High latitudes mean ice, cold, no repair facilities, no rescue.
The hull must be strong, impact- and abrasion- resistant, adapted to cold and easy to repair at sea.
There is no magic material for high latitudes. Sooner or later, any material can be damaged, broken or perforated.
However, several materials provide an adequate safety margin, provided that they are adequately implemented.
2023 02 17 - Danco Island, at anchor. 3 knots current, icicles heavier than the boat bumping and scratching the hull.
“The hull material determines the robustness”
Not so. The thickness of the hull plates and the way they are supported are at least as important.
Compare two types of aluminum boats: the Vinson of Antarctica, designed for ice, has close-spaced stiffeners. The Strongall boats, designed for travel, have thick plates and no stiffeners.
“The more rigid the material, the stronger it is”.
Not so. In the Titanic, they used the best steel available at the time. In the cold, the hull plates became brittle.
“The harder the material, the stronger it is” .
Not so. Glass panels resist to scratches but they don´t stand impacts. Polycarbonate is easy to scratch but it stands strong impacts.
“Fiberglass is less robust than metal”.
Not so. Increase the thickness and you reach any level of robustness. And it is surprisingly long-lasting.
The professional fishing boats made of thick fiberglass are the best example. After 40 fishing seasons, their hull is still in good condition.
High ductility steel should be employed, e.g. quality H36. Otherwise the hull plates may break in case of impact. In addition, you need specific alloys for cold, otherwise the plates become brittle, e.g. CH36 or DH36. Advantages High surface hardness, i.e., abrasion and scratches resistant High mechanical strength, e.g., 21kg/mm2 to cause damage to stainless steel, 36kg/mm2 for H36 steel. Fatigue strength: good. Repeated stress like waves has little long-term effects. High stiffness,i.e., in theory, you can use thin hull plates and few hull stiffeners. About thickness Thin hulls are difficult to weld and they tend to depress permanently between stiffeners (the " hungry horse look"). Typical hull thickness for polar sailboats: lower limit 6mm, up to 15mm for extra resistance (e.g., MV Astra). Drawbacks Weight: steel is very heavy (density 7.80 to 7.85). For instance 6mm thick plates weight about 47 kg/m2. In light weather, a steel boat needs a lot of motoring, and a lot of fuel to move its weight. Only large sailboats (e.g. global challenge, 72ft) can overcome the weight handicap. Oxidation. Steel is prone to oxidation, galvanic, electrolytic, and ordinary rust. Steel needs a good protective coating and anodes. Repairs at sea. Submarine epoxy putty or fast underwater cement for small leaks, SOS for large holes.
Aluminum is widely used for expedition sailboats and fast service boats. The usual marine alloy is the 5083. Advantages Weight: aluminum is 2.8 times lighter than steel (density 2.66-2.8). Impact strength: very good. The plates bend, the bows twist but they generally remain watertight. However, no material is 100% safe. I have seen aluminum hulls with deep grooves and multiple leaking after touching rocks. Fatigue strength: good. Repeated stress like waves has little long-term effects. Other features Surface hardness: much below steel, similar to epoxy resins. The hulls often present scratches and pokes. Mechanical strength: high, slightly lower than steel, e.g., 19 kg/mm2 for aluminum vs. 21kg/mm2 for stainless steel. Stiffness: only 30% of steel, in other terms Aluminum is flexible. This means that the hull plates must be thick and supported (stiffeners). About thickness and stiffeners Typical thickness for strong expedition sailboats is 15mm at bottom. The plates weight about 40kg/m2. Given the relative flexibility and deformability of aluminum, it is safer to use close-spaced hull stiffeners. Drawbacks The aluminum is prone to galvanic and electrolytic oxidation. If there are too few anodes, the hull may be damaged in depth. Repairs at sea: like steel, submarine epoxy putty for small leaks. SOS for large holes.
Very strong boats can be constructed with fiberglass. For instance fishing boats made of thick fiberglass are in good shape after more than 40 years. Build quality is essential. Using chopped fiber instead of roving (woven), too much resin, dry spots, bubbles etc. cause weakness and low internal cohesion. Advantages Weight: typical fiberglass composites are 5 times lighter than steel, (density 1.70 vs. aluminum 2.8, steel 7.8). Fiberglass allows for performant boats (light-medium displacement hulls). Low maintenance, easy repair. New layers of fiberglass, sand milling, and the hull is as good as new. Other features Surface hardness: similar to aluminum for epoxy resins, poor for polyester (about 8 times softer). Impact strength: variable. With a well-built, thick hull, the damage will probably be limited to delamination and/or a small hole. For poor quality hulls, there will be extensive delamination, large fractures and/or large holes. Mechanical strength: high. Fiberglass composites can stand higher loads than steel and aluminum, provided that the load is static (see below). Stiffness: low (40% of aluminum, 10% of H36 steel). Fiberglass is very flexible. You need thick hull plates, supported by stiffeners. About thickness Robust expedition sailboats typically have hulls 15mm to 20mm thick at bottom, thinning progressively towards the top. Thickness brings rigidity, robustness, resistance to fatigue and most important safer behavior in case of impact. Drawbacks Bending strength: poor. Fiberglass do not stand excessive bending. Fatigue strength: poor. Repeated efforts like waves weaken the hull. In both cases, the solution is: thick and rigid hull plates. No bending, no fatigue. Repairs at sea. Submarine epoxy putty for small leaks, plates of plywood, sikaflex or similar, and conical screws for large holes.
Polyester NO, Epoxy YES
In general epoxy resins are mechanically stronger than polyester and their surface hardness is about 8 times higher. However, the main difference for high latitudes is the water resistance. Whereas epoxy is watertight, polyester absorbs water. This means that deep scratches have no effect on epoxy but cause internal damage to polyester, e.g., osmosis. Note: vinyl ester is a good substitute for polyester: although less strong than epoxy it is water-resistant... and cheaper.
Carbon fiber: NO
Carbon fiber is superior to steel and any other material in terms of mechanical strength, fatigue strength, and weight. It allows building stiff and light hulls, perfect for ULDB (ultra light displacement boats). Drawbacks Carbon would be the perfect material except that it does not stand impacts. They generally cause extensive delamination, sometimes beyond repair. At higher impact energy, the plate explodes.
Sandwich materials: NO
In a sandwich, two thin layers of a strong material (e.g., fiberglass) enclose a thick core made of material like balsa wood, rigid foam... Sandwich materials are very light and easy to repair. Their stiffness can be arbitrarily high: the thicker the core, the more rigid the sandwich. Their surface hardness is that of the resin, generally epoxy. Drawbacks The impact strength is low. Under an impact, the core is often crushed and the skins "peel off", they separate from the core. In addition, because the skins are thin, deep cuts and scratches may reach the core (damaging for balsa wood, hollow honeycomb etc.).
In general, compared to steel, the mechanical strength of wood is less than 40%, the bending stiffness less than 10%. Note that these figures vary widely with the type of wood and the quality of the planks. This does not mean that wood is a weak material, just that the planks have to be thick (e.g., 40mm) and the stiffeners closely spaced. Drawbacks Possible flooding. The pressure of the ice can move the planks and damage the joints. Impact strength is rather low. Whatever its thickness, wood tend to break (tested up to 60mm thick). The wood must be protected from parasites, even in cold waters. So you need facilities for periodic hull maintenance, cleaning and coating.
Plywood: NO if unprotected
Plywood is light (typical density 0.5), relatively stiff, and resistant to mechanical fatigue. It is a good material to build ultra light to light displacement boats. Drawbacks Impact strength low. Plywood does not stand impacts whatever the thickness (tested). Under impact, it breaks and there is a flooding. The surface hardness is low. Because the inner layers of plywood are generally soft woods, there may be deep scratches External protection (fiberglass or aramide) prevents from scratches and immediate flooding, but it does not improve impact resistance (tested). A double protection, inside and outside, improves the impact strength of plywood. It becomes impact-resistant (tested), see below why. Repairs at sea: even for large holes, a piece of plywood, sikaflex or similar and conical screws.
Laminates are composed of multiple layers (plywood, wood, rigid foam..) separated by layers of fiberglass. For instance large unsupported curved beams can be made of multiple layers of wood alternating with layers of fiberglass. The fiberglass layers support the internal stress that appears during bending, whereas a plain material would break. The external layers of fiberglass increase the stiffness, like in a sandwich. Advantages Stiffness: high, similar to sandwiches. The thicker, the more rigid. Mechanical strength: high. Similar to fiberglass for axial efforts, but a laminate also stands transversal efforts and twisting. Impact strength: high, and safe behavior (tested). The multiple layers of fiberglass prevent flooding. Low maintenance and easy repair. Similar to fiberglass. Other features Surface hardness: that of fiberglass-epoxy, i.e. similar to aluminum. Weight: depends of the number and type of layers, but in general rather light, a mix of the weight of plywood and fiberglass. Drawbacks Cost of production: many heterogeneous layers mean a lot of work Repairs at sea. Submarine epoxy putty for small leaks, plates of plywood, sikaflex or similar, and conical screws for large holes. An example of laminate for high latitudes Sonabia 2 uses the following: internal glass epoxy 2mm thick, plywood 10mm thick glass epoxy 2mm thick, PVC foam 30mm thick and external glass epoxy 6mm at bottom to 2mm (19.5mm at the bow)
Summary of important features.
Stiffness / flexibility The stiffness (Young modulus) characterizes the force to apply to stretch or compress the material. What does it mean? Any material is somehow elastic: it can stretch or compress before returning to its initial length. The Young modulus measures the effect of axial forces on the length of the material. The Young modulus allows calculating the bending stiffness, i.e. how much a plate of the material bends under the effect of transverse forces. The Young modulus also tells us how thick the hull must be to reach a desired stiffness. Against intuition flexible materials can make rigid hulls, just by increasing the thickness. Mechanical strength The mechanical strength characterizes the pressure to apply to break the material (tensile strength, compressive strength) or cause permanent damage (yield strength). What does it mean? It defines how much the material resists to traction or compression. Indirectly it allows calculating the bending strength, i.e., the transverse effort that will cause a permanent damage The mechanical strength is independent from the stiffness. Some flexible materials are strong whereas some stiff materials are not. Fatigue strength The fatigue strength characterizes the amount of repeated efforts that a material can support before break or permanent damage. What does it mean? The fatigue strength is expressed as a combination of amplitude and repetitions but it basically characterizes a lifespan. For instance a hull plate is exposed to waves. At each wave, although it is hardly visible, the plate bends and straightens. This causes microscopic damage (¨fatigue cracks¨) that have a cumulative effect. However, it is possible to build fatigue-resistant hull plates with short-living materials: increase the thickness and place stiffeners to reduce the bending. Impact strength The impact strength characterizes the impact energy necessary to break or damage permanently the material. What does it mean? It represents the capacity of the material to stand instant, locally concentrated efforts. Impact strength is measured by means of destructive tests where the parameter is the energy, e.g., dropping a weight from a given height. The impact strength increases with the mechanical strength, but you cannot say much more: the type of effort is different. For instance carbon fiber has a high mechanical strength but a low impact strength. Energy absorption The energy absorption characterizes how a material can absorb energy without breaking. What does it mean? During an impact, the more energy is absorbed, the less energy is left to cause complete destruction. In materials like aluminum, high ductility steel, kevlar (aramide), the energy is used to deform the material before the rupture. On the contrary, materials that do not absorb energy break immediately, e.g., carbon fiber, high grade steels, plywood... Rupture behavior The rupture behavior characterizes what happens when the material breaks or is permanently damaged. There are several types of rupture behavior: deformation, internal damage, through fractures. The damage can be local or widespread. Rupture behavior is known by real experience and experimentation, not by theory. However there are 4 rules of the thumb: 1) ductile materials are first deformed; 2) heterogeneous and composite materials are first damaged internally; 3) rigid materials break badly; 4) all materials break completely under high energy impacts. Surface hardness The surface hardness characterizes how the material resists to penetration and cuts. What does it mean? hard materials resist well to scratches and abrasion whereas soft materials are damaged whatever the superficial coating (primer, painting, anti fouling). There is no universal scale but the following ranking is safe: 1) iron(far ahead) 2) aluminum and resins 3) wood and 4) plywood. For each material there are many variations (alloys, types of resin, hard vs soft wood...). Finally, the resin is determinant for composites and impregnated wood and plywood (impregnation means that the resin penetrates in depth). Epoxy resins provide a surface hardness similar to aluminum alloys. Resistance to cold The resistance to cold characterizes how the material stands direct exposure without protective coating. The hull material is exposed to low air temperatures and to cold water because of the cuts and scratches caused by ice. Some materials become brittle in cold, e.g., high grade steels wood and plywood if they are soaked and the internal water freezes (e.g., deck). Ease of repair The ease of repair covers two aspects: emergency repairs and restoration of the hull in its initial state. Emergency repairs are relatively easy when the damage is local. Stop the flooding and patch the material. Emergency repairs turn difficult when the damage is widespread. Thus materials with a bad rupture behavior like carbon should be discarded. Restoration is easy for composites (sandmill and add new layers), plywood (scarf) and wood (replace the planks). Restoration is more difficult for metals: the damaged plates are weakened and should be replaced (cut and weld). Using putty is merely cosmetic. Weight In modern yacht design, the intrinsic weight of the hull material is determinant for the architecture and the performance of the boat. This is why steel is generally discarded. However for high latitudes the significant factor is the weight of the hull that provides sufficient strength. This takes into account the density of the material, the plate thickness and the stiffeners.