Flash Cards
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Tonnage
Two Primary Types of Tonnage | Volume & Weight
Volumetric Tonnage | Used to determine the earning capacity of vessels, port fees, dock fees,
dry docking charges, etc
100 cubic feet = | 1 ton
Gross Tonnage | Internal volume of vessels hull from main deck down to keel with no exempted spaces
Net Tonnage | Remaining tonnage after non-earning spaces are removed from the Gross Tonnage
Non-earning spaces | Doublebottoms Forepeak and Aftpeak (if for water ballast only) Poop, Bridge and
Forecastle (if fitted with tonnage openings) Shelter deck (if fitted with tonnage
openings) Passenger spaces on the deck above the uppermost continuous deck
Other miscellaneous spaces including companionways, skylights, wheelhouses,
vents and some water closets
Deductions | Crew and working spaces, Machinery space (principle deduction)
Strength of Materials
4 general categories of steel items used in ship building | Beams, Plates, Columns, shafts.
Trochoidal Wave | Imaginary wave with crests equal to the length of the vessel and height 1/20th the
length of the vessel
Trochoidal Wave is used to | determine the maximum load on the hull girder, and scantlings required
for construction
Load | The total force acting on a structure, usually expressed in pounds or tons
Stress | The force per unit area, usually expressed in pounds or tons per square inch
Strain | The distortion resulting from stress
Tensile Stress | The resistance of a material to a force tending to tear it apart. <—o—>
Tensile Stress Formula |
Ts = Pull = P
Area
A
Compressive Stress | The resistance of a material to breaking under compression. —>o<—
Compressive Stress Formula | Cs = Pull = P
Area
A
Shearing Stress | The resistance of a material connecting two layers of material being forced to slide
along each other. <=o=>
Shearing Stress Formula |
Ss =
Pull
= P
Area of the rivet
A
Ultimate tensile strength of mild steel | 28 to 32 tons per square inch
Ultimate shearing Strength of mild steel | 22 tons per square inch
Steel flattens when compressed at about | 18 tons per square inch
Slide12 - 22
When referring to ships, the 2 very broad phases of strength | Local Strength, Hull-girder Strength
Local Strength | The strength of individual parts of a ship.
Hull-girder Strength | The strength of the ship as a whole
Class Societies | Class societies are the regulator that actually determine the scantlings of such
construction components as beams, stiffeners and shell plating. Most are insurance driven and are
based on catastrophic loss in the past.
Bending Moment | A moment of a force about any line is the product of the force times the
perpendicular distance to that line.
Example A
100 lbs is located 4’ from the end of a board imbedded in a wall
B.M. = Weight x Distance
B.M. = 100lbs x 4’
B.M. = 400 ft-lbs
Example B
100 lbs is located 8’ from the end of a board imbedded in a wall
B.M. = Weight x Distance
B.M. = 100lbs x 8’
B.M. = 800 ft-lbs
Example C
200 lbs is located 4’ from the end of a board imbedded in a wall
B.M. = Weight x Distance
B.M. = 200lbs x 4’
B.M. = 800 ft-lbs
Beam | Horizontal strength members loaded vertically
As compression and tension are opposite forces, there must be a layer in the beam where the forces are
neutral.
The zero point is located along the center of gravity (centroid) of the beam.
If a beam of similar size is made up of 5 individual layers that are free to slip over one another, the top 2
layers would not be under compression as they are allowed to slip, the bottom 2 layers are not under
tension. The middle will not be neutral.
This beam will carry only 1/5th the load of a solid or laminated beam.
The depth of a solid beam controls its resistance or strength. Another way of considering this is the
farther from the neutral axis the compression and tension edges are, the stronger the beam.
Example:
Formula:
Relative Strength=(D2/D1)2
where D1 is the depth of the smaller of the 2 beams considered.
Using a 2” x 4” and a 2” x 8” we can see
(8/4)2= 4 or the 2” x 8” beam is 4 times stronger than the 2” x 4” beam
In the beam with 5 individual members, each 1” thick, compared to a similar sized solid beam 5” thick,
the same formula applies
(D2/D1)2=Relative Strength
(1”/5”)2= 1/25th as strong for each piece
The five pieces considered together will be 1/5 as strong
Because the maximum force (compression and tension) on a beam is concentrated at the upper and
lower edge, the material of the rectangular beam can be re-distributed to the upper and lower edges, it
will gain a great deal of strength without gaining weight,
There are 5 factors that determine the size of a beam:
1) Type and amount of load on the beam
2) Distance between supports
3) Type and efficiency of end connections
4) Number of supports
5) The material the beam is constructed of.
Type and amount of load on a beam
A) Concentrated load
B.M.= WL/4
W= weight
L= distance between supports
Example: 2 tons x 10 ft/ 4 = 5 ft-tons
Type and amount of load on a beam
B) Uniform loan
B.M.= WL/8
W= weight
L= distance between supports
Example: 2 tons x 10 ft/8 = 2.5 ft-tons
Distance between supports ( often referred to as span)
A) Deflection of a rectangular free-end beam varies as the cube of the span
Example: A 10’ span has a deflection of 1”
What will be the deflection on a beam with a 20’ span?
(S2/S1)3= (20’/10’)3= 8 inches
Distance between supports
B) Strength of a rectangular free-end beam varies inversely as the span
Example: A 10’ span will support 10 tons
How much weight will a 20’ span support?
Relative strength= span A/span B
10’/20’=.5
10 tons x .5 = 5 tons
Type and efficiency of end connections
A) A fixed-ended rectangular beam will support twice as much concentrated load as a freeended beam
B) The deflection of a fixed-ended rectangular beam is 1/4th that of a free-ended beam
Effects of number of supports
The greater the number of supports in a given distance, the shorter the span. A shorter span
means a smaller bending moment
The beam in the next slide is a homogeneous material and rectangular, it is symmetrical
The ends of the beam are considered free as they are not imbedded in the wall
A load applied to the center of the beam will cause a deflection. The upper surface will shorten, the
lower surface will lengthen, and the middle will remain neutral.
The upper surface must be under compression.
The lower surface must be under tension
Material the beam is constructed of
Most ship construction is mild-steel.
Other materials used include:
Stainless Steel
High tensile steel
Aluminum
Columns | A strut placed such that it is loaded vertically (also referred to as a stanchion or pillar)
Columns are usually symmetrical (round)
Shafts | A shaft subjected to a twisting moment is said to be in torsion
The twisting moment is referred to as torque
Torque (lb-ft)= horsepower x 5252/RPM
The higher the RPM, the less torque
Example:
A 5,000 horsepower 80 RPM engine will require about the same size shaft as a 20,000
horsepower 320 RPM engine
Torque= 5,000 x 5252/80=328250 lb-ft
Torque= 20,000 x 5252/320=328250 lb-ft
Continuity of strength
Vessels must be constructed such that stresses may be gradually and continuously dissipated.
No part should be oversized or undersized
A discontinuity or change in shape will cause a concentration of stresses and may result in a
failure
Welding
fusion welding | most common
electric arc
6000 degrees F.
1930’s | welding starts to replace old method of joining material together
WWII | welding almost completely replaces other methods
Covered Electrode (welding rod) | flux provides shield (from air)
prevents brittle weld (glass)
Electrode (welding rod) | welding rod provides filler metal
Notches | main cause of cracks aboard ships discontinuity
hatch and port opening
or a faulty weld
Concentration of stresses can be reduced by | Use of Rounded corners and better resisted with
double plates
Secondary cause of fractures | low temp.
unusually high bending moments
heavy seas
Keels
Keels | backbone or spine that ties together transverse bottom members and helps distribute
loads over large area
dry-docking effect on keel | keel rests directly on keel-blocks and absorbs much of the weight of
the vessel
seaway effect on keel | keel absorbs a large portion of the stresses produced by the hull-girder
action
Four Types of Keels | bar keel
flat plate keel
box keel (duct keel)
bilge keel(not really a Keel)
Bar Keel (hanging keel) | advantage -
protects vessel when grounded
first point of contact
also helped to reduce rolling
disadvantage-
increases draft without increasing its
displacement
usually increases with increase in draft
TPI | Tons Per Inch Immersion amount of weight it takes to sink a vessel one inch at a given
draft
Flat Plate Keel | reduces draft, utilizes increased shell plate thickness allowing it to better
withstand dry-docking and grounding loads forces, extends entire length of vessel
keel is of heavier size (scantling) at mid-ship section
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longitudinal bending moments ( greatest mid-ship)
I-beam | flat plate keel + center vertical keel(son) + rider plate
Transverse Bulkheads | assist in supporting keel and bottom by transforming long flexible
girder into shorter length
Box (Duct) Keel |
act as conduit for wires and electrical cables
allows access into other areas of double bottom