(Ranger Hope © 2008, contains edits of material courtesy of A.N.T.A. publications)
CHECK YOUR PROGRESS NAME:
1. Explain the causes of deterioration of timber hulls and structures.
2. List the precautions to be taken to preserve timber
3. Explain the causes of corrosion in vessel structures.
4. Describe two ways of preventing corrosion.
5. Describe in general terms the surface preparation required prior to painting the surface.
6. List the survey items that require a vessel to be removed from water.
7. Describe the method of removing a vessel from the water using a patent slip.
8. Describe the method of removing a vessel from the water using a graving dock.
9. Describe the method of removing a vessel from the water using a floating dock.
10. Describe the method of removing a vessel from the water using a Synchrolift.
11. Describe the method of removing a vessel from the water using a travel lift.
12. Describe the method of removing a vessel from the water by careening.
13. Describe the method of removing a vessel from the water by heaving down.
14. Describe the general precautions to be observed in a dry dock.
15. List the undocking checks that should be carried out prior to returning a vessel to the water.
16. Explain the difference between preventative and corrective maintenance.
In the following questions, circle the most correct answer.
17. Saltwater encourages corrosion because
a) it is an excellent conductor of electricity
b) it is denser than fresh water
c) it dissolves paint
18. Sacrificial anodes are usually found
a) near the propeller
b) inside cargo holds
c) on the main deck
19. The drying of paint can be retarded by
a) high temperatures
b) low humidity
c) pollutants in the air
20. Propellers and rudders are inspected during
a) annual surveys
b) two yearly surveys
c) four yearly surveys
21. Life saving and fire-fighting appliances are surveyed during
a) annual surveys
b) two yearly surveys
c) four yearly surveys
22. Anchors and cables are surveyed during
a) annual surveys
b) two yearly surveys
c) four yearly surveys
23. When a vessel is removed from the water using a graving dock, the critical moment for stability occurs
a) just before the water level starts falling
b) just before the vessel settles on the keel blocks
c) just after the dock is completely dry
24. Prior to dry-docking, the dockmaster should be informed about
a) the vessel’s last port of call
b) the location of echo sounder transducers
c) the vessel’s maneuvering characteristics
25. One advantage of planned maintenance is that
a) it involves less paperwork
b) there are fewer breakdowns and repairs
c) the vessel has to be dry-docked less often
1. Deterioration of timber is caused by fungal attack and marine insect attack.
2. The following precautions will keep the risk of fungal and insect attack to a minimum.
· Ensure good ventilation throughout the boat, particularly when it is lying idle.
· Make sure rainwater cannot get in.
· Prevent condensation by ventilation. Where it is unavoidable eg. on insides of windows, use water-repellent preservative on woodwork.
· Use a water soluble preservative in the bilge water. A cheap and effective one can be made by dissolving 0.65 Kg of borax and 0.45 Kg of boric acid in 4 litres of hot water. This mixture is non-corrosive and harmless to animals.
· Inspect the vessel’s timbers for decay regularly, at least every 6 months. If decay is found act at once, a few weeks in summer is enough for major damage to be done.
· Use a preservative from a variety of preservatives that have been developed for the successful treatment of timber for decay resistance.
· Use a proprietary poison for extermination of marine insects.
3 Chemical corrosion is the attack of metals by solutions of acids or alkalines which will chemically combine with the metal to form entirely new products. The material can be considered as being dissolved in the solution. Such attack is usually caused by spillage of liquids such as battery acids, galley refuse, or in toilet areas.
Electrochemical corrosion is the most common type of corrosion. It is caused by very small electrical currents flowing between one metallic area to another. These electrical currents cause the material which is being corroded to change to a completely different substance; for example, steel changes to rust.
4. a). By providing a piece of material which will corrode in preference to the vessel. Such a substance is usually found attached to the hull near the propeller or attached inside a tank, in the form of a sacrificial anode.
b). By coating the surface with a substance such as paint. Paints prevent the electrolyte from coming into contact with the metal.
5. In general, surfaces can be prepared for painting by:
· Cleaning and, where necessary, degreasing the surface.
· Removal of corrosion products (rust) and/or defective paint coats by abrasive blast cleaning, mechanical or hand cleaning.
· Using rust converters/inhibiters.
· Roughening of the old paint where needed.
· Removal of dust.
· Drying the surface.
6. Inspection of hull externally, sea cocks and valves, bilge injection valves and overboard discharge valves, propellers, rudders and underwater fittings, screw and tube shafts.
7. The patent slip is basically a sloping, reinforced concrete runway which extends well below the low water mark. On the slip itself is built a set of railway tracks set well apart. Wheeled carriages run on these tracks and depending on the size of the vessel being dry docked, carriages can be linked together to form a single unit. Cradles are fitted onto these carriages with keel blocks on the centre line atop the carriage. The entire assembly is made up to suit the vessel being dry docked.
The vessel is maneuvered onto the cradle under its own power and is secured with “springs”. As the vessel settles onto the cradle bed, wedges are inserted to keep the vessel upright. The entire assembly is slowly winched up the slipway. As the vessel takes to the keel blocks, securing beams are drawn tight and any shores, if required, are fitted. The vessel now secure in its cradle on the carriage is slowly winched out of the water.
8. The graving dock is excavated from the land and closed to the sea by means of a large watertight door or gate known as the “Caisson Gate”. The edge of the dock bottom beneath the gate is referred to as the sill. The dock bottom has a very rigid construction and is usually made of reinforced concrete. The dock bottom always has a slight slope towards the sill to aid drainage. The sides of the dock are usually terraced to enable side shoring.
Along the centre line of the dock are blocks of concrete topped up with timber. They form the keel blocks. Two parallel rows of blocks on either side form the bilge blocks or side blocks. Depending on the size of the vessel and the shape of the underwater hull, the blocks are repositioned to suit the particular vessel.
On the sides of the dock at ground level are rails on which winches travel along the length of the dock. Wires from the winches are used, two on the forward beam and two on the after beam, to help position the vessel over the keel blocks when the vessel is brought in. Cranes are used for heavy lifting.
When the vessel is in position the lock gates are shut and pumping out commences. A diver may be employed to ensure that the vessel’s keel is in line with the keel blocks. As water is pumped out the diver keeps checking that the vessel is taking to the blocks as planned. Sometimes blocks are shifted so that maintenance can be done on a sea chest valve, drain plug, etc.
9. The basic structure of the floating dry dock is a very strong and rigid double walled “U”. The bottom is constructed very similar to the bottom structure of ships. The sides of the dock are vertical wing tanks. Keel blocks and bilge blocks are laid on top of the double bottomed structure. The whole dock forms a floating, watertight structure which can be submerged by flooding the double bottom and wing tanks.
The vessel to be dry docked is simply floated into the dock and positioned above the keel and bilge blocks by use of mooring lines. Shores are fitted to provide support and as the dock tanks are pumped out the dock rises until the pontoon deck is dry.
10. The vessel is floated in over a submerged platform and is then lifted clear of the water by raising the platform. The synchrolift is a land-based platform which is lowered into the water by a series of synchronised winches lining either side of the dock.
11. A narrow dock is excavated and then opened to the sea. The vessel to be lifted manoeuvres slowly into the dock and secured temporarily with mooring lines while a mobile straddle carrier is positioned above the vessel. Broad slings which will eventually distribute the weight of the hull are then put in place. The weight is taken up by the slings. The moorings are released and the vessel is lifted clear of the water. The straddle crane, under its own power, carries the slung vessel to a suitable position in the shipyard, where it is lowered on blocks and shored and the slings removed.
The main advantage of this system is that many vessels an be docked at the same time and the slipping facility is not laid up for the duration of the vessel’s stay.
12. This method does not require a slipway or dry dock, so it is suitable for repairs in an isolated area or in an emergency. The only requirement is a tidal range greater then the vessel’s draught.
The vessel is driven to a flat, cleared section of the beach or river bank and positioned parallel to the shore or bank, to give even support along its length as the water level falls and rises. The bank should not be too steep, and must be clear of obstructions. The vessel must fall up hill if flooding on the incoming tide is to be avoided. It may be positioned between poles driven into the bed or simply weighted to fall up hill. Hawser lines may be tied to solid sections of the vessel, eg. the foot of a mast, and secured to points on-shore to help prevent the vessel from falling downhill. When the water level is low enough, shoring is installed on the downhill side to prevent rolling over.
13. In this method, a vessel is heeled over, while afloat, by means of tackles set up between its masts and another ship, or shore attachments. This method is not as successful as careening in exposing the hull, but since the vessel is afloat, there is little hull stress, and the dangers, through touching the bottom, or damage to the hull and the intakes, are minimal. It must be remembered that by heeling a vessel you increase its draft, and you should be sure that there is sufficient under-keel clearance for the job.
· Echo Sounder, Log, and Sonar transducers should be covered with grease and then masking tape to prevent them from being painted over.
· Remove drain (docking) plugs from all tanks that need to be drained. Put them in a safe place and keep a written record of which plug goes where. Ensure that plugs are all replaced prior to flooding the dock or entry into the water.
· Ensure that safe access is provided to and from the vessel.
· Ensure that fire safety precautions are adhered to.
· Ensure that all tanks, void spaces etc are opened, vented and ready for inspection by surveyors at the appropriate time.
· Ensure that all pollution control requirements are met. (Remember that sewage and garbage cause pollution too.)
15. Ensure that:
· all docking plugs have been replaced
· all intake gills/grates have been replaced
· all transducers are uncovered and wiped clean
· all tanks are boxed up (manhole/inspection covers are replaced)
· anchors are secured
· all loose gear is secured
· new paint is dry to manufacturer’s specifications
· there is sufficient water depth to unslip
· there is no distribution of weights on board that would cause the vessel to take up an unwanted list or trim when it takes to the water.
· preferably the vessel should be in the same stability condition for undocking as it was at docking.
16. Preventative maintenance is aimed at preventing failures or discovering a failure at an early stage. Corrective maintenance is aimed at repairing failures that were expected, but were not prevented because they were not critical for safety or economy.