The transom on a small boat typically supports an outboard motor. The transom transfers the energy generated by your motor to the rest of the hull, moving your boat through the water. By turning the helm, you change the angle that the motor is pushing from, and that redistribution of energy against the transom turns your boat.
Because the transom is under a great deal of stress, it is thicker than the rest of the hull. Not only does the transom support the boat's engines, but it also must withstand the extreme forces generated while turning your boat. To compensate for that excessive stress, the transom is the strongest part of your boat's hull.
If the transom becomes weak or damaged, it should be repaired or replaced.
Many older boats were constructed by laying fiberglass over wood at the transom. If you have a fiberglass boat that was built before 1990, closely examine the condition of the transom. Wood-cored transoms have a metal or plastic strip on top of the transom that is held in place by screws. Remove this strip to inspect the wood for signs of rot. Any softness in the transom should raise a red flag. It can be repaired, but it is an expensive and labor-intensive job.
To repair a transom with rot in its wood core, you must first remove the rot. After all the rotten material is removed, replace it with new marine-grade plywood and epoxy the repair in place.
Transducers and motor brackets require mounting holes. Any time you put a hole in your transom, you create a new weak point and a potential source of trouble. Always be sure to drill holes to exactly the right size, as overly large bolt holes can result in excessive wear on the transom as the bolts move back and forth. Use 3M's 5200 marine sealant in the screw holes to limit any water penetration.