Not every bright mark is a fish. Wakes, sidelobes and the surface itself all draw echoes that look like something, and knowing the usual suspects saves you a lot of chasing.
At a glance
- Wakes are strong reflectors and block the sound beyond them
- Sidelobes can paint false echoes over shallow hard bottom
- At 0 degrees of tilt the sea surface dominates in rough water
- Lower transmit power, not gain, for hard shallow bottoms
About 7 minutes. Part of Read the water in the CSH-10 course.
Wakes
The wake of your boat or another vessel is a strong reflector at a narrow tilt, and the air bubbles in it absorb sound. On the screen it shows as a thick line, and the water on the far side of it is poorly covered. When a school lies behind a wake, a different heading or a higher tilt can change the geometry.
Sidelobes
A sonar beam has a main lobe and weaker side lobes. Over a shallow, hard bottom, a side lobe can return a strong echo of its own, which appears as a false mark at a distance where nothing is. Furuno also notes that poor transmission through the soundome can cause the same effect. If false echoes appear only over rocky shallows, suspect sidelobes before suspecting fish, and tell your installer if it looks like the dome rather than the bottom.
The surface and the bottom
At 0 degrees of tilt about half the beam's energy goes up, and the sea surface dominates in rough water. A small increase in tilt lifts the beam off the surface. At the other end, 30 to 40 degrees of tilt shows the whole bottom echo, which can hide fish close to it, and 10 to 20 degrees puts half the beam on the bottom. The tilt lessons explain how to pick.
Cleaning it up
For a shallow hard bottom, Furuno's advice is to lower transmission power rather than cut gain, which keeps weak echoes visible while taming the strong ones. An irregular bottom echo in rough seas or at long range is expected; step the range down. Hull noise is a different matter: Furuno's troubleshooting table points to re-greasing, which belongs with the service lesson.
Check yourself
Three questions
Sources
- CSH10 Operator's Manual: Sections 1.4, 2 and 8
- CSH10 Explanation of User Settings: Transmission power
This lesson summarizes Furuno's public documents in our own words. Limits and settings differ by model and software version, so your own manual and your installer have the last word. OmniCSH is an independent project, not a Furuno program.