Boat Buying Tips: Stringers are the Key to Hull Strength - 07/05/2017

Stringers Give the Hull its Strength

Stringers are the Key to Hull Strength
Shiny gelcoat can hide a multitude of sins, but no worry with this Formula 37 Performance Cruiser. She's one of the best-built boats out there, with molded stringers secured by Plexus adhesive which is stronger than the fiberglass laminate itself.

What's Under Her Skin?

Without X-ray vision, or by cutting a sample core out of the hull, no one, not even a surveyor, can tell what's in the laminate. Boat buyers have to rely on the builder's reputation.

BoatTEST can happily report that we hear of very few cases of boat hulls delaminating -- of any brand -- these days. It used to happen, and occasionally still does, but it is not something that most buyers need to be worried about.

The earliest fiberglass boats had very thick hulls because no one really knew what the composite of polyester resin and encapsulated glass fibers could take. Many of the early hulls were so thick that they will probably still be around for the next coming of dinosaurs. And, those hulls were heavy.

Over the years, recreational boat hulls have gotten thinner and thinner, and builders learned how strong the material really was. As the hulls got thinner, stringers and frames became even more important.

A boat’s stringer system is one of the most important elements of a boat.

Consumer Caveat. Interestingly, there are no ABYC or CE standards on how boat hulls and stringers should be built. There is plenty of published guidance written by engineers as fiberglass boat building enters its seventh decade of active building, but none of it is mandatory for recreational boats. Nevertheless, more is known now about what fiberglass can and can’t do than ever before.

Stringers are the Key to Hull Strength
These stringers have been laid-up in molds, then glassed into the hull. Some builders core the stringers with foam, or add solid support under the engines. Others don't -- it's hard to tell just by looking. These longitudinal stringers and transverse supports, along with bulkheads, are what gives boat hulls their strength.

The Stringers

Looking at the bottom-support structure provides clues to the real quality of construction. A fiberglass laminate is very flexible, so all fiberglass boats rely on longitudinal stringers and athwartships frames and bulkheads to maintain the hull's designed shape. While it's impossible to see into the laminate, the hull support structure is accessible, and provides a clue to overall quality.

Stringers are the Key to Hull Strength
This worker is spreading a high-tech adhesive prior to the molded support grid being dropped into place. This glue, usually Plexus, creates a stronger bond than fiberglassing the grid into place – which used to be the practice.

The stringers and frames together create a grid-like arrangement that's attached to the hull in some way. The fiberglass stringers can be molded over plywood -- satisfactory when done correctly – but more likely these days they are laminated over pvc foam or laid up in a separate mold and bonded into place.

Stringers laid-up in situ over foam cores (or plywood) is labor-intensive and has to be done carefully. Stringers molded separately and glassed or glued into place using sophisticated adhesives is the best way, in our opinion. For one thing, it is exactly repeatable. Sometimes the grid is molded as part of a liner and dropped into the hull. In this way, the liner serves two purposes and saves weight.

Stringers are the Key to Hull Strength
Glued in place, this grid will be covered by a molded fiberglass liner, making it possible to access the grid in only a limited number of places. Not usually a problem, hull supports installed like this one rarely fail.

Most new boats built today have a fiberglass grid stringer system that is actually made in its own special mold. This eliminates the need for a plywood form over which to laminate the plywood. The thickness of shape of the stringers can be well controlled, and they will all come out of the mold the same. But not so long ago many builders used plywood – even the best ones.

Older Boats. At one time or another virtually all builders used plywood to create fiberglass “hat” sections, frames and substructures. So, when buying a used boat, the thing to focus on is not whether or not there is plywood in the stringers, but whether or not that wood has been completely encapsulated.

Consumer Caveat. The wood does not supply the strength of the stringers – strength comes from the fiberglass over it, its shape, and the overall grid of fiberglass is what is important.

Stringers are the Key to Hull Strength
Typical plywood stringer system forms ready for the fiberglass laminate to be laid over it.

Used Boats -- Beware

Used boats that have plywood stringers are common, and the older the boat the more prevalent is the use of plywood. When buying a used boat make sure the wood is fully encapsulated in fiberglass fabric and resin, with no bare wood showing. Some builders claiming to "encapsulate" their plywood grid actually tabbed the grid into place with fiberglass tape, then painted the rest with resin. This is no good -- it should be covered with fabric, which locks the plywood into place, makes the structure stronger, and generally prevents rot.

Any perforations in the grid -- to pass wiring or plumbing, for example -- should be sealed with resin, and have a gasket to prevent chafe. Bare or paint-coated wood suggests that lots of corners have been cut and consumers should move on.

Stringers are the Key to Hull Strength
The builder of this center console is proud of it stringer system which it calls a “structural grid” – somewhat similar to an egg crate box. It is molded outside and placed in the hull with adhesive.

Advantage -- New Boats

New boats cost more than boats of the past -- in constant dollars. The reason is that the materials used are better than before, and today virtually every builder uses vinylester resin as a skin coat to prevent water migration through the fiberglass -- and blisters. Vinylester resin was not used widely in boats until the 1990s, so most boats older than that are at risk of getting blisters.

Few builders today use plywood in hat sections or in frames over which glass is laminated.

Questions to Ask

When inspecting the bilge of a prospective new boat purchase, look at the stringers.

  • Are they robust?

  • Are there many or a few?

  • Are they wide or narrow?

  • Are they filled with flotation foam or are they empty?
  • Ask the boat dealer if the stringers we laminated in place, or where molded outside the boat? Were they glassed in or fixed with the use of a chemical bond. Chances are he will have a picture or a drawing of the system.


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