Question

What is a tender ship.

A vessel with a long slow lazy rolling period.

Why is it important in a tender ship to keep the D/B tanks pressed up?

When the GM is comparatively small, for example 0.16m to 0.20m the righting moments at small angles of heel will also be small.

The ship will thus be much easier to incline and will not tend to return so quickly to the initial position.

The time period will be comparatively long and a ship, for example 25 to 35 seconds, in this condition is said to be ‘tender’.

This condition is not desirable and steps should be taken to increase the GM by lowering the effective centre of gravity of the ship.

A time period of 15 to 25 seconds would generally be acceptable for those on board a ship at sea.

Hence it is important to keep the D/B tanks pressed up.

Question

What factors influence the frictional resistance of a ship and what formulae is used to calculate this resistance.

As the ship moves through the water, friction between the hull and the water causes a belt of eddying water adjacent to the hull to be drawn along with the ship, although at a reduced speed.

The belt moves aft and new particles of water are continually set in motion, the force required to produce this motion being provided by the ship.

The frictional resistance of a ship depends upon:

(i) The speed of the ship

(ii) The wetted surface area

(iii) The length of the ship

(iv) The roughness of the hull

(v) The density of the water.

Wm Froude developed the formula:

R_f=fSVⁿ N

Where

f is a coefficient which depends upon the length of the ship

L, the roughness of the hull and the density of the water.

S is the wetted surface area in m²

V is the ship speed in knots.

n is an index of about 1.825.

Question

If resistance ά S V2 and S ά D2/3, derive the Admiralty Coefficient formula.

Coefficient, Admiralty.

A coefficient used in power estimating.

The Admiralty coefficient is the cube root of the square of the displacement in tons times the square of the speed in knots divided by the indicated or shaft horsepower.

The valve of the Admiralty coefficient is practically identical for similar ships at corresponding speeds.

Is based on the assumption that for small variations in speed the total resistance may be expressed in the form:

Question

Effect of GM on rolling

GM and rolling period

GM has a direct relationship with a ship's rolling period.

A ship with a small GM will be "tender" - have a long roll period - an excessively low or negative GM increases the risk of a ship capsizing in rough weather.

It also puts the vessel at risk of potential for large angles of heel if the cargo or ballast shifts. A ship with low GM is less safe if damaged and partially flooded because the lower metacentric height leaves less safety margin.

For this reason, maritime regulatory agencies such as the IMO specify minimum safety margins for sea-going vessels.

A larger metacentric height, on the other hand can cause a vessel to be too "stiff"; excessive stability is uncomfortable for passengers and crew.

This is because the stiff vessel quickly responds to the sea as it attempts to assume the slope of the wave.

An overly stiff vessel rolls with a short period and high amplitude which results in high angular acceleration.

This increases the risk of damage to the ship as well as the risk cargo may break loose or shift.

In contrast a "tender" ship lags behind the motion of the waves and tends to roll at lesser amplitudes.

A passenger ship will typically have a long rolling period for comfort, perhaps 12 seconds while a tanker or freighter might have a rolling period of 6 to 8 seconds.

The period of roll can be estimated from the following equation.

Where g is the gravitational constant, k is the radius of gyration about the longitudinal axis through the center of gravity and GM is the stability index.