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PROPULSION SYSTEMS | SAILING BOAT PROPELLERS | FIXED PITCH PROPELLERS | ACCESSORIES  | COMPLETE SET
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Nozzles
Nozzles
 
 

Introduction | Application | Advantages | Application Limits | Nozzle types
 
Introduction
 
 
 
Application
 

Nozzles are necessary when a maximum thrust is required at low hull speed. The tug is a good example as well as a fishing trawler whilst fishing at low speed : they need nozzles. Nozzles can be in a fixed position with a rudder placed on their aft part. Or nozzles can be directed : in this case they work as Rudders.

 
 
Advantages
 

When you compare a thrust curve on a propeller nozzle design, with a thrust curve on a fixed propeller in open water, you can note an important difference. This difference decreases when the vessel speed increases (see schem 1).

 
 
 
Application Limits
 

Every accessory fitted below the water line increases the drag coefficient. Thus the advantage of the nozzle decreases with boat speed - as the drag becomes more important.
As a rule this limit point is between 12 and 20 knots according to the vessel type and the tonnage on board.

 
 
Nozzles types
 

Different tests have proved that a mathematical relationship exists between the propeller diameter D and nozzle lenght E. The knort nozzle 19 are the most currently used but other profiles are available.