PRINCIPLE OF OPERATION OF A PROPELLER-NOZZLE SYSTEM

The action of a shroud in bringing about the propulsive gain is easy to see in a qualitative sense and, as a result of its action, subsidiary changes are necessary in the design of the propeller, not only in pitch, but also in diameter.

From the desktop of

Leonardo Montoya

The flared entry of the shroud causes the water to be accelerated more than would otherwise be the case ahead of the plane of the propeller. Whereas the flow toward the propeller normally takes up its own natural streamline shape, which changes with change in propeller loading, the shroud, by providing rigid boundaries to the flow, forces the inflow to follow a defined course. The shroud is therefore an interfering body, which causes an increase in velocity at the plane of the propeller.

Anything which interferes with natural flow to cause a change in velocity at the plane of the propeller upsets the regime, and causes a force to act upon the interfering body. Where the flow at the propeller is reduced, this force will be deductive from the propeller thrust, but where velocity is increased, the force developed will be additive to the propeller thrust. This is the case with a properly designed shroud. The total thrust produced is thus formed of the addition of two distinct and separate thrusts, one the normal propeller thrust, the other the reactive force which is developed upon the shroud. The propeller/nozzle combination must therefore be viewed as a single propulsive unit, and not separately.

At high slips, the reactive force is very large, accounting for barely half of the total thrust. As slip is reduced, the reactive force is also reduced in proportion. It will be clear that the reactive force is not transmitted to the hull by way of the thrust block, but by the connection of the nozzle to the hull. Because of the increased velocity at the propeller plane, the thrust of the propeller is actually less than without the nozzle, despite the overall thrust increase, so that the thrust block is less loaded than otherwise.

Reference:  Strategy of propeller design (Hannan)

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