Ship Structure Flashcards
Mid-section of a ship, identify and describe the function of the main components
must draw
Transverse bulkhead of a ship
&
Water tight hatch with Coaming
Fore deck bow arrangement. To prevent Panting
Reduce Panting
• Panting Stringers (horizontal plates welded to the sides of the vessel).
• Panting Beams (transverse beams extending from side to side).
• Transverse Bulkheads.
• Thicken Shell Plate.
• Deepen Floors
Sailing vessel
Sailing Boat, Mast, and Rigging
Fore and back stays keep the mast upright longitudinally and side stays (aka shrouds)
support it transversely. These wires must be tensioned with adjustable rigging screws to
hold the mast without bending it. The rigging acts like the strings of a bow with the mast
as the arrow, trying to force its way into the bottom of the boat. The rigging pulls upwards
wrapping the hull around the mast. The wind creates a sideways force on the mast, so the
weather shrouds (those on the upwind side) take the strain. The thrust of the sail causes
the boat to move ahead, so trying to push the mast over the bow. This is counteracted by
the back stays.
The mast is mounts on a thick keel plate which runs into a strong Stem and Stern post.
The mast mounts through a mast ring, a reinforced frame running from the keel around
the sides of the boat and across the deck so that the mast is supported at the bottom and
at deck level. The shrouds either side of the mast attach to this ring via chain plates which
spread out the load. The stays connect to the keel at the bow and stern posts in a similar
fashion.
The mast is mounts on a thick keel plate which runs into a strong Stem and Stern post.
The mast mounts through a mast ring, a reinforced frame running from the keel around
the sides of the boat and across the deck so that the mast is supported at the bottom and
at deck level. The shrouds either side of the mast attach to this ring via chain plates which
spread out the load. The stays connect to the keel at the bow and stern posts in a similar
fashion.
Diagrams & Definitions
Load/Summer Displacement
(W or Δ)
Mass of the vessel fully loaded with cargo, stores, etc.
floating at her assigned freeboard.
Reduce Racking
Reduce Racking
• Transverse Bulkheads
• Web Frames
• Beams
• Beam Knees (brackets at the corners).
Reduce Panting
Reduce Panting
• Panting Stringers (horizontal plates welded to the sides of the vessel).
• Panting Beams (transverse beams extending from side to side).
• Transverse Bulkheads.
• Thicken Shell Plate.
• Deepen Floors.
Panting stress
Panting Stress
The hull squeezes in and out like a bellows due to the fluctuating pressures on the hull as
the ship moves through waves. The effect is most pronounced at the ends when the ship is
pushing through waves and is pitching heavily. The large pressure changes at the bow and
stern over a short time create considerable stresses in these areas.
Racking Stress
Racking Stress
When a ship is rolling, the deck tends to move laterally relative to the bottom structure.
This also tends to move the shell vertically on one side relative to the other. This
deformation is called ‘racking’. Racking is usually caused by the rolling action of the vessel,
particularly in beam seas. A vessel which is very ‘stiff’ is particularly prone to racking
stresses.
Pounding Stress
Pounding Stress
Occurs in the bottom shell and framing forward when a vessel drives into head seas. It
creates a loud bang followed by vibration as the stresses transfer through the hull. Most
severe if a lightly ballasted ship slams into the trough of a wave. If the area of impact with
the water is far enough back, then it will not have the fine shape of the bow. All ships are
prone to pounding and the most vulnerable area is the forward third of the hull.
Reduce Pounding stress
Reduce Pounding stress
• Alter Course (do not drive directly into waves).
• Reduce Speed (do not fly into waves).
• Increase Scantlings (structural elements, like a honeycomb).
• Thicken Shell Plating.
• Plate Floors.
Vibration stress
Vibration Stresses
Ship hulls vibrate in sympathy with the seaway and propeller causing pulsating vibrations
to run through the vessel. Stresses re caused in local areas by machinery and over-all from
the effects of Pounding and Cavitation. This occurs when excessive aeration on the ship’s
propeller blades lead to bubbles of air becoming superheated. This leads to a pressure
difference on either side of the blade. The bubbles implode and start to quickly erode the
blades speeding up the effect of cavitation.
Reducing vibration stress
Reducing Vibration
• Alter Course (try not to drive with waves dead ahead or astern).
• Adjust Speed (do not encounter waves with the same frequency as your roll
period).
• Increase Scantlings (wherever heavier items such as engines are located).
• Fit and maintain Efficient Propellers (to reduce the effects of cavitation).
Centre Girder
1) Centre Girder: Large steel I section girder running the length of the ship and resting
on the Keel Plate. It gives longitudinal strength to the ship and helps to resist Hogging
and Sagging (note it is not the keel).
Plate Floor
2) Plate Floor: A vertical plate running across the ship which transfers weight from the
deck and pillars to the keel giving transverse strength to the structure. It supports the
bottom plating and transfers the weight of the ship into the surrounding water. Some
have Lightening holes to reduce the overall weight. They may also have Manholes for
access and Limber holes for drainage.
Hull/Shell Plating
3) Hull/Shell Plating: The skin of the ship, designed to keep the water out and to support
the structure. The upper and lower ‘strakes’ are thicker to give greater strength
where the stress from Hogging and Sagging is greatest.
Transverse Frame
4) Transverse Frame: The ribs of the ship giving support to the plating, deck and the
deck beams as well as transferring the load from the deck down to the bottom.
Beam Knee
5) Beam Knee: Supports the join between the deck beam and the transverse frame;
resists Racking and helps to transfer the load from the deck into the frames.
Deck Girder
6) Deck Girder: Supports the deck and secures the deck beams adding longitudinal
strength to resist Hogging and Sagging.
Deck Beam
7) Deck Beam: Supports the deck and transfers the load from the deck into the
transverse frames. Adds transverse strength to resist Racking.
Pillar
8) Pillar: Support the deck beam, transferring weight to the plate floor.
Stringer
9) Stringer: Horizontal longitudinal girder giving support to the frames and longitudinal
strength to the structure.
Side Girder
10) Side Girder: Support the floors and give longitudinal strength to the ship, reducing the
effects of Hogging and Sagging.
