What Is Reserve Buoyancy? ( Ships Stability )

What Is Reserve Buoyancy? ( Ships Stability )

Reserve buoyancy is a key concept of ships stability lessons. Without which you simply cannot understand practical operation of ship and its seaworthiness. The term reserve buoyancy refers to the volume of enclosed space or the part of ship above the waterline that can be made watertight.

In short it is the watertight volume of the ship above its waterline providing additional buoyancy. Thus many a times it is also termed as the free board.

The distance between the top deck plate to the waterline on the ship side. When we add more weight to the ship (cargo or crew); it tends to sink downwards as it looses some of its buoyancy. It is this reserve buoyancy which then converts into buoyancy and stabilizes the ship preventing it from sinking.

A ship would simply sink if the lose in buoyancy due to extra weight added to the ship unless when excess of reserve buoyancy is available to compensate for the losses.

But before you learn in detail about the reserve buoyancy and its critical role in ship stability. Let us first give you a quick reversion of the parent topic “buoyancy”.

What Does Buoyancy Mean?

It is the upward force exerted on the submerged or partially submerged object in a fluid. It is equals to the weight of the volume of fluid displaced by the object. In short it is the ability of an object to float caused by the upthrust force which when is equals or more then its weight will float.

This is the same reason why you feel lighter in a swimming pool or a ship floats but nail sinks. The buoyant force or buoyancy is caused due to fluid pressure acting on the opposite side.

Consider an object A which is submerged in a water to a depth of “h” distance:

What Is Reserve Buoyancy? ( Ships Stability ) - Explanation of buoyancy with the pic

Now here the varying water pressure with depth can be given as:

P = ɣd

Where γ = specific weight of the water or any other fluid given by formula


and d is used for the depth.

Then the pressure of the liquid acting on both the top and bottom surface of the block will be:

What Is Reserve Buoyancy? ( Ships Stability )

Now here pressure force acting on the bottom is more than the pressure on the top. Since the force due to pressure is written as “F” the total force acting downward on the top surface can be given as:

FDown = PTopA = ɣ x h x A

Similarly the total force acting up from the bottom surface will be:

FUp = P BottomA = {ɣ x (h+y) x A} – {ɣ x h x A}

What Is Reserve Buoyancy? ( Ships Stability ) - Explanation of buoyancy with the pic

Now the net force acting on the object will be given as:

FNET = FUP – FDown = ɣ x y x A = ɣ x Volume Displaced ( Vd )

Thus we get the buoyant for equation.

Principle Of Flotation

According to Archimedes principle when we submerge a substance / object in fluid; there is apparent loss in its weight due to upward buoyant force.

This upward force is equal and opposite to the weight of the water it displaced. When a ship displaces water volume of more or equal weight in comparison to its own weight; the ship will float and this concept is called the principle of flotation. So, a 100,000-tonne ship must displace at least 100,000 tonnes of water to stay afloat.

There is two key force acting on a floating body, weight and buoyancy. For a ship to be stable and stay in equilibrium the center of gravity and buoyancy must be in a straight line.

Suppose the shipping weight is distributed uneven and is tilted on one side along the longitudinal axis; then the point at which the original vertical line meets the line passing through the center of gravity and buoyancy is called the metacenter.

Now the ability of the ship to return to its equilibrium position depends upon the center of gravity and metacenter. If the metacenter lies above the center of gravity; then the ship will regain its equilibrium.

But when metacenter lies below the center of gravity; the weight and buoyant force leads the ship to topple ( overturn ) causing the ship to sink. To better understand the basic law of Archimedes principle and buoyancy behind this phenomenon simply refer to my old post.

Why Reserve Buoyancy Important?

A ship at sea has to face different forces including the rolling, pitching, heaving, pounding along with the major buoyant and propelling forces. Especially on bad weather reserve buoyancy becomes a major deciding factor how easily it can sail across in tough conditions.

In such conditions the easiest way to increase the reserve buoyancy of the ship is by closing all the watertight and weather tight doors of the ship.

The reserve buoyancy of the ship is determined the capacity to which the ship can be loaded maintaining compliance to the load lines and other maritime rules.

A ship would simply sink if the loss in buoyancy due to the extra weight added to the ship unless when an excess of reserve buoyancy is available to compensate for the losses. There is a need for reserve buoyancy to account for the risks involved with accidents like grounding and collision.

When a ship undergo collision or grounding exposing one or more compartments or section of the ship. It lost a significant amount of its buoyancy thus improving the risk of collapse.

Now since the amount of load on ship remain more or less the same the reserve buoyancy decreases. Then as an emergency step captain will order to shut all watertight doors in an effort to maintain the reserve buoyancy of the ship; in short keeping it intact and continue sailing.

How To Calculate Reserve Buoyancy

For any ship, the reserve buoyancy lost is the total weight lost or the equivalent amount on water flooded in a compartment. When hull ruptures the ship lists heavily to one side making it more prone to sinking.

Watertight doors, shell plating, armored bulkheads, and segregated compartments help maintain the reserve buoyancy of that vessel. The reserve buoyancy for the damaged ship can be calculated for the following formula which then came in handy in preparing proper upright plan.

ΔRB = Δ0Δ – ρ x g x ∇fwΔsp

Where, Δ0 is the total buoyant force intact i.e not impacted by the damage, ∇fw is the volume of the water displaced in flooding and Δsp as the buoyancy of the salvage ship part or pontoon.

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