Calculate the Fresh Water Allowance & Dock Water Allowance II Change in Ship's Drafts due to Density

Summary

In this educational video, Captain Ron explains how to calculate a ship's freshwater and dark water allowances, which affect the ship's draft when moving between waters of different densities. He discusses the concept of effective density, relative density, and provides methods to determine these allowances, including mathematical formulas and stability information from manuals. The video also covers how changes in draft are calculated and applied.

Takeaways

• 🛳 The video is about determining a ship's freshwater and dark water allowance, which are changes in the ship's draft due to different water densities.
• 💧 Density is defined as the mass per unit volume and is crucial for understanding how a ship floats in water with varying densities.
• 🌊 Freshwater has a density of 1000 kg/m³ or 1.0 ton/m³, while saltwater has a density of 1025 kg/m³ or 1.025 tons/m³.
• 🌌 Dark water is water with a density between 1001 kg/m³ (1.001 tons/m³) and 1024 kg/m³ (1.024 tons/m³).
• ⬇️ When a ship moves from saltwater to freshwater or from higher to lower density water, it sinks slightly, increasing the ship's volume underwater.
• ⬆️ Conversely, moving from freshwater to saltwater or from lower to higher density water causes the ship to rise, decreasing the underwater volume.
• 🔢 Relative density is the ratio of the density of a substance to that of freshwater, and it helps in understanding changes in a ship's draft.
• 📏 Freshwater allowance is the change in draft when a ship moves between saltwater and freshwater at its summer displacement.
• 📘 Two methods to find freshwater allowance are provided: mathematical calculation using the formula or extracting the value from the stability information manual.
• 📐 Dark water allowance is calculated similarly to freshwater allowance but considers the transition between saltwater and dark water or vice versa.
• ⚖️ The formula for dark water allowance involves the freshwater allowance, the relative densities of the departure and arrival waters, and the difference between saltwater and freshwater densities.
• 📊 The script provides examples of calculating changes in draft and arrival drafts for different scenarios involving freshwater, dark water, and saltwater.

Q & A

What is the main topic of Captain Ron's video?

The main topic of the video is how to determine a ship's freshwater and dark water allowance, which is the change in draft due to different water densities.

What is the density of freshwater in kilograms per cubic meter?

The density of freshwater is 1000 kilograms per cubic meter.

What is the density of saltwater in kilograms per cubic meter?

The density of saltwater is 1025 kilograms per cubic meter.

What is the term used for water densities between 1001 kg/m³ and 1024 kg/m³?

Water with densities between 1001 kg/m³ and 1024 kg/m³ is referred to as dark water.

What does the term 'relative density' refer to in the context of the video?

Relative density is the ratio of the density of a substance to that of fresh water, used to compare different water densities.

How is the relative density of saltwater calculated in the video?

The relative density of saltwater is calculated by dividing the density of saltwater (1025 kg/m³) by the density of fresh water (1000 kg/m³), resulting in 1.025.

What is the freshwater allowance in the context of a ship's draft?

Freshwater allowance is the number of millimeters the mean draft of a ship changes when it moves from saltwater to freshwater or vice versa, while loaded to its summer displacement.

What are the two methods mentioned in the video to find the freshwater allowance?

The two methods to find the freshwater allowance are by using a mathematical formula with the ship's summer displacement and saltwater TPC, or by extracting the value from the stability information manual.

What is the purpose of the load line marks on a ship's hull?

The load line marks on a ship's hull indicate the maximum safe draft for different conditions and are used to determine the ship's load carrying capacity and stability.

How is the dark water allowance calculated in the video?

The dark water allowance is calculated using the freshwater allowance, the relative density of the departure and arrival waters, and a constant denominator which is the difference between the relative densities of saltwater and freshwater.

What is the significance of calculating the arrival draft when a ship changes water density?

Calculating the arrival draft is important to ensure the ship maintains stability and safety, as the draft will change due to the difference in water density.