Types of Hulls Used For Vessels
Every marine vessel has a specific hull suited for that particular ship or boat. Over centuries, different kinds of crafts were constructed, leading to the evolution of different hull designs and types. Check out the different ship-hull types and boat hulls in this article.
Boats have been used by humanity since times immemorial. The earliest constructions included logs of wood tied together. As technology progressed, so did the size and nature of vessels. Huge ships came into the picture in the medieval era. They were constructed for carrying materials, preparing for wars, and so on.
Different crafts like sailboats or cargo ships have unique hulls; however, all hull designs have the same underlying principles.
Types of Hulls
First and foremost, let us briefly recapitulate the physics of flotation again. Any floating object is in a state of physical equilibrium due to a countering force offered by the water medium. Buoyancy or buoyant force equals the weight of volume displaced by the object. Known as displacement, it acts in a direction opposite to the object’s weight. Hence, the resultant force acting on the body equals zero. This is famously known as Archimedes’ principle.
Based on this principle of floatation, all known vessels, irrespective of size, type, purpose, complexity, operation, etc., can be categorized into three broad categories based on the kind of hulls used.
Displacement Hull: This is the simplest, earliest, and most basic hull type. Most vessels have displacement hulls, including canoes, large tankers, fishing boats, ocean liners, tugboats, or offshore support vessels. Resistance and friction define speed, depending on the partially submerged hull’s wet surface and the waves’ resistance.
Such vessels have a linear weight-buoyancy relationship; the heavier and larger the ship, the more the resistance and power required to increase speeds. Thus, a large vessel like a tanker or bulk carrier has a powering capability proportional to the hull size and displacement. Displacement hulls can be of monohull or multi-hull configurations. They have relatively fuller hull forms and are more acceptable for cruises, containerships, fighter vessels, etc.
Planing hull: They are fast vessels partially supported by the buoyancy force. The rest of the hull remains above the water surface due to the hydrodynamic lift generated. As their speed increases, the dynamic pressure increases with increasing aft draft, which helps propeller immersion, creating a forward lift. They have typical hull forms.
Semi-displacement hull: Midway between the above two, semi-displacement hulls generate dynamic lift to a certain extent. Their hull forms are similar to displacement types, and most of the weight is supported by buoyancy.
Configuration of Hulls
While we have discussed the main types of hulls, let us now look at the basic configurations of hulls available.
Monohull: Most vessels like barges or large carriers have a single hull of displacement, semi-displacement, or planning type.
Multi-hulls: A vessel having two or more hulls is multi-hulled. When there are two hulls, it is known as a catamaran, while trimarans have three hulls. Both the hulls support the superstructure and bridge deck of catamarans. Trimarans have a central hull and a secondary hull on either side.
Multi-hulled vessels are stable as the weight is distributed among the hulls. The greater the distance between the hulls, the more stable a vessel is. Another critical factor is the underdeck clearance between the two hulls. Large crossbeams, girders, and decks laterally join all the hulls, or an immense superstructure, if present.
SWATH: SWATH stands for Small Waterplane Area Twin Hull. It is a unique, modern twin-hull form but quite different from traditional catamaran configurations. For SWATH configuration, the hulls are characterized by two identical structures on either side and a remarkable under hull clearance visible above the water surface. While seeing the vessel, one can easily pose the question, given almost a hollow hull, where is the required buoyancy derived from? The answer ironically lies below!
The slender projections on either side extend underneath the water surface to large, tubular, broad cross-sections that account for the required displacement necessary to keep the vessel afloat. This typical design provides a two-fold advantage: Reduction of wave motion, drag, and resistance effects at the surface due to a narrow hull cross-section interspersed by an ample clearance; and more efficient stability.
Since the bulkier portion of the hull or structural pontoons lie well below the waterline, they are entirely free from surface effects like waves. SWATH is used in pleasure yachts, ferries, oceanographic research vessels, support vessels, small military vessels like patrol ships for coast guards, etc. However, they are not employed for larger vessels due to complex design and space limitations for cargo, heavier equipment, and machinery. Because of their typical design, they are strictly displacement type.
Common Hull Shapes
Ship hulls are primarily derived from traditional boat hulls. However, most hulls for larger passenger and cargo vessels are simple monohulls for cost-effectiveness and to avoid complexity in design. But depending on the speed and other requirements in terms of cargo, utilities, size, etc., they are altered in terms of fineness or different hull coefficients. Modern ship designs have a complex combination of the following hulls in varying degrees, optimizing voyage requirements, ship purpose, function, size, type, and so on.
Now let us look into some classical types of primary hull forms :
Round-Bottom Hulls or Bilge hulls are the most common forms of hulls present since the early days. Everything from a primitive fisherman’s boat or a canoe to a typical fishing trawler tends to have round-bottomed hulls. These hulls are seen almost everywhere and comprise a perfectly symmetric configuration with rounded curvatures that blend at the base region. They are typically displacement types and intended for slow to moderate speeds. However, because of their form, they encounter lesser drag forces and can offer higher rates in better sailing conditions.
Round hulls help seamlessly traverse all kinds of water bodies without much hindrance. They are efficient against wave resistances, owing to their increased submerged hull surface. However, owing to their shape, they can pose problems in stability given a high depth or draft. Often, they tend to roll in unfavorable conditions. This is not an issue for small boats or vessels, but the design parameters are carefully optimized for larger ocean-going vessels.
Round-bottom hulls are less prone to grounding. Furthermore, they offer an incredible amount of underdeck volume for cargo stowage, accommodation, and other amenities. These hulls are also known as bilged hulls because of their smooth curvature at the corners or bilge region.
Flat-Bottomed Hulls: One of the oldest and simplest types of hull curvatures, flat-bottomed hulls have a flat base, devoid of any angle, curvature, rise, etc. A barge or small fishing boat is the most classic example of a flat-bottomed hull. They are suitable for lakes, ponds, and rivers, with calm waters and shallow drafts. In wave or choppy conditions, they not only impose a high drag but also cause problems like bottom pounding and slamming. Another significant issue with such vessels is the risk of grounding. They are perfectly displacement type and are slow. However, some high-speed river crafts are flat-bottomed.
V-Bottom Hulls are opposites of a flat or a round hull. They virtually have no bottom as the two symmetric sides of the hull intersect at the lowest point at sharp oblique angles, making its cross-section look like a simple V. They are suitable for high-speed vessels like planning crafts. They are ideal for rough conditions with high wave-breaking efficiency without the slightest effect on their high speeds. Because of their shape, they have increased hydrodynamic lift capabilities and are very apt for planning vessels. They have high propulsive requirements and are hence, equipped with high-power engines. Of course, they have less underdeck hull volume because of their shape and form. Though hydrodynamically very efficient, they also can pose problems in stability and are risky during banking or turning at high speeds.
Chined Hulls: Most modern ship designs derive from chined forms. Though slightly complex, these hulls have a solid angle or angles at the lower bottom region of the hull. Chines can be soft or hard. A hard chine has more curves and swift, rapid changes in curvature. A soft chine is composed of lower angles or a combination of smaller tips to give a smoother form.
A round-bottom hull can be considered a perfectly soft chine with a tiny number of small angles. Similarly, a flat-bottom hull is a hard chine with a zero angle!
S-Shaped Hulls are similar to rounded hulls but with a significant difference. As shown, the side curvatures join to form a sharp, pointed apex. Each hull side resembles a crooked S or a spline form. These hulls were prevalent during the old times.
You might also want to read-
- Crane Barges Explained – Everything You Wanted To Know
- When were boats invented?
- Introduction To Different Types Of Yachts
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Subhodeep is a Naval Architecture and Ocean Engineering graduate. Interested in the intricacies of marine structures and goal-based design aspects, he is dedicated to sharing and propagation of common technical knowledge within this sector, which, at this very moment, requires a turnabout to flourish back to its old glory.