This just popped into my head, so here I come to ask.
Why does many submarine sonar use cylindrical arrays? I find it weird that some of the arrays are actually facing the submarine hull itself. Wouldn't it be more efficient to just make a half-cylinder array so all the sonar waves are sent outwards to detect targets/obstacles?
There are half cylinder arrays too.
Sonar arrays act like active phase arrays, each element has a receiver and transmitter unless it's passive then it's all receivers. The array beam forms and focuses on one hemisphere. If the beam is to be formed, steered and focused at three 'o'clock, you can use the elements from 12 o'clock to 6. But if the beam has to steered past 3' you will notice you will have less elements available for this. So a bow cylindrical or spherical sonar is strongest at the frontal hemisphere from 9' to 3'.
If you sent sound waves simultaneously from all elements in all directions, the sound doesn't get far without being focused and steered into a beam.
You will notice that a half cylinder does not present as much elements to focus and steer a beam to the sides as a full cylinder.