Jet Propulsion Principle:( BOATS AND SHIPS PROPULSION)

žThe driving forward of a body by means of jet of gas or fluid.ž

The reaction of high velocity jet issuing from the nozzle provides the necessary thrustž
žIs employed in propelling the ships, aircrafts and missiles.

Used for flights, to hydraulic jet propulsion for high speed boats and pleasure craft.

PRINCIPLE OF JET PROPULSION
ž
Ø Works on principle of Newtons third law of motion.

Ø For example as in the  fig, a blown up toy balloon with its neck closed shows no tendency to move because air inside is  pressing equally in all direction.

Ø If the neck is opened suddenly, balloon shoots away. The escaping air relieves pressure at neck and there is a reaction from the air opposite the neck.

ØIt is the air pushing against the inside front wall of the balloon that propels it forward.

JET PROPULSION OF SHIPS AND BOATS

ØMakes use of hydraulic pressure of water taken in at forepart of the vessel.

Ø Passed  through the vessel to be accelerated  at different stages by the use  of turbines or other pumping means.

Pressure of water passing through vessel will result in a discharge jet of increased velocity. Hence results in motion of boats or ships.

Inlet orifices at right angles to the ship motion

As intake is right angle to direction of motion,inlet velocity=0

ØPropulsive force exerted on ship=density of water * a *(V+u)* V

Ø kinetic energy supplied by jet per second = ½ density of water * a* (V+u)^3

ØEfficiency of propulsion =  2Vu/(V+U)^2

where,

 outlet velocity = relative velocity of jet with respect to ship , Vr

 V= absolute velocity of issuing jet

  u= velocity of moving ship

  Vr=V+u, relative velocity of jet with respect to ship

   a= area of orifices

Inlet orifices facing direction of ship


In this case water enters with a velocity equal to the velocity of ship (u).
Ø
ØKinetic energy supplied by jet= ½ density of water * a * Vr * [Vr^2-u^2]

ØEfficiency of propulsion= [density of water *a* Vr * V*u] / [ ½ density of water * a * Vr {Vr^2-u^2}]

Where, u= velocity of ship

            V=absolute velocity of issuing jet

            Vr= V+u , relative velocity of jet with respect to ship

            a= area of orifices

Fig: Jet propulsion of ship