Marine Engineering Study Materials

Stress on Ships

The modern ship is made up steel plating, section and builds up girders so connected as to provide adequate strength in all parts to withstand the forces acting on the ship under all condition of service.

The forces acting on a ship may be static or dynamic. The static forces are due to the difference in the weight and buoyancy, which occur through out the ship. The dynamic forces are cause by the motion of the ship at sea and the action of the wind and wave.

These forces create: 

1. Longitudinal stress
2. Transverse stress
3. Local stress

The greatest stress set in the ship as wholes are due to the distribution of load along the ship, causing longitudinal bending.

Longitudinal Stress

• The forces are two in number, the weight of the ship and all that it carries acting downwards and the vertical component of the hydrostatic pressure.
• Depending upon the direction in which the bending moment acts the ship will Hog or Sag.

Hogging

• If the buoyancy amidships exceed the weight due to loading or when the wave crest is amidships, the ship will Hog, as a beam supported at mid length and loaded at the end.

Sagging

• If the weight amidships exceed the buoyancy or when the wave trough amidships the ship will sag, as a beam supported at a ends and loaded at mid length.

Transverse Stress

• A transverse section of amidships is subjected to static pressure due to the surrounding water as well as  internal loading due to the weight of the  structure, cargo, etc.

The parts of the structure, which resist transverses, are

1. Transverse bulkhead.
2. Floor in the double bottom.
3. Bracket between deck beam and side frame, together with bracket between side frame and tank top plating, or margin plate .
4. The pillars in hole and tween deck.

Local Stress

These are created by such item:

1. Heavy concentrated load like boiler, engine etc.
2. Dead cargo such as timber
3. Hull vibration
4. Ship resting on block on a dry dock (Static Stress)

Dynamic Forces

The dynamic effects arise from the motion of the ship itself. A ship among waves as three linear motions.

1. Surging: The forward and aft linear motion (along x) of a ship is called surging.
2. Heaving: The vertical up and down linear motion (along y) of a ship is called heaving.
3. Swaying: The side to side linear motion (along z) of a ship is called swaying.
4. Rolling: The rotational motion of a ship about longitudinal axis is called rolling.
5. Yawing: The rotational motion of a ship about vertical axis is called yawing.
6. Pitching: The rotational motion of a ship about transverse axis is called pitching.

When the ship motions are large particularly in pitching and heaving, considerable dynamic forces can be created in the structure.

Panting

• As wave passes along the ship they cause fluctuation in water pressure which tends to create in and out movement of the shell plating.
• This is particularly the case at the fore end.
• The rules of the classification societies required extra stiffening, at the end of the ship, in the form of beams, brackets, stringer plate, etc. in order to reduce the possibility of damage.
• This in and out movement is called panting.

Slamming or Pounding

• In heavy weather when the ship is heaving and pitching, the fore end emerges from the water and reenters with a slamming effect which is called  pounding.
• Extra stiffening require at the fore end to reduce the possibility of damage.

Apparent Slip

• Since the propeller work in water, the ship speed Velocity will normally be less than the theoretical speed.
• The difference between the two speeds is known as Apparent slip and is usually express as a ratio or percentage of the theoretical speed.

Real slip or true slip

• This is the difference between the theoretical speed and the speed of advance, express as a ratio or percentage of the theoretical speed.
• The real slip is always positive and it dependant of current.

Speed Length Ratio used for Froude’s Law of Comparison

Above expression is known as speed length ratio.

Where,

Rr = residual resistance

L  = linear dimension of vessel

V  = speed of vessel

1. At corresponding speeds the wave-making characteristic are the same.
2. At high speed the speed length ratio is high and the wave making resistance is large.
3. A ship is therefore consider slow or fast in relation to it’s speed length ratio.

If ratio is below 1.0 the ship is said to be slow. If ratio is above 1.5 the ship is said to be fast. Thus a speed of 15 knot would be regarded as slow for a ship 225M long, but fast for a ship 100M long.

Reference:

Oral Guide by – MIN ZAR TAR