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.

Outlines

00:00

🚢 Understanding Ship's Freshwater and Dark Water Allowance

Captain Ron introduces the concept of freshwater and dark water allowances, which are crucial for determining a ship's draft changes when transitioning between waters of different densities. He explains the importance of water density, with freshwater at 1000 kg/m³ and saltwater at 1025 kg/m³, and introduces the term 'relative density'. The video aims to educate viewers on how a ship's draft changes when moving from saltwater to freshwater or vice versa, affecting the ship's buoyancy and the volume of trapped air (traps). The load line marks on a ship are also discussed as a reference for these changes.

05:03

📐 Calculating Fresh Water Allowance for Draft Changes

This paragraph delves into the specifics of calculating the freshwater allowance, which is the change in a ship's draft when moving from saltwater to freshwater at its summer displacement. Two methods are presented: a mathematical formula using the ship's displacement and the tropical freshwater TPC (Transverse Period of Change), and referencing the stability information manual. An example calculation is provided, demonstrating how to find the arrival draft in freshwater by adding the freshwater allowance to the departure draft in saltwater.

10:05

🌊 Determining Dark Water Allowance and Arrival Draft

The final paragraph focuses on dark water allowance, which is the draft change when a ship moves between saltwater and dark water or vice versa. Dark water is defined as having a density between freshwater and saltwater. The calculation of dark water allowance is explained using a formula that involves the freshwater allowance and the relative densities of the departure and arrival waters. Two examples are given to illustrate how to calculate the change in draft and the arrival draft in different scenarios, considering whether the ship is moving to a berth with a higher or lower water density.

Keywords

💡Density

Density is defined as the mass per unit volume of any given liquid or substance. In the context of the video, it is crucial for understanding how a ship's buoyancy changes in water with varying densities. The script discusses freshwater and saltwater densities, which are key to calculating the ship's freshwater and dark water allowances. For instance, freshwater has a density of 1000 kg/m³, while saltwater is denser at 1025 kg/m³.

💡Freshwater Allowance

Freshwater allowance refers to the change in a ship's draft when transitioning from saltwater to freshwater or from water with a higher density to lower density. The script explains that this allowance is essential for determining the ship's draft changes without any additional weight being loaded or unloaded. An example given is that if a ship moves from saltwater to freshwater, it will sink slightly, resulting in an increased draft.

💡Dark Water

Dark water is water with a density between that of freshwater and saltwater, ranging from 1001 kg/m³ to 1024 kg/m³. The term is used in the script to describe water densities that are neither freshwater nor saltwater, and understanding these densities is important for calculating the dark water allowance, which affects the ship's draft when moving between different water densities.

💡Relative Density

Relative density is the ratio of the density of a substance to that of freshwater. It is used in the script to simplify the comparison of different water densities. For example, the relative density of saltwater is calculated as 1.025 by dividing its density (1025 kg/m³) by that of freshwater (1000 kg/m³), which helps in understanding how the ship's draft will change in different water conditions.

💡Load Line Marks

Load line marks are molded on the ship's hull to indicate the maximum safe draft for various conditions and water densities. The script mentions specific marks like the tropical freshwater (TF), fresh water (F), tropical (T), summer (S), and winter North Atlantic (WNA) to illustrate how these marks are used to determine the ship's draft changes when moving between different water densities.

💡Summer Displacement

Summer displacement is the weight of the ship when it is fully loaded for summer conditions. The video script uses this term to establish the basis for calculating the freshwater and dark water allowances, as these calculations assume the ship is loaded to its summer displacement when transitioning between water densities.

💡Draft

Draft refers to the depth a ship sinks in the water, measured from the waterline to the bottom of the hull. The script discusses how the draft changes when a ship moves between different water densities, either increasing or decreasing, which is essential for determining the freshwater and dark water allowances.

💡Stability Information Manual

The stability information manual is a document that contains important data for a ship's stability, including the freshwater allowance. The script mentions using this manual to extract the freshwater allowance value, which is then used in calculations to determine the ship's arrival draft in different water conditions.

💡TPC (Tonnage Per Centimeter)

TPC is a measure used in the script to calculate the freshwater allowance mathematically. It represents the amount of water displaced by the ship per centimeter of draft. The script explains using the saltwater TPC value for the summer load draft to find the freshwater allowance.

💡Dark Water Allowance

Dark water allowance is the change in a ship's draft when transitioning between saltwater and dark water or between different dark water densities. The script provides a formula and examples to calculate this allowance, which is essential for determining the ship's arrival draft when moving between these water conditions.

Highlights

Introduction to determining a ship's freshwater and dark water allowance and change in draft due to different water densities.

Explanation of effective density on a ship's drafts and its impact on buoyancy.

Definition of density as mass per unit volume and its relevance to ship buoyancy.

Density values for freshwater (1000 kg/m³), saltwater (1025 kg/m³), and dark water (1001-1024 kg/m³).

Impact of water density changes on ship's draft: sinking in lower density water and rising in higher density water.

Introduction to relative density and its calculation method.

Calculation example of relative density for saltwater.

Explanation of freshwater allowance and its definition in terms of ship's draft change.

Description of ship's load line marks and their significance in determining freshwater allowance.

Two methods for finding freshwater allowance: mathematical formula and stability information manual.

Example calculation of freshwater allowance using a ship's departure draft and relative densities.

Explanation of dark water allowance and its definition in terms of ship's draft change.

Formula for calculating dark water allowance and its components.

Example calculation of dark water allowance and its impact on ship's draft when transitioning between saltwater and dark water.

Clarification on whether the change in draft is an increase or decrease based on water density transition.

Final calculation of arrival draft considering freshwater and dark water allowances.

Note on disregarding changes in draft due to fuel and other consumptions in these calculations.

Conclusion and call to action for viewers to subscribe and enable notifications for new content.

Transcripts

hi guys I'm Captain Ron in this video I

will show you how to determine the

ship's freshwater and Dark Water

allowance or the change in draft since

the ship sail from different water

densities

but before I proceed to some

calculations in determining the

freshwater allowance and Dark Water

allowance I will first discuss the

effective density on the ship's drafts

if you like this video guys kindly click

the Subscribe button and hit the

notification Bell to notify you when new

videos are uploaded

density is the mass per unit volume of

any given liquid or substance so the

ships presumed that she always floats in

water that has a density within the

range of fresh water and salt water

the freshwater density is 1000 kilograms

per cubic meter or 1.0 ton per cubic

meter the salt water density is 1025

kilograms per cubic meter or 1.025 tons

per cubic meter and water between the

densities of 1001 kilogram per cubic

meter or 1.001 ton per cubic meter to

1024 kilograms per cubic meter or 1.024

tons per cubic meter is called Dark

Water densities

if the ship will proceed from salt water

to fresh water or from water with higher

to lower density there will be a sinkage

of a few centimeters which will result

in an increase in the ship's traps on

another hand if the ship will proceed

from fresh water to salt water or from

water with lower to higher density the

ship will rise for a few centimeters

which will result in a decrease in the

ship's traps

the term relative density is also used

instead of density it is defined as a

ratio of the density of the substance in

question to that of fresh water to find

the relative density of the water

substance relative density is equal to

the density of water or substance

divided by the density of fresh water

let us find the relative density of salt

water which is a density of 1025

kilograms per cubic meter so the

relative density of salt water is equal

to

1025 kilograms per cubic meter divided

by the density of fresh water which is

1000 kilograms per cubic meter then

cancel the unit kilograms per cubic

meter so the answer is

1.025 if the given density of any liquid

or substances in tons per cubic meter

let's say in this example the given

density of salt water is 1.025 tons per

cubic meter so just divided it by the

density of fresh water in tons per cubic

meter then cancel both units and the

answer is

1.025 so to find the relative density of

any liquid or substance we simply

divided it by the density of fresh water

then cancel the units relative density

has no existing units to simplify it to

find the relative density just take out

the existing unit of the given density

of a substance in tons per pubic meter

and that's the value of relative density

so let us proceed to freshwater

allowance fresh water allowance defines

as the number of millimeters in which

the mean traffic changes when she passes

from salt water to fresh water or from

fresh water to salt water when she is

loaded to its summer displacement

these is a ship's load line marks molded

in the midship part of the ship's Hull

important starboard side TF means

tropical freshwater f for fresh water

tea for tropical as for summer W for

winter and below the winter load line

Mark is the wna or winter North Atlantic

I will not go deeper into the discussion

of load lines since our topic in this

video is of fresh water and Dark Water

allowance

so to understand freshwater allowance

let's say the ship is floating in salt

water the relative density is 1.025 and

she is loaded to its summer displacement

if we inspect the ship's load line the

water line will be on the upper edge of

the s-load line Mark then she proceeds

to a berth in which the water has a

relative density of 1.0 or she arrived

in fresh water if we inspect the load

line marks again the water line is on

the upper edge of the airflowed landmark

in this case there is an increase in the

ship's traps by several number of

millimeters as she passes from salt

water to fresh water without adding or

loading any weight on board the ship the

change in the ship's draft when she

passes from salt water to fresh water is

called fresh water allowance

on another hand let's say the ship is

floating in fresh water in which the

relative density is 1.0 and she is

loaded to its summer displacement if we

inspect the ship's load line the water

line will be on the upper edge of the

airflowed landmark and when she proceeds

to a berth in which the water has a

relative density of 1.025 or when she

arrived in salt water then as we inspect

the load line marks again the water line

is on the upper edge of the s-load line

Mark in this case the ship Rises and the

effect is a decrease in the ship's traps

by several number of millimeters as she

passes from fresh water to salt water

without discharging any weight on board

the ship the change in the ship's draft

when she passes from fresh water to salt

water is called fresh water allowance

in this case I disregard the change in

the traps due to fuel and fresh water

consumption

there are two methods to find fresh

water allowance first by solving it

mathematically using the formula

freshwater allowance in millimeters is

equal to Summer displacement divided by

four times TPC in salt water

the TPC to be used in finding fresh

water allowance is the saltwater TPC

value for the summer load draft

the second method of finding the fresh

water allowance is by extracting the

value from the stability information

manual this is a photo of a load lying

taken from the ship's stability manual

in which the fresh water allowance is

231 millimeters

I will use this fresh water allowance

value in finding the ship's arrival main

draft let's say the ship departs from

salt water in which the relative density

is 1.025 with a summer load draft of

9.80 meters and she proceeds to a berth

in which the relative density is 1.0 the

ship's fresh water allowances 231

millimeters determine her arrival draft

in fresh water disregarding the change

in the draft due to fuel and other

consumptions

so this is the solution to find the

Rival draft departure draft is 9.800

meters applied to change in drafter

fresh water allowance which is 231

millimeters but we will convert

millimeters first into meters before we

can apply it to the departure draft to

convert millimeters divided by 1000 is

equal to meters so 231 millimeters

divided by 1000 is equal to 0.231 meter

we can now apply the freshwater

allowance which is 0.231 meter but we

need to determine if the change in the

draft will be added or subtracted to the

departure draft remember if the ship

departs from salt water and proceeds to

fresh water there will be a sinkage and

the effect will be an increase in the

ship's traps so we will add the fresh

water allowance and the arrival draft is

10.031 meters

but if the ship departs from fresh water

and proceeds to salt water there will be

a rise and the effect will be a decrease

in the ship's draft so subtract the

freshwater allowance from the departure

draft to find the arrival draft let's

proceed to talk water allowance

duck water allowance defines as the

number of millimeters in which the ships

mean draft changes when she passes from

salt water to dark water or from dock

water to salt water when she is loaded

to its summer displacement I will show

you some examples of how to calculate

the dock water allowance

calculate the ship's change in draft and

the arrival draft if she departs from

salt water with a summer load draft of

10.300 meters and proceeds to a berth

where the relative density of Dark Water

is

1.014 the ship's fresh water allowances

246 millimeters

the first step is to calculate the

change in draft or dark water allowance

the formula is dock water allowance is

equal to freshwater allowance times the

relative density of salt water minus the

relative density of Dark Water divided

by the relative density of salt water

minus the relative density of fresh

water the denominator in this formula is

constant it is always a difference

between salt water and fresh water

densities in this case I use relative

density so the denominator is .025 if

you use density which is in kilograms

per cubic meter the denominator is 25.

allowance is equal to 246 millimeters

times

1.025 minus 1.014 divided by the

constant which is .025 duck water

allowance now is equal to 246

millimeters times .44 and the answer is

108.24 millimeters

so we need to determine if

108.24 is an increase or decrease in the

ship's draft if the ship from Salt Water

proceeds to dock water with lesser

density there will be a sinkage and the

effect will have an increase in the

ship's draft so the ship's mean draft

will increase by

108.24 millimeters

so the next step is to calculate the

arrival draft the departure draft is

10.300 meters at the dock water

allowance of

108.24 millimeters since it is an

increase in the ship's draft but in

order to add we need to convert

108.24 millimeters into meter to convert

just divided it by one thousand the dock

water allowance now is 0.108 meter so

the arrival draft is

10.408 meters

if the ship departs from dock water and

proceeds to salt water subtract the dock

water allowance to find the arrival

draft

this is the next example calculate the

change in draft and the arrival draft

the ship is loaded to its summer

displacement and she will depart from

the birth where the Dark Water relative

density is

1.007 and she will go to another birth

where the topwater relative density is

1.018 her departure draft is 10.520

meters with the fresh water allowance of

253 millimeters

the first step is calculate the change

in draft or dock water allowance so the

formula is dock water allowance is equal

to fresh water allowance times the

relative density of arrival dock water

minus the relative density of departure

dock water divided by .025 just take the

difference between the departure and

arrival dock water or subtract the

Lesser from the grader

allowance is equal to 253 millimeters

times 1.018 minus 1.007 divided by .025

dock water allowance since 253

millimeters times .44 and the answer is

111.32 millimeters

so she sails from a lower density to a

higher density the ship will rise and

the effect on ship's draft will decrease

the ship's draft will decrease by

111.32 millimeters

so the final step is to calculate the

arrival draft the departure draft is

10.520 meters then subtract the dock

water allowance of

111.32 millimeters since the effect is

decreased in the draft just divide

111.32 millimeters by 1000 to convert

into meters in order to subtract from

the departure draft the dock water

allowances 0.111 meters so the arrival

draft is

10.409 meters

the change in drafts due to fuel and

other consumptions has been disregarded

in these calculations that's all for now

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