FIBERGLASS BOATBUILDING: Internal Hull Structures

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Fiberglass sailboat under construction

We’ve already discussed how a fiberglass laminate is created: what fabrics and resins are used, molds, the problem of blisters, and how cores can be used to make a laminate both stronger and lighter. Now we’ll consider how a simple fiberglass boat hull can be reinforced and strengthened by the structural elements within it. This is necessary, because in fact the molded glass hull of any boat much larger than a dinghy is not normally rigid enough to withstand much abuse. Without internal structures to help stiffen it, a large hull’s laminate would otherwise have to be unreasonably thick and heavy.

The most basic sort of structural reinforcements are called floors and stringers. Floors are transverse sills in the bottom of a hull on which cabin soles are traditionally installed. Besides stiffening the bottom of the hull, floors provide critical support to the root of a sailboat’s keel where it meets the hull. Stringers, meanwhile, are lateral fore-and-aft beams that are installed along the bottom of a hull. Instead of traditional floors and stringers many modern shallow-bilged boats have a unitary grid, sometimes called an “egg crate,” which consists of structural beams running both laterally and transversely across the bottom of the hull.

Interior fiberglass hull structures
Structural elements found inside fiberglass boat hulls

Bulkheads, partitions, and other structural components of a boat’s interior accommodations and furniture also play an important role in stiffening a hull. Bulkheads are particularly critical, as they can simultaneously provide support to the deck overhead, the bilges below, and the sides of the hull as well.

A primary concern with internal structural components is how they are attached to the hull. On some modern vacuum-bagged or resin-infused boats an egg-crate grid is molded into the bilge as part of the primary hull layup, which is an excellent practice. The traditional procedure, however, is to bond, or tab, internal components in place with strips of fiberglass tape after the hull has been molded. These secondary adhesive bonds are weaker than primary chemical bonds. To create a superior secondary bond the surfaces involved must be properly prepared. In many cases the component being tabbed to the hull is made of wood (often it is plywood), in which case the wood grain must be sealed beforehand or it will suck resin out of the tabbed joint when the fiberglass tape is laid down and wet out. Surfaces on both the structural part and the hull itself should be scratched with sandpaper or a grinding disk to give the resin texture to bite onto; they should also be wiped down with solvent before any glass or resin is applied.

The area of the bonded surfaces must also be large enough to absorb loads on the joint. The rule of thumb is there should be at least a 2-inch margin of tabbing either side of any joint, though a minimum of 3 inches is better, particularly on bulkhead joints. Discrete parts such as grids, floors, and stringers located in the bilges of a boat should be completely glassed over so they don’t absorb any oil or water. Limber holes should also be cut through structures in the bilge so water can flow freely and easily to the lowest point in the hull, where a bilge pump can evacuate it. It may also be necessary to cut access holes through these parts to accommodate wiring or plumbing. The interior surfaces of all such holes must be carefully sealed so they don’t absorb any water or oil passing through them.

Bulkhead bonds in fiberglass boats
Bulkheads bonds are stronger if radiused with a fillet (right)

Particular care should be taken with any bonded joint that forms a sharp right angle. The danger here, especially with parts like bulkheads or lateral partitions that transfer loads all the way from the deck to the hull, is that so called “hard spots” will be created. These are areas where abruptly imposed structural support within a hull amplifies the total amount of stress created when the area is subject to load. Even where hard spots are created by isolated minor structures such as interior cabinetry, significant stress can result if there is an abrupt impact or collision in the area. The best analogy is that of a stick broken over a knee. The narrow fulcrum of the knee focuses stress in a single area and greatly decreases the load required to break the stick. Bend that same stick over a wider surface–a barrel, say–and there is much less stress. A greater load can be imposed without the stick breaking.

To avoid hard spots it is best if any perpendicular structure bonded to a hull does not actually meet it. Instead, there should be a small gap filled with a softer material like foam, balsa wood, or putty. The joint should also be nicely radiused with a wide fillet. This serves both to reduce stress in the area and to strengthen the bond generally, as the transition from one bonded surface to the other is more gradual. The wider the radiused angle, the stronger the bond will be and the less stress it will experience.

Hull liners

Properly installing an interior hull structure can be very labor intensive. Any economy of scale realized by popping multiple bare hulls from the same mold can be quickly negated by the attention to detail required to properly finish a hull’s interior. This is probably the one phase of boat construction where builders have tried hardest to streamline their procedures. Their key weapon is the molded hull liner, which is simply another large fiberglass part incorporating elements of a boat’s interior that is inserted into a hull.

The larger the part, the bigger the savings in terms of work and effort. A truly comprehensive one-piece hull liner can include not only a structural bilge grid, but also all major furniture components from the bow to the stern. Bulkheads and partitions in these cases are not bonded directly to the hull, but are fitted and glued into pre-molded slots in the hull liner and overhead deck liner or, alternatively, are bolted to special flanges in the liner.

Fiberglass hull liner bonds
Hull liners simplify interior hull construction, but may compromise structural integrity

A liner can’t provide much structural support unless it is firmly bonded to its hull in as many places as possible. The usual practice is to lay down beds of adhesive putty (adhesive “splodges”) or thickened resin in appropriate spots, then set the liner down on top of these. This relatively light bond should then be improved by tabbing the liner to the hull with glass tape anywhere there is access to contact points between the two parts. Such access, however, is always limited, and work spaces are often cramped and awkwardly situated.

In the end, it is never possible to create as strong a structure as is formed when all individual components are bonded piece by piece directly to the hull. If the hull is unduly stressed, the liner may break free in some areas. I have heard more than one tale of mass-produced boats failing like this in strong weather. Such damage can be difficult to detect and is always difficult to repair. It may involve cutting away and then rebuilding large portions of the liner in situ, which may prompt an underwriter to declare the boat a total loss.

The best practice is to create the hull liner in small sections and install the parts separately. Ideally, support for the bottom of the hull, usually a grid of some kind, is laid in first. One-piece grid pans are often used, but it is best if the grid is built up in place with each part bonded directly to the hull. Bulkheads and hopefully partitions should also be bonded directly to the hull. Separate interior liner sections can then be laid in place around the bulkheads and on top of the grid. It is easier to create strong bonds between the hull and these smaller, more discrete parts; the bulkheads and bilge structure will also both offer more support to the hull than would otherwise be the case.

Small hull liner section
Smaller discrete hull liner sections can be bonded to a hull more easily

Another disadvantage to a hull liner, no matter how it is installed, is that it limits or precludes access to the hull once it is in place. This makes it hard or impossible to repair damage to the hull from within the boat without first cutting away the liner. If the hull is breached while underway, a liner makes it harder to both find and staunch any leak, which is why some cautious cruisers always carry a heavy tool such as an ax or crowbar for quickly tearing away a liner in an emergency.

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4 Responses
  1. Amazon

    Thank you very much, lovely bit of info, I have just bought 23′ Clovic Watson (with no GRP or boating experience) and need to put a floor in the shower/ toilet room. I had a feeling there would be issues with bonding to the hull …onwards … more importantly … must look at engine mountings before engine arrives, thanks again, S xxx

  2. Dan

    I’m preparing a Reliance 44 for offshore sailing, and part of that included replacing the cabin sole. Once I got a good look at the “grid” holding the current sole I became concerned. Now reading this article I realize that my concern is well placed. The “grid” has been so chopped up and modified over the years that it’s practically non-existent. By the time I’m done with this boat I will have practically built it from scratch!

  3. With less than 2 months sailing experience, I built a 50 foot 23 ton cutter yawl. Just my bride to be and myself. After almost solid daily sailing and cruising for over 42 year the Daedalus is still going strong, never had a blister. As an introduction here’s a six minute aerial video of my Daedalus https://www.youtube.com/watch?v=i07qL_N22_c I launched her August 23, 1977 in Santa Barbara California.
    I have hand built many things, self studied architecture , became a licensed architect also a US Merchant Marine Master with a passenger sailing endorsement. I have designed and built several boats restored boats, exotic cars all on hand to mouth finances designed and built a bridge The following are portions of my daily log; . In 1974 I had very little sailing experience. I designed; hand built a 50 foot sail boat the Daedalus. The following are excerpts of chapter 11 then later further chapters 12 through 16 if y’all are a bit interested. Thank you Capt. Fred.

    Little did I expect to find a partial plywood mold disintegrating in a back yard? The bow and aft end were mostly missing. That old useless mold got my juices going and I could see thru all that mess a very beautiful potential for a hull. The basic shape for this sailboat hull was there and with a lot of bracing and bending, I figured I could do something with it
    . At that time in history people were building thousands of boats up and down the entire West Coast from Canada to San Diego. I found a build it yourself boat yard on Gutierrez Street in Santa Barbara and rented a little corner of the yard.

    About 8 other boats were under construction. Some were under construction for as long as 10 to 15 years, Also, transferring ownership that many times. One fellow boat builder, an Engineer, who said he went through two wives and families already while building his boat, really scoffed at me when I said I would be done in about three years. He asked if I was going to buy a boat kit, which were available in different stages of construction. “No from scratch”, I answered. When he saw the old mold, he raised one eyebrow. His sympathy for me was obvious. At the yard we all became friends and trusted and watched out for each others stuff.

    Thousands of boats were under construction on the West Coast and old wooden boats were being demolished by the hundreds of thousands worldwide. Beautiful bronze parts were available. Fiber glass materials were relatively inexpensive and distressed sales were all over the place.

    I heard that 10,000 ferro cement boats were built on the West Coast. Sadly, presently there are only a few of them left. The steel mesh armature and sea water make a nice battery that just ate them up. My engineer friend in my boat yard was building out of cement, and he launched at the same time as I launched the DAEDALUS. His 60 foot Ferro cement settled in at about 12 inches above the waterline, to just below the portholes. That is what mindset is all about! However, he just raised the waterline and kept the port holes sealed.

    The miracle I experienced building the DAEDALUS was mystifying to say the least. When I needed something, whatever it was, resin, a bronze fitting, the mast, whatever it was. It just seemed to materialize. I found it shortly somewhere. It is almost like something was watching over me and always has. In adventures yet to come, at sea especially, you’ll see what I mean about being watched over. I’m not superstitious or religious, I’m just sayin…

    The mold was restored and I fashioned a bow and stern section as I saw fit. I painted the mold with a can of light colored paint, I found lying around. Then I heavy waxed the mold. BTW, I built scaffolding that went all around the inside of the mold without touching it, out of some used 2X10 lumber I found. Now I could reach it all and store supplies and cans and buckets of resin around. I did make a few mistakes, but a major one was not to take a pictorial record of the whole procedure. Big mistake! I did build a kind of shed over, with very valuable used sails I had salvaged. I did not know how valuable they were. https://flic.kr/p/uvzS11

    A beautiful old 1930’s 50 foot sloop named the “LAST STRAW”, designed by the most famous yacht builder, Herreshoff,

    https://www.surveymonkey.com/s/2JS8GSZsank. It had just left Santa Barbara heading north, to be refitted in San Francisco; she hit the rocks at dangerous Point Conception and was destroyed. Somebody dove and cut off the enormous lead keel, she floated on to the beach. Some farmer said it was his beach and he chained sawed that beautiful yacht into pieces, right through another skylight and some other precious parts.

    Frantically I was in the fray trying to save what I could from the LAST STRAW. For $1,000 I saved almost all the major bronze, including all the winches, cleats, sails, beautiful famous Herreshoff teak and copper hatches and thousands of other parts, anchors lines, turnbuckles etc., etc. I was stoked; the antique stainless gimbaled 4 burner propane stove alone was worth almost a $1000. The DAEDALUS was becoming a reality.

    There was a furniture factory on on side of the boat yard. Up on the 2nd floor there was one lonely 3 foot wide aluminum sliding window. Out of curiosity I got my ladder and looked in. It was right next to the DAEDALUS. It was a pine paneled room with a sink, refrig, double bed, carpet floor. Hey, what else do we need? Ok, a toilet, which was in the factory. Wow! I spoke to the amiable owner and he rented the room to us and we became kind of his night watchmen and friends. He had just kicked out a girl friend he had secreted in that room. What luck we loved it and moved a ladder to the window and that was our front door to our cozy home for a couple of years. It was great. Friends and relatives visited us there from all over the Country. We ate lots of tortillas and drank lots of rum in that boat yard.

    I have several more chapters, but at this point, I’m 87 years old I use a walker and My body can’t keep up much any more. I fin it difficult to sail so I designed an old man’s boat not too tippy something that will allow me to enjoy the wind and the water : https://www.flickr.com/gp/77284431@N02/ih2Lnd

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