This section should explain the basic elements of the different stability
matters and enable the user to understand the principle and effect of
free liquid surface its importance and practical precautions to avoid free
surface as much as possible understand GZ and its importance know how to read
stability curve know how G moves when mass is moved on board calculate GZ at
small and large angles of he'll have knowledge of the general stability
requirements if the ship heels the center of buoyancy
B moves to be one because the shape of the underwater hell is changing the
force working at B one is still vertical and straight through the metacenter m on
the line drawn between B 1 and n we now get a new point called Z Z is placed for
a straight angle from the line b 1m through g the distance between G and Z
is a measurement of the forces trying to keep the ship from healing
when adding a mass which is not in the centerline of the ship the ship will
heal due to the heeling moment created by the added mass the heeling moment is
dependent on the weight of the mass and its distance on the ship's centerline G
moves towards the added mass when moving a mass so to the centerline of the ship
the ship will heal due to the heeling moment created by the moved mass the
heeling moment is dependent on the mass and moved distance the writing lever GZ
is the horizontal distance between vertical forces acting through b1 and G
the stability depends upon the length of GZ and the buoyancy force how to
calculate GZ at small angles of heel this is a typical GZ curve for a
specific ship in one certain condition to find the GZ value at one angle of
heel we use the following procedure
start at a given angle of heel so straight up until we meet the GZ curve
we can read the GZ value by going horizontally out to the axis where the
GZ values are written in meters if we increase the distance between m and G we
will see the bits have a great effect on improving the GZ values the result is
better stability if the ship has a large GM value the ship will roll rapidly the
movements can be very rough cargo and equipment may become an lashed in
extreme conditions much damage could be done to the ship and crew a GM that
isn't very high or too small causes less strain on the cargo and equipment
lashings and the ship in general the rolling is slower and the movement more
gentle a small GM will result in little strain on the cargo and equipment
lashings and bulkheads the rolling is very slow and the movement is very nice
and even a negative GM may result in a constant
list if exposed by an external force this is because G was higher than M but
after healing the GM might be positive again due to the changed follow you with
the underwater hell a negative GM may also result in capsizing G is higher
than M and GM will not be positive before after the capsizing at large
angles of heel the metacenter will move out of its original position and we will
have to use a more complex formula to calculate the writing lever G Z this is
simplified by the following formula when moving a mass already on board G
most parallel to the movement of the mass
when calculating heavy lift the principles are the same of when adding a
mass the arm used to calculate the heeling moment is now the same as a
transverse distance from the center line to the end of the lifting gear when the
surface of a liquid is free to move the effect is that G raises towards M G M
decreases the liquid will of course always move to the lowest side the
effect is mainly dependent of the breaths for space where liquid is
present this is because the free surface effect is proportional to the breaths
with one longitudinal bulkhead the effect of the free surface is reduced to
one force with two bulkheads the effect is reduced to 1/9 the more bulkheads the
less effect practical precautions in order to reduce the effect of free
surface minimum number of daily service tanks in use where possible tanks either
full or empty bilges pumped out freeing ports side kettles scuppers etc clear
and unobstructed the healing moment caused by free
surface is very dependent of the shape of the tanks in square tanks the effect
is largest at approximately 50 percent full these are the general stability
requirements that are related to the topics dealt with in this module