Transoms, some solid, many cored, and it is difficult if not impossible to tell them apart at a glance. Many smaller sailboats have very thin solid fiberglass transoms with barely enough integrity to support the aft shroud chainplate and stay. In larger sailing vessels the transoms are typically solid and structurally capable of carrying the applied load.
Many production power boats with stern drives have a cored transom, which usually consists of plywood sandwiched between inner and outer layers of fiberglass laminate.
During production the transom is fabricated in the mould with the lay-up schedule including the plywood as an integral part of the transom. Typically the engine mounts, many are made from plywood covered with fiberglass, are also installed at this stage of production. Once frames and supporting structures are completed the hull is removed from the mold. The next step includes the engine installation. A section of the transom, (hole through the transom), is cut out to facilitate the various transom assemblies. Transom assemblies are the units which are secured to the transom and support the stern drive unit. When the hole is cut to fit the transom assembly the circumference of the hole shows exposed fiberglass laminate and plywood. The plywood, in many cases, does not get sealed properly. As this hull penetration is partially or in many cases completely underwater it is vulnerable to the elements and ultimately water infiltration to the plywood core is the next step. How fast the plywood rots away depends upon several factors, they include; the climate, the various types of insects indigenous to the area, how long the boat is in the water each year, and how many hours the boat is under way each year, and lastly, how the boat is stored, protected, and ventilated during winter lay-up. It usually takes many years for severe damage to occur in this area, but not always. I recently inspected a three year old Bayliner with twin out drives that had actually pulled away from the structure about an inch. The owner called me to check out the transom because he did not recognize the new protrusion at the stern drive that had somehow occurred over the winter.