Why Are 4 Blades Better Than 3?

when it comes to ships propellers have you noticed that some of three blades some are four and others might even have five or six then we get to ships like the Olympic that were fitted with a full-bladed central propeller with three bladed props on either side of course that's led to the mystery of the Titanic Central propeller was it three-bladed or was it four the ship's plans always had it down as a three-bladed yet her sister the Olympic started life with a full-bladed one swapped to a three-bladed urinary fit in 1912 and then refitted her full-bladed one during another refit sometime before 1919. there are no pictures of Titanic's propellers and the central one hasn't been found on the wreck so really no one knows but I think it was probably a case of Harland and wolf wanting to experiment to find out what was best either way that leads us to the question and the subject of today's video Why do ships have different numbers of blades on their propellers and what's best in the early days of mechanical propulsion ships basically just mechanized the ancient practice of rowing with massive rotating paddle the thing is just a casual look tells us that it's rather inefficient you waste so much power with the paddles breaking the water surface on Entry spilling water during the first part of the stroke and then lifting water and finally circling back around to start the stroke again this is why in April 1845 an early design of screw propeller thrashed the existing paddle design in a famous tug of war between the warships HMS Rattler and HMS alecto in an idealized scenario a screw propeller works by accelerating a cylinder of water backwards which due to Newton's third law exerts an equal and opposite force on the ship pushing it forwards in its most basic form you can think of a propeller a little like this Archimedes Screw As It rotates water is drawn through following a single rotation shows us that the water caught by the Leading Edge will be drawn along this far vary the angle of attack and for each rotation you alter the distance over which that water is drawn a steep Wrangle of attack is going to move the water further so it's going to be harder work to turn the screw in theory theoretically move a ship faster of course we know that a ship's propeller doesn't look like this but by simply shortening the Archimedes screw we get what looks like a single bladed propeller it still moves water and we still have the principle of a steeper angle of attack moving the water further but remember this is only an idealized scenario in reality a steeper angular attack will also induce a vortex in the water wasting energy at its extreme the water is going to be flung outwards and the ship attached to the propeller isn't going to go anywhere this is our first compromise we need a steep angle of attack to generate thrust but not so steep that too much energy is wasted generating that Vortex so that's great but what about actually moving a ship a slowly turning single blade really isn't going to do much you need to speed it up but again there are limitations the ship's Machinery will only be able to tolerate a certain rotational speed before the engines and bearings are overloaded not to mention that a single blade is going to wobble quite a lot and even if you do balance it with counterweight it's going to generate substantial vibrations as the pressure points past the hull every rotation and then we get to the real limitation with high speed props cavitation cavitation is the process where the lower pressure water behind the blades evaporates and creates Vapor Pockets remember low pressure means evaporation happens at low temperatures so you drop the pressure low enough and it's going to evaporate at Sea temperature anyway that Vapor pocket is carried around a bit until it encounters a higher pressure area where it implodes and releases so much energy that can damage the steel of the propeller it's a very complex phenomenon but we can summarize it by saying that a slower rotational speed will reduce cavitation so although our single bladed propeller is very efficient is going to reach physical limitations before it can generate enough power to be useful on a ship you need more blades more blades mean that there are more leading edges in contact with the water so the propeller is directly transferring energy to more water particles it can rotate lower while generating the same thrust as a propeller with fewer blades rotating faster that slower RPM reduces wear on the mechanical parts and reduces the chance of cavitation it even reduces the vibrations felt on the ship because the power is spread over more blades reducing the forces generated by each one but although more blades sounds great it does come at a cost the closer the blades are together the more they disturb the water for subsequent blades so the lower the overall efficiency and as the propeller is going to generate more thrust it's going to be harder to turn it faster so its maximum speed is going to be lower again it's a compromise more blades equals more power to give an RPM reduce wear on Machinery reduced ship vibrations and less cavitation but it comes at the cost of reduced efficiency and a slower maximum speed for a given engine power coming back to the Titanic mystery and the Olympics changing props we can suddenly see the logic behind experimenting with different numbers of blades swapping out the middle four bladed propeller for a three-bladed one could have allowed it to be more efficient and spin at a higher RPM leading to a slightly higher top speed they probably then found more vibration at the AFT end and maybe even that it took longer to reach the top speed due to less overall grunt from the prop this will be why a few years later Olympics experimental three-bladed propeller was again switched out to be full-bladed even today propeller theory is an incredibly complicated area of Science and the only way we have of determining the best propeller for any given ship is experimentation albeit we can use computers rather than needing to test out full-size props on ocean liners modern ship propellers are now designed bespoke for the ship they're going to be on to be as efficient as possible given the water flow around the ship's Hull the Machinery characteristics and the desired operational parameters of the vessel usually three four or five blades is about right although there are ships that can have up to nine or ten and of course sailing ships will only have two simply to reduce resistance while the engine is not in use propeller Theory really is fascinating and I hope you've enjoyed this quick introduction to the topic remember the directors commentary for this video has just gone live in the community so members with that perk can go and check it out for anyone interested in becoming a member check the description to find out more

Channel: Casual Navigation

Views: 1,415,291

Keywords: casual navigator, marine, shipping, casual navigation, maritime explaination, merchant navy, sailing, marine animation, propeller blades

Id: 2cnKzCTJC_8

Length: 6min 11sec (371 seconds)

Published: Fri Mar 10 2023