Unlike through-hull models, in hull transducer models do not require a hole in the boat's hull. Instead the unit is placed inside the hull and transmits and receives signals from there. This type of fish finder works only with fiberglass hulls and as with other methods, placement is critical. Usually a flat spot in the aft bilge area is the best location, but check with a professional. Enter the realm of The "Shoot through" transducer.
There are several ways to attach a shoot through model. They can be bolted on or some can be fiberglassed or epoxied to the hull. There are also some shoot through models that are encased in a special enclosing. A professional can help you choose the right method for your fish finder and your boat.