Construction Of Different Bows Types On Ships
There are a various number of factors that affect the construction of a vessel. Ship owners will try to build a vessel as cheap as possible, whilst meeting the standard safety requirements, so that they can make a profit from their business. The prominent part of the vessel is the bow. When constructing the bow of the vessel it is important to use the correct and strongest steel. This is important as the bow of the vessel takes the brunt of the waves. Generally Grade D and E steel are used for the bow of the ship. Grade E is used for icebreaker ships.The bow of the vessel must not only be strong enough to resist pressure and flexing, but it must also accommodate work spaces. There should be sufficient work room for mooring and anchoring stations and the chain locker. There should also be room for trimming tanks. Many vessels are now equipped with bow thrusters resulting in the bow being constructed differently. There are different types of bows used by vessels. These include the bulbous, X bow, Ax bow and bow doors. Vessels that proceed at high speeds and vessels with high block coefficients generally use bulbous bows. The X bow is generally used on offshore vessels.
Bulbous Bow
The bulbous bow is the most popular type of bow used on vessels. The formation of the bulbous bow was discovered accidentally when carrying out model testing of naval ships in the USA. In 1912 the bulbous bow was introduced to USN naval ships. It wasn’t until 1929 that the bulbous bow was introduced to merchant vessels.
When carrying out the construction of the bulb the designer must evaluate if the length of the bulb is adequate to the considered speed for the vessel. Other factors are also evaluated such as the overpressure generated at the sides of the bulb is adequate and that the bow flare does not increase too much the height of the wave at the bulb. The designer must as also take into account the additional construction cost in relation to the expected operating costs. The shape, size and design are different on other ships. It is often used an extra ballast tank, resulting in enhancing trim and increasing the draught of a vessel in heavy weather. Another option is leaving it as a void space. The bulbous bow is likely to become damaged from floating obstructions and therefore it should be inspected thoroughly when the vessel enters dry dock. The bulbous bow has many advantages. The bow reduces the resistance from waves. It also results in significant reductions of the required propulsion power when manoeuvring at certain speeds. It works as a bumper in the event of a collision. It allows the installation of bow thrusters at the fore end of the vessel, thus making it more efficient. It reduces pitching which is viewed as important for cruise ships. It also allows for more ballast water capacity forward. It is also particularly useful for vessels that engage in ice navigation.
X Bow
The X bow is the latest development in bow construction. The design was based on the Norwegian Ulstein design. The design results of an inverted bow shape resulting in a smooth and continuous bow shape. It is a unique design and was first introduced in 2006. It was designed to improve seakeeping at high speeds and to make it easier to navigate in the North Sea. The bow has been proven to be easier to build than the conventional bow. When construction of the bow was being carried out, numerous tests were also carried out. These tests include bow impact pressures, acceleration levels, wavedrift, heaving etc. These tests resulted in numerous findings. Due to lesser bow impacts, acceleration levels are lower. Spray water at the bow is reduced. This means that the X-bow is much more suited for Arctic conditions than other bow types.The X bow has many advantages. Due to its inverted shape is increases fuel efficiency, thus reducing emissions. The design also provides smoother movement and less slamming, thus resulting in a more comfortable work environment and preventing a shift in cargo. It also costs less to construct an X bow compared to a bulbous bow as labour costs were reduced by up to 15% and the cost of assembly was cut by 50%.
Bow Doors
Bow doors are a common feature found on Ro-Ro vessels. Bow doors should be fitted above the freeboard deck. There are a variety of different types of bow doors such as horizontal sliding doors, vertical moving doors and ramp-door combinations. Inside bow doors the inside has a flat edge with a rubber seal to make them watertight. The door absorbs the forces from the waves. Therefore they are required to follow strict requirements concerning strength, locking systems, seals and security. There is a watertight door positioned behind the bow door but they must separate from away from each other. This is normally achieved by positioning the watertight door behind the collision bulkhead.
The doors, once they are opened, must be provided with mechanical means to be fixed in an open position. If a bow door gives access to vehicle decks they must be provided with a remote control station from an area above the freeboard deck. This area must be an area where they can observe the opening and closing of the bow door. There should also be someone observing the bow door to prevent unauthorised access.The doors must be equipped with lights and alarms, when in operation, on the bridge and on the operating panel. This is to ensure that the doors are closed, locked and secured in the proper position. A water leakage detection system must also be equipped and provided with an audible alarm and video footage to alert the engine room and the bridge if any leakage occurs through the bow doors.A major advantage with bow doors is that the design is cheaper and easier as there are less parts that can fail. However they can be expensive to construct and add to the vessel in dry dock or a ship building yard Doors that are equipped with arms can be opened outwards. Therefore these types of doors can be fitted on any ship. On Ro-Ro’s vehicles can leave the vessel easier and much quicker through bow doors. This helps a vessel load and unload faster, thus reducing the amount of time the vessel remains in port.
Ax-Bow
The ax-bow was developed by the Enlarged Ship Concept (ESC). The axe-bow designed to increase the length of the hull without increasing the functions of the vessel. The extra length has resulted in an improvement of seakeeping and has resulted in less resistance required. The axe-bow is mainly used by warships, the coastguard and offshore vessels. The ax-bow has a vertical stem and an extremely deep V-shaped bow. To increase the buoyancy of the vessel, the sheer rises notably and the keel slopes downward to the stem. The bow is very high, to help prevent a wet deck, and deep in order to reduce the risk of slamming.
The ax bow has many advantages. The bow has resulted in vessels being able to reach a speed of up to 35 knots. There is less fuel being used, resulting in a decrease of ship emissions.
Conclusion
When choosing a bow type for a vessel, the ship owner should consider the type of vessel they want to build. A bulbous bow is the most popular bow type chosen. Whilst it is more expensive to build, it is the more reinforced than other bow types, therefore adding more protection to the vessel in case of collision. Navigation in ice becomes easier due to the presence of the bulb. However due to the extra length added by the bulb, a vessel may become restricted in where she can berth in various ports. Bow doors are only suited for Ro-Ro vessels, particularly Ro-Ro’s engaged on short voyages. As discussed previously, the addition of a bow door results in loading and unloading cargo faster. This is beneficial for the ship owner as the less time the vessel spend in port, the more money they make. The X bow is still relatively new to the industry. However many cruise companies are now in discussion about adding the X bow instead of the usual bulbous bow. Due to the X bow increasing the vessels speed, it makes it ideal for cruises ships as there is less likelihood of it becoming delayed due to bad weather. The bow is also ideal for Arctic conditions, and with many cruise ships partaking in cruises there, it makes it ideal.
The Ax bow is primary used on warships however the yacht industry is now considering adding ax bows to their vessels. The Ax bow will be beneficial to the yacht industry because the bow of the fast speeds and long journeys undertaken by yachts. However the length of the yacht will have to be increased for an Ax bow to be added.
