SH Lesson 6 Interaction Bank Effect

Summary

This ship handling lesson delves into the effects of shallow water on ships, including scot and bank effect, and their impact on steering and maneuverability. It explains how water acceleration under the hull leads to increased sinkage or 'squat', affecting ship draft and trim. The lesson also covers the bank effect, where a ship's stern can be drawn towards the bank due to pressure differences. It discusses interactions between vessels during head-on passing and overtaking, highlighting the importance of understanding these effects for safe navigation in confined waters.

Takeaways

• 🚤 **Squat Phenomenon**: As ships approach shallow water, the water's inability to compress causes acceleration of water molecules beneath the hull, leading to a drop in pressure and an increase in draft known as 'squat'.
• 📏 **Draft Increase**: The apparent increase in draft due to shallow water is called 'squat', which can be estimated by adding 10% to the draft or 0.3 meters for every 5 knots of speed.
• 🔄 **Ship Trim**: The distribution of a ship's buoyancy affects how it trims; a forward center of buoyancy causes head trimming, while an aft center causes stern trimming.
• 🌊 **Bank Effect**: The hydrodynamic pressure distribution around a ship can cause it to be drawn towards the bank due to low pressure areas forming behind the pivot point.
• 🚦 **Interaction with Banks**: When a ship is near a bank, it may require constant corrective rudder action to counteract the strong force of the low-pressure area.
• 🛳 **Vessel Interaction**: During head-on passing, ships experience four phases of interaction, including repulsion, balance, attraction, and separation, each requiring specific helm responses.
• 🔄 **Overtaking Dangers**: Overtaking maneuvers can be dangerous due to the prolonged nature of the interaction, with particular risk during the phase where the bow of the overtaking ship overlaps the stern of the overtaken ship.
• 🚢 **Size Matters**: The interaction between vessels of different sizes is significant, with smaller vessels being more affected by the pressure changes produced by the larger vessel's hull.
• 💡 **Speed Reduction**: Reducing speed is a key strategy to mitigate the effects of squat and bank effect, as well as to improve steering control in shallow waters.
• 📉 **Steering Challenges**: Shallow water can make steering more difficult due to hydrodynamic forces acting against the rudder's turning movement, potentially requiring emergency speed reductions for effective control.

Q & A

What happens to the water underneath a ship's hull as it approaches shallow water?

As a ship approaches shallow water, the water underneath the hull gets squeezed, causing the water molecules to accelerate due to Bernoulli's theorem, which results in a drop of internal pressure.

What is the term used to describe the apparent increase in a ship's draft due to shallow water effect?

The term used is 'squat', which refers to the sinking of the ship as it displaces more water to maintain buoyancy in shallow water.

How does the ship's center of buoyancy influence the direction of trim when experiencing squat?

If the center of buoyancy is forward of midships, the ship will trim by the bow. Conversely, if it is aft of midships, the ship will trim by the stern.

What types of ships are more likely to trim by the head when experiencing squat?

Ships with fat bows and finer sterns at summer draft, such as large oil tankers and cape size bulk carriers, are more likely to trim by the head.

What is the effect of high speed in shallow water on a ship's steering ability?

High speed in shallow water can adversely affect a ship's steering ability, as the increased water acceleration under the hull leads to increased sinkage, making it harder to control the ship's direction.

How can the effects of squat be estimated in shallow water?

Squat can be estimated by adding 10 percent to the draft or 0.3 meters for every 5 knots of speed.

What is the bank effect and how does it influence a ship's movement?

The bank effect is the hydrodynamic pressure distribution around a forward-moving ship that can cause the stern to be sucked towards the bank. It increases with speed and can be severe if the ship is too close to the bank.

How can a ship handler counteract the bank effect?

A ship handler can counteract the bank effect by slowing down and steering towards the bank, which may help to balance the ship running parallel to the bank.

What are the four distinct phases of interaction between two vessels in a head-on passing situation?

The phases are: 1) Repulsion due to positive pressure at the bow, 2) Balancing of pressure at the bow, 3) Attraction due to reduced pressure between hulls, and 4) Sterns drawing together due to negative pressure.

How does the interaction between a large and a small ship during overtaking affect the smaller vessel?

The smaller vessel is affected more by the interaction due to the larger vessel's pressure field, which can cause the tug to be drawn towards the larger ship, requiring constant corrective action to avoid collision.

What is the Venturi effect as it relates to the interaction between a tugboat and a larger ship?

The Venturi effect develops between the tug's bow and the larger ship's side, causing the tug to turn in towards the larger ship due to the reduced pressure in this area.