09/13/2013

OVERVIEW OF HULL LAMINATES, HULL CORING AND FIBERGLASS HULL CONSTRUCTION

By David E. Walters, David Walters Yachts, Inc.

Most buyers in the market for sailing or motor yachts consider fiberglass construction their first choice.  However, many are not familiar with the differences in hull construction.  The two primary options include solid or cored construction.  For both these options, there are multiple techniques.  Building methods include conventional hand lay-up, vacuum bagging, and resin infusion.  Material choices include woven roving, bi-directional, double-bias, uni-directional and tri-axial. These materials are most commonly fabricated with E-glass yarn because of its moderate cost, strength and stiffness, good chemical resistance and ability to maintain strength properties over a wide range of conditions.  Alternative, more costly fibers are S glass, carbon fiber and Kevlar.   Resins used are general purpose, vinylester and epoxy. 

The earliest and most common fiberglass construction was a solid laminate with conventional woven materials and general purpose polyester resins.  During these years there was limited knowledge of what laminate thickness should be, resulting in many overbuilt, heavy hulls.  Solid construction is still very popular with many production and custom builders and, with the experience and knowledge gained, solid hulls are now not as heavy without sacrificing strength and stiffness.  Primary reasons for the improvements to the solid laminate are more sophisticated materials, resins and methods of construction.  Less resin is needed for bi-directional and uni-directional materials.  The quality of resins has significantly improved with wide use of vinylester, especially on the outer laminates to better restrict water intrusion.  Vacuum bagging and resin infusion are widely used to further improve the glass-to-resin ratio. 

Over 30 years ago yacht designers and builders started using core materials for hulls in order to reduce the total weight, especially in the topsides.  Cored hulls have become very popular, especially for power boat construction.  The most popular core materials are balsa and PVC cross-link foam, including Klegecell, and Divinycell.  Another popular foam core is Airex, which is made of PVC but is linear in its chemistry (not cross-linked).  Core-Cell is another foam core that has become widely used in the past 15 to 20 years.  It is similar in properties to Airex but is made with different SAN chemistry.  The advantage of using core is the reduction in weight by using thinner laminates on each side of the core. The most common core materials are 1” thick and have knife cuts through approximately 2” x 2” blocks, attached to a scrim of larger sections.  The reason for the knife cuts is that it allows the sections to be applied to curved surfaces and allows resin to cover the walls of each block resulting in the best bond to the glass fabrics and mat.

In addition to weight saving, cored construction has the added advantage of providing hull thermal and sound insulation properties, and significantly reduces the need for multiple longitudinal stringers which are required in the construction of a solid hull.  The quality of the installation of the core is very important to prevent separation between the core and glass materials, and to ensure full resin coverage of the core material including the sides of the blocks.  Vacuum bagging has reduced the labor and increased the quality of cored construction.  A well-executed hand lay-up and core installation still produces some of the highest quality yachts.  Vacuum bagging the core has become very popular and ensures full penetration of the core material and a good bond to the fiberglass.

What is most important when core is used below the waterline, is for the builder to remove the core and replace it with solid glass where a thru-hull fitting is to be installed.  This allows the thru-hull fitting to be sealed and tightened without compressing the core.  This is necessary to prevent water intrusion into the core which can be a serious problem, especially with a balsa cored boat.  A surveyor can identify high moisture in the core or laminate using a moisture meter.  Lower levels of moisture are common and tapping of the area can identify if there is a problem that needs to be repaired.  However, it should be noted that moisture meters are not always reliable particularly on older cored hulls.  If the moisture meter is giving high readings on an older quality-built yacht, it is worth drilling a ¼” hole through the interior laminate to confirm the condition of the core.  During a recent survey of a 51’ sailboat showing high readings on an Airex-cored hull, an exploratory hole proved the core was completely dry.

To summarize, solid and cored fiberglass hulls that have been designed and built by experienced professionals using proven materials, resins and procedures provide their owners a low-maintenance, reliable structure.  It is worth asking if the hull laminate, with or without core, was designed to comply with a standard, such as Lloyds or ABS (American Bureau of Shipping) guidelines and if the standard was for inland, coastal or unlimited offshore use.

Boats that are built in Europe or are exported to Europe must meet the EU boatbuilding standards which are defined by the European Recreational Craft Directive, or RCD.  The RCD specifies written and certified compliance with the International Standards Organization's (ISO) boatbuilding standards, which are very stringent, much more so than any standards in the US, and even more rigorous than Lloyds or ABS.

David Walters is a member of the YBAA Board of Directors and is president of David Walters Yachts, Inc., located in Ft. Lauderdale, FL.