Fig. 9 The following letter in relation to his screw propellers was written by Colonel Stevens to the well-known scientist, Dr. Robert Hare, of Philadelphia:Different photographic illustrations of the engine, etc., its boiler, and also of this machinery placed in the boat lately constructed of the size of the original boat of 1804, are shown in Figs. 1, 7, 8 and 9.
Colonel Stevens' plan for working twin screws by a single cylinder is the most simple one that could be devised. The reaction of the connecting rods against each other at their junction with the piston-rod acting as a parallel motion, or as slides would do, to keep the rod in alignment. When the screw propeller came into use, after a lapse of nearly forty years, this plan of a single cylinder for twin screws was revived in this country and abroad, being known in France as the "Etoile Engine." The valves on this twin-screw engine are formed by two-way cocks, a modification of the single-way cocks used by Savery and Newcomen, one cock at each end of the cylinder, answering both for the admission and the exhaust of steam.
The valve motion is derived from a crank on the inboard end of one of the propeller shafts. This crank works a rack, the teeth of which mesh into those of wheels on the plugs of the two-way cocks; this motion being similar to the toothed rack and segment of a wheel which Watt used, in one of his first engines, to raise his conical valves.
The boiler is one form of the multi-tubular boiler of Colonel Stevens. It has 28 copper tubes, each 1.5 inches in diameter and 18 inches long, 14 tubes projecting from each side of a rectangular chest. The grate is placed at the end of one set of tubes, and the flame passes around these tubes and then under the chest and around the tubes at the other end to the smokestack.
Fig. 9 The following letter in relation to his screw propellers was written by Colonel Stevens to the well-known scientist, Dr. Robert Hare, of Philadelphia:
HOBOKEN, Nov'r 16th., 1805.
DEAR SIR:
"I have received your favor of the 8th inst. communicating an outline of Mr. Weeden's' contrivance for propelling boats. The attention you have manifested on this occasion merits my warmest acknowledgments, as it evinces the interest you feel in the success of my project. But although I am persuaded I should derive no advantage from Mr. Weeden's contrivance, yet I shall ever preserve a due estimation of your kind intentions.
With the surprising effects of the Chinese scull, I have long been acquainted." (He alludes to a mill wheel, then called the tub wheel, the Chinese scull or the flutter wheel: one form of which, developed afterward, is the turbine.) "And it is now five or six years ago that I made an attempt to apply a steam engine, the construction of which was admirably adapted to the purpose, to working a system of sculls attached to the stern of a boat-but failed of success from the inefficiency of the power in the engine I employed owing principally to the imperfection of the boiler. But, my dear sir, I am fully satisfied, both from theory and practice, that my present mode of applying the power of a steam engine to propelling boats is in various respects preferable to any possible adaptation to the purpose, of this scull with an alternate movement.
You may recollect the description I gave you when I first had the pleasure of seeing you at Hoboken. To the extremity of an axis passing nearly in a horizontal direction through the stern of the boat, is fixed a number of arms with wings like those of a windmill or smoke jack. These arms, are made capable of ready adjustment, so as that the most advantageous obliquity of their angle, may be attained after a few trials. The principle of an oblique stroke is the same here, as in the scull-but the continuity of movement in the wings, gives them greatly the advantage over the alternation in the sculls; both in the loss of time, and the resistance of the fluid in the change of motion: besides that, this change of motion must give to the boat a wriggling movement; and it has also a tendency to elevate and depress the stern of the boat. The sculls, would also be liable to be affected by the swells in rough water, and, like the paddles I had some thoughts of using, would be an awkward appendage to the stern of the boat. The consideration which determined me, when I saw you last, to make a trial of the paddles, was merely to avoid the necessity of giving the boat a draught of water too great for passing the over-slough near Albany; but this objection to the use of wheels, I expect to obviate by an increase of the number of them, and consequent dimunition of their diameter. Indeed, it is absolutely necessary to have at least two, revolving in opposite directions, to prevent the tendency to rotation which a single "heel gives to the boat. Since you were here, I have made a fair experiment on the wheel, compared with oars. Two men were placed at two cranks, by ,which a wheel in the stern of the boat was turned; and with a stop watch, the time of passing over a given distance was precisely ascertained. After making a sufficient number of trials, the wheel was taken off and the same men were furnished with oars. The result of repeated trials, was a few seconds in favor of the wheel. It is unnecessary to observe, that the wheel must have worked to much disadvantage. The proper angle of obliquity, was not attended to; besides, the wings were made with a flat surface; whereas, a certain degree of curvature was necessary. And in order to give a due submersion to the wheel, the axis was inclined at least 30 or 40 degrees below the horizontal line. The machinery too, was put up in a very coarse manner.
One very important consideration in favor of these wheels is the facility with which they can be defended from all external injury, by placing them in the stern thus:
My foreman promises me to have the engine agoing in the boat in about two weeks from this time. I shall embrace the first opportunity of acquainting you with the result of my experiments. I am dear sir with esteem and regard yours etc."
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