Know-how of Marine Propeller Types and Casting |
Posted: June 13, 2017 |
A marine propeller is a rotating fan, which helps to drive the ship forward. The main engine generates and transmits the power, which is then converted to a thrust. The resultant thrust gives momentum to the water, thus moving the ship forward.Traditionally, marine castings were done using the Randupson process using Portland cement as a binding agent. However, now boats are made of an alloy of stainless steel and aluminium, to which alloys of nickel, aluminium and bronze might also be added. The construction of a marine propeller also involves attaching blades to the hub, which are classified by the number and pitch of blades.
1. Marine propellers classified by the number of blades The efficiency of propellers for marine propellers depends on the minimum number of blades. Two-blade propellers are usually used in yachts and high-speed racing boats. However, due to the need of extra strength and subjection to heavy loads, merchant ships do not run on two-blade propellers. a) 3-blade propellers Three-blade propellers are generallymade of aluminium alloy, but can be made from other metals as well. They are the most commonly used propellers on an outboard, sterndrive and inboard engines. They give a high-speed performance with excellent acceleration. Moreover, its manufacturing costs are inexpensive when compared to propellers with more number of blades. On the downside, low-speed handling is practically inefficient. b) 4-blade propellers Four-blade propellers are generally made of stainless, but can be made from other metals as well. They are used on large boats, particularly commercial fishing boats. They also give a high-speed performance and are good at low-speed handling, therefore better at rough seas.They are strong and durable with a high-fuel economy, and higher manufacturing cost as compared to 3-blade propellers. c) 5-blade propellers Five-blade propellers can be made of copper, stainless and aluminium alloy.They offer maximum for the boat to move forward, handle excessive propeller overloading and good for restricted diameter aperture. They are also known for their efficiency and high-speed performance, and is extremely durable. Due to minimum vibration, they can handle rough waters very well. Manufacturing costs can be very high.
The pitch of a marine propeller refers to the displacement the propeller makes per revolution. Thus, propellers can be classified on the basis of the pitch of blades as follows. a) Fixed pitch propeller Fixed pitch propellers are made of copper or aluminium alloy, and are used in ships with poor manoeuvrability requirements. The blades are permanently attached to the central hub, as the blades are made through precision casting. The design does not have any mechanical or hydraulic components as found in the Controllable Pitch Propeller (CPP), these propellers are reliable and durable. It gives high-speed performance with efficient acceleration. However, low-speed handling is inefficient. Though the manufacturing, installation and operational costs are lower than CPP, the manoeuvrability is not as good as CPP. b) Controllable Pitch Propeller Controllable Pitch Propellers can be made of steel alloy, and are used in ships with frequent sailing routes and work with multiple operating conditions. It has high-speed performance, and is strong and durable. They can withstand harsh weather conditions or operational requirements. It has a high-fuel economy and has a high manufacturing cost. Therefore, there are marine propellers designed for every specific application. There are propellers available in various Marine Casting, number and pitch of blades. It is better to consult a professional to select the best marine propeller for your specific purpose.
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