56 BULX-ETIN 127, UI^ITED STATES NATIONAL MUSEUM. 



Full-size copy of iron screw propeller used by Col. John Stevens in 1804. 



In a letter written by Stevens to Dr. Robert Hare of Philadelphia, 

 Pa., dated November 16, 1805, the following data appear: 



" To the extremitj^ of an axis passing nearly in a horizontal direc- 

 tion 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 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 thought of using, would be an awkward 

 appendage to the stern of the boat. The consideration which de- 

 termined 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 overslough near Albany, but this objection 

 to the use of wheels I expect to obviate by an increase of the number 

 of them, and consequently a diminution 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 wheel 

 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 re- 

 sult 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 

 advantage. The proper angle of obliquity was not attended to; be- 

 sides, the wings were made with a flat surface, whereas, a certain de- 

 gree of curvature Avas necessary. And in order to give a due sub- 

 mersion to the wheel, the axis was inclined at least 30 or 40 degrees 

 below the horizon line. The machinery, too, was j)ut up in a very 

 coarse manner." 



This propeller blade is made in accordance with the description 

 given in the letter to Doctor Hare. The broken shank on the hub 

 shows the propeller to have been when last used the short two- 

 bladed screw propeller of the present day, with the blades separately 

 attached, and the two other opposite holes in the hub show that it 

 was made to be also used as a four-bladed propeller. The blades 



