Hydrophone hunting - German U-boat counting screws

[Canovaro]

Originally Posted by SubConscious

I'm a complete novice at topics like engineering and acoustics, so please pardon if I ask questions that may have answers that are obvious to others, but I can't help but wonder how one could ever determine even a good estimate of speed from RPMs.

Perhaps my problem is that I can't differentiate propeller sounds from the engine noise. I would think that even knowing the exact ship, engine noise could be impacted by wear, damage, refitting, lubrication levels, and how the draft of the ship. My guess is that propeller RPMs would be influenced by the size of the propellers, shape of the blades, wear/damage of the blades, the draft of the ship and the weather (e.g., an unloaded ship in heavy seas could experience the propellers breaching the surface of the water - I would guess the RPMs would be higher without water resistance and also difficult for a sub to hear).

I'm intrigued. How does one count RPMs and estimate speed?

I guess you can hear the engine sounds much better than the prop screws. The chugging sounds in SH3, isn't that the engine sound rather than the prop? I bet it is.