This equation states that the volumetric flow into the system must equal the volumetric flow out of the system. IfA1 is greater thanA2, thenV2 must be greater thanV1. In other words, if the input cross sectional area is greater than the output cross sectional area, then the exit fluid velocity will be greater than the input fluid velocity. Hence, more water is moved and thrust is increased for the same input power and torque.
Put in simpler terms this is what happens: One cross sectional area is greater than the other and since the fluid density is constant then the water must accelerate from one to the other. Hence the water is already moving faster as it reaches the propeller than it would on a conventional open propeller. Therefore more water is moved and more thrust created for the same input power and torque.
The nozzle consists of a ring of aerofoil section which forms a nozzle surrounding the propeller. The suction of the propeller causes an acceleration of flow in the mouth of the nozzle and hence a drop of pressure in this region. Since the pressure on the outer part of the nozzle remains relatively unchanged, there is a resulting differential in pressure, which acting on the projected annulus of the nozzle, gives the additional forward thrust.
Maureen May
