The slip of a propeller is the difference between the distance the ship is moved by the propeller and the distance it would move if the water was solid. Slip is usually expressed in the percentage that the distance the ship actually travels bears to the distance she would have travelled if there had been no slip. From 10 to 20 per cent. is lost in slip.
A screw of increasing pitch is one in which the angle of the face of the propeller blade to the axis of the shaft increases as the thread recedes from the shaft, or from the centre to the circumference of the blade, or in both directions.
In a uniform pitch the angle of the blade to the propeller axis is the same at all distances from the axis.
An example of a screw of uniform pitch would be a piece of angle iron wound around a parallel shaft. If wound on a tape shaft, the largest diameter being nearest to the ship’s stern, it would have an increasing pitch. If wound around a parabola, the pitch would vary at every point in its diameter and thread.
A thrust bearing is a journal bearing provided with a number of corrugations or collars fitting with corresponding corrugations or recesses in the thrust block, the area thus provided serving to resist the end thrust placed by the propeller upon the shaft.
It must be freely lubricated by ways leading to each collar or corrugation, and so situated that it is accessible for examination. It is sometimes at the end of the first length of shaft aft of the engine.
A stern tube is a sleeve enveloping the aft end of the propeller shaft to protect it from the sea water, which would corrode it. At the aft end of the stern tube is a gland and stuffing box. At the inner end, which extends to the aft bulkhead, it has a flange which is bolted to the bulkhead.
The bearing area of the shaft and stern tube are lined with brass (about half an inch thick) to prevent their oxidation from the action of the sea water.
A lignum vitæ bearing is a wooden bearing generally fitted to the outer end of the stern tube in propeller engines, or to the outer ends of the paddle shaft of paddle engines. It consists of strips of lignum vitæ dovetailed into the bearing or bush, and running lengthways of it. These strips are prevented from working out by a check plate at each end of the bearing.
Screw propellers may be fastened to their shafts in several ways, as by a key or feather sunk in the shaft, and projecting into a keyway in the propeller bore, and a nut on the end of the shaft with a safety pin outside the nut, or by a key passing through the boss of the propeller, and a safety pin or plate upon the key.