Advanced Marine Vehicles

Many of us are familiar with the principle behind aircrafts - The Bernoulli's Principle. The difference in pressures above and below the aerofoil section of the wing creates lift . But I doubt whether many are apprised of the fact  that the same principle has been applied to ships also over the years! In fact some of the fastest warships in history were able to achieve such speeds by making some specialized sections attached to their hulls "fly" inside the water just like the wings of an aircraft.Racing fascinates us in every medium be it be on land , air or water. Ever wondered what makes the racing boats capable enough to cross speeds of 300km/hr?. Read the article to gain valuable insight about more such developments in making of Advanced Marine Vehicles.

The ocean environment is dynamic in nature and the conventional displacement hull forms are not best suited for all circumstances. Many new hull forms have been developed over the years to serve different purposes. 

High Performance Mono Hulls

WHAT IS A MONO HULL?

A Monohull is basically a boat or a ship with a single hull making it the most extremely used, efficient, versatile hull up to a Froude no. of (1).

Most high speed small monohulls have recently been converted to hard chine structures. They are now provided with greater beam and reduced length, thus giving improved performance in calm water and leading to high vertical accelerations in a seaway. Their ride has been improved by using higher deadrise angles leading to a deep V form.

Ø  For getting a higher Froude no. we need to lift the hull out of the water, by planing the hull.

Ø  Destriero and Jupiter are the names of two famous planing monohulls. Planing is the method of operation for a naval vessel in which its weight is predominantly supported by hydrodynamic lift, instead of hydrostatic lift (buoyancy).

Ø  We can make a hull go faster by making it extremely slender and using a very narrow beam. But if we take it to extreme, it results in an unstable ship, hence some sort of change has to be done to get stability.

Ø  Trimarans belong to a class of vessels properly called Stabilized Monohulls. They are characterized by the extreme slenderness of the main hull.

CATAMARANS

A catamaran is a multi-hulled watercraft which has two parallel hulls of equal size. It is a geometrically-stable craft, which derives its stability from its wide beam, instead from a ballasted keel but from a monohulled sailboat. “Catamaran” is abstracted from a Tamil word "Katt-umaram" which basically means logs tied together.

Catamarans typically have less hull volume, higher displacement, and shallower draft (draught) than monohulls of comparable length. It provides large upper deck areas for passenger facilities in ferries for helicopter operations.

The hull separation in a catamaran is about 1.25 times the beam of each hull which helps in its stability and helps to take the manoeuvrability of these vessels to a greater extent. Catamarans are available in all sizes from small (sailing or rowing vessels) to large (naval ships and cargo ferries). 

WAVE PIERCING CATAMARAN

Catamarans have faced many problems in the past mainly related to ride facilities. So as to make the ride quality smooth, “PHILIP CHRISTIAN HERCUS” the famous Australian naval architect came up with the wave piercing hull form.This new hull form design takes the help of a narrow protruding beam piercing the waves instead of rising above one another.

Incat Tasmania ,is an international manufacturer of  high speed crafts, mainly wave piercing catamarans found by “BOB CLIFFORD” and “PHILIP HERCUS”. The company excels in building large vessels using aluminium as a construction material, wave piercing and water-jet propulsion technology. The vessels have been constructed up to a length of 112 metres, with a size of 10,800 Gross Tons and having  cruising speeds up to 58 knots (107 km/h).

Image courtesy: https://photos.marinetraffic.com/ais/showphoto.aspx?mmsi=770576272

Built at Incat's shipyards in Hobart, Tasmania, “FRANCISCO”, this is the world's fastest catamaran ship. With a cargo of over 1,000 passengers and 150 cars, the 99-meter catamaran can travel at speeds of up to 58 knots or 107 km per hour.

HYDROFOILS

The term "hydrofoil" may be sub-divided into two meanings:

> A hydrofoil is a foil which can operate in water. They are same in appearance and purpose when related to aerofoils.

> The term "hydrofoil" is often used to refer the floating vessels that use  the hydrofoil technology.

The term "hydrofoil" is commonly referred to the wing-like structure which is clamped onto the struts below the hull, or across the keels of a catamaran in a variety of boats, which helps to lift the boat out of the water during forward motion, thus resulting in reducing the drag force on hull. Hydrofoils can be artificial, for example the rudder or keel of a boat, the diving planes on a submarine, a surfboard fin. As soon as a hydrofoil-equipped marine craft increases its speed, the hydrofoil elements below the hull develop or produce enough lift force to raise the hull up, out of the water. This results in a great reduction in drag force of the hull, along with a corresponding increase in speed and efficiency in operation with respect to fuel consumption.

Image courtesy: https://upload.wikimedia.org/wikipedia/commons/thumb/5/56/Hydrofoil_types.svg/450px-Hydrofoil_types.svg.png

SURFACE EFFECT SHIP

A Surface Effect Ship (SES) or Sidewall Hovercraft is a naval vessel which comprises of an air cushion or pillow, just like a hovercraft, and also twin hulls, as a catamaran. When the air cushion is in practice, a small part of the twin hulls lie in the water. When the air cushion is turned off, the full weight of the vessel is balanced by the buoyancy of the hull pair.
The “SES” has acquired two advantages over a normal hovercraft in case of open sea operation. It encounters more resistance when it slips sideways and it operates in air or sea, and it can use water jets for propulsion since the inlet nozzles are always covered by water.
Surface effect ships are a special type of vessels which are a combination of hovercrafts and catamarans.  They are so designed that they can use both air cushion and conventional propulsion as and when required. This helps them to attain a greater speed on sea water. The main advantage of the surface effect ships is that they retain their stability in any kind of water conditions on the basis of being equipped with both – dual hulls and air springs. The maximum speed that can be achieved is around 80 knots.

Video Courtesy : YouTube

ACVs or HOVERCRAFT

The ACV or Air Cushion Vehicle is a vessel, which is capable of moving in land, sea, snow, mud or other surfaces. It does not have the side hulls of the SES or catamaran. It has got an air cushion like structure, which makes it an amphibious watercraft. It allows very low drag force, thus ensuring speeds to establish high Froude Number. (Froude Number=4)

ACVs are generally noisy, making it a bit uncomfortable, and mechanically complex, but they do possess unique features, such as the ability to fly up over the beach. Large hovercraft have successfully served on the English Channel for over 25 years.

According to my knowledge, I have seen some written materials which state that the hovercraft was also used for airport‐to‐airport service across San Francisco Bay.

Imagecourtesy: https://slideplayer.com/slide/4311856/14/images/32

WING IN GROUND-EFFECT(WIG)

When a normal aerofoil is operationally active in a region close to the ground, its normal pressure distribution is disturbed. Pressure tends to develop to a higher level under the wing and adds to the normal dynamic lift of the aerofoil. This enhanced lift force is popularly known as the ground effect. Wing in Ground-effect (WIG) aircraft, can be referred as "flying boats" made in purpose to operate just above the wave crests in order to avoid occasional water contact during flight. WIG aircraft possess one or more wings which possess three orders of magnitude larger than the foils of hydrofoil craft. When a WIG aircraft has accelerated to a sufficient high velocity through the water,then the aerodynamic lift generated by the wings helps to lift the aircraft entirely out of the water. By remaining close to the water's surface, WIG aircraft faces significantly less resistance than it would face at higher altitudes because of the aerodynamic lift.

Imagecourtesy:https://www.maritimeexecutive.com/media/images/article/Photos/Vessels_Small/Cropped/Wingship%2016x9.jpg

THE SUSTENSION TRIANGLE

The "sustention triangle" is a very popularly used device for categorizing different ship types. It is an imaginary or conceptual device used for the better understanding of the floatation process of boat. Traditional or normal ships float because they are immersed in water and get an up-thrust or buoyant force due to Archimedes' principle. It occupies the lower left corner of the triangle. There are other ways to hold ships up. As in case of hovercraft, for example, where the ship is lifted on a bubble of air.

Hydrofoils and hydroplanes are both dynamic lift marine vessels. It thus occupies the topmost corner of the triangle.

Some vessels occupy the intermediate positions on the edges of the triangle. For example, an SES (mixture of catamaran and hovercraft).

THE SUSTENSION CUBE

It is also an imaginary, conceptual “design space” which consist of mutually orthogonal axes. The result of this consideration leads directly to the sustention cube.

The Contents of the Sustention Cube

The corners are defined by combing the following pairs, to produce eight points:

> Passive or Active

> Hydro or Aero

> Static or Dynamic

Thus the eight corners are:

> Passive Hydrostatics- Conventional ships and barges.

> Passive Hydrodynamics- Hydrofoils.

> Passive Aerostatics- Blimps

> Passive Aerodynamics- WIGs

> Active Hydrostatics- Hovercraft

> Active Hydrodynamics

> Active Aerostatics

> Active Aerodynamics


Article By- Suryadip Ghosh