Behind the stern, in a foamy stream. Drawing, description

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Behind the stern, in a foamy stream. Children's Science Lab

Children's Science Lab

A nimble racing scooter and a half-kilometer long tanker, a nuclear submarine and a river tug - these different ships and dozens of others move in and under water with the help of a propeller. Nine modern self-propelled vessels out of ten are propeller-driven.

Behind the stern, in the foamy stream

Hydrobiologists have studied hundreds of inhabitants of rivers, lakes and oceans - from shrimp to squid, from sprat to whale, carefully studied the ways of their movement in the water. Not one of them has been endowed by nature with anything resembling a propeller. Like the wheel, the main mover of the modern fleet has no analogues in nature. The ship's screw is a descendant of the screw created by the brilliant Archimedes for lifting water. If we also recall the law of Archimedes, it turns out that shipping today is twice obliged to the great Syracusan ...

In 1793, the French mathematician Ponton suggested using a propeller to propel ships. Three decades later, the Czech inventor I. Ressel created the first screw design - the auger. And in 1836, an accident led to the fact that the screw gave way to the screw of today's shape - the bladed one. During the tests, a long, like a meat grinder, the auger broke off, and the ship went faster.

In 1849, the advantages of the propeller were proven in a fair fight. The English steamships, the propeller-driven Niger and the wheeled Basilisk, both with 400 hp engines, connected by cables, set off in opposite directions. It was something like a tug of war. For more than an hour, the screw ship pulled its rival at a speed of 1,5 knots.

True, after this competition, paddle steamers were built for more than one decade. But most likely, by inertia ...

Globtik Tokyo, the leader of the world tanker fleet, has on board engines with a total capacity of 45 hp. s., the power of the engines of the icebreaker "Ermak" - 000 42 l. s., passenger liner "Queen Elieabet-0" - 2 liters. With. But getting energy is not everything: it must be used.

The ship moves by throwing water flows, in other words, all self-propelled ships are hydromets. The wheels discard water by pressing on it with a blade - a plate. With a screw, this process is more complicated. Each of its blades serves as a kind of wing. When the blade rotates in the water column, a rarefaction zone is formed on one surface of the wing, and a compression zone on the other. The pressure difference creates the lifting force of the hit, and the sum of the lifting forces of the blades is the propeller stop. Unlike an ordinary screw, the head of which moves with each revolution by a step of the screw line, the propeller, as it were, removes the thread, throwing back a lot of water. The greater this mass, the stronger the emphasis and the better the screw.

It would seem that to increase the emphasis, it is enough to increase the speed. But at the same time, it is no longer a crushing, but a rupture of an invisible water thread. The pressure at the leading edge of the blade drops even more, and the water boils, forming numerous bubbles. As soon as the bubble flies out of the zone of low pressure, it collapses: it is compressed by the water column. The death of each bubble is accompanied by a microexplosion.

Explosive blows of bubbles on the propeller blade cause not only noise and vibration. The protective oxide film breaks off the metal, cavitation erosion begins. The propeller blade, subject to cavitation, resembles a target riddled with bullets.

Cavitation occurs at certain propeller revolutions. To get rid of this extremely unpleasant phenomenon, you need to reduce the speed. But then the thrust of the screw will fall, its thrust.

The fight against navigational metal erosion led to a paradoxical technical solution: they decided to increase cavitation. Propellers with blades of a special profile were created. At ultra-high speeds, navigation bubbles began to cover the entire working surface of the blade, forming a huge steam bubble. Increasing the speed of such a screw almost does not change the pressure in the vapor bubble, and the pressure behind the blade and the overall stop increase. Such screws are called supercavitating. For ordinary ships, they are ineffective, but are indispensable when you need to reach speeds in excess of 40 knots. For example, the Soviet gas turbine hydrofoil "Typhoon" is equipped with a supercavitating propeller.

The blades spin in a continuous round dance. It is enough for one of them to "lose time" for the entire propeller to vibrate, and behind it the entire feed. It happened that due to the vibration of the propeller it was necessary to rebuild the ship. That is why the screws are balanced with special care, the shape of the blades and their inclination are verified with special templates, the surface is polished to a mirror finish at least once a year. If, when examining a screw that has worked for some time, potholes deeper than one millimeter are found on its surface, they are puttied with epoxy compounds and polished again to a mirror finish.

A particular problem is the choice of metal for the propeller. Due to severe corrosion in sea water, conventional steels are practically unacceptable. Suffice it to say that on the icebreaker "Murmansk" for two years of operation, each helical blade lost 200 kilograms in weight. This is from special alloy steels!

In recent years, more and more ship propellers are made of brass or bronze. The corrosion resistance of brass is one hundred times greater than that of ordinary steel. But in sea water, brass is also susceptible to corrosion - zinc is washed out of it. Areas with a low zinc content are covered with cracks, the strength of the blades decreases. A small crack opens and closes with each turn of the screw, corrosion products grind its edges, expand it. And then the moment comes when the metal does not withstand and breaks ...

Brass is not a very durable metal. For high-speed propellers, manganese aluminum or nickel-aluminum bronze is more often used, the strength of which is close to the strength of alloy steels, and the resistance in sea water is several times higher than that of brass. Screws weighing more than 50 tons are cast from such bronzes for modern container ships and supertankers.

However, such alloys do not withstand collision even with light ice. Therefore, for icebreakers it is necessary to manufacture propellers from alloyed stainless steel, which includes copper, manganese, nickel, titanium and a number of other additives.

A story about modern materials for ship propellers would be incomplete without mentioning plastics. Ships already have cast nylon propellers. But only on small boats. Even nylon blades edged with steel sheets cannot withstand the gigantic mechanical loads that are carried by the propellers of large ships. But the propeller has existed for a hundred and fifty years, and the first experiments with plastic blades began only about ten years ago ...

Authors: M.Korotkiy, M.Nayding

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