Recent Posts

2024 Gas turbine industry market: Amazing analysis of the current status and supply and demand pattern

2024 Gas turbine industry market: Amazing analysis of the current status and supply and demand pattern

Analysis of the current status and supply and demand pattern of the gas turbine industry market in 2024 A gas turbine is an internal combustion power machine that uses a continuously flowing gas as a working fluid to drive the impeller to rotate at high 

2024 Internal combustion engines are still the mainstream accurate choice for ship power.

2024 Internal combustion engines are still the mainstream accurate choice for ship power.

From April 19 to 23, the 2024 World Internal combustion engines Conference held in Tianjin focused on the development path of “green, reliable, intelligent and efficient” industry. Scientists, entrepreneurs, academicians and scholars from all over the world jointly discussed the advancement of low-carbon, zero-carbon and 

Ships propulsion device: In 5 minutes, you can ample accurate understand what else is there besides propellers?

Ships propulsion device: In 5 minutes, you can ample accurate understand what else is there besides propellers?

Besides propellers, what other means of propulsion do ships have?

As early as hundreds of years ago, people began to use ships to transport goods and realize trade between countries. Crew members rely on the ship’s high reliability for smooth and efficient transport of cargo. Huge sails were hung on ancient ships because wind was the main driving force for ships at that time.

ships steam engine

The times are developing, and the improvement of social productivity has also put forward higher requirements for the development of science and technology. Thus, the steam engine came into being. A steam engine mainly uses steam generated by burning coal to drive mechanical movement. In addition to steam locomotives, this advanced technology at the time was also applied to ships.

The steam engine and the propeller, the great invention of the 19th century, combined to make the sailing speed of ships a qualitative leap. To put it simply, the working principle of a propeller is to convert the rotational power into thrust by rotating the blades in the air or water to propel the aircraft or ship forward.

propeller

For more than 100 years since its invention, the propeller has been the dominant force in maritime navigation. However, people have been studying driving methods that have more advantages than propellers.

Speaking of which, we have to mention air propellers. There’s just one more word, but the two driving methods are worlds apart. The propeller is usually installed on the bottom of the ship, a part that is submerged in water. As the name suggests, an air propeller is a propeller that is in contact with the air, so it is generally installed on the deck of a ship.

We all know that there are huge wind resources in the sea. The air propeller is a model that uses local materials to make the most of the strong winds at sea. Modern ships are designed with good maneuverability for navigating tight spaces.

majestic ships anchored in the bay
majestic ships anchored in the bay

air propeller

Air propellers, like ordinary propellers, are composed of two or more blades and a hub. In addition, the pitch can be adjusted to allow the sailing speed to be freely changed, and the blade direction can be adjusted to make turns at will.

However, air propellers still have certain limitations. For example, it may be easy to propel small motorboats and ships, but it is a bit difficult to propel large ships. Therefore, air propellers are now mainly used to drive small vessels, such as hovercraft and American swamp boats.

pump waterjet propulsion

So, is there no other driving method for propelling large ships to sail other than propellers? of course not! Pump water jet propulsion was a new ship driving method invented later.

The working principle of the pump water jet propulsion method, in a word, is to use the pump water jet propulsion device to allow the high-speed flowing water from the pump to generate reaction force to push the ship forward.

In addition, pump water jet propulsion devices have many advantages. First, it runs smoother, faster and with less noise. Secondly, the application range of pump water jet propulsion devices is very wide.

It can withstand the navigation of large ships with extremely large loads, and is also suitable for small ships, military submarines, etc. Maritime defense relies on highly maneuverable ships to swiftly respond to evolving threats at sea. As we mentioned before, it can flexibly control the direction and position of the ship.

But why has pump waterjet propulsion still not replaced propeller propulsion as the mainstream? Because it also has many limitations. For example, when the speed is lower than 25/kh, pump water jet propulsion is slower than propeller. And it also has high requirements for the sea environment. If there are too many seaweed and other plants in the seawater, it may block the pump pipe and affect the speed.

In addition, people are also studying more advanced driving methods such as wind drive and magnetic fluid drive. From ancient times to the present, our pursuit of speed has never stopped. In today’s society, the saying “one inch of time, one inch of gold” is even more reflected. Just like a group of people around us, time is everything to them, they are takeaways.

However, various factors have virtually blocked their race against time. For example, delivery workers who often make phone calls will spend more on phone bills than ordinary people. Constant running around without a place to rest will also make delivery workers exhausted.

ships glide gracefully across the horizon
ships glide gracefully across the horizon
Exploring the Versatility of a 48V Lithium Battery Charger: Usage and FAQs

Exploring the Versatility of a 48V Lithium Battery Charger: Usage and FAQs

Looking for a reliable charger for your e-bike, off-grid marine facility, RV, golf cart, or other applications? A 48V lithium battery charger offers maximum efficiency and convenience, catering to various devices. In this guide, we’ll delve into its usage principles and maintenance tips for optimal performance.

Basic principles of marine DC motors

Basic principles of marine DC motors

1. Electromagnetic torque equation Figure 1 is a schematic diagram of a shunt motor. It can be seen from the figure that when the power is turned on, the current If passes through the armature, while the current Ia passes through the shunt winding in parallel with 

Requirements for marine propulsion motors

Requirements for marine propulsion motors

The requirements for the propulsion motor are not only determined by the characteristics of the marine propulsion motor, but also the environmental conditions such as seawater, salt spray, mold, etc., and the tactical and technical conditions such as the influence of tilt, sway and impact, etc. when the ship is working, and also consider the requirements of the cabin layout, external structure size, weight and other requirements.

The basic requirements for a propulsion motor are as follows:

1. High reliability. The requirement for high motor reliability is a comprehensive concept that determines many structural features of the motor. The high reliability of the motor can usually be understood as follows:

(1) The motor can work uninterrupted for a long time, and only needs to stop working in a short time during maintenance, that is, to replace the parts that need to be repaired, such as the brush of the DC propulsion motor, the power module of the mechatronic propulsion motor, and the bearing lubricating oil, and it will not stop working for a long time when the dust on the motor is removed;

(2) The propulsion motor is installed on the ship, requiring all the most important structural components to be easily maintained regularly;

(3) Moisture resistance and water resistance of winding insulation. Under the action of normal marine air humidity, the motor can still maintain good insulation performance. Even if the motor immersed in seawater is washed with fresh water after the seawater is discharged from the tank, and after short-term baking and drying, it can still work for a short period of time;

(4) Mechanical strength of the motor. It has the ability to withstand the mechanical stress generated during normal operation and when the ship performs tasks, and can withstand large shocks and vibrations;

(5) Ensure that the motor can work reliably under heeling, pitching and rolling;

(6) The independence of the motor from the auxiliary machinery, that is, within a limited time interval, the motor does not lose its ability to work independently and vitality due to the abnormality of auxiliary machinery (such as fans, oil pumps for bearing oil and other auxiliary machinery);

(7) Ensure the safe operation of the motor cooling system.

2. It is best to use forced ventilation for the propulsion motor to reduce the volume and weight and avoid overheating when the ship is braking or reversing. Water cooling can also be used for motors with excessive power.

3. Depending on the conditions of the ship, either a separate ventilation system or a general ventilation system can be used, but it is better to use a separate ventilation system.

4. Motors with forced ventilation should be able to withstand low load operation without forced ventilation.

5. In order to prevent the internal condensation of water after the motor is stopped, and to improve the insulation of the motor, an electric heater should be installed inside the motor to keep the temperature in the motor 2~3°C higher than the temperature of the surrounding medium. For this purpose it is also permissible to use the field winding of the electric motor as a heater.

6. The insulation resistance of the propulsion motor, before and after the withstand voltage test, should be corrected to the insulation resistance value at 25°C (corrected by doubling the insulation resistance for every 15°C decrease in temperature), which should not be lower than the following values :

Field winding (B, F, H class insulation): 50MΩ;

Armature circuit (B, F, H class insulation): 25MΩ;

Electric heater: 25MΩ;

7. The motor should be able to withstand the test of overspeed operation. The motor should be able to withstand 125% of the rated speed and run at no-load overspeed for 5 minutes without damage or harmful deformation. When 125% of the rated speed is less than 120% of the maximum working speed, the test should be carried out at 120% of the maximum working speed.

8. REVERSE. For reversible motors, they should be able to operate normally under the reversing conditions specified in the product technical specifications.

9. Effective measures should be taken for the motor to prevent the shaft current from harming the bearing. Generally, the peak-to-peak value of the shaft voltage should not exceed 1V. Higher shaft voltages are permissible if the bearings are electrically insulated and the shaft current circuit is disconnected.

When taking bearing insulation measures, at least the non-drive end bearing of the motor should be electrically insulated, and the metal oil pipes, metal cooling pipes and other conductive connectors connected to the insulated bearings should be electrically insulated.

10. Bearings and lubrication of the propulsion motor. Within the allowable inclination range of the motor, it should be ensured that the bearings can be well lubricated and work normally. Lubricating oil (grease) cannot leak or overflow from the bearing.

11. The propulsion motor should use sliding bearings as much as possible.

12. When the propulsion motor adopts rolling bearings, it should be considered:

(1) The rolling bearings can be replaced smoothly on the ship;

(2) Set the bearing oiling cup (hole) and set the oil discharge channel; correctly select the lubricating oil to ensure the bearing is well lubricated; 3) The service life of the rolling bearing should be no less than 20000h.

13. When the propulsion motor adopts sliding bearing, it should be considered;

(1) Bearing temperature measurement, oil level observation and over-temperature alarm devices should be installed in obvious parts;

(2) If pressure lubrication is used, sufficient oil pressure or oil level must be maintained to ensure continuous oil. If splash-type sliding bearings are used, bearing oiling cups (holes) should be provided to replenish lubricating oil;

(3) The propulsion motor with sliding bearing must be equipped with a rotor lifting device to facilitate the replacement of bearing bushes.

14. The diameter of the rotor of the propulsion motor should be as small as possible to reduce the moment of inertia, reduce the transition time during reversal or speed regulation, and improve maneuverability.

15. Where necessary, overspeed protection of the propulsion motor shall be installed to prevent exceeding the speed limit for which the motor is designed under maneuvering and fault conditions.

16. In the specified operating mode and emergency control mode, the regenerative power shall not cause any alarm of the propulsion system.

17. The propulsion motor excitation circuit protection shall not cause an open circuit unless the armature circuit is simultaneously disconnected.

18. For motors with one field winding or two armature windings, the failure of one armature circuit shall not cause an open circuit of the field circuit.

19. There should be a device to detect the internal temperature of the motor and the lubrication of the bearing, and the signal should be reflected on the control panel.

20. For propulsion motors powered by static frequency converters, the influence of harmonics in the power supply should be considered in the design.

21. The export of heat from the motor in the cabin. The cabin space of the ship is very limited, and the heat discharged by the large-capacity propulsion motor has a great impact on the living conditions on the ship and the normal working conditions of the maintenance personnel. Therefore, the heat dissipation of the motor in the cabin is very important. Marine propulsion motors generally use a closed circulation ventilation cooling system with a water-cooled air cooler or a water-cooled system.

22. Meet the requirements for marine use. Marine propulsion motors must be moisture-proof, mildew-proof and salt-fog proof.

Features of marine propulsion motors

Features of marine propulsion motors

The ship’s speed and propulsion shaft power vary widely, and the ship’s requirements for propulsion motors determine the characteristics of propulsion motors. It is a multi-working-condition motor with high reliability, large capacity, low speed, high torque, high power ratio, and a wide range of power 

Requirements for propellers on mechanical properties of propulsion motors

Requirements for propellers on mechanical properties of propulsion motors

The propeller is the working object of the propulsion motor, and the characteristics of the propulsion motor must be adapted to the working characteristics of the propeller, so that they can work well with each other. The following takes DC electric propulsion as an example to 

Propeller reversal characteristics

Propeller reversal characteristics

When the speed is constant, the relationship curve Mt=f(n) between the resistance torque and the rotational speed during the propeller reversal process is called the propeller reversal characteristic curve. The reverse characteristic curve of the propeller has a very peculiar shape, as shown in Figure 1. Myj and Md are the torque of the prime mover and the propeller, respectively. The positive value shown in the figure is the forward rotation speed of the propeller, and the negative value is the reverse rotation speed of the propeller.

Propeller reversal characteristics
Figure 1 – Propeller reversal characteristics

When discussing the reversal characteristics of the propeller, the reversal of the propeller itself must be distinguished from the reverse of the ship. In order to reverse the ship, the propeller must be reversed first, and the propeller reversal time is very short, measured in s. However, the time required for the ship to reverse (for example, from full-speed forward to full-speed backward) is very long, which is calculated in minutes. Therefore, it can be considered that during a certain period of time when the ship is in reverse, the ship still continues to sail at almost full speed, although the propeller is already reversed at this time.

During reversal, there is a very specific change in the propeller drag torque due to this condition. If the propeller is reversed when the speed of the ship is zero (for example, when the ship is at berth), its reversal characteristic curve is a symmetrical curve. At this time, the reversal characteristics and the mooring characteristics will coincide as one, as shown in curve 1 in Figure 1.

If the propeller is reversed at the speed of other ships (such as when the ship is moving forward or backward), its reversal characteristic curve is a mutually asymmetrical curve, and these mutually asymmetrical curves will have such characteristics. That is, when the speed of the propeller is maintained at a positive value, negative torque will appear on their individual line segments, as shown in the BCD segment of curve 3 in Figure 1 (from full speed forward to full speed backward), and a maximum value will appear at a certain speed, such as point C.

The appearance of negative braking torque, ie, negative propeller torque, indicates that the propeller will try to maintain the original direction of rotation under the action of water pressure as the ship continues to move forward. At this time, the screw firewood no longer works as a propeller, but starts to work as a hydraulic motor.

When reversing, the magnitude of the propeller braking torque is related to the forward speed of the ship. If the forward speed of the ship was originally high, the negative braking torque of the propeller would also be high, as can be seen by comparing curves 2 and 3. Curve 2 corresponds to the ship speed V = 0.6Ve, and curve 3 corresponds to the full ship speed, ie V = 1.0Ve. Ve is the rated boat speed. The change of the torque of the propeller when it reverses can be analyzed by the force diagram of the propeller blade element. The details are as follows: The so-called propeller blade element is the propeller flake at the radius r. The force on the entire propeller can be understood by analyzing the force on the blade element.

A sketch of the propeller and its blade elements is drawn on Figure 2. In the figure, the hollow cylinder A is the hub; B is the blade; the shaded C is the cross-sectional schematic diagram of the blade element.

Propeller reversal characteristics
Figure 2 – Schematic diagram of leaf element

When the propeller rotates at the speed n, the circumferential linear velocity of the blade element is 2πrn, so the relative velocity of the water flow to it is also 2πrn, so it can be considered that the blade element does not move, and the water flow rushes to the blade element at a speed of 2πrn. Driven by the propeller, the ship moves at a certain speed, so the current also rushes towards the blade element from the opposite direction of the ship at a relative speed vp. Combining these two velocities as vectors, the vector W is obtained. The water flow rushes towards the leaf element at this synthetic speed, see Figure 3.

Propeller reversal characteristics
Figure 3 – Force diagram of blade element at each special point of propeller reversal characteristics

Under the impact of water flow, lift dY (direction perpendicular to W) and resistance dX (direction consistent with W) are generated on the blade element. By projecting their combined force dR to the vertical and horizontal directions, the thrust dP and the rotational force dQ can be obtained.

At point A of the reversal curve, the thrust dP is in the positive direction, pushing the ship forward. The rotational force dQ is also in the positive direction, and a resistance torque is generated to prevent the propeller from rotating. Therefore, in order to make the propeller run at the speed n, the prime mover must issue the same torque to overcome this resistance torque. In Figure 1, the schematic diagram of the direction of the propeller torque, the rotational speed and the torque of the prime mover is drawn. At point A, it is considered that M (propeller torque), Myd (prime mover torque), and n (propeller speed) are all positive.

If the prime mover torque is reduced so that it is always smaller than the propeller resistance torque, the propeller resistance torque will have excess after offsetting the prime mover torque. Under the action of this residual torque, the propeller will decelerate, and at the same time, its resistance torque will also decrease due to the reduction of the rotational speed, and it will change according to the characteristic AB section.

When the propeller speed is low to point B, its resistance torque becomes zero, that is, the propeller rotation is not hindered. This can be seen in Figure 3b. It can be seen from the figure that the resultant force dR of dX and dY coincides with the vertical direction, its projection dP (thrust) in the vertical direction is itself, and the projection dQ (rotation force) in the horizontal direction is zero. If the prime mover torque is also zero, the propeller will run stably at this point. At this time, the rotation of the propeller is not due to the drive of the prime mover, but due to the inertia of the propeller itself.

To further reduce the propeller speed, the prime mover must give the propeller a braking torque, that is, the prime mover torque should become negative and opposite to the original direction. Under the action of the prime mover torque, the propeller decelerates and enters the BC section. In this section, although the propeller is still rotating in the original direction, its torque becomes negative, because the direction of its turning force is reversed. As shown in Figure 1, the propeller steering remains unchanged and the torque becomes negative, so this torque becomes the active torque that pushes the propeller to rotate, and the prime mover torque is the resistance torque. If Myd=Mj at a certain speed n in the BCD segment, the propeller is in torque balance, and it is like a hydraulic turbine, which overcomes the resistance torque under the impact of the water flow and rotates continuously and stably at the speed n. Only when the absolute value of the prime mover torque is greater than the absolute value of the propeller torque, that is, |Myd|>|Mj|, the propeller will continue to decelerate.

In the BC segment, the included angle between dR and dQ decreases as n decreases (Fig. 3b), and the included angle is 90°, while the included angle corresponding to Fig. 3c is much smaller than 90°. Therefore dQ increases as n decreases. At point C, dQ reaches the maximum value, and Mj also reaches the maximum value. When n further decreases, since W decreases, dR also decreases, therefore, dQ decreases, and Mj also decreases. In this way, a peak torque appears at point C, forming a peculiar shape of the propeller reversal characteristic.

When the speed of the propeller is reduced to zero (point D), the torque of the propeller is not equal to zero, as can be seen from Figure 3d, the torque dQ has a certain value at this time, which means that under the impact of water flow, an active torque is generated on the propeller, trying to maintain the original rotation direction. To keep the propeller stationary at this point, there must be an equal amount of drag torque from the prime mover. In order to accelerate the propeller in the opposite direction, the negative torque of the prime mover must be further increased. When the propeller accelerates in the opposite direction, the relationship between its torque and rotational speed is shown in the DE section. In this section, even if the speed is very low, the torque that the prime mover should send is quite high, otherwise it is not enough to overcome the propeller torque. Therefore, in the process of propeller reversal, the prime mover works very heavy.

If the propeller is reversed at lower ship speeds, its reversal characteristic is above that of full ship speed. For example, in Figure 1, characteristic 2 (corresponding to ship speed V=0.6Ve) is above characteristic 3 (corresponding to ship speed V=Ve, that is, full ship speed). When reversing from zero ship speed (this is reverse starting), the characteristic takes the shape of curve 1. This characteristic curve is the reverse anchoring characteristic, which is completely symmetrical with the anchoring characteristic during forward starting.

During the reverse rotation of the propeller, the speed of the boat does not actually remain constant, but decreases continuously. Therefore, the propeller torque does not change according to a certain characteristic curve, but continuously transitions from the lower characteristic curve to the upper characteristic curve, corresponding to the ship speed at each moment.

In the following discussion, we will generally ignore the friction loss of the shaft and consider the propeller power and torque as the motor power and torque.

Free sailing characteristics and mooring (anchoring) characteristics of propellers

Free sailing characteristics and mooring (anchoring) characteristics of propellers

Propeller characteristics refer to the relationship curve between propeller torque, power and rotational speed, that is, M=f(n), P=f(n) curves. The most commonly used are the following three typical characteristic curves: (1) Free navigation characteristics My=f(n), Py=f(n); (2) Mooring characteristics or anchoring characteristics Mz=f(n), Pz=f(n); (3)