[Iceman]

From Eagle site: I've always heard 10% signal loss shooting through the kayak hull, but have no source. It comes down to detail at higher depths.

Thru-hull transducers are inserted through a hole drilled in the hull. These typically have a long stem that slides through the hull and is held in place with an equally large nut. If the hull is flat, this is the extent of the installation. However, if the transducer is to be mounted on one side of a "vee" hull, then a fairing block must be made from wood or plastic that allows the transducer to be mounted vertically. Thru-hull transducers are typically used on inboard powered hulls so the transducer can be mounted in front of the rudders, propellers, and shafts.

Shoot-thru-hull transducers are epoxied directly to the inside of fiberglass boat hulls. The sound is transmitted and received through the hull of the boat - but at the cost of some loss of sonar performance. (You won't be able to "see" as deep with a shoot-thru-hull transducer as one that's mounted on the transom.) The hull has to be made of solid fiberglass. Don't attempt to shoot through aluminum, wood, or steel hulls. Sound can't pass through air, so if there's any wood, metal, or foam reinforcement, it must be removed from the inside of the hull before installing the transducer. Another disadvantage of the shoot-thru-hull transducer is it can't be adjusted for the best fish arches. Although there are disadvantages to a shoot-thru-hull transducer, the advantages are considerable. One, it can't be knocked off by a stump or rock since it's protected inside the hull. Two, since there is nothing protruding into the water flow, it generally works quite well at high speed if it is mounted where a clean laminar flow of water passes over the hull. Three, it can't be fouled by marine growth.