C. C. Herbert
The question of the proportions and action of propellers is, on the whole, perhaps the least satisfactory of all with which the marine engineer has to deal. The dimensions of a propeller for any given purpose are usually almost entirely a matter of trial and experiment; as calculations along this line are not only complex, but are not always entirely satisfactory.
The principal features of a propeller may be taken to be:-Diameter, Pitch, Blade Area, Speed of revolution and Slip. The Diameter is that of the circle described by the tips of the blades. The Pitch, considering the propeller to be a portion of a screw, is the amount which it would advance in one turn, supposing it to travel in a solid medium. The Blade Area is the actual area of all the blades.
The Speed of Revolution is customarily reckoned in turns per minute. The Slip is the difference between the amount which the propeller actually advances per turn and the amount which it would advance if turning in a solid medium. For example, if the pitch of a screw is 30 in. it would advance 30 in. at each turn if there were no slip. Suppose that it only advances 20 in. per turn, then the slip is 10 in. per turn, or as it is usually figured in percent, or 33 1/3 percent. As a further example, suppose a propeller of 30 in. pitch turning 300 turns per minute drives a boat at the rate of 6 miles per hour. The advance of the propeller in feet per minute is 30/12 x 300 = 750, while the advance of the boat is 6 x 5,280/60 = 528 ft. per minute. The slip is then 750-528 = 222. or as a percentage, 222/750 = 29.6 percent. It might seem at first thought that a perfect screw propeller would have no slip; but this is a practical impossibility; it is also theoretically impossible for a propeller to work without slip.
The most important dimension from the standpoint of the absorption of power, is the blade area. A certain blade area may be obtained by a relatively wide blade on a small diameter or by a narrow blade on a relatively large diameter. In the former case the area of the blades bears a greater proportion to the area of the circle through the tips than in the latter case. There are certain limits for this proportion of blade to disc area for well designed wheels beyond which it is not well to go. These are as follows:
For two blades
.20 to .25
For three blades
.30 to .40
For four blades
.35 to .45
This means that for a 24 in. diameter propeller, whose disc area 452 sq. in. the blade area for a three bladed wheel will very from
.30 x452 136 sq. in. to
.40 x 453 181 sq. in. depending upon the power to be absorbed by it. The blade area should not, for ordinary use, be made greater than these proportions as the blades then become so wide as to interfere one with another. Of course, where a propeller must, for shallow draft, be of unusually small diameter, the proportion of blade area must be increased, but at the expense of some loss of economy. Strictly speaking, for a well balanced propeller, the blade area fixes the amount of power which the propeller can deliver, while the pitch, combined with the turns per minute governs the speed. As a matter of fact, for the average propeller the two are closely related, each having a certain influence upon the other. To illustrate, a propeller may have a small blade area and so great a pitch that the blades act somewhat like fans and simply churn the water, offering great resistance and absorbing the power, but doing little effective work. In this case while the power of the engine is absorbed, but little effort is exerted towards driving the boat. This propeller would be improved by decreasing the pitch to a reasonable figure and increasing the blade area to take up the power.
The opposite case is shown by a propeller of large blade area and very small pitch where the blades are almost flat. Here the blades tend to simply revolve edgewise through the water and the power is absorbed by the surface friction. In this case the engine can turn up to a high rate, but has little effect on the motion of the boat. This propeller will be improved by increasing the pitch and reducing the blade area.
There are certain well defined limits for the proportion of pitch to diameter; for instance, in the 24 in. propeller mentioned above the ratio of pitch to diameter is -- = 1 1/2. The pitch should not be less than the diameter nor as a rule greater than 1 1-2 times it.
The blade area is the most important feature, as if this is of the correct amount to absorb the power, the pitch will, within certain limits, take care of itself. This explains why most engine builders can furnish a certain propeller with a certain engine without regard to the conditions under which it is to be used. As a rule the same propeller will be furnished for a heavy working boat, which can only be driven at a low speed, as for a light, high speed launch, and it will appear to work equally well in both cases. This is due to the difference in the slip; in the first case the wheel is working with a large slip, and in the latter case ewith a moderate or low slip, but with a fair efficiency in both cases, provided that the blade area is of proper amount.
An average slip for a good working propeller Is usually taken at from 10 to 20 percent. A propeller may work efficiently at a high slip, but the revolutions of the engine may then be unnecessarily high. A slip of over 30 percent will usually indicate that a different wheel would probably give better results. The shape of the after end of the hull also influences the slip, a very full run hindering the flow of the water and increasing the slip. The remedy for this is a larger diameter, to reach out into the clear water beyond. To measure the blade area of a given propeller the center line is drawn down the middle of the blade and the length of the bladed divided into several equal spaces. At these divisions lines are drawn across the blade as shown in the 3ketch. The widths of the blade at each of the lines is measured, all the widths are added together and multiplied by the distance between the cross lines, giving the orea of one blade. This is then multiplied by the number of blades for the total area.
To find the pitch of a propeller it is laid upon a flat surface with the shaft exactly vertical. The pitch at any point, as A, may be found as shown in the lower sketch; A, C is the width across the blade; C, b is a vertical line at one edge and a, b is the width on a horizontal line. It is plain that in the distance a, 6, along the circumference, the advance of the propeller is b, c. The circumfeoence of a circle passing through A is 6.28 X 0, A. Now the pitch will bear the same relation to b, c that the circumference of the circle through A bears to a b or
Pitch / b.c = circumference / a.b = pitch = b c x circ. /a b
The pitch at different points of each blade is likely to be different, in which case the average pitch is used. It is hoped that these few hints may be of some help to those who may be unfortunate enough to have unsuitable or poorly designed propellers.