Although Shannons all have a classic yacht appearance with low freeboard, properly proportioned transom, and good spring to the hull sheer, Shannon Yachts is generally acknowledged in the boat building industry for its use of sophisticated fiberglass construction. The most important feature of Shannon's fiberglass work is the use of composite core construction techniques. Composite core construction uses a layer of structural foam sandwiched between two thicknesses of laminated fiberglass. A composite hull can be both lighter and stronger than a conventional hull made with only solid fiberglass laminates. Cored hulls can remove unwanted weight above the waterline, and they have tremendous impact strength to absorb a blow from a piling, another boat or a grounding. Solid laminate hulls are heavier in the topsides and when hit tend to fracture and fail along the filament lines of the laminate. Shannon hulls use 1/2"-1" semi-rigid PVC closed-cell linear foam material. Linear foams do not shear internally under impact, as do the less expensive cross-linked PVC foams. Extensive testing has proven that foam core materials have better memory than balsa wood cores, enabling them to spring back into shape after a concussion. Unlike balsa wood, foam cores do not allow water migration and rot if water penetrates into the core material from a skin fracture. Shannon does not use balsa wood in hull or deck construction.
Shannon hulls are hand laid-up in one piece, so there is no centerline seam where two half hull sides are joined together. One piece hull construction also allows for continuous transverse fiberglass laminates for added strength. The Shannon hull laminate schedule calls for an initial layer of NPG iso gelcoat followed by a layer of 1.5 ounce mat set in vinylester resin. Next is a layer of 2mm Coremat to prevent print-through of the roving materials underneath set in iso resin. The laminate schedule then calls for multiple layers of biaxial unidirectional filament 2408 roving. The foam core material is then set in Corebond mastic from the shear to the turn of the bilge, with solid laminate at the keel. This mastic produces greater adhesion between the core and the laminate than does the use of resin. The core is followed by multiple layers of 2408 roving, with additional reinforcements of 1708 45/45 bias roving, Kevlar/glass hybrids, woven roving and epoxy resin in high stress areas like the stem, chainplates, rudder post, bulkhead attachments, and internal hull flange. The use of the isopthalic gelcoat and a vinylester resin veil-coat in combination with Interprotect 2000 epoxy barrier undercoats applied before the bottom paint completely protects against osmotic blistering.
Shannon does all the fiberglass work in its own dedicated lamination building with a controlled environment to insure proper curing. More importantly, Shannon's crew of experienced laminators treat their work as the most significant part of every Shannon. The result is a fiberglass hull and deck able to deal with the rigors of the sea for several lifetimes. When strength and durability (but not cost) are considered, the fiberglass materials and methods used by Shannon are undeniably those best suited for an offshore cruising sailboat of the highest quality. Shannon's ten year warranty on the hull and deck fiberglass laminate attests to the conviction that Shannon's fiberglass work is the finest in the industry.
Hull to deck joint and one piece hulls
The strongest and most watertight hull to deck joint is achieved by a substantial inboard flange along the top of the hull mold at the shear. This flange makes a substantial base so the deck can be bonded and thru-bolted to the hull. An inboard hull flange on a one piece hull like a Shannon yacht requires laying-up the hull inside a two piece mold that is separated upon completion to allow release of the hull from the mold. This is a labor intensive method of construction. In order to have the considerable benefit of an inboard flange, but not the difficulty of laying up inside the mold, some boat builders lay-up the two halves of a hull separately and then join the pieces by tabbing the hull together down the seam. As the seaming of a two piece hull can make the centerline section (which is the area subject to the most stress) the weakest part of the hull, this is an unacceptable method at Shannon. Each yacht hull is laid up as one piece in the bolted-together mold which allows the fiberglass laminates to be continuous athwartships, with overlaps of double thickness on the centerline.
The hull to deck joint is bonded with structural adhesive/ bonding compound and mechanically fastened with 3/8" stainless steel bolts, washers, and aircraft lock nuts on 16" centers. The deck is further secured by thru-bolting the teak toe rail on the deck with 5/16" bolts on alternate 16" centers, and then thru-bolting the genoa track on the toe rail with 1/4" bolts on 4" centers. As the bolts are staggered, the deck is thru-bolted to the internal hull flange at least every eight inches. In over two decades, no Shannon has had a hull to deck joint structural failure, in spite of some significant accidents and hurricane damage.