Recent Updates

Monday, 13 May 2019

JET PROPULSION OF SHIPS

We have seen various topics such as  force exerted by a jet on vertical flat plate,  force exerted by a jet on stationary inclined flat plateforce exerted by a jet on stationary curved plateforce exerted by a jet on a hinged plate,  force exerted by a jet on a curved plateforce exerted by a jet of water on a series of vanes and force exerted by a jet of water on a series of radial curved vanes in our recent posts.  

Now we will see here the basics of jet propulsion of ships with the help of this post. Before going to study the basics of jet propulsion of ships, we must have information about the principle of jet propulsion

We have already discussed the principle of jet propulsion, in our previous post, where we have discussed the following terms as mentioned here.  
  • Jet Propulsion definition 
  • Jet propulsion of a tank with an orifice 
  • Condition for maximum efficiency 
  • Equation for maximum efficiency 

Now it’s time to discuss the basics of jet propulsion of ships, so let us take one cup of coffee and read this post. 

Jet propulsion of ships 

As we know that Jet propulsion means the propulsion or movement of the bodies such as ships, aircraft, rocket etc., with the help of jet. The reaction of the jet, coming out from the orifice provided in the bodies, is used to move the bodies. 

We will be interested here to study the movement of ships with the help of jet. A ship will be driven through water on the basis of principle and application of jet propulsion. 

A jet of water, which is discharged at the back of ship, will exert a propulsive force on the ship. Ship will have centrifugal pumps which will withdraw water from the surrounding area from sea. This water will be discharged through the orifice, provided at the back of ship, in the form of jet. 

Hence the reaction of jet, coming out at the back of the ship, will propel the ship in the opposite direction of the jet. 

There will be two way through which centrifugal pump will take water from surrounding sea 

Through inlet orifices, orifices which are at right angles to the direction of motion of the ship
Through inlet orifices, orifices which are facing the direction of motion of the ship

Case I –
Jet propulsion of the ship when inlet orifices are at right angles to the direction of motion of the ship

Following figure, displayed here, indicates a ship which is having the inlet orifices at right angles to the direction of motion of the ship. 

Propulsive force exerted on the ship, F = ρ a (V + u) x V

Work done per second, W = ρ a (V + u) x V x u 


Where,
V = Absolute velocity of jet of water coming at the back of the ship
u = Velocity of the ship
Vr = Relative velocity of jet with respect to jet = (V + u)

Case II –
Jet propulsion of the ship when inlet orifices are facing the direction of motion of the ship

Following figure, displayed here, indicates a ship which is having the inlet orifices facing the direction of motion of the ship. 

In this case, the expression for propelling force and work done per second will be identical as we have seen in case I i.e. when orifices are at right angles to the direction of motion of the ship.
Energy supplied by the jet in case II will be different. 

Do you have any suggestions? Please write in comment box. 

We will see another topic i.e. Introduction to hydraulic machines, in the subject of fluid mechanics, with the help of our next post.  

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google   

Also read  

1 comment: