Tank (hull tube)
An omni sonar term, in plain English.
The tube through the bottom of the hull that the transducer passes through. Furuno's 8-inch-class tubes are 190 mm inside diameter.
See it in action
The free lessons show where this shows up on a real screen: start with the learning path, try it yourself on the omni trainer, or look it up on real CSH-10 captures in the field guide. Owners and captains talk settings every day on the forum.