(a) Explain why high tensile steel may be used in area of high stress.
(b) State the advantages of the use of aluminium alloys in the construction of superstructures
(a) Use of High Tensile Steel
- Steels having higher strength than mild steel
- High tensile steel use in more highly stressed regions of large tankers, container ships and bulk carries.
- Higher tensile steels used for hull construction purposes manufactured and tested in accordance with classification requirements.
- Use of higher strength steels allows reductions in thickness of deck, bottom shell, and framing fitted in midships portion of larger vessels.
- Used in constructing masts and rigging fittings, desirable to reduce scantlings.
- Weld-ability of higher tensile steels is an important think their application in ship structures.
The higher tensile steels distinguished by grade mark followed by letter H. i.e.
AH for grade A application
DH for grade D application
EH for grade E application
(b) Advantages of the use of Aluminium Alloys
- Use of aluminum alloys is
- More advantages than use of steel
- About one-third of weight of steel for same volume of material.
- Reduce of 60% of weight of steel for same structure.
- This reduction in weight
- Improve stability of vessel, mainly in upper region of structure
- Lowering centre of gravity, Increased metacentric height.
- Pure aluminum has a low tensile strength and little use for structural purposes.
- Corrosion resistance of aluminum is good, but careful maintenance and insulation from joining steel structure
- Pure metal is alloyed with small percentage of other materials to give higher tensile strength.
- For shipbuilding purposes, alloys with 3% to 5% are commonly used.
Define the deck of a vessel and sketch for the followings.
(a) Deck transverse
(b) deck plating plan
- Decks at different levels for various function, they may be either watertight decks, strength decks, or cargo and passenger accommodation decks.
- Watertight decks fitted to maintain the watertight integrity of the hull
- Most important is freeboard deck uppermost continuous deck exposed to weather & sea which permanent closing all opening
- Non-watertight decks may be fitted to provide platforms for passenger accommodation and cargo loading.
- Weather decks usually cambered. Horizontal decks fitted in some ships, particularly if carried containers and desire regular cross-sections
- Strength deck forms the upper flange of the main hull girder
- Short lengths of internal deck or flats are horizontal as a rule.
(a) Deck Transverse
- Decks arranged in plate panels with transverse or longitudinal stiffening, and local stiffening in way of any openings.
- Longitudinal deck girders support the transverse framing.
- Deep transverses support the longitudinal framing as shown in fig.
- The purpose of plating is to keep out water and tie together the ship’s frame work.
- Resisting longitudinal bending stresses, stronger amidships than the ends, particularly at the deck and bottom.
- In long ship, shearing stresses occur near neutral axis
- To strength hull, at about half-depth of ship, in neighborhood of one-quarter of length from each end.
(b) Deck plating plan
- Figure shown is simplified partial plan of deck plating
- Plans show all the plates in the hull, drawn to scale, many other details, including frames, floors, deck edges, stringers etc
Describe discontinuities in the vessel structure and sketch
(a) Forecastle deck plating break
(b) poop deck plating break.
Discontinuities in the vessel structure
- A discontinuity refers to any break or change in section, thickness or amount of plating material.
- Compensation made for any discontinuities in shell or deck plating resulting from doors, hatchways, etc.
- Compensation is of particular importance where loss of longitudinal material results.
- Where changes in amount of plating material occur at bulwarks, the change gradual and well radiused.
- All openings have well radiused corners and doubling plates or thicker insert plates may be fitted.
- Any sharp corner produce a notch, after stressing, could result in a crack.
(a) Forecastle deck plating break
- Ends of superstructures are major discontinuities in the ship structure.
- Side plating forming part of the superstructure must well radiused at the ends towards the side shell.
- Longer structures such as bridges and forecastles require considerable strengthening at the ends.
- Classification society rules require, increase in thickness -upper deck sheerstrake by 20% & Deck plating at superstructure ends
- Classification society rules, all ships fitted with a forecastle or provides a minimum bow height.
- Side plating of the forecastle, continuation of the shell plating, is thicker than end plating.
- Adequate arrangements for stiffening of the forecastle plating provided.
(b) Poop deck plating break
- The poop front adequately plated and stiffened as for the bridge front.
- Internal stiffening provided by webs and partial bulkheads as required, particularly where deckhouses located above.
- After end of poop, being exposed, requires a more substantial construction than that of the aft ends of other structures.
Describe the following terms of structural members in ship construction andsketch the respective drawings.
(a) Keel
(b) centre girder
(c) collision bulkhead
(d) sheer strakes
(a) Keel
(1) It is the back bone of the ship and principal member of a ship’s construction.
(2) It runs along the centre line of the bottom structure.
(b) Centre Girder
- It is the girder situated in the centre of transverse section lying in fore and aft longitudinal position to make separate port and starboard side double bottom tanks.
(c) Collision bulkhead
- It is foremost major watertight bulkhead, which extends from bottom to the uppermost continuous deck.
- Location not less than 5% and not greater than 8% of the ship’s length from the fore end of the load waterline.
- To limit entry of sea water in event of head-on collision.
(d) Sheer strakes
- It is the upper strake of plating adjacent to the strength deck.
- Sometimes called “top strake”.
- Being in a highly stressed region, it has a greater thickness than other strakes of side shell plating.
- It is necessary to avoid welded attachments to this strake or cutouts which would introduce stress raiser
(a) Draw the simple shell expansion plan.
(b) With reference to your plan, indentifying the plating.
(c) Regarding the deck plating plan, How to arrange the corner of hatch way to release the stress.
(a) Simple shell expansion plan
- Strakes of shell plating distinguished by letters from keel outwards and plates in each strake are usually numbered from aft to forward.
- E.g. plate D5 would be the 5th plate from aft in the 4th strake from the keel.
(b)
Garboard strake
1. Garboard strakes are strake immediately adjacent to keel on either side of keel.
2. Garboard strake being strake “A” in figure.
Stealer Plates
1. At end of ship, width of plates is decreased.
2. To save making plates too narrow at end of ship, it is usual to run two adjacent strakes into one. This is done by a stealer plate.
3. As shown in figure, Plate B4 is a stealer plate, since it runs B and C strakes into each other.
4. Stealer and plates beyond it, always take name of lower strakes number.
Sheer strakes
Sheer strikes are the upper strakes of shell plating on either side, next to the upper deck. They are Strakes J.
(c)
- At corner of hatch way, insert plate is fitted to release the stress.
- At the radiused corners, insert plating at corners radiused or elliptical.
Sketch and describe arrangement of funnel uptakes for motor vessel giving details of method of attachment of funnel and hold support is provided
Funnel uptake
- Funnel form outer casing of thickness 6 mm to 8 mm steel plates, stiffened internally by angles or flat bars running vertically
- Funnel connect to deck by boundary angle, free end on top stiffened by molding, half round in section
- Support given by means of wire stays, attached by lugs to funnel and deck capable tightened by means of rigging screws
- Access is water tight door, open from both sides
- Platform about 1 m high inside funnel
- Uptake pass through holes cut in platform without any connection, have sliding ring arrangement ot permit expansion
- To care of expansion bellows fitted
- Uptake end at top funnel connect to upper platform by means of angle iron or ring
- Silencer fit to engine uptake support on its own seat
- Ladders and grating provide access for maintenance and inspection
