat.
The water intake plate is fitted to the boat’s
bottom and the unit is installed.
The transom plate is bolted over the jet drive
nozzle and gasket.
Finally the engine is connected to the impeller
drive shaft. In this example a jack shaft is used to connect the engine with the
jet drive unit.
Drawings and some material courtesy of Hamilton Jet.
See Capt. John Wenz explain how Yamaha jet drives work --
engine jet drive video by Yamaha --