Why bigger propeller for lower speed ship, and smaller propeller for high speed ship?
Sinooutput want to explain this questions from tech. Aspect and from experience.
The principle of a ship’s propeller is the same as that of a helicopter wing, relying on thrust or lift generated by rotation. The force generated is proportional to the density of the medium, the square of the speed is proportional, the area is proportional, and there is also a lift coefficient. The density of water is 800 times than that of air, so the propeller area can be much smaller than that of a helicopter.
Generally, the propellers used in high-speed boats are very small, but there are 2 or 4 propellers, but the operation is inconvenient. The larger the ship, the greater the resistance and the greater the thrust required. For some large ships, such as 100,000-ton oil tankers and bulk carriers, and 300,000-ton oil tankers, the diameter of propellers can reach 4, 5 meters or more.
The huge buoyancy of the water offsets the weight of the ship, so the power requirements of the propeller are reduced accordingly. In fact, the propellers of large ships are still very large, ranging from 1 to 10 meters in diameter, weight up to tens of tons, and the number of propellers of large ships is not one, usually more than two.
For example, a 300,000-ton super large oil tanker is a twin-shaft twin-propeller, each with a five-blade propeller with a diameter of 10 meters (equivalent to a three-floor building) and a weight of 75 tons.
The requirement for kinetic energy is different from that of civil ships, so the number of propellers will be increased according to the combat requirements of the ship. For example, the US Navy’s 100,000-ton Nimitz-class nuclear-powered aircraft carrier uses four-shaft four-propellers, each with a five-blade propeller having a diameter of 6.4 meters and the weight is 30 tons.
In addition, when choosing a suitable propeller, it depends on the size of the ship, engine output power, high waterline, and fluid resistance of the ship. Generally speaking, they are high-speed small propellers and low-speed large propellers. The former is suitable for ships with smaller tonnage at 800 rpm. The latter is 300-450 revolutions per minute. If a medium-sized freighter with a smaller engine power is generally 300 revolutions, in fact, the higher the speed is not necessarily the better, it mainly depends on the output power of the engine and the structure of the propeller whether it will waste power. Assuming that even if the engine power is very high, the speed is very high, the blades are also large, but the blade design is not suitable for high-speed water flow structure is meaningless.
So each propeller must be customized according to your boat drawings, then could make sure it better suitable for your boats, because each boat design is different.
Sinooutput Group LTD. Usually help customers overseas to design the propellers, each propeller is strict from design department and get approve from the marine bureau, then issue to customer confirmed by sending CAD format to combined the whole ship CAD drawings to come out the actual mould to check and test, then start to produce.