free ebooks

A Catechism of the Steam Engine by Bourne

By repeated experiment with the dynamometer


by virtue of a larger thrust exerted by the screw in pressing forward the shaft and with it the vessel, but by the gravitation against the stern of the wave of water which the screw raises by its rapid rotation. This wave will only be raised very high when the progress of the vessel through the water is nearly arrested, at which time the centrifugal action of the screw is very great; and the vessel under such circumstances is forced forward partly by the thrust of the screw, and partly by the hydrostatic pressure of the protuberance of water which the centrifugal action of the screw raises up at the stern.

579. _Q._--Can you state any facts in corroboration of this view?

_A._--The screw vessel will not preponderate if a screw and paddle vessel be tied bow to bow and the engines of each be then reversed. In, some screw vessels the amount of thrust actually exerted by the screw under all its varying circumstances, has been ascertained by the application of a dynamometer to the end of the shaft. By this instrument--which is formed by a combination of levers like a weighing machine for carts--a thrust or pressure of several tons can be measured by the application of a small weight; and it has been found, by repeated experiment with the dynamometer, that the thrust of the screw in a screw vessel when towing a paddle vessel against the whole force of her engines, is just the same as it is when the two vessels are

maintaining an equal speed in calms. The preponderance of the screw vessel must, therefore, be imputable to some other agency than to a superior thrust of the screw, which is found by experiment not to exist.

580. _Q._--Has the dynamometer been applied to paddle vessels?

_A._--It has not been applied to the vessels themselves, as in the case of screw vessels, but it has been employed on shore to ascertain the amount of tractive force that a paddle vessel can exert on a rope.

581. _Q._--Have any experiments been made to determine the comparative performances of screw and paddle vessels at sea?

_A._--Yes, numerous experiments; of which the best known are probably those made on the screw steamer Rattler and the paddle steamer Alecto, each vessel of the same model, size, and power,--each vessel being of about 800 tons burden and 200 horses power. Subsequently another set of experiments with the same object was made with the Niger screw steamer and the Basilisk paddle steamer, both vessels being of about 1000 tons burden and 400 horses power. The general results which were obtained in the course of these experiments are those which have been already recited.

582. _Q._--Will you recapitulate some of the main incidents of these trials?

_A._--I may first state some of the chief dimensions of the vessels. The Rattler is 176 feet 6 inches long, 32 feet 8-1/2 inches broad, 888 tons burden, 200 horses power, and has an area of immersed midship section of 380 square feet at a draught of water of 11 feet 5-1/2 inches. The Alecto is of the same dimensions in every respect, except that she is only of 800 tons burden, the difference in this particular being wholly owing to the Rattler having been drawn out about 15 feet at the stern, to leave abundant room for the application of the screw. The Rattler was fitted with a dynamometer, which enabled the actual propelling thrust of the screw shaft to be measured; and the amount of this thrust, multiplied by the distance through which the vessel passed in a given time, would determine the amount of power actually utilized in propelling the ship. Both vessels were fitted with indicators applied to the cylinders, so as to determine the amount of power exerted by the engines.

eBook Search
Social Sharing
Share Button
About us is a collection of free ebooks that can be read online. Ebooks are split into pages for easier reading and better bookmarking.

We have more than 35,000 free books in our collection and are adding new books daily.

We invite you to link to us, so as many people as possible can enjoy this wonderful free website.

© 2010-2013 - All Rights Reserved.

Terms of Use | Privacy Policy | Contact Us