The “Push/Pull” Concept
To understand this concept, let us freeze a propeller just at the point where one of the blades is projecting directly out of the page. This is a right-hand rotation propeller, whose projecting blade is rotating from top to bottom and is moving from left to right. As the blade in this discussion rotates or moves downward, it pushes water down and back as is done by your hand when swimming. At the same time, water must rush in behind the blade to fill the space left by the downward moving blade. This results in a pressure differential between the two sides of the blade: a positive pressure, or pushing effect, on the underside and a negative pressure, or pulling effect, on the top side. This action, of course, occurs on all the blades around the full circle of rotation as the engine rotates the propeller. So the propeller is both pushing and being pulled through the water.
- The fore and aft thrust, is a force which causes a ship to move ahead or astern through the water. It is most efficient when the ship is moving ahead.
- [Full Astern Axial Thrust = Half ahead Axial Thrust]
- The stopping distance of a ship depends upon axial thrust
TRANSVERSE THRUST/ SCREW`S EFFECT
- Sideways thrust of the propeller blades as they rotate which produce turning effect
- The upper blades meet less resistance because it is near the water surface and lower blades experience greater reaction to motion of propeller. The upper blade can’t easily cancel out the opposite effect of the lower blades.