Most modern general purpose CHIRP depth and fish finders can reliably operate down to ~900ft (274m).
These instruments work by sending sonar pulses through the water and listening for the return. Water and the bottom absorbs energy from these pulses which creates an effective limitation of how deep you can scan.
Most fish finders will have multiple transmission modes
- High Frequency (~200khz): Provides a high definition and accurate indication of what is in the water down to ~600ft (~182m). Able to detect small targets.
- Low Frequency (~50kHz): Lower definition signal more suited to deeper ocean water, better suited for larger fish and finding the bottom. Can have incredible depth.
Factors that can influence the max depth include:
- Transmitting output power (300 RMS watts to 4kW): The larger and more powerful the signal, the deeper you can go
- Transducer (under water antenna) type & mounting
- Cone Angle: Tighter cone (narrow straight down) will operate deeper than a wider cone angle that has better coverage of what's in the water around the boat.
- Quality of the Instrument: A higher quality unit will generally have more sensitive circuity and better processing capability allowing for a better image in deeper water.
