Hotel Service Unit 16 Q1

A bilge keel is a long, flat projection that runs along the length of a ship’s hull, usually on both sides near the turn of the bilge (the curved section where the bottom of the hull meets the side). It’s primary function is to reduce the ship’s tendency to roll from side to side.  

1. Bilge keels – Wärtsilä

2. Bilge keel – Wikipedia

How it works:

• When a ship rolls, the bilge keel on the lower side dips into the water, creating additional drag or resistance against the rolling motion.
• This resistance helps to dampen the roll, making the ship’s motion more stable and comfortable for those on board.   1. Bilge keel – Wikipedia en.wikipedia.org

Advantages of bilge keels:

• Improved stability and comfort, especially in rough seas.
• Reduced stress on the ship’s structure due to less rolling.
• Relatively simple and cost-effective to install compared to other stabilization systems.
• Minimal impact on the ship’s internal arrangements.   1. Bilge keel – Wikipedia en.wikipedia.org

Disadvantages of bilge keels:

• Slightly increased drag, which can reduce the ship’s speed and fuel efficiency.
• May not be as effective in extreme rolling conditions compared to active stabilization systems.
• Can increase the risk of grounding in shallow waters due to their protrusion from the hull.

Other uses of bilge keels:

• In some warships, bilge keels were designed to provide additional protection against torpedoes by deflecting them away from the hull.   1. Bilge keel – Wikipedia en.wikipedia.org
• On smaller vessels, such as yachts, bilge keels can allow the boat to sit upright when drying out on a beach or mudflat.   1. Bilge keel – Wikipedia en.wikipedia.org

Overall, bilge keels are a simple yet effective passive stabilization device that contributes to a smoother and more comfortable ride for ships in various sea conditions. While they may not be as powerful as active stabilization systems, their low cost, simplicity, and minimal impact on ship design make them a popular choice for many vessels.

Method of Attachment:

One of the most common methods for attaching a bilge keel to a vessel’s hull is welding. This involves:

1. Preparation: The hull plating in the bilge area is carefully cleaned and prepared to ensure good contact for the weld. The bilge keel itself, typically fabricated from steel plates, is also prepared and shaped to fit the hull contour.
2. Positioning: The bilge keel is positioned along the turn of the bilge, running longitudinally (lengthwise) along the hull. Careful measurements and alignment are crucial to ensure its effectiveness.
3. Welding: The bilge keel is then welded to the hull plating using continuous or intermittent welds, depending on the design and strength requirements. The welds are typically inspected and tested for quality and integrity.
4. Fairing: After welding, the area around the bilge keel is faired and smoothed to reduce any drag or turbulence caused by its protrusion.

Reason for Longitudinal Positioning:

The longitudinal positioning of the bilge keel, parallel to the ship’s centerline, is crucial for its effectiveness in reducing roll motion. Here’s why:

1. Maximized Surface Area: When the ship rolls, the bilge keel on the lower side dips into the water, creating resistance against the rolling motion. The longitudinal positioning maximizes the surface area of the bilge keel in contact with the water, increasing this resistance and dampening the roll more effectively.
2. Counteracting Roll Axis: The roll axis of a ship is typically located near its centerline. By placing the bilge keels along the length of the hull, they are positioned furthest away from the roll axis, creating a larger lever arm and increasing their ability to counteract the rolling motion.
3. Minimal Impact on Pitching: While the primary function of bilge keels is to reduce roll, their longitudinal placement also minimizes their impact on the ship’s pitching motion (fore and aft movement). This helps maintain better control and stability in various sea conditions.
4. Structural Considerations: Longitudinally positioned bilge keels can also contribute to the overall structural strength of the hull by acting as additional longitudinal stiffeners.

Alternative Attachment Methods:

While welding is the most common method, other techniques may be used in certain cases:

• Bolting: Bilge keels can be bolted to the hull using strong brackets and fasteners. This method may be preferred in some cases, particularly for retrofitting bilge keels to existing vessels or for smaller boats.
• Adhesive Bonding: In some applications, especially with composite or fiberglass hulls, bilge keels may be attached using strong adhesives. However, this method requires careful surface preparation and bonding techniques to ensure a secure and durable connection.

Overall, the longitudinal positioning of bilge keels is crucial for maximizing their effectiveness in reducing roll motion, enhancing stability, and improving the overall seakeeping performance of the vessel.

Let’s compare the advantages and disadvantages of fitting a bilge keel against other methods of stabilization:

Advantages of Bilge Keels:

• Simplicity and Cost-Effectiveness: Bilge keels are relatively simple structures, easy to manufacture and install, making them a cost-effective option for reducing roll motion.
• No Internal Components: Unlike active stabilization systems, they don’t require complex machinery or take up valuable internal space within the vessel.
• Minimal Maintenance: Once installed, bilge keels require little to no maintenance, reducing operational costs and complexity.
• Passive System: They work automatically without the need for power or complex control systems.
• Effective in Moderate Conditions: They provide significant roll reduction in moderate sea conditions, improving comfort and stability for passengers and crew.
• Can aid in beaching/grounding: On some smaller vessels, they can help the vessel sit upright when resting on the seabed.

Disadvantages of Bilge Keels:

• Limited Effectiveness: They are less effective in extreme sea conditions or for vessels with high centers of gravity.
• Increased Drag: Bilge keels create additional hydrodynamic resistance, leading to a slight decrease in speed and fuel efficiency.
• Risk of Grounding: They protrude from the hull, increasing the risk of grounding in shallow waters.
• Not Suitable for All Vessels: May not be ideal for high-speed vessels or those requiring maximum maneuverability.

Comparison with Other Stabilization Methods:

• Active Fin Stabilizers:
  • Advantages: Highly effective in reducing roll even in severe conditions, can be retracted when not needed.
  • Disadvantages: Complex and expensive, require power and maintenance, occupy internal space.
• Anti-roll Tanks:
  • Advantages: Relatively simple and reliable, can be integrated into the ship’s structure.
  • Disadvantages: Require additional space for tanks, less effective than fin stabilizers, can affect stability in certain loading conditions.
• Gyroscopic Stabilizers:
  • Advantages: Very effective in reducing roll, no external appendages.
  • Disadvantages: Extremely heavy and expensive, require significant power, occupy large internal space.

Choosing the Right Method:

The ideal stabilization method depends on various factors, including the type and size of the vessel, its intended use, operating conditions, and budget considerations.

• Bilge keels are a good choice for smaller vessels, those operating in moderate sea conditions, and when cost-effectiveness and simplicity are priorities.
• Active fin stabilizers or gyroscopic stabilizers are often preferred for larger vessels, luxury yachts, or those operating in harsh environments, where maximum roll reduction is desired.
• Anti-roll tanks can be a good compromise between simplicity and effectiveness, especially for vessels with limited space or power constraints.

In conclusion, bilge keels offer several advantages in terms of cost, simplicity, and passive operation, making them a popular choice for many vessels. However, their effectiveness is limited compared to active stabilization systems, and they may not be suitable for all types of vessels or operating conditions.