Do Ships Float Higher In Saltwater?



Sailing in rivers and open sea, you may notice that your ship is floating higher in saltwater or what is more important, the draft is smaller. This makes navigation safer and in some cases even reduces fuel consumption and increases speed.

On the other hand, if you enter a river or port with fresh water, your ship or boat will float lower, and your draft will increase. Ignoring the bigger draft may bring you to an unpleasant situation – touching the sea ground or even grounding. Let’s take a look at why it happens.

How Does Density Affect A Ship’s Draft?

In seawater, ships and boats float higher than in freshwater, because the weight of the boat is equal to the weight of water displaced by the boat.

The density of seawater is higher and a smaller amount of seawater is to be displayed to keep a ship afloat. Therefore, the boat floats higher in seawater than in freshwater, and at the same time, with an increase in the density of water, the draft of the vessel decreases.

Conversely, with a decrease in the density – a ship’s draft increases. The change in ship’s draft due to changes in water density can be calculated using the formula:

The formula of a ship's draft changes due to changes in water density
The formula of a ship’s draft changes due to changes in water density

The amount by which the ship’s draft decreases when moving from freshwater to seawater with a density of 1.025 t/m³ is called the freshwater allowance and is usually measured in millimeters.

For each vessel, this correction shall be indicated on the Ship’s Load Line Certificate and can be easily found on Plimsoll Disk on the ship midship on the port and starboard side.

Ship's draft mark and Plimsoll disk
Ship’s draft mark and Plimsoll disk

The load line marked on both sides of the ship shows the minimum freeboard the ship can have in seawater with a density of 1.025 t/m³ and is marked with the letter S.

When a ship is loaded in a freshwater port, the load line may be sunk by an amount equal to the freshwater allowance. Upon transition to seawater with a density of 1.025 t/m³, the ship’s draft will decrease by the value of this correction, and the ship will have a load line draft.

When loading in a port where the water density is more than 1,000 t/m³ but less than 1,025 t/m³. the amount by which the load line can be sunk is called the Dock Water Allowance. Use this formula to calculate Dock Water Allowance:

Dock Water Allowance formula
Dock Water Allowance formula

Dock Water Allowance or the draft correction calculated using the formula above is in centimeters.

Example: The vessel’s draft on the load line is 4.50 m. Freshwater correction is 126 mm. The density of water at the berth is 1.006 t/m³. Calculate how much the draft can be increased so that with the transition to water with a density of 1.025 t/m³. the vessel had a draft of the load line.

Dock Water Allowance Calculation Example
Dock Water Allowance Calculation Example

The load line can be sunk by 9 centimeters.

How does temperature affect water density?

It should be noted that with a decrease or increase in water temperature, its density changes. The density of freshwater is 1.00000 only at 4°C. At temperatures close to 4°C the density changes very little and using most of the instruments are hardly identified.

Therefore, if the ship is in freshwater, the temperature of the water must be taken into account, since at high temperatures freshwater has a density below 1,000 t/m³. If this is not taken into account in the calculations, then the difference between the true and calculated displacement can be very significant.

Freshwater density table at different temperatures:

Temperature (°F/°C)ρ, Density kg/m3ρ, Weight pounds/ft3

When the vessel is in seawater, the correction for the seawater temperature is not taken into account and it is necessary to be guided only by the readings of the hydrometer.

A hydrometer is a device for measuring the density of a liquid. Modern hydrometers are usually glass and the measurement scale is graduated in kg/m³. The value of the density of the liquid is read from the division of the scale, which is on the same level as the water level and not the meniscus of the liquid. To clarify, perform the reading of the hydrometer scale at the lowest point of the surface of the liquid.

For measurement, use a container with a diameter of at least 50 mm. Outboard water samples must be taken from both sides in the midship area from a depth equal to half the ship’s draft, as soon as possible after the draft has been removed.

After that, It should be noted that the density of water in ballast tanks is measured with the same hydrometer when determining the amount of cargo from drafts, so clean or flush the hydrometer with the water you will measure.

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