Method of controlling rotational speed of propeller of ship to reduce cavitation

1 . A method for controlling a rotational speed of a propeller of a ship to reduce cavitation, the method comprising:

generating cavitation information about the cavitation occurring during rotation of the propeller;

measuring a rotational angle of the propeller in real time after generating the cavitation information; and

changing a rotational speed of the propeller using the generated cavitation information and the measured rotational angle of the propeller such that an occurrence of the cavitation is reduced,

wherein the cavitation information comprises information about a rotational angle of the propeller at a time of the occurrence of the cavitation,

wherein the generating of the cavitation information comprises:

measuring fluctuating pressure applied to a stern of the ship in water, the fluctuating pressure being caused by the cavitation occurring on surfaces of blades of the propeller during the rotation of the propeller;

measuring the rotational angle of the propeller to generate the cavitation information; and

generating the cavitation information by using the measured fluctuating pressure applied to the stern of the ship and the measured rotational angle of the propeller,

wherein the generating of the cavitation information further comprises:

time-synchronizing information about a first fluctuating pressure applied to the stern of the ship in water near a descending propeller blade with information about the rotational angle of the propeller, the first fluctuating pressure being caused by the cavitation occurring on a surface of the descending propeller blade during the rotation of the propeller;

generating first cavitation information about the cavitation occurring around the descending propeller blade using the time-synchronized information about the first fluctuating pressure and the measured rotational angle of the propeller;

time-synchronizing information about a second fluctuating pressure applied to the stern of the ship in water near an ascending propeller blade with information about the rotational angle of the propeller, the second fluctuating pressure being caused by the cavitation occurring on a surface of the ascending propeller blade during rotation of the propeller;

generating second cavitation information about the cavitation occurring around the ascending propeller blade using the time-synchronized information about the second fluctuating pressure and the measured rotational angle of the propeller; and

generating the cavitation information including the first cavitation information and the second cavitation information.

2 . The method according to claim 1 , wherein the measuring of the fluctuating pressure comprises:

measuring the first fluctuating pressure applied to the stern of the ship in water near the descending propeller blade, the first fluctuating pressure being caused by the cavitation occurring on the surface of the descending propeller blade during the rotation of the propeller; and

measuring the second fluctuating pressure applied to the stern of the ship in water near the ascending propeller blade, the second fluctuating pressure being caused by the cavitation occurring on the surface of the ascending propeller blade during the rotation of the propeller.

3 . The method according to claim 1 , wherein the first cavitation information is propeller rotational angle range information when a value of the first fluctuating pressure applied to the stern of the ship in water near the descending propeller blade is equal to or greater than a set value, or propeller rotational angle information when the value of the first fluctuating pressure applied to the stern of the ship in water near the descending propeller blade is a peak value, and

wherein the second cavitation information is propeller rotational angle range information when a value of the second fluctuating pressure applied to the stern of the ship in water near the ascending propeller blade is equal to or greater than a set value, or propeller rotational angle information when the value of the second fluctuating pressure applied to the stern of the ship in water near the ascending propeller blade is a peak value.

4 . The method according to claim 1 , wherein the changing of the rotational speed of the propeller comprises:

calculating speed control information comprising information about a speed change range within which the rotational speed of the propeller is to be changed and information about a speed variation using the generated cavitation information; and

changing the rotational speed of the propeller using information about the measured rotational angle of the propeller and the calculated speed control information.

5 . The method according to claim 4 , wherein the speed change range included in the calculated speed control information comprises a propeller rotational angle range included in the first cavitation information when a value of the first fluctuating pressure applied to the stern of a ship in water near the descending propeller blade is equal to or greater than a set value and a propeller rotational angle range included in the second cavitation information when a value of the second fluctuating pressure applied to the stern of the ship in water near the ascending propeller blade is equal to or greater than a set value, and

wherein the speed variation included in the calculated speed control information is a value within 5% of a reference speed of the propeller.

6 . The method according to claim 4 , wherein the speed change range included in the calculated speed control information comprises a range of ±10 degrees of the rotational angle of the propeller included in the first cavitation information when the value of the first fluctuating pressure applied to the stern of a ship located in water near the descending propeller blade is a peak value and a range of ±10 degrees of the rotational angle of the propeller included in the second cavitation information when the value of the second fluctuating pressure applied to the stern of the ship in water near the ascending propeller blade is a peak value, and

wherein the speed variation included in the calculated speed control information is a value within 5% of a reference speed of the propeller.

7 . The method according to claim 4 , wherein the rotational speed of the propeller is changed using an inverter and a motor by rotating the propeller at a speed higher or lower than a reference speed by an amount equal to the speed variation in speed change ranges included in the speed control information, and rotating the propeller at the reference speed in ranges other than the speed change ranges.

8 . The method according to claim 4 , wherein the rotational speed of the propeller is changed using an inverter and a motor by rotating the propeller at a speed higher than a reference speed by an amount equal to the speed variation in a speed change range corresponding to a first-half rotation range of the propeller among speed change ranges included in the speed control information and rotating the propeller at a speed lower than the reference speed by an amount equal to the speed variation in a speed change range corresponding to a second-half rotation range of the propeller among the speed change ranges included in the speed control information, or the rotational speed of the propeller is changed using the inverter and the motor by rotating the propeller at a speed lower than the reference speed by an amount equal to the speed variation in a speed change range corresponding to the first-half rotation range of the propeller among the speed change ranges included in the speed control information and rotating the propeller at a speed higher than the reference speed by an amount equal to the speed variation in a speed change range corresponding to the second-half rotation range of the propeller among the speed change ranges included in the speed control information, and

wherein the first-half rotation range of the propeller is 0 to 180 degrees, and the second-half rotation range of the propeller is 180 to 360 degrees.