Modern Seamanship

1. CHAPTER III
MECHANICALAPPLIANCES
ON SHIPBOARD
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2. MECHANICAL
APPLIANCES
• Mechanical appliances refer to
the machinery, tools, and
equipment that are used on
board ships to facilitate various
tasks, such as cargo handling,
anchoring, and mooring,
among others. These devices
use mechanical principles to
provide force, motion, or
control.

3. The Composition and
Resolution of Forces
• The derivation of a single resultant from
several distinct forces is called “The
composition of Force”
• It is found also that we may reverse the
above process, and from a single given
force, find two or more forces acting along
given lines which may be substituted for
the one original force without any change
in the effect produced.
• This process is called "The Resolution of
Forces" and the forces resulting from it are
"Components" of the original force from
which they are derived.

6. • The two triangles are similar in the sense of having the
same relations on the sides as between the corresponding
sides of the other. This means that the tension on the
topping lift is the same proportion of o c, the downward
pull of w and that the length of the toping-lift is of the
mast. This proportion is expressed by the following
proportion.
Tension on Topping-lift = length of topping-lift
Weight Length of Mast
• The rule applies when the boom is stepped at a distance
from the mast, extending the line of the boom until it
intersects the mast and using the lengths given. (fig. 5,
Plate 28)
Similarly, we may deduce the rule:
Thrust on boom = Length of boom
Weight Length of mast
• The lengths of the mast and boom are derived from the
triangles of Figures I and 5.

8. • The most important idea is that for a fixed
length of mast and boom, we cannot change
the thrust on the boom by any change in its
angle, but we can vary the tension on the
topping lift within wide limits.
• This tension depends upon the length of the
topping lift and grows less, and less topping
lift is topped up until it is as nearly vertical
as it can be made.
• A short boom, nearly level, gives a minimum
strain on the boom and a maximum strain on
the lift, while a long boom, well topped up,
gives a maximum strain and a minimum
strain.

9. • Suppose the boom is half along as the mast.
• The thrust on it will be one-half the weight, no matter
what the angle of the boom may be.
• The tension of the topping lift will be about 1 1/4 times
the weight when the boom is level and rather less than
3/4 of the weight when the boom is topped up to 45
degrees.
• In figure, I, b, plate 25, the tension on the guys are
found by this method.
• If we vary the lead of the guy, we shall find that, by
bringing it closer to the mast, as in figure 3, we increase
the tension on it, exactly as we do in the case of the
topping lift when we bring this down the mast and make
it fast only a little distance above the heel of the boom.

10. • There is an important difference between the case in which
the derrick is used with a fixed elevation and that in which it
is to be topped up or lowered with weight hanging from it.
• In the first case, the only demand upon the parts of the
topping lift is that arising from the downward pull of the
weight, this pull is resolved along the line of the topping lift
as above described.
• In the second case, there is added to this an important
percentage due to the resistance of the friction, as is
explained in the chapter on tackles.

12. THE SPAN
• Supposed first that this is rigged between two masts
for plumbing a hatch. If the two parts of the span
make equal angles with the vertical, they will bear
equal strains; Otherwise, the one which hangs more
nearly vertically takes the greater strains.
• The span can be calculated using the following
formula:
• Span = Load / Load Capacity x 100%
• Load capacity refers to the maximum load that the
crane or lifting device can handle.
• The load is the weight of the load being lifted.

13. Factors affecting the span:
• The weight of the load and the distance between the attachment
points are the primary factors that affect the span.
• Other factors that can affect the span include the size and strength
of the wire rope or rope, the angle of the rope or wire, and the
weather conditions.

14. THE LEVER
• Another important principle of mechanics that
are involved in work on ship-board is that of the
lever A lever is a rigid bar movable about a fixed
point called the fulcrum
• The lever ratio is the ratio of the distance from
the fulcrum to the effort to the distance from the
fulcrum to the load.
• The lever ratio can be calculated using the
following formula:
Lever Ratio = Distance from Fulcrum to Effort /
Distance from Fulcrum to Load

15. TYPES OF LEVERS
• There are three types of levers: first class, second class, and third class.
• In a first-class lever, the fulcrum is located between the effort and the load.
Examples of first-class levers on board ships include the steering wheel and the
brake lever.
• In a second-class lever, the load is located between the fulcrum and the effort.
Examples of second-class levers on board ships include the anchor windlass and
the cargo winch.
• In a third-class lever, the effort is located between the fulcrum and the load.
Examples of third-class levers on board ships include the oar and the rudder.

16. THE TACKLE
• There is no mechanical appliance of
greater importance than the tackle. It is
important to note that in all mechanical
appliances by which power is multiplied,
the gain in power is purchased by a
proportional sacrifice of speed.
TYPES OF TACKLES:
• There are two types of tackles: single and
double.
• A single tackle consists of one rope or
chain and one or more pulleys.
• A double tackle consists of two ropes or
chains and two or more pulleys.

17. Calculation of mechanical
advantage:
• The mechanical advantage of a tackle is the ratio
of the load lifted to the effort applied.
• The mechanical advantage can be calculated using
the following formula: Mechanical Advantage =
Load / Effort
• The more pulleys a tackle has, the greater the
mechanical advantage.