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Friday, October 18, 2013

Design of Ship Hull Structures

Spray painting

Spray painting is a painting technique where a device sprays a coating (paint, ink, varnish, etc.) through the air onto a surface. The most common types employ compressed gas—usually air—to atomize and direct the paint particles. Spray guns evolved from airbrushes, and the two are usually distinguished by their size and the size of the spray pattern they produce.
Airbrushes are hand-held and used instead of a brush for detailed work such as photo retouching, painting nails or fine art.
Air gun spraying uses equipment that is generally larger. It is typically used for covering large surfaces with an even coating of liquid.
Spray guns can be either automated or hand-held and have interchangeable heads to allow for different spray patterns. Single color aerosol paint cans are portable and easy to store.(http://en.wikipedia.org/wiki/Spray_painting)

Using a Compressed Air Paint Spray Gun

A compressed air sprayer is the best way to rapidly put down a flat, even coat of paint. The paint is mixed with compressed air, which atomizes your paint. Compressors, which are powered by either electricity or gasoline, take in air and compresses it in a cylinder, diaphragm or rotating blades, then expels it in the form of high pressure air.

There are compressors which deliver pressurized air directly to the spay gun, and other types which store it in a metal receiver tank which delivers air on an as needed basis. Either one can be used.
How does a Paint Spray Gun Work

As you pull the trigger, a needle valve opens in the nozzle tip of the gun where the paint comes out.

The nozzle tip has an air cap that directs compressed air into the stream of paint coming from the gun’s reservoir container. In a paint spray gun, paint is delivered through either gravity feed, suction feed, or pressure feed.

With the gravity feed, you have a paint container that is mounted on the top of the gun; paint drips down into the nozzle. For suction feed guns, you have a specialized air cap producing low vacuum in the fluid tip.

Paint is delivered via overall atmospheric pressure from a container on the bottom of the gun. Compressed air spray guns are of the pressure feed type, compressed air is used instead of the atmospheric pressure/vacuum system.
Getting Ready

Since overspray is inevitable, you must protect areas surrounding the surface being painted.

Give a straight line to the edges of the surface with masking tape, laying newspaper behind the edge. For larger areas, plastic tarp or sheeting can be used; staple, tape or weight it down to fix in place.

Paint should be thinned before spaying. It will smooth out better on the surface and the nozzle fluid tip will be less likely to clog up. To thin, add the proper solvent for the paint type, stirring to a smooth consistency that easily runs from the stirring stick’s surface.

Strain any older paint if it has developed a skin. Straining it through a cheesecloth several times will remove any lumps or particles.
Spraying Paint

First, adjust the gun to get the best shape and density of spray. To do this, the air control screw knob is turned clockwise to make the cone of spray narrower, or counter clockwise to widen the cone. Increasing the cone size requires adjusting the paint flow, using the fluid control screw knob.

The shape that gives the most coverage is a fan pattern. It is created by the position of the air cap horns; vertically set they produce a vertical fan, and horizontally set produce a horizontal fan pattern.

Hold the sprayer gun 8 to 10 inches from the surface to be painted.

Depress the trigger and move the gun in steady parallel passes from side to side.

At the end of each pass, direct the spray off the work onto the newspaper or plastic tarp, while at the same time releasing the trigger. This prevents paint build-up at the sides of the pass.

Give each pass a slight overlap with the previous one.

Be careful to keep the gun the same distance from the surface all through the pass. Also make sure you keep the pass parallel to the floor (it you are painting a wall, for example) throughout the pass, rather making your spray in an arc motion.
Spray Paint Problems

If your spray gun is spitting intermittently, you need to stop and fix it or you will get uneven coverage.

Spitting is caused by a needle valve with dried out packing rings. The dried out packing lets air seep into the fluid passages, or dirt to enter the fluid top seating.

Disassemble the nozzle and apply a drop or two of machine oil to the packing. With a shop rag dampened with paint thinner, clean the fluid tip, seating and spray gun body in that area. Reassemble the nozzle tip.

Spattering of paint is caused by either too much air pressure for the paint or not enough atomization. It will produce a speckled and uneven looking painted surface. You need to adjust your air pressure level to eliminate this problem.

An uneven spray pattern that deflects or is heavy on one side and light on the other is usually the fault of uneven pressure due to an air hole blockage. You need to clean out the nozzle tip as outlined above.
Clean Up

When you are finished, any remaining paint should be poured out of the container. Spray the proper thinner solvent for the paint type through the gun until it comes out clear.

The container should then be wiped clean with a rag dampened in the same solvent. Disassemble the air cap and fluid tip. Wipe them clean, making sure all air passages are clear of paint and debris.(http://www.onlinetips.org/spray-gun)

Anti-fouling paint 1

Anti-fouling paint or bottom paint is a specialized coating applied to the hull of a ship or boat in order to slow the growth of organisms that attach to the hull and can affect a vessel's performance and durability.
Hull coatings may have other functions in addition to their anti-fouling properties, such as acting as a barrier against corrosion on metal hulls, or improving the flow of water past the hull of a fishing vessel[1] or high-performance racing yacht...
In the Age of Sail, sailing vessels suffered severely from the growth of barnacles and weed on the hull, called "fouling." Thin sheets of copper or Muntz metal were nailed onto the hull in an attempt to prevent marine growth. One famous example of the traditional use of metal sheathing is the clipper Cutty Sark, which is preserved as a museum ship in dry-dock at Greenwich in England. Marine growth affected performance (and profitability) in many ways.
The maximum speed of a ship decreases as its hull becomes fouled with marine growth.
Fouling hampers a ship's ability to sail upwind.
Some marine growth, such as shipworms, would bore into the hull causing severe damage over time.
The ship may transport harmful marine organisms to other areas.[2]

The inventor of the anti-fouling paint was Captain Ferdinand Gravert, born in 1847 in Glückstadt (Schleswig-Holstein, now in Germany but then Danish), who sold his chemical formula in 1913 at Taltal, Chile.
Captain Alex Gravert has valuable documentation about this.

Modern anti-fouling paints
In modern times, anti-fouling paints are formulated with toxic copper, organotin compounds, or other biocides—special chemicals which impede growth of barnacles, algae, and marine organisms.

"Hard" bottom paints, or "non-sloughing" bottom paints, come in several types. "Contact leaching" paints "create a porous film on the surface. Biocides are held in the pores, and released slowly."[3] Hard bottom paints also include Teflon and silicone coatings, which are too slippery for growth to stick. SealCoat systems, which must be professionally applied, dry with small fibers sticking out from the coating surface. These small fibers move in the water, preventing bottom growth from adhering.[3]

following this link
http://en.wikipedia.org/wiki/Anti-fouling_paint

the manner in which a boat's hull is affected by fouling

Cleaning the hull by fouling takes money, time, labour and euipment, maily used water pressured, the process precedes the following steps for applying the bottom paint or the antifouling coating., and thsi should be done only after the ship is being docked in the shipyard. Despite the standards procedures each cleaning project is unique and should be treated likewise organizing the operations, organizind human resouces, sincronising thei efforts with suppliers and time availble for each stage.