A difficulty which paddle-vessels have to contend with is that of securing a proper immersion of the floats. For a vessel in smooth water the immersion of the top edge is usually calculated at about one-eighth of the breadth of the float; but for a vessel intended for general sea service, an immersion of not less than half the breadth of the float is allowed, that is to say, the float at its moment of deepest immersion has a height of water above it equal to half its diameter. If the float goes much deeper the efficiency of the wheel becomes impaired. This is a point which has to be taken into consideration in designing paddle-boats, so that the maximum power shall be available when the vessel is fully laden, and shall not be much lessened when the vessel is running light. The earliest steamers suffered greatly in this respect as their designers had not discovered the right size of wheels or floats to suit the hulls. A loaded vessel consequently went very slowly owing to the great depth to which her floats were immersed. To overcome this difficulty an ingenious system of what can best be called reefing was invented. Affixed to the axle of the wheel was a rod with an arrangement of cogs at the end, and these fitted into a series of teeth in rods affixed to the floats, so that it was a simple matter to expand or contract the effective diameter of the wheel by altering the position of the floats as required. The same result has sometimes been obtained by a system of levers, but the toothed wheel business was the older. It was tried on a few of the earlier boats on the Clyde, not always, however, with success. A peculiarity of some of the larger paddle-wheels in use in America is that they are not only of much greater size than those in use in Great Britain in proportion to the size of the boat, but they have a proportionately less immersion and the wheel is constructed in a very different fashion. The floats, instead of being of one piece, as here, are constructed of three narrow fixed strips, two of which are on the same radius but have a space between them equal to the breadth of the third strip, which is placed a few inches behind the vacant space. It is contended that this method disturbs the water less than the broad float and increases the propelling efficiency. Probably the most notable instance is the great wheel of the _Sprague_.