Needing Structure
If the hull and deck were only  composed of gelcoat and fiberglass   layers, the whole boat would flex tremendously in minor seas, and it could never support heavy outboard engines. Adding a foam-core stringer system and transom greatly improves structural integrity.

Historically, builders crafted these pieces from wood. But before today's marine-grade wood and improved waterproofing processes, water could get to the wood, if it was not properly encapsulated. Boats with rotting wood transoms angered consumers and caused builders to shy away from the product.  

Many of today's stringer systems and transoms are made from foam — either urethane or PVC. You'll see brand names of pre-made foam such as Divinycell or Klegecell; foam may come in liquid form, too. Heavier urethane is generally needed to achieve the same properties and goals as PVC foam, Reeves says. Some transoms may be made with ceramic poured into a mold in liquid form. In theory, it may fill more of the space — leaving fewer air pockets — because of its composition, but generally weighs more than foam.

Outside companies make some of these parts — such as Prisma beams by Compsys — and sell them to boatbuilders. Such beams are made in a mold and usually include an interior layer of fiberglass surrounded by foam; then they're enveloped with another layer of fiberglass — often a biaxial mat.

Smaller boats may have two longitudinal stringers; larger boats are more likely to have four. Most companies create latitudinal cross members to join the stringers, forming a more rigid, boxlike configuration. The stringers are bonded to the hull and reinforced with fiberglass tape.

The transom board may be affixed with putty to the back wall of the hull and then clamped in place and covered in fiberglass. The stringers and transom are connected with fiberglass tape, creating one support system to distribute the load.

Once a builder completes the hull, stringers, transom and deck, and installs the boat's systems, the hull and deck are mated together and finished, producing a vessel that meets the designer's goals.
 
"There are no magic numbers," Reeves says, explaining that what works for one boat of a particular size that's used for a certain application surely does not work for a dissimilar boat. "Speed, weight and performance drive the lamination process."