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A frame to support a hull ashore.



Related Terms

ARCH BRACE

A curved brace, usually used in pairs to support a roof frame and give the effect of an arch.

CHOCK, BOAT

A cradle or support for a lifeboat.

AIMING SCREWS

Spring-loaded screws designed to secure headlights to a support frame and permit aiming of the headlights in horizontal and vertical planes.

AUTOMOTIVE SUSPENSION

The springs and related parts intermediate between the wheels and frame of an automotive vehicle that support the frame on the wheels and absorb road shock caused by passage of the wheels over irregularities.

PANTING FRAME

The frames in the forward and after portions of the hull framing to strengthen against shell panting

CATHODIC PROTECTION

Protection of a ship's hull against corrosion by the use of impressed electric current or by sacrificial anodes.
Cathodic protection has been credited with considerable success in this field. Ships equipped with this type of protection may have some or all of their tanks fitted with a number of sacrificial metal anodes. These anodes consist of magnesium or other suitable metals. To obtain the best results the tanks containing the anodes have to be cleaned so that the steelwork is free of grease and oil residue. After cleaning, clean salt water ballast is pumped into the tank until it is full and the tank is left for several days.

The salt water sets up a galvanic couple. Galvanic current attacking the sacrificial anodes conveys a protective coating from the anode to the steelwork around it. The anodes waste away and eventually have to be replaced.

Unfortunately, cathodic protection has a number of drawbacks. It is only effective when the tanks containing the anodes are ballasted for a reasonable period of time. Obviously, this is not always possible. In certain trades, the ballast passage may be of limited duration and in any case, work has to go on, tanks have to be cleaned, dirty ballast changed, and so on. Because of this, some ship-owners and tanker companies only fit cathodic protection in the ballast tanks which fall within one of the vessel's regular ballast patterns. To protect the other tanks alternative measures are taken. Sometimes the interior of the tank is sandblasted or shot blasted to clear the interior surfaces of all scale and the special protective paints are applied.

Another factor that must not be overlooked when considering corrosion in oil tankers, is the effect of relatively high humidity or dampness in empty cargo tanks on the ballast passage. Corrosion from this source is considered to be sufficient to justify the installation of a ventilation system, the purpose of which is to pass warm dry air into the cargo tanks. The reduction in humidity automatically results in the reduction of corrosion. Vessels fitted with this type of system report that it is also an excellent means of keeping empty tanks gas free.

cathodic protection

Impressed current systems have been moving gradually towards better materials and increased efficiency. Diodes of silicon have superseded selenium in some applications and although these must be protected against static discharge, the changeover is generally justified in terms of cost. One advantage with these devices, of course, is that they can be used at high temperatures and are therefore, suitable for process plants of different kinds.

Cathodic protection is applied to most major structures where materials (metals) are exposed to adverse conditions where electrolytic action is likely to take place. Although they require a power supply, the demand is low and compared with other methods of protection the maintenance cost is small.

But, there is one potential drawback which follows as a natural corollary of the system and that is, it is as harmful to supply too large a current as it is to supply too little. Changes in temperatures, surface conditions of the metals and conductivity of the electrolyte demand that the current, and therefore the potential difference, should be changed.

It is for this reason that there has been a move towards automation in recent years, where conditions are automatically monitored and the power supply regulated to match them exactly through, for example, thyristor control system.

Automated systems of this kind are particularly applied to ships and power stations and are being used on a modest scale in these fields.

But recently cathodic protection has moved beyond the narrowly defined areas of metal corrosion and has moved into the protection of metals against other forms of attack.

CARGO BATTENS

Strips of wood fitted inside the frames to keep cargo away from hull steelwork. Also called sparring

BILGE BRACKET

A vertical transverse flat plate welded to the tank top or margin plate and to the frame in the area of the bilge.

DRAFT

The depth of the ship below the waterline measured vertically to the lowest part of the hull

CANT FRAME

A frame connected at the upper end to the cant beams (See beams, cant.)

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