For the main engine of the ship, the maximum power of the main engine of the ship under the rated speed, within the specified normal maintenance period of the main engine, and under standard environmental conditions, is regarded as the continuous power or rated power.
If the factory-calibrated power of the main engine is not the rated power under the marine standard environment, it should be corrected according to the marine standard environment in actual use.
From the perspective of power plant design, considering the economy and maintenance of the main engine, it is generally necessary to leave a margin for the power of the main engine, so that the main engine can adapt to the power reduction due to long-term operation, and the pollution and deformation of the ship after long-term use. Factors such as the swaying and turbulence of the ship in the wind and waves.
For general transport ships, a 10% margin is often selected;
For heavy load conditions such as fishing boats and tugboats, a 15% margin is often selected.
The power after deducting this margin can be used as the usual power.
2. Propeller received power:
The power emitted by the main engine can only be transmitted to the propeller through the reverse reduction gearbox, intermediate shaft, propeller shaft, etc. Gearboxes, shafting, etc. will lose a part of power due to friction loss, and accessories such as the transmission oil pump of the gearbox itself will also lose a part of power. These are called transmission efficiency.
3. Main engine rated speed:
It is the speed at which the diesel engine emits the rated power for continuous operation under standard ambient conditions.
4. Trial speed:
When the ship is on trial, in calm sea, the wind force is not greater than Beaufort level 2, deep water channel, and the average speed of the forward and reverse horizontal speed at the rated speed of the main engine.
The above concepts are clarified, we can then discuss how to match the ship, engine and paddle.
For general ships, the lower the speed of the propeller, the larger the diameter, the more efficient the propeller.
Therefore, ships with medium and high-speed diesel engines as the main engine are generally used. The speed of the propeller must be reduced by means of a reduction gear box, in order to increase the diameter of the propeller.
But this is not absolute. The diameter of the propeller cannot be increased blindly. There should be a certain appropriate gap between it and the hull. If the gap is too small, the hull will vibrate and affect the normal operation of the equipment. Seriously, it even affects the structural strength of the hull, and the welds are cracked.
