Ship Hogging and Sagging
On long ships, such as very large tankers and bulk carriers, those responsible for loading the ship have to take care to avoid straining the ship’s hull.
If too much weight is placed amidships the ship will SAG. As the ship cannot submerge her load line mark amidships she will not be able to load her full cargo.
If excess weight is placed at the ends of the ship and not enough in the middle the vessel may HOG. If a ship in such a condition were loaded with a full deadweight cargo, her load line marks amidships would indicate she could carry more cargo.
In the old days, it is said that this was done deliberately. With large modern vessels this distortion can be feet rather than inches, and apart from the obvious strain on the hull and the problems already mentioned, it may also increase the draft which is often critical for large ships getting in and out of port.
Ships are fairly flexible structures and the bending may not do much permanent harm; but if bent severely the ship may become permanently distorted, which is obviously undesirable from many points of view.
To help ships’ officers and those responsible for making the necessary calculations avoid this bending they must, of course, be supplied with the necessary information, gadgets, and calculators. Any such longitudinal stresses will be aggravated by the ship pitching when ends on to the waves.
What is Hogging and Sagging of Ships?
Hogging and sagging are terms used to describe the deformation of a ship’s hull due to the distribution of weight and the forces acting upon it. These terms are often associated with the bending of a ship’s hull along its length.
- Hogging: Hogging occurs when the middle part of the ship’s hull is higher (arched) than the bow (front) and stern (rear) sections. It typically happens when excessive weight is concentrated in the center or amidships of the vessel. The weight causes the center of the ship to bend upward, creating a hogging effect. This can be problematic because it puts excessive strain on the ship’s structure and can lead to structural failures, such as cracks or fractures.
- Sagging: Sagging, on the other hand, is the opposite of hogging. It occurs when the bow and stern sections of the ship’s hull are lower than the middle part. Sagging typically happens when there is excessive weight distributed towards the ends of the ship. The weight causes the bow and stern to bend downward, resulting in a sagging effect. Like hogging, sagging can also lead to structural issues and compromise the integrity of the ship’s hull.
Both hogging and sagging can be influenced by various factors, including the distribution of cargo, equipment, or ballast on board the ship. Additionally, external forces such as waves, rough sea conditions, or improper loading can also contribute to hogging or sagging.
It is crucial for shipbuilders, naval architects, and ship operators to carefully consider the weight distribution and loading conditions to prevent excessive hogging or sagging. Regular inspections and maintenance are also necessary to detect and address any signs of deformation in the ship’s structure, ensuring its safety and integrity during operations.
How to determine whether a Ship is Sagging or Hogging?
To determine whether a ship is sagging or hogging, you can follow these steps:
- Observe the ship’s hull: Stand at a distance where you can see the entire length of the ship’s hull. Look for any noticeable visual deformations along the length of the vessel.
- Check the waterline: Observe the ship’s waterline when it is loaded and floating in calm water. If the bow and stern sections appear lower than the middle part of the hull, the ship is sagging. Conversely, if the middle part of the hull appears higher than the bow and stern, the ship is hogging.
- Use a measuring tool: If you want a more precise measurement, you can use a measuring tool such as a straightedge or a laser level. Place the measuring tool along the length of the ship’s hull, ensuring it is in contact with the surface. Check if there are any gaps between the tool and the hull at different sections. If there is a gap in the middle section and the ends touch the hull, it indicates hogging. If there are gaps at the bow and stern sections and the middle touches the hull, it indicates sagging.
- Conduct inclining experiment: Another method used by naval architects is the inclining experiment. In this experiment, the ship is purposely tilted to one side by adding weights or shifting cargo. The resulting changes in the ship’s trim can provide valuable information about its hogging or sagging tendencies. However, this method requires specialized equipment and expertise.
It’s important to note that determining sagging or hogging is not always straightforward and may require input from naval architects, structural engineers, or experienced ship surveyors. These professionals can conduct detailed inspections and measurements to assess the ship’s structural integrity accurately.
If you suspect that a ship is experiencing sagging or hogging, it is crucial to address the issue promptly. Contacting the relevant authorities or professionals with expertise in ship design and naval architecture will help ensure the safety and seaworthiness of the vessel.
Ship Hogging and Sagging Calculation Example
The preliminary draft measurement of your vessel is as follows: the forward draft is 10.0 meters, while the aft draft is 10.50 meters. The amidship draft is recorded at 10.30 meters.
To proceed, we must determine the mean draft. Let us combine the forward and aft drafts: 10.0 + 10.50 equals 20.5 meters. Then, we divide the sum of the two drafts by 2: 20.5 divided by 2 gives us a mean draft of 10.25 meters.
Now, let us assess whether our ship is experiencing hogging or sagging. Based on the provided drafts for the forward and aft positions, the waterline at amidships should be at 10.25 meters. However, the actual draft at amidships measures 10.30 meters, indicating a sagging condition. Hogging refers to a beam that curves upwards in the middle, while sagging describes a beam that curves downwards.
Next, we calculate the mean of the mean draft considering the sagging and hogging conditions. Given the Length Between Perpendiculars (LBP) as 178 and the values for Deadweight Forward Draft (D.F.DRAFT) and Deadweight Aft Draft (D.A.DRAFT) as 1.53 and 8.31 respectively, we determine the Length Between Midship Perpendiculars (LBM) as LBP minus the sum of D.F.DRAFT and D.A.DRAFT, resulting in 168.16.
For the 1st condition (sagging), with a trim of 0.5 meters, we correct the forward draft as 10 minus (0.5 multiplied by 1.53 divided by 168.16), giving us a corrected forward draft of 9.99545 meters. Similarly, the corrected aft draft is determined as 10.5 plus (0.5 multiplied by 8.31 divided by 168.16), resulting in a corrected aft draft of 10.52470 meters. The mid draft remains unchanged at 10.30 meters. Subsequently, the mean of the means (M.O.M) is calculated as (6 times the mid draft plus the forward and aft drafts) divided by 8, yielding a value of 10.29001875.
Moving on to the 2nd condition (hogging), with a trim of 0.5 meters, the forward draft is corrected using the same formula as before, resulting in a corrected forward draft of 9.99545 meters. Likewise, the corrected aft draft is determined as 10.5 plus (0.5 multiplied by 8.31 divided by 168.16), giving us a corrected aft draft of 10.52470 meters. The mid draft is recorded as 10.20 meters. The M.O.M is calculated similarly as in the previous condition, resulting in a value of 10.21501875.
How to prevent Ship Hogging and Sagging?
To prevent ship hogging and sagging, the following measures can be taken:
- Proper weight distribution: Ensure that the cargo, ballast, and equipment are distributed evenly throughout the ship. Pay attention to the ship’s load plan and follow the recommended weight distribution guidelines provided by the ship’s manufacturer or naval architects. Avoid excessive concentration of weight in one area, especially amidships or towards the bow or stern.
- Trim optimization: Maintain the ship’s trim within the recommended range. Trim refers to the longitudinal balance of the ship, ensuring that it remains level along its length. Deviations from the recommended trim can contribute to hogging or sagging. Adjust the distribution of weight and ballast to achieve the desired trim.
- Consider ship design and stability: During the ship’s design phase, naval architects carefully consider factors such as the ship’s hull shape, structural strength, and stability characteristics. It is essential to adhere to the design specifications and ensure that the ship is constructed according to approved standards and regulations.
- Regular inspections and maintenance: Implement a comprehensive inspection and maintenance program to monitor the ship’s structural integrity. Regularly inspect the hull, frames, bulkheads, and other structural components for signs of deformation, cracks, or corrosion. Timely detection and repair of any structural issues can prevent hogging or sagging from worsening.
- Load monitoring: Continuously monitor the ship’s loading conditions during cargo operations. Utilize onboard load monitoring systems, such as load cells or strain gauges, to measure the distribution of weight and stresses on the ship’s structure. These systems can provide real-time feedback, allowing for adjustments to be made if any imbalances or excessive loads are detected.
- Training and awareness: Ensure that the ship’s crew, particularly those involved in cargo operations, are adequately trained and aware of the importance of proper weight distribution and loading procedures. Encourage a culture of safety and adherence to guidelines to minimize the risk of hogging or sagging incidents.
- Consult professionals: Seek the expertise of naval architects, marine surveyors, or structural engineers to assess the ship’s design and provide guidance on weight distribution and loading procedures. These professionals can offer valuable insights and recommendations specific to the ship’s characteristics and operational requirements.
By implementing these preventive measures, shipowners, operators, and crew members can minimize the risk of hogging and sagging, ensuring the structural integrity and safety of the vessel during its operations.
What is the effect of Ship Hogging and Sagging?
The effects of ship hogging and sagging can have significant implications on the ship’s structural integrity and overall safety. Here are the main effects of each:
- Increased stress on the ship’s structure: Hogging puts excessive stress on the ship’s midsection, particularly on the deck and bottom plating. This can lead to structural deformations, including cracks, fractures, or even buckling.
- Reduced buoyancy amidships: The upward bending of the hull reduces the buoyancy in the middle section of the ship. This can result in decreased stability, compromising the ship’s ability to withstand external forces such as waves, wind, or cargo shifts.
- Potential cargo damage: Hogging can cause cargo stored in the middle section of the ship to experience increased pressures or uneven distribution of weight. This can lead to cargo shifting, damage, or even loss.
- Impaired maneuverability and performance: The altered shape and compromised stability caused by hogging can affect the ship’s maneuverability, making it more difficult to control and potentially reducing its performance in terms of speed, fuel efficiency, and response to helm commands.
- Increased stress on bow and stern sections: Sagging places additional stress on the bow and stern areas of the ship’s hull. This can lead to structural issues, including deformation, cracking, or even failure of these sections.
- Reduced freeboard: Sagging causes the bow and stern to be lower in relation to the waterline. This decreases the freeboard, which is the vertical distance between the waterline and the ship’s deck. Reduced freeboard can compromise the ship’s stability, seaworthiness, and safety, especially in rough sea conditions.
- Potential water ingress: The lower bow and stern sections can be more prone to shipping water, especially during heavy seas or when encountering waves. This can lead to water ingress, affecting the ship’s buoyancy, stability, and potentially causing damage to cargo or onboard systems.
- Impaired propeller immersion: Sagging at the stern can result in the propeller partially or fully coming out of the water, affecting propulsion efficiency, maneuverability, and potentially causing cavitation or damage to the propeller.
In both hogging and sagging scenarios, the structural integrity of the ship is compromised, potentially leading to catastrophic failure if not addressed. It is crucial to detect and mitigate hogging and sagging issues through proper design, weight distribution, maintenance, and regular inspections to ensure the safety and seaworthiness of the vessel.