Of all the plans of propelling, the undershot water wheel, with radiating floats attached to the arms of the wheel, is that which is most generally employed, on account of its extreme simplicity, its strength of construction and its little liability to derangement. The proportions most generally adopted are to make the diameter of the wheel equal to four times the length of the stroke, and the depth of each paddle about one-eighth of the diameter. In steamers intended to ply chiefly on rivers, the width of the paddles is commonly about one half the diameter of the whole, but in sea-going steamers the breadth is commonly about one-fifth less.

Although the undershot water wheel is generally deemed superior upon the whole to any other apparatus for propelling steam vessels, it still possesses several defects, the chief of which are the waste of power caused by the oblique action of the floats, and the swell caused by the back water, especially in narrow and crowded rivers, where small boats and deeply laden barges are frequently swamped. The same cause also, more perhaps than any other, has opposed the adoption of steam in canal navigation, as the swell caused by the paddles has been found to destroy the banks rapidly.

These defects of the ordinary wheel, which are universally admitted to a certain extent, although their importance is differently estimated, have been felt from the commencement of steam navigation, and the attention of the mechanical world has been occupied to devise modifications of it, or substitutes for it, which shall be free from them. In fact, it has become a sort of mechanical hobby, and perhaps more patents have been taken out for improvements in propelling, than for any one subject, the steam engine only excepted.