High thrust at low speed
If high thrust at low speeds is important for you, then a nozzle is the best solution. The lifting action of the nozzle profile tends to pull the nozzle forward. How does this work?
On rudder propellers with a nozzle, the velocity of the water flow is increased and the pressure is reduced. As a result, the thrust and the torque of the propeller decreases slightly, while a circulation with inward-directed force takes place. This force provides for a forward-acting element, giving the nozzle a positive thrust which is generally larger than the reduced thrust of the propeller. Due to the small clearance between the nozzle and the tip of the propeller blade, the tip vortices are reduced thereby increasing efficiency. However, the frictional resistance of the nozzle increases as the speed increases, and will eventually become greater than the additional thrust. The nozzle is thus ideal for providing greater pulling power for heavy loads, but less suited for high speed vessels.
The advantages summed up of a nozzle compared to an open propeller:
Air intake at shallow draft can be avoided by placing an anti-vortex plate above the propeller.
Two types of nozzle
Veth Propulsion uses two types of nozzles: VG40 and VOB50 (optimized nozzle 19A). On both types the nozzle is suspended on three points, and the lining is made completely of stainless steel. The top of the nozzle is flanged and bolted, making it relatively easy to disassemble.
Depending on your application, you can choose for the best option.
The length is 40% of the propeller diameter
The length is 50% of the propeller diameter
Combination of high thrust and low resistance
The inlet of the nozzle (modelled after the 19A) is slightly enlarged in order to improve the flow to the propeller. Due to the enlarged entry this nozzle of offers more resistance, so if the vessel speed is important then a VG40 nozzle is recommended. This type of nozzle is often used with tugs. Bollard pull efficiency is 2-3% higher than a VG40 nozzle.
Suitable for higher speeds
Suitable for higher thrust at low speed (Bollard Pull).