Measuring flow noise
The upper water layers of the sea contain a myriad of small air bubbles created by the breaking waves. When the hull moves through water it will cause a disturbance, and this will generate friction. The friction zone is called the flow boundary layer. All objects sticking out from the hull, or dents in the hull, will disturb the flow and will increase the thickness of the boundary layer. When the flow speed is high, the turbulence can be violent enough to destroy the integrity of the water. Small voids or cavities in the water will occur and this is called cavitation.
Prerequisites
Neither tools nor instruments are required.
Context
To measure the noise, observe the noise values provided on the Noise page in the Diagnostics dialog box.
Tip
If you record all the raw data during the noise test, you can repeat the test later using the replay file.
You may wish to make the noise/speed curve under realistic conditions. This means that all relevant vessels systems are in normal use. If you experience very high noise levels, you may wish to repeat the test with "silent ship" settings.
Note
When the transducer is fully lowered, the speed of the vessel must not exceed the maximum speed permitted for the hull unit. This maximum speed depends on the physical size of the transducer, the mechanical construction of the hull unit, and how far down the transducer has been lowered. The System Protection page allows you to define a limit that triggers an alarm if the maximum speed is exceeded. The System Protection page is located in the Installation dialog box.
Image
Illustration:
Read the noise value from the noise circle. Use the mean value of the readouts at 0 degrees (straight up).
Procedure
1 Make sure that you have sufficient water depth below the keel before you lower the transducer.
2 Lower the transducer to its bottom position.
3 Select the default settings.
a On the Main menu, select User Settings.
b Observe the Factory Settings list.
c Select the setting you want to use.
d Select Activate Selected Setting.
e Select OK to apply your changes and close the User Settings dialog box.
4 At the bottom of the display presentation, select a suitable presentation mode, and activate the Horizontal view.
5 Make the following preparations.
a On the Main menu: Set Range to 1500 m.
b On the Operation menu: Set Tx Power to Off.
c On the Active menu:
1 Set Pulse Type to CW Medium.
2 Set Frequency to 20 kHz.
3 Set Vertical TX Sector to Narrow.
4 Set Noise Filter to Off.
5 Set the Bottom Filter Threshold to Off.
d On the Setup menu: Open the BITE (Built-In Test Equipment) dialog box.
6 Select Noise to open the page.
7 Establish a separate communication line with the bridge to verify the vessel speed during the test.
8 Measure the flow noise versus the vessel speed.
a Ask the bridge to increase the vessel speed to the maximum permitted speed for the hull unit.
b Wait until maximum speed has been obtained.
c Ask the bridge to abruptly turn off all engine power and - if possible - change the pitch of the propellers to zero.
d Read the noise value from the noise circle.
e For every knot the speed is reduced, read the noise.
(If you find it more convenient, type the data directly into the spreadsheet.)
f Fill in the result table.
9 When all the measurements have been made, type the data (speed and noise) into a spreadsheet to create the curve.
10 Hoist the transducer back up to its upper position.
Result
Speed/Frequency/Noise
Speed Frequency Noise level
20  

 
19  

 
18  

 
17  

 
16  

 
15  

 
14  

 
13  

 
12  

 
11  

 
10  

 
9  

 
8  

 
7  

 
6  

 
5  

 
4  

 
3  

 
2  

 
1  

 
Sea state:  

Water depth:  

Use this table to record the values. Print it out and make multiple copies. Alternatively, you can type the values straight into a spreadsheet.