10.2 Rivers, Estuaries and Restricted Water
Define:
- Shallow water
- Squat
Explain:
- How Squat is dependent on speed of the vessel, block coefficient and the width of the channel
- Reduction in under keel clearance resulting from rolling and pitching and heel or list
- How to round bends in a channel with a current in either direction, taking account of the effect of wind
- Use of an anchor to assist in rounding a bend
- How to turn short round in a narrow channel, with or without a wind, and current.
Calculate:
- The approximate sinkage due to squat
Shallow Water:
Shallow water refers to areas where the depth of the water is relatively low, posing potential navigational challenges. The exact definition of shallow water may vary depending on local regulations, but it generally implies a depth that restricts vessel maneuverability, requires careful navigation, and may necessitate additional precautions.
Squat:
Squat is the phenomenon where a vessel's draft (submerged depth) increases as it moves through the water, particularly in restricted channels or shallow water. This increase in draft is caused by the pressure difference created between the bow and stern wave systems as the vessel moves forward. Squat is influenced by several factors, including the speed of the vessel, block coefficient (a measure of hull form), and the width of the channel.
Speed: Squat is directly related to the speed of the vessel. As the speed increases, the pressure difference between the bow and stern waves also increases, leading to a deeper draft.
Block Coefficient: The block coefficient, which represents the fullness of the vessel's form, can affect squat. Vessels with larger block coefficients generally experience greater squat.
Channel Width: In narrow channels, the confinement of water can amplify squat due to the closer proximity of the vessel to the channel banks.
Reduction in Under Keel Clearance:
Under keel clearance refers to the vertical distance between the deepest point of a vessel (keel) and the seabed. In restricted waters, factors such as rolling and pitching (motions of the vessel), as well as heel or list (lateral inclination), can reduce the under keel clearance. These motions cause the vessel to dip or tilt, potentially decreasing the available clearance between the vessel's keel and the seabed.
Rounding Bends in a Channel with Current and Wind:
When rounding bends in a channel with a current in either direction, the effect of wind must be considered. The combined forces of current and wind can push or steer the vessel off its desired track. To counteract these forces, the helmsman should apply appropriate rudder and engine controls to maintain the vessel's heading and maneuver safely around the bend.
Use of an Anchor to Assist in Rounding a Bend:
In certain situations, the use of an anchor can assist in rounding a bend in a narrow channel. Deploying the anchor from the bow or stern can provide additional control and help counteract the effects of current or wind. By adjusting the anchor cable length and applying the appropriate tension, the vessel's heading and speed can be better managed during the maneuver.
Turning Short Round in a Narrow Channel with or without Wind and Current:
When turning short round in a narrow channel, the vessel must navigate within limited space. If wind or current is present, it can exacerbate the challenge. To navigate effectively, the helmsman must apply proper helm commands, engine controls, and thruster assistance to execute a tight turn while considering the influence of wind and current.
Approximate Sinkage due to Squat Calculation:
Calculating the approximate sinkage due to squat involves considering the vessel's speed and the characteristics of the channel. Specific formulas or tables provided by shipbuilders, naval architects, or hydrodynamic experts can be used to estimate the expected squat and resultant increase in draft based on the vessel's characteristics and operating conditions. These calculations help determine the required under keel clearance to navigate safely in shallow waters.
It is important to note that the precise calculations, procedures, and guidelines for maneuvering in rivers, estuaries, and restricted waters may vary depending on local regulations, port authorities' requirements, vessel type, and specific circumstances. Vessel operators should refer to applicable maritime regulations, industry standards, and the guidance of local authorities to ensure safe navigation in these challenging environments.
The formula for calculating ship squat can vary depending on the specific mathematical model used. Ship squat is a complex phenomenon influenced by various factors, such as vessel speed, block coefficient, channel width, and depth of water. Different mathematical models and empirical formulas have been developed to estimate ship squat based on these parameters.
One commonly used empirical formula to estimate ship squat is the Holtrop-Mennen squat formula. This formula provides an approximation of the additional draft caused by squat. It is expressed as:
ΔT = k * V^2 * B / D
Where:
ΔT is the additional draft (squat)
k is an empirical coefficient
V is the vessel speed
B is the vessel's block coefficient
D is the vessel's draft
It is important to note that the values of the empirical coefficient k may vary depending on the specific model and data used. Additionally, this formula provides an approximation and may not capture all the complexities of squat accurately. More advanced mathematical models and computational fluid dynamics (CFD) simulations can provide more precise predictions, but they require more detailed vessel-specific data and computational resources.
When calculating ship squat, it is advisable to consult reliable sources, such as naval architects, hydrodynamic experts, or specialized software tools that consider the specific vessel characteristics and operating conditions. These resources can provide more accurate estimations of squat based on the vessel's design and the specific channel and environmental parameters.
ROTI
Describe:
- Use of constant rate of rate and constant radius turn in restricted waters
Ships Rate of Turn Indicator (ROTI) is a navigation instrument that provides real-time information about the rate at which a vessel is turning. It helps the navigator monitor and control the vessel's maneuvering during turns, especially in restricted waters where precise maneuvering is essential. The ROTI display typically indicates the rate of turn in degrees per minute (°/min).
The use of a constant rate of turn (CRT) and constant radius turn (CRT) techniques in restricted waters can assist in safe and predictable maneuvering.
Constant Rate of Turn (CRT):
The CRT technique involves maintaining a steady rate of turn throughout the maneuver. By setting a specific rate of turn on the ROTI, such as 3°/min or 5°/min, the navigator ensures that the vessel turns at a consistent rate. This allows for better control and predictability during the turn, especially in confined areas or when navigating through narrow channels.
The navigator adjusts the rudder angle and engine controls to maintain the desired rate of turn. The ROTI provides real-time feedback on the vessel's actual rate of turn, allowing the navigator to make necessary adjustments to maintain the desired rate.
Constant Radius Turn (CRT):
The CRT technique involves executing a turn with a fixed radius. It is particularly useful when navigating through channels or areas where maintaining a specific distance from navigational hazards or restricted zones is critical.
To perform a CRT, the navigator uses the ROTI to monitor the vessel's rate of turn and adjusts the rudder angle and engine controls to maintain a consistent rate of turn while keeping the vessel on a predetermined radius. This technique helps ensure that the vessel follows a consistent path, maintaining a fixed distance from nearby obstacles or hazards.
By employing these techniques, the navigator can navigate through restricted waters with greater precision, reducing the risk of grounding, colliding with other vessels, or encroaching into prohibited areas. The use of ROTI provides real-time feedback on the vessel's turning rate, enabling the helmsman to make timely adjustments to maintain the desired maneuvering parameters.
It is important to note that the specific techniques and parameters used for maneuvering in restricted waters may vary based on vessel type, and the specific characteristics of the waterway. The navigator should consult vessel-specific maneuvering procedures to ensure safe and efficient navigation in restricted waters.