On the tablet on the wall of the Inland Towage Hall of the National Dutch Towage Museum the visitor will find a clear explanation about the various types of ship propellers. The author of this information is mr. H. Hensen.
Another word for propeller is "screw". This is a perfect indication of a ship's screw, also for a tugboat's screw. Indeed, a ship's screw operates very much the same as a screw in a nut, be it that a ship's screw operates in water and not in a nut.
Three characteristics are relevant for the function of a conventional ship's propeller: the shape of the blades, the number of blades and the position of the blades. A tugboat uses a different propeller model than for example a cruiseship or a tanker.
Propellers can be manufactured with 2, 3, 4, 5 or 6 blades. The fewer the number of blades, the higher the propeller efficiency will be. However, for reasons of strength, propellers which are to be subjected to heavy loads cannot be manufactured with only two or three blades.
The blades of a propeller can be fixed or adjustable in angle. The first type is a so called "fixed pitch propeller" (fpp) and the second a "controllable pitch propeller" (cpp).
It is clearly visible whether a screw is adjustable or on the contrary has fixed blades. An adjustable screw has a thick hub, and the controllable pitch blades are mounted on the hub with bolts. A fixed screw is cast in one block and normally made of a copper alloy.
The distance the screw travels in one rotation, is called speed and depends on the pitch of the blades. Water is liquid, so some loss will occur. This is called slip.
Another difference in screws is the rotation direction. Looking from behind at a screw that will move the ship forward when the blades turn clockwise (so to the right), it is a right handed screw. In case of a left turning screw it is called a left handed screw. Most of the single screw ships are fitted out with a right handed screw.
Propellers often rotate in a shaft, a so called nozzle. This provision has initially been developed to reduce damage to slopes, campshots and suchlike on canals and rivers. But soon it appeared that a nozzle had a positive influence on the effectivity of the screw; the ship speeded up. Ever since the nozzle has been applied on a lot of ships. Dependent on the requirements of the craft, different types of nozzles have been developed. The name Kort nozzle is often used, after the inventor's name.
There are also totally different propulsion systems. We mention the Voith Schneider propeller and the JET propulsion. Moreover, many ships nowadays like cruise ships and tugboats make use of so called azipods.
All this has been made visible on the wall tablet in the museum.