Hull: Bulwark Against the Seas
Sailing in high seas was never a task for the faint at heart. Giant waves toss the ship up and down while ferocious, howling winds and angry raindrops lash out with full might. Life hangs in balance as the deck beneath your feet shivers violently and the skies literally turn dark while it is still day time.
You can tide over such hostile waters only with nerves of steel. Of course, it helps if your ship has a hull of steel or other tough materials to complement your steely nerves. Forget rough seas, even calm seas are lethal because seawater is amongst the most corrosive of environments.
Hulls are the most important technical and economical design parameter of a ship. They determine its safety, stability, and structural integrity while making up around 20% of the ship’s total cost.
Shipbuilding Steel is the most common hull material. Other materials include aluminum and titanium alloys, fiberglass, composite materials, and wood. Workmanship though is equally important. Skilled workmen will build wonders out of humble wood while unskilled craftsmen will make a mess out of the finest aerospace-grade composites.
Following properties are important for hull-building materials:
- Strength is the resistance to permanent deformation and breaking. It can be:
- Yield Strength: stress i.e. force per unit cross-sectional area at which materials are unable to recover their original shape
- Ultimate Strength: stress at which materials break
- Strength Efficiency is strength relative to density. Equivalent to the Strength-to-Weight ratio, it is an excellent measure of structural efficiency
- Stiffness is the resistance to deformation under load. A material may be strong enough not to break, but may not be stiff enough and will bend making is useless. Deflection depends on the material stiffness and the situational geometry
Young’s Modulus measures a material’s stiffness. It is the ratio of stress applied on the material to the strain produced
- Stiffness Efficiency is stiffness relative to density
- Cold Resistance
Fiberglass and steel have great strength while aluminum alloys and wood possess lesser strength. However, fiberglass has higher strength efficiency and so does wood and aluminum with heavier steel falling behind. All steels have high values of Young’s Modulus while aluminum, fiberglass, and wood have lower values. The stiffness efficiency of all these materials is nearly same.
Wooden Hull Internal Ribs
Stiffness increases with increasing thickness of the cross section in the direction of the load. Doubling the thickness of a rectangular beam makes it eight times stiffer. But making the sections thicker adds to the weight. This is precisely why we use I-beams – increasing thickness without adding much weight.
Considerations when choosing materials:
- Steels are heavy and corrosion-prone but hard, easy to fabricate into large hulls, and abrasion resistant
Cold-Proof Welding Hull Steels perform well up to temperatures of -600C. They find application as icebreaker hulls and lighter-abroad ships. They are highly process-able and resistant to layered failures and cracking
High-Strength Clad Corrosion-Resistance Steel makes hulls of ice-ships and icebreakers. The material increases serviceability, eliminates corrosion-erosion wear, reduces required quantity of metal per structure, and lowers loads on hulls arising from breaking away of ice-floes and heavy sticking
High-Strength Non-Magnetic Steels make good hulls with high-strength, non-magnetic properties, reliability, durability, compatibility with pearlitic steels when operating in corrosive media, and top mechanical properties over wide temperatures
Nitrogen-Alloyed Steels provide high-strength, absolute corrosion resistance, flexibility, and non-magnetic behavior due to the unique combination of the atoms of iron and carbon with nitrogen. Pearlitic Steels are two-phased steels with alternating layers of alpha-ferrite (88% weight) and cementite (12% weight)
- Fiberglass is the most environment-friendly hull-building material. Its chemical and biological stability translates into least emission of chemicals into water and air. It is also perhaps the strongest material, is inexpensive, and even unskilled labor can mass-produce fiberglass hulls
It does however require enormous initial investment making it compatible only with large scale production. Plus, it hides second-rate workmanship making buyers apprehensive
Trawlers, yachts, high-speed airfoil and hydrofoil crafts, and sweepers typically use fiberglass hulls
- Aluminum is lighter, easier to work, and corrosion resistant vis-à-vis steel. It has saved many riverboats from sinking because it deforms on impact, it does not break. However, it is tougher and more expensive to weld aluminum
Hydrofoil and airfoil ships, high-speed crafts, and aerostatic vessels use aluminum-alloy hulls
- Wood is visually captivating but piles up maintenance bills. Wood-Fiberglass composites are inexpensively maintained but are suitable only for short or one-off production
All said and done, there is no single material with all the desirable properties. The application determines the choice of material and there is always a trade off. Fiberglass and composites hold great promise for advances in material science have always furthered human development.