2018년 2월 2일 금요일

Water jet propulsion

Water jet propulsion

Water jet propeller for detachment works

Four water jet propellers provided at the stern of Nachchan World

Cover for sudden deceleration and reverse for the water jet nozzle is getting off

Water Jet Promotion (Waterjet Sushin) is one of the promotion methods of. It is a system that obtains driving force by jetting a high pressure water flow behind. In English-speaking countries, "Pump- Jet) "is also called.

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Overview

Water jet or hydro jet propulsion gains propulsion by vigorously discharging the water pumped up from the bottom of the ship from the rear nozzle at high pressure operating by the main machine (power source). In an underwater ship, the outer circumference of the screw part is covered with a circular shroud or the like. Propulsion efficiency is inferior to screws, but it is suitable for navigation at high speed, so small ones are gasoline engines, larger ones are high speed and powered.

Application example

For leisure, it is adopted for high speed ,, etc. Also, it is adopted also for diver support ships because there is little danger of being involved in the screw. Also, since the outer circumference of the screw part is covered with a circular shroud or the like, it is said that "it is also quiet and excellent", and nuclear submarines with a margin for output supplement low propulsive efficiency and are being used. Since the propulsion efficiency was low in the past, there was a limitation on the cruising distance, but in recent years it has been adopted for many of newly developed, focusing on high speed and quietness. Most of them are onboard engines, but there are examples of adoption as propulsion devices.

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Characteristic

Structure of pump jet propeller for "jet ski"

"Torpedo" Pump jet propeller seen from the rear

Mechanism of water jet 1. During forward movement 2. Backward movement

By combining the two injection directions, forward, backward, lateral movement and in-situ turning can be performed.

Although it is more difficult to design and manufacture than general, it is difficult to reach with a screw ship 40 - 50 (about 74 - 93) to enable high-speed navigation in the propulsive system. There is no protruding portion at the bottom of the ship, navigation on shallow water surface is possible, there is no need to change the direction of the ship by changing the jet direction of the nozzle. In addition, sudden braking using the mechanism of the nozzle is possible. In addition, ultra-fast navigation that is impossible with the screw propeller is possible.

On the other hand, there are drawbacks such as poor directional stability and steerability during low-speed navigation, inferior to screw propellers and poor in low speed range. As an image, there is a thing called "a boat moving forward by vigorously expelling water flow from the nozzle", but the larger the difference between the nozzle flow velocity and the hull velocity, the less efficient it is. From the viewpoint of efficiency, it is far more efficient to generate "a large amount of water flow that is slightly faster than the hull speed" rather than "to inject a small amount of water swiftly" for the same acceleration (a general screw propeller is more effective than this principle , It is made to slowly rotate large feathers). Therefore, in order to efficiently reach the target speed from the stationary state, it is necessary to smoothly raise the engine output and slowly accelerate it.

By the way, in the speed range of about 25 knots, the efficiency of the screw propeller reaches 65%, whereas the water jet is only about 45%. However, in the case of water jet, there are no hull protrusions but it becomes smaller, so it is necessary to comprehensively consider the difference in efficiency as a whole ship. In the case of the propeller, the propeller increases as the rotation speed increases, but it can not be avoided. In general, the efficiency is reversed in the speed range of 30 knots or more, and the water jet is better, so it was intended for high speed use There are many adopted cases such as.

In order to increase the speed, in addition to a diesel engine for steering at low speed and a propeller, there are some equipped with a water jet.

A famous ship

  • (JetFoil)
    • /
    • Pleasure boat

footnote

1.

2. ****This is solved by changing the orientation of the nozzle.

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Post Date : 2018-02-02 17:00

This article is taken from the Japanese Wikipedia Water jet propulsion

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