473

MODERN STEAM PRACTICE.

hypothenuse is the line that will delineate the thread when woundround the wood. Thus a point on the thread, when working in ariut with a corresponding thread, will move forward the extent ofthe pitch in a single revolution. In this way the screw propellermay be termed an endless screw, working into an endless nut—theocean. Thus supposing the pitch of the screw is 20 feet, and thedistance the vessel has to go forward 4000 feet, the screw propellerwould require to make 200 revolutions to move the vessel thatdistance. The action of a cork-screw is similar to the action ofthe screw propeller: with each revolution of the screw a certainprogressive movement is made through the cork, due of course tothe pitch or spiral of the screw; but as the screw propeller movesin a yielding fluid, sometimes the action ceases by the thread ofthe watery nut becoming as it were broken for a time, and thepropeller revolves without imparting any forward motion to thevessel. This is termed the “slip” of the screw, and varies fromabout 10 to 20 per cent. Again we have what is termed “negativeslip,” the screw moving through less space (even although workingas in a solid nut) than the speed of the vessel, the vessel as it wereoutstripping the screw and dragging it through the water. Thisnegative slip occurs only in vessels having full after lines; in thosehaving a clean run aft it is never feit. The question of the bestpitch for the screw propeller depends on the lines to be given tothe ship, and shows the necessity for the builder and the engineerworking hand in hand. When a maker of marine engines contractsfor the machinery of a vessel built by another firm, he shouldalways be provided with the full lines of the ship, so as to meet therequirements of the particular case. Some screw propellers aremade to feather, by which means the best pitch for propelling thevessel cheaply is arrived at; but it is not advisable to carry thispractice too far, for then the blade of the screw presents neithermore nor less than the action of an oblique board. And althoughwe may be able to get a little more speed out of the vessel by thisdeviation from a truly scientifically constructed propeller to suitthe known run of the vessel as found, surely it is preferable toarrive in the first instance at the pitch to be adopted, which canonly be successfully found by studying the lines of the ship, andcomparing them with former practice. And when we do get agood result from the best form of vessel yet constructed, it can beused as the bäsis for further improvements, increasing the speed of