As our Sabre 38 Express hull starts down the assembly line the first items installed are the stringers and the main engine room bulkheads. These components form the skeleton of the structural reinforcement of the hull and the stringers will also be the supports for the engines. And since the engines are as big as 480 horsepower each and weigh about one ton each with their gears attached, you want to be sure the stringers are well built.
Stringers are made of two materials. The parts which are both forward and aft of the engine room are made of foam, the parts under the engines are built up plywood laminations and the spots where the engine is going to be bolted to the stringer have steel caps. The reason for going from foam to wood is to stop vibrations from traveling down the stringers to the rest of the boat.
The internal material is a form over which multiple layers of structural Eglass is laid, overlapping from side to side. The foam is closed cell so that water cannot penetrate the stringer and Sabre places PVC tubes in the limber holes for further assurance against water penetration. Most important to remember here is that the overlapping fiberglass is the structure, the foam is simply the form.