8.1. Abstract
The vessel’s structure is made of many elements, some transverse and some longitudinal. A watertight outer skin, called plating (or planking) encloses all these structures and makes a barrier for the surrounding water. All the elements which build a structure must be continuous: figure # 40 shows that the longitudinal stringers of the bottom correspond with the structures of the bulkheads and of the deck. The same applies to the transverse sections, where the floors, the frames, the beams build up a continuous ring. The vessel’s structure is therefore made of “rings” which tie the ship in both planes.
Please note the names of the structure parts: they must become familiar. Figure # 41 shows a similar structure during a vessel building. The weight of the structure is only from 25 to 35% of the total weight of the ship: there is a substantial dissimilarity between different building materials, but the range only involves a small percentage of the whole displacement of the vessel. In all vessels, but for the wooden constructions, the tanks are structural, hosted in a double bottom structure. Metal built ships (either out of steel or aluminium alloy) must have plugs (that can be opened from outside) below the fuel tanks, as all repair works which need welding must follow a complete and careful emptying of the fuel tanks. The maritime authority checks the vessel before the works start and releases a “gas free” certificate” only in case the tanks are empty and clean. These plugs must be clearly shown on a dedicated drawing.
Fig. 40
Fig. 41
8.2. The wood
Wood, as boatbuilding material, has been neglected since a long time, and it’s a real pity. It requires well-seasoned wood, qualified manpower, great experience, and long construction time.
Besides, the wood structure is quite large and cuts space for the arrangements. Finally, a wood vessel requires a lot of maintenance. Yet, no other material has the same appeal and scent.
8.2.1. Wood: a live substance
Wood is a live material: its characteristics change considerably depending from the area where the trees grow, from the period of the year of their felling, from the way the logs are sawn into planks, from the drying system and from the seasoning. Planks sawn parallel to the log’s axis do contort (shrinks and twist) more than planks sawn with a “quarter cut”, that is to say with a 45° angle (see figure # 42). The wood should be preferably seasoned in an open space, exposed for enough time to the inclemency of the weather. This way the wood gradually loses the interstitial and intercellular moisture, till it reaches the ideal humidity, that is to say roughly 12% of its weight. Like this wood reaches its dimensional stability. For commercial reason and to cut time, wood is often oven seasoned: a method that sometimes brings some bad surprise. Commonly used woods are: oak, douglas fir, pine, cypress, and naturally the classic mahogany and teak. Each of these species is fit for some specific parts of the boat.
Fig. 42
8.2.2. The building techniques
Wooden vessels, alike all other boats, are made of transverse and longitudinal structures. The mechanical properties of wood (its ultimate tensile strength is @ 60/65 N/mm2) are not comparable with the steel ones (ultimate tensile strength @ 480 N/mm2) or with those of aluminium alloy (ultimate tensile strength @ 320 N/mm2). As a consequence the dimensions of the floors, of the frames, of the stringers etcetera must be larger, heavier and would cut space inside the boat. The parts of the structure are joined by means of marine glue, screws, bolts and nails. Naturally all the metalware is made either of copper or of stainless steel.
8.2.3. The strake planking
Solid wood planks, opportunely shaped, are fixed on the structure. It’s the oldest and more classic building method and it’s still used for small boats, mainly in emerging Countries. It’s quite strong and heavy. It needs a careful caulking to be waterproof.
8.2.4. The clinker
The planking strips are put one above the other: it’s typical of small boats, i.e. the Dinghy 12’ (see figure # 43).
8.2.5. The cross laminated wood
The building starts from the inner structures: some of them permanent, such as bulkhead and floors, and some provisional, like several transverse frames that shall be later demolished. From five to six layers of wood sheets are glued upon this framework: the sheets thickness is only 4 to 5 millimetres while their width is 300 to 500 millimetres. The wood is normally mahogany. The sheets cross at a 30° angle. It’s an elastic and light structure, fit for round hulls, such as sailboats.
Fig. 43
8.2.6. The marine plywood
Marine plywood can only be used for hard-chine boats, as the sheets only bend following a conic or cylindrical generatrix and don’t yeld. The structure is quite heavy.
8.2.7. The unfit wood composites
There are two main composites which are totally unfit for boat building purposes. One is the domestic plywood, because the glue which is used would give away with the sea humidity. The other is the medium density fibreboard: it’s a cheap material, heavy, inconsistent.
8.3. The light alloy
Light alloy is commonly called “aluminium” but it’s’ not correct. Actually marine industry uses an alloy made of aluminium, magnesium, manganese, silicon and other metals even though the percentage of aluminium is by far the highest. Marine industry uses two light alloys: one belongs to the ISO 5,000 series, namely AlMg4.5Mn, and the other to the ISO 6,000 series, in particular AlMgSiCu. The 5,000 series alloys are fit for welding, while the 6,000 series are either glued or riveted or both. The ISO 5083 alloy is the most commonly used: it’s sold in plates and bars. It can have different annealing status which give diverse mechanical characteristics: the most commonly used are H32, H 321 and H 111. Light alloy needs inert gas welding and qualified manpower. Light alloy boat building has many advantages and only two handicaps. Aluminium is a self-protecting metal: its exposed face oxidises with a very strong ceramic surface. It doesn’t rust and doesn’t degrade. It’s quite light: let’s remember that its specific weight is @ 2.66, while steel is @ 7.85. The mechanical characteristics of aluminium aren’t, of course, the same of steel (see 8.2.2 for the ultimate tensile strength): therefore all structures of an aluminium alloy vessel must be oversized, compared to a steel building. The weight saving (mind: for the structure alone) is roughly 40%, compared to steel. Building a steel hull and aluminium alloy decks is convenient for some types of ships: the weight is less and the centre of gravity is lower.
Steel and aluminium cannot be directly welded. Therefore they are coupled by means of a bi-metallic special welding system. One commercial brand is “Detaclad”: it’s made of a steel sheet and an aluminium sheet, one above the other, joined by explosion. For what above said, it would seem that
aluminium alloy was the best possible boat building material. It would be so, if it were not for two issues. One is the cost: aluminium is much more expensive than steel. Even though in terms of weight there’s a saving, at the end the construction is more costly. A pity because it’s even cleaner than steel and easier to handle. As for the second problem, that is to say corrosion, please go back to 6.1.1 and see the electrolytic scale of metals, or galvanic series. You might notice that aluminium’s position isn’t much fortunate, not a very noble metal. Crucial precaution must therefore be used: no nobler metals should come in contact with the aluminium structure. AISI 316 stainless steel is mainly used.
Just out of curiosity, AISI stands for American Iron and Steel Institute. Figure # 41 shows a light alloy construction.
8.4. The steel
Steel used for boatbuilding is commonly Fe 42, where Fe is its official name in the periodic table and 42 is its ultimate tensile strength in Kg/mm2, equal to 412 N/mm2. Steel is an old building method: several years ago the structures used to be riveted, while nowadays they’re only welded.
Ship building in steel is classic for large vessels, where the structure weight doesn’t influence the total weight of the ship and her performance. As a matter of fact, steel construction is one of the heaviest, simplest and cheapest. Unfortunately steel quickly oxidizes and corrosion keeps working below the surface rust layer, up to the point that whole thin sheets of steel drop off.
8.5. The fibreglass
This term includes various building methods such as fibreglass, sandwich and composite. I personally deem this material as devoid of any appeal, smelly, amorphous, and lacking of any particular quality. Its ultimate tensile strength is from 150 to 200 N/mm2. Yet it’s essential for mass production. The cost of boats dropped since fibreglass was used, which allowed many people to approach yachting and boats. Every new fibreglass boat building begins with the construction of scale one-to-one wood models of hull and decks. Cheap wood is used, such as pine or poplar: at the end of the works it will be cast away. These models are the exact picture of what the finished boat shall look like: all details and particulars must be carefully foreseen and inserted at this stage: for example the manholes necks, the windows housing shapes, the raised areas for the mooring equipment installation, the anti-skid weave areas etcetera. The surface of such models must be perfect, without bumps or hollows and must be mirror polished, like a new car body. It’s plain that every flaw will show on each boat built afterwards. The construction of the moulds starts as soon as the models are finished, checked and approved. A detachant chemical product is sprayed on the models, and then the gelcoat is applied. Gelcoat is a hard resin, fit for surfaces. Generally it’s black or red, because these colours enhance all possible small flaws. Layers of resin-soaked glass fibre are then applied above the gelcoat. The weight of these glass woven is quite variable: lamination starts with a very light one, called “mat” , made of short fibres, arranged at random, glued together. More layers are applied afterwards, increasing the woven weight, till several heavy roving are laid. “Roving” is made of long and regularly crossed fibres, sawn together. It’s mandatory to use a sufficient weight of mat before laying the roving, unless the mould surface (and later the boats skin) shows a horrible weft in relief.
The Registers of Shipping rule the glass weight per square metre, both for the moulds and for the actual boat building. The weight changes depending on many parameters and on the different vessel’s areas. Similarly the Registers of Shipping rule the percentage of glass and resin which the panels are made of. As for the rule of the thumb, the glass weight shouldn’t be less than 30% of the total weight.
A steel structure is now fastened to the outer part of the mould, to avoid deformation. The moulds and the wood model are taken apart, the wood model is thrown away, the moulds are mirror polished and are now ready for the boats production. The moulds are the female, or negative, mark of the boat.
The completion of the process above described with a few lines actually needs a long time. The resins are either polyester of epoxy: in any case they harden by means of a catalyst. The completion of the catalysis needs a “cure time” which goes from several days to some weeks. Once the moulds are ready, the building procedure is the same as above ditto: first the gelcoat (this time it shall have the chosen final colour of the boat), then the mat and so on. When the shell is made the structures (floors, frames stringers etcetera) are installed. The structures are made of laminated fibreglass beams, with a
“C” shape, built of fibreglass above an inner core, such as wood, hard paper, polyurethane foam: the core isn’t part of the structure, it’s only a useful shape. Parts of the arrangement can be built of fibreglass, such as floors, showers trays, beds structure and much more. Fibreglass doesn’t’ require maintenance, doesn’t rust, doesn’t rot, doesn’t chip, doesn’t dent, and doesn’t need repainting. It’s the ideal building material from the point of view of the customer: it’s cheap and long lasting. It’s the ideal building material for the Builder: it’s easy to build and doesn’t create problems. Yet I don’t love it.
8.6. The ferrocement
I write about it only because it exists but, also out of personal experience, I wouldn’t suggest it for pleasure crafts building. It’s a cheap method which was used during Wold War Two to build cargo ships (the “Liberty”, see figure # 44) following the steel shortage, as this metal was mainly bound for military constructions. The vessel’s structure is made of small diameter steel bars, the same used to reinforce concrete pillars, and steel net with square or hexagonal knit. The net is fastened by means of steel wire to the longitudinal and transverse steel bars network. Once the load-bearing structure is finished, the concrete is placed. Workers apply the concrete from outside and from inside at the very same time, carefully filling all hollows and pressing the concrete from both sides. The procedure is very similar to the reinforced concrete construction, but no gravel is added to the concrete: only portlandite and sand. The specific weight of such a structure is @ 3: it’s not bad, mainly when compared to steel. But the very high thickness which is necessary do frustrate this advantage.
Ferrocement doesn’t rust, doesn’t rot, doesn’t smell and it’s easily repairable. On the other hand it easily transmits vibrations and noise and every modification to holes and passageways is impossible once the concrete hardens. Fairing the hull and deck is a time consuming procedure and paper sanding is about impossible. Yet it’s probably the cheapest ship building method.
Fig. 44
8.7. The fairing and the painting
Only fibreglass boats don’t need painting. All the remaining yachts, ships, vessels need paint. Let me recall an old, short and nice rhyme which tells about a boat:
She scorns the man whose heart is faint And doesn’t show him pity.
And like a girl she needs the paint To keep her looking pretty.
Let’s start from the bare surface, be it either steel, aluminium alloy or wood. The first thing to do is to build a scaffolding structure all around the vessel, which could take some time. All surfaces need to be thoroughly cleaned: there must be no traces of grease or dust. Often the paint supplier specifies that a first layer of epoxy interface needs to be sprayed before the filler is used. Then an applicator smooths all surfaces by applying some filler, which thickness should not exceed a few millimetres. But naturally it depends upon the initial quality of the construction. Once the filler is hard enough (it’s actually a resin which needs a catalyst), the painter sands the filler, leaving it in the hollows and scraping it away above the bumps. It usually is a time consuming procedure. At the end the surfaces must be cleaned again and then the first layer of paint can be applied, only when the fairing work is satisfactory: it’s called a “primer” and is an interface between the filler and the paint.
Two more layers of paint follow: a so called “undercoat” and finally a “topcoat”. I suggest you to contact a plaster and paint supplier and discuss the painting procedures with its technical staff. The painting process can be pretty different depending upon the material that the vessel is made of. The supplier will also decree the environmental conditions to respect while painting: mainly humidity, temperature, thickness and timing. It’s plain from the “Acknowledgements” paragraph that our favourite paint supplier is Jotun Marine Paints and Coatings. We choose the paint for a vessel for its colour, but another criteria is the “gloss”, which measures the opacity grade of a surface: 100 is extremely shiny, 0 is absolutely matt. Both values are purely theoretical (see also figure # 89). Please remind that the darker the paint and the higher the gloss, the easier is spotting all minor flaws on a surface.
I wish to suggest that the colours choice criteria shall follow this policy: after the customer’s and the designer’s indication, several samples of the colours shall be painted on aluminium sheets, 30 by 40 cm and supplied to the customer and his consultants for approval. With the colour chosen among them, a 1 m. by 1 m. aluminium sheet shall be painted. Once the colour is approved in writing by the customer and/or his consultants, two wide samples, 2 m. by 4 m. shall be painted on the ship (hull and deck) for final approval in writing. Sometimes a colour sample on a small plate doesn’t look the same when it’s painted on the vessel: it might be the light incidence or a change in the mood… I might therefore now introduce
Principle number nine: spoken words fly, written words remain.