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I-Beam. A rolled shape, generally of mild steel, having a cross section shaped like the letter I. In ship work it is used for bulkhead stiffeners, girders, etc. The size is denoted by dimensions of cross section and weight per running foot.

Ice Lining. A term applied to doubling plates fitted on the bow of vessels at the waterline for the purpose of protection against ice.

Impedance. The apparent resistance in ohms of an alternating current circuit, i. e., it is that quantity which, when multiplied with the total current in amperes, will give the impressed e. m. f. in volts. It is a term applied to alternating current circuits and is due to resistance and inductance, to capacity and resistance or to all three.

Impulse Reaction, Turbine. See Turbine, Impulse Reaction..

Impulse Turbine. See Turbine Multiple Stage, Impulse; Turbine Single Stage, Impulse.

In and Out System. See Plating, In and Out System.

Inboard. Towards the center; within the vessel’s shell and below the weather decks.

Inboard Profile. A plan representing a longitudinal section through the center of the vessel, showing heights of decks, locations of transverse bulkheads, assignment of various spaces and all machinery, fittings, etc., located on the center or between the center and shell on the port side.

Incandescent Lamp. See Lamp, Incandescent.

Inclining Weights. Known weights placed on board a vessel for use in obtaining a slight list when performing an inclining experiment.

Inclinometer. A sensitive, accurate instrument, invented in 1842, with magnetic needle and graduated ring dial, for measuring the force of magnetic attraction exercised by the earth, also the direction of the attraction relative to the plane of the horizon.

Increaser. See Reducer.

Independent Piece. A timber bolted to the forward part of the stem above the water line.

Indicated Horsepower. See Horsepower, Indicated.

Indicator. An instrument designed to measure and record the variation in the cylinder pressure of steam engines, pumps, etc., throughout the entire length of stroke.

Indicator Card. A diagram showing the variation in pressure in steam cylinders or pumps throughout the entire stroke. This diagram is made upon cards or paper by the indicator pencil.

Indicator Cock. A cock located in the indicator pipe line for the purpose of controlling the supply of steam to the indicator.

Indicator, Direction and Revolution. A device fitted on the bridge and designed to show immediately the actual engine movement, direction, and speed so as to minimize the serious results possible from a misunderstanding of orders.

Indicator Pipe. A small pipe connection with the ends of a steam cylinder fitted for the purpose of supplying steam to an indicator.

Inductance. The property of an electric circuit, which results in an electro-motive force being induced in it due to a change in the valve of the current it is carrying, is called its self-inductance, and the process is called self-induction. When two conductors or circuits are so related that a change of current in one results in an electro-motive force being induced in the other, they are said to have a mutual inductance and the process is called mutual induction.

Induction Coil. An electrical device consisting of a primary and secondary coil on a soft iron core. The most common form of induction coil consists of a primary coil of a few turns of coarse wire wound on the core and connected to a voltaic cell through a vibrator and a secondary coil of a large number of turns of fine wire wound upon the primary coil and carefully insulated from it.

Induction Fan. See Blower.

Inertia, Moment of. See Moment of Inertia.

Initial Condition. The datum condition from which the variations characteristic of any other condition are measured, or with which they are compared.

Initial Stability. The stability of a vessel in the upright position or at small angles of inclination. It is usually expressed by the metacentric height.

Injection Pump. See Pump, Injection.

Injector. An apparatus or fitting designed to force feed water into a boiler against the ordinary boiler pressure. The injector does not take the place of the boiler feed pump but is additional thereto. In principle the injector is an instrument for the mixture of live steam at high velocity with a stream of cold water. This results in the condensation of the steam and the imparting of great velocity to the water. The pressure thus built up is sufficient to open the check valve and force water into the boiler by overcoming the boiler pressure. As constructed in practice, the injector has almost perfect efficiency as a boiler feeder. The only heat loss involved being due to radiation from the injector itself and its connections.

Inner Bottom. A term applied to the inner skin or tank top plating. The plating over the double bottom.

Inner Bottom Plate. A term applied to any of the plates in the tank top.

Inner Bottom Plating. See Tank Top Plating.

Inner Keel. See Keel, Inner.

Inner Post. A reinforcing timber bolted to the forward side of the stern post.

Inner Skin. See Skin, Inner.

Inner Stern Post. See Stern Post, False or Inner.

Inside Strake. See Strake, Inside.

Inspectors. Men who examine and test the structure or machinery in order to ascertain if it meets the contract requirements.

Insulation, Electric. A poor conductor of electricity, i. e., rubber, fiber, mica, marble, slate, etc.

Insulation, Heat and Cold. Several considerations enter into the proper insulation of pipes and boilers for sea service, which are not always considered of equal importance for installation ashore. The subject naturally divides itself into two heads: covering for the high pressure main and auxiliary steam pipes and boilers, and covering for plumbing, service pipes, etc. In selecting the former class of covering, we have not only to consider the relative heat saving science of the material, but also its weight per foot and its ability to resist hard usage, constant vibration, the pounding of heavy seas, steam and water leakage and even possible submersion. The effect of a properly efficient pipe and boiler insulation is first to save coal or other fuels. By doing so it not only saves the labor of the firing squad but also materially increases the cruising radius of the ship and permits the use of smaller bunkers. A second but equally important result is to secure an ample supply of hot, dry steam to the engine at all times. Every engineer knows the discomfort, loss of time, and the potential danger of a water logged steam supply, especially in the turbine engine room. In order to secure the greatest possible amount of energy per pound of fuel, the insulating material must have the highest efficiency. The most important characteristic of a good insulation is its capacity to resist the factors of heat.

Insulator, Electric. A support for electric wires or cables made of good insulating material.

Intercommunicating Telephone. A telephone set used as one station in an intercommunicating telephone system. See Intercommunicating Telephone System.

Intercommunicating Telephone System. Two or more telephone sets so arranged as to enable the calling party to signal the party desired and to complete the talking circuit between the calling party’s phone and any phone in the intercommunicating system by pushing a "Push Button" and without the assistance of a switchboard operator or any mechanical or electrical switching mechanism.

Intercostal. Occurring between ribs, frames, etc. The term is broadly applied, where two members of a ship intersects, to the one that is cut. A girder composed of short members running between and attached to continuous members.

Intercostal Floor. See Floors, Intercostal.

Intercostal Girders. See Girders, Intercostal.

Intercostal, Keelson. See Keelson, Intercostal.

Intercostal Plates. See Plates, Intercostal.

Intercostal Stringer. See Stringer, Intercostal.

Interior Communication. Electrical, mechanical or voice tube systems installed aboard a vessel to provide means of communication between the important stations like the bridge, engine and boiler rooms, and also inter-communication between the officer’s staterooms, etc.

Intermediate Beam. See Beam, Intermediate.

Intermediate Frame. See Frame, Intermediate.

Iron. Described under Steel and Iron.

Iron Wire Rope. See Rope, Iron Wire.

Isherwood System. A system of ship’s framing patented by Isherwood. This system contemplates closely-spaced, light longitudinal frames supported on widespread, transverse members of comparatively great strength.

Isochronous Oscillation. Swinging or rolling back and fourth, each swing or oscillation occupying the same.

Isogonic Lines. These show magnetic variation as determined by the Inclinometer. "Isogonic" means literally the indication of equal angles or magnetic variation.

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