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Articles in category "Ship Propulsion"

Propulsion - Voith-Schneider

In our Propulsion- series we now discuss a second system, Voith-Schneider.

Voith Schneider propulsion - © VoithThe Voith-Schneider propeller is a specialized marine propulsion system. It is highly maneuverable, being able to change the direction of its thrust almost instantaneously.

Over 80 years ago this ship's propulsion system, the only one of its kind in the world was developed by Voith from an idea by the Austrian engineer Ernst Schneider. It allows thrust of any magnitude to be generated in any direction quickly, precisely and in a continuously variable manner. It combines propulsion and steering in a single unit.

From a circular plate, rotating around a vertical axis, a circular array of vertical blades (in the shape of hydrofoils) protrude out of the bottom of the ship. Each blade can rotate itself around a vertical axis. The internal gear changes the angle of attack of the blades in sync with the rotation of the plate, so that each blade can provide thrust in any direction, very similar to the collective pitch control and cyclic in a helicopter.

Voith Schneider propulsion
© H.G. Hogeboom

Unlike a Z-drive (where a conventional propeller is tilted on a vertical rudder axis) changing the direction of thrust with a Voith-Schneider drive merely requires changing the pattern of orientation of the vertical blades. In a marine situation this provides for a drive which can be directed in any direction and thus does away with the need for a rudder. It is highly efficient and provides for an almost instantaneous change of direction. These drives are becoming increasingly common in work boats such as fireboats and tugboats where extreme manoeuverability is a very useful characteristic.

Z-drives (and Kort nozzles) have both advantages and disadvantages when compared to cycloidal drives. The Z-drive is less efficient and slower to maneuver, but is likely to be significantly cheaper in both the long and short term. A choice is made on the basis of perceived performance requirements. Also the blades of the drive protrude from the bottom of the hull making shallow water operation a problem.

A low accoustic signature favors the device's use in minesweepers.

sources: Voith, Answers.com

Propulsion - The Kort nozzle

We intend to highlight some of the various propulsion systems frequently used on tugs. Today we start with the Kort nozzle.

Patent - drawing Kort nozzle

The term "nozzle" is derived from the Middle English noselle, meaning "nose". Interestingly, "nozzle" also remains in the English language as a slang term for "nose."

The Kort nozzle is a shrouded, ducted propeller assembly for marine propulsion invented by Ludwig Kort. The hydrodynamic design of the shroud, which is shaped like a foil, offers advantages for certain conditions over bare propellers.

Kort nozzles or ducted propellers can be significantly more efficient than unducted propellers at low speeds, producing greater thrust in a smaller package. For the bollard pull it may produce as much as 50% greater thrust per unit power than a propeller without a duct. Tugboats are the most common application for Kort nozzles as highly loaded propellers on slow moving vessels benefit the most.

The additional shrouding adds drag, however, and Kort nozzles lose their advantage over propellers at about ten knots (18,52 km/h).

Kort nozzles may be fixed, with directional control coming from a rudder set in the water flow, or pivoting, where their flow controls the vessel's steering

sources: Wikipedia, Everything 2, US Patent no. 2,139,594