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Characteristics of Marine paints that protect ships hull against corrosion at sea
Paint characteristics onboard
Paint consists of pigment dispersed in a liquid referred to as the ‘vehicle’.
When spread out thinly the vehicle changes in time to an adherent dry film.
The drying may take place through one of the following processes.
(i) When the vehicle consists of solid resinous material dissolved in a
volatile solvent, the latter evaporates after application of the paint, leaving
a dry film.
(ii) A liquid like linseed oil as a constituent of the vehicle may produce a
dry paint film by reacting chemically with the surrounding air.
(iii) A chemical reaction may occur between the constituents of the
vehicle after application, to produce a dry paint film. The reactive ingredients
may be separated in two containers (‘two-pack paints’) and mixed
before application. Alternatively ingredients which only react at higher temperatures
may be selected, or the reactants may be diluted with a solvent so
that the reaction occurs only slowly in the can.
Corrosion-inhibiting paints for application to steel have the following
vehicle types:
(1) Bitumen or pitch: Simple solutions of bitumen or pitch are available
in solvent naphtha or white spirit. The bitumen or pitch may also be
blended by heat with other materials to form a vehicle.
(2) Oil based :These consist mainly of vegetable drying oils, such as linseed
oil and tung oil. To accelerate the drying by the natural reaction with
oxygen, driers are added.
(3) Oleo-resinous :The vehicle incorporates natural or artificial resins
into drying oils. Some of these resins may react with the oil to give a
faster drying vehicle. Other resins do not react with the oil but heat is
applied to dissolve the resin and cause the oil to body.
(4) Alkyd resin: These vehicles provide a further improvement in the
drying time and film forming properties of drying oils. The name
alkyd arises from the ingredients, alcohols and acids. Alkyds need
not be made from oil, as an oil-fatty acid or an oil-free acid may be
used.
(5) Chemical-resistant :Vehicles of this type show extremely good resistance
to severe conditions of exposure. As any number of important
vehicle types come under this general heading these are dealt with
individually.
(a) Epoxy resins :Chemicals which may be produced from petroleum
and natural gas are the source of epoxy resins. These
paints have very good adhesion, apart from their excellent
chemical resistance. They may also have good flexibility and
toughness where co-reacting resins are introduced. Epoxy
resins are expensive owing to the removal of unwanted side
products during their manufacture, and the gloss finish may
tend to ‘chalk’ making it unsuitable for many external decorative
finishes. These paints often consist of a ‘two-pack’ formulation,
a solution of epoxy resin together with a solution of cold curing
agent, such as an amine or a polyamide resin, being mixed
prior to application. The mixed paint has a relatively slow curing
rate at temperatures below 10 °C. Epoxy resin paints should not
be confused with epoxy-ester paints which are unsuitable for
underwater use. Epoxy-ester paints can be considered as alkyd
equivalents, as they are usually made with epoxy resins and oilfatty
acids.
(b) Coal tar/epoxy resin: This vehicle type is similar to the epoxy resin
vehicle except that, as a two-pack product, a grade of coal tar pitch
is blended with the resin. A formulation of this type combines to
some extent the chemical resistance of the epoxy resin with the
impermeability of coal tar.
(c) Chlorinated rubber and isomerized rubber: The vehicle in this case
consists of a solution of plasticized chlorinated rubber, or isomerized
rubber. Isomerized rubber is produced chemically from natural
rubber, and it has the same chemical composition but a different
molecular structure. Both these derivatives of natural rubber
have a wide range of solubility in organic solvents, and so allow a
vehicle of higher solid content. On drying, the film thickness is
greater than would be obtained if natural rubber were used. High
build coatings of this type are available, thickening or thixotropic
agents being added to produce a paint which can be applied in
much thicker coats. Coats of this type are particularly resistant to
attack from acids and alkalis.
(d) Polyurethane resins: A reaction between isocyanates and hydroxylcontaining
compounds produces ‘urethane’ and this reaction has
been adapted to produce polymeric compounds from which paint
film, fibres, and adhesives may be obtained. Paint films so produced
have received considerable attention in recent years, and since
there is a variety of isocyanate reactions, both one-pack and twopack
polyurethane paints are available. These paints have many
good properties; toughness, hardness, gloss, abrasion resistance,
as well as chemical and weather resistance. Polyurethanes are
not used under water on steel ships, only on superstructures, etc.,
but they are very popular on yachts where their good gloss is
appreciated.
(e) Vinyl resins :Vinyl resins are obtained by the polymerization of
organic compounds containing the vinyl group. The solids content
of these paints is low; therefore the dry film is thin, and more
coats are required than for most paints. As vinyl resin paints have
poor adhesion to bare steel surfaces they are generally applied
over a pretreatment primer. Vinyl paint systems are among the
most effective for the underwater protection of steel.
(6) Zinc-rich paints: containing metallic zinc as a pigment in sufficient
quantity to ensure electrical conductivity through the dry paint
film to the steel are capable of protecting the steel cathodically. The
pigment content of the dry paint film should be greater than 90 per
cent, the vehicle being an epoxy resin, chlorinated rubber, or similar
medium.
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Ropes are made of short fibres that are spun into yarns,
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And the strands are spun or braided to make the finished
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Synthetic man-made ropes and hawsers
Although natural fibre ropes are still widely used throughout the marine industry, they have been superseded by synthetic fibres for a great many
purposes. Not only do the majority of synthetic ropes have greater strength
than their natural fibre counterparts, but they are more easily obtainable
and at present considerably cheaper.
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Natural fibre rope
All natural fibre rope is manufactured from manilla, sisal, hemp, coir, cotton
or flax fibres.The process of manufacture consists of twisting the fibres
into yarns and turning the yarns in an opposite direction to establish the
strands.
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Lay of Ropes and hawsers - Small Stuff descriptions
The lay of rope is a term used to describe the nature of the twist that produces the complete rope .The most common form of rope at sea is known as ‘hawser laid rope’ comprising three strands laid up right- or lefthanded.
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- Stresses in ship structures and how to mitigate
Heavy weights tend to cause a downward deflection of the deck area supporting the load .This subsequently produces stresses, with
consequent inward and outward deflections of supporting bulkheads,
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- Anchoring safe practice
Prior approaching an area for anchoring ships master should investigate fully a suitable anchoring position and conduct a planned approach including speed reduction in ample time and orienting the ships head prior anchoring to same as similar sized vessels around or stem the tide or wind whichever is stronger . Final decision to be made on method of anchoring to be used , the number of shackles , the depth of water, expected weather and holding ground. .
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- MacGregor single-pull weather-deck hatch cover
Hatch covers are used to close off the hatch opening and make it
watertight. Wooden hatch covers, consisting of beams and boards over
the opening and covered with tarpaulins, were once used but are no
longer fitted. Steel hatch covers, comprising a number of linked steel
covers, are now fitted universally. Various designs exist for particular
applications, but most offer simple and quick opening and closing,
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- Cargo holds access arrangement
The access shall be separate from the hatchway opening, and shall be by a stairway if possible. A fixed ladder, or a line of fixed rungs, shall have no point where they fill a reverse slope
.
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Washing is always carried out after the compartment has been swept. Drying time for washed compartments must be allowed for, before loading the next cargo; this time will vary with the climate, but two to three days must be expected.
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- Strength and stability of the Lifting appliances
The vessel's structure, crane, derrick or other lifting device and the supporting structure should be of sufficient strength to withstand the loads
that will be imposed when operating at its maximum load moment .
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- Lifting appliances - Maintenance, testing, controls & safety measures
When there is any suspicion that any appliance or item of
equipment may have been subjected to excessive loads, exceeding the Safe
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should be taken out of service until it can be subjected to a thorough
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- Safe operation of Lifting appliances and gears
All lifting operations must be properly planned, appropriately
supervised and carried out to protect the safety of workers.
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- Derricks for lifting cargo on board
Derricks for lifting cargo on board is required to be of adequate strength and stability for each load, having regard in particular to the stress induced at its mounting or fixing points , securely anchored, adequately ballasted or counterbalanced and supported by outriggers
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- Deck cranes
Deck cranes have a number of advantages, the rigging
time being negligible, and the crane is able to pick up and land permitted
loads anywhere within its working radius. The safe working loads of cranes
is generally of the order of 10 to 15 tonnes and larger cranes are available
capable of lifts from 30 to 40 tonnes..
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- Characteristics of Marine paints
Paint consists of pigment dispersed in a liquid referred to as the ‘vehicle’.
When spread out thinly the vehicle changes in time to an adherent dry film.
The drying may take place through one of the following processes..
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- Protection by Means of Paints
It is often assumed that all paint coatings prevent attack on the metal
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- Role classification societies maintaining seaworthiness of vessels
classification societies publish rules and regulations which are principally concerned with the strength of the ship, the provision of adequate equipment, and the reliability of the machinery .
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