In-hull (a.k.a. shoot-through) transducers are epoxied directly to the inside of the hull. These are only used in fiberglass hulls. In-hulls will not work with wooden aluminum, wood, or steel hulls, or in foam sandwich or hulls that have air pockets. Any wood, metal, or foam reinforcement must be removed from the inside of the hull.

With an in-hull transducer, the signal is transmitted and received through the hull of the boat. As a result, there is considerable loss of sonar performance.

In other words, you won’t be able to read as deep or detect fish as well with an in-hull transducer as with one that’s transom mounted or thru-hull mounted.

Fiberglass hulls are often reinforced in places for added strength. These cored areas contain balsa wood or structural foam, which are poor sound conductors. The transducer will need to be located where the fiberglass is solid and there are no air bubbles trapped in the fiberglass resin. You’ll also want to make sure that there is no coring, flotation material, or dead air space sandwiched between the inside skin and the outer skin of the hull.