The Majestic is propelled by four of these propellers, two on each side astern. Turning at 180 revolutions a minute they utilize 80,000 horse power. One might think that propellers would waste much of this power—that is, that they might turn in the water without pushing the ship forward very much, as a wood screw sometimes turns round and round without getting a grip on the wood into which the carpenter wishes to drive it. This is true to some extent when a ship is first starting, but once the ship is in motion a properly designed propeller will be 95 per cent. efficient—that is, it will go as far in 100 revolutions as if it ran without any “slip” for 95 revolutions.
A propeller is measured by its diameter, just as an electric fan is measured. A propeller ten feet in diameter is one whose blades, measured from the centre of the shaft, are five feet long. Another and equally important measurement is the “pitch”—that is, the distance forward the propeller would travel in one revolution if it were running through a solid. Take a wood screw and look at it carefully. You will find that the threads run around it in a spiral. Mark a spot on one thread, and then trace the thread around the screw until it again reaches the side you marked. It will have advanced toward the point, and the direct distance between the place you first marked and the place you have arrived at would be the “pitch.” As a propeller blade travels in a path similar to the thread of a screw, its pitch is similarly measured.
The four propellers on the Majestic are built of manganese bronze and each carries four blades. They are 16½ feet in diameter, and their pitch is 14 feet 11½ inches.
Two of these propellers turn one way and the two on the opposite side turn the other. This is to offset the tendency to swing the ship out of its line of travel, which would be very apparent if all the propellers turned one way. Ships with one propeller feel this action very plainly.
Such huge propellers as the Majestic carries, and such great power as her engines develop, necessitate the use of heavy shafts, which are the great round steel rods that connect the engines and the propellers. These shafts run from the centres of the propellers through the ship’s stern to the engine room, and in the Majestic are 16½ inches in diameter. Where they enter the ship there must be a very carefully built “stuffing box” and bearing which will prevent the entrance of water. Once each shaft has passed this bearing it runs for a considerable distance through a “shaft tunnel,” which is a long, low, narrow compartment through which men may walk in order to inspect the shaft and see that the bearings, which are spaced at intervals along the “tunnel,” are properly oiled. Finally the shaft reaches an apparatus called the “thrust block.” This is a simple but highly important arrangement. To explain its use it is necessary to go back to the propeller.
When the engines are in motion and the propellers are being turned they develop a great “push” against the water, and it is this push that makes the ship move. If, however, something were not done to take up the push, the propellers would slide the shafts lengthwise through their bearings, and the end of the shaft attached to the engine would press so hard against it that it would push the engine from its base, or at least would wear the bearings out, and the engine would be damaged.
In order to receive this “thrust,” as it is called, the “thrust blocks” are installed. There are several designs, but they all accomplish the same task in a similar fashion.
THE MAURETANIA
A British liner of the Cunard Line.