The Limits Of The Speed Launch
There is at present no distinct definition of a speed launch, although the advent of this peculiar craft has been heralded for several years. As it is now, almost any launch, whether large or small, capable of making ten miles per hour, is honored by the prefix "speed." unless it is built outright like a scow or barge. But it is a well known fact that a large vessel may be driven with comparative ease at a speed that it would be impossible to attain in a small boat. hence the length of the launch plays a very important part in its speed, as a small, short launch should be credited with its higher relative speed, which is always proportional to the square root of the length on the load water line.
The writer would therefore propose the following definition: The speed launch is a vessel built to offer the smallest possible resistance when running, capable of a rate of speed, in miles per hour, equal to twice the square root of its length on load water line.
For instance, a 25-foot launch must make at least 10 miles to be called a speed launch, as the square root of 25 is 5, and 2 times 5 equals 10. For a 36-foot launch 12 miles is the lower limit, and so forth. And to call a 60-foot boat, making only 14 miles, a speed launch, as I saw in an advertisement some time ago, it is certainly to stretch the limit a little. 15 1/2 miles would be the proper speed for a 60-foot launch, if this definition is accepted. The speed limit is by no means arbitrary but derived from the relation between the length and resistance of a vessel. At this speed, the launch has just passed safely over the inmost critical point as regards its resistance.
"Speed" has a peculiar charm of its own, quite apart from its business aspect of saving of time. One of the most pleasant impressions of speed is to glide along swiftly over the calm surface of the water, among a beautiful scenery. This mode of traveling has gained many admirers and created a widespread interest in the modern high-speed vessel. The invention of the internal combustion engine has made the small speed launch a possibility, owing principally to the lightness, compactness and small cost of such engines compared with steam engines.
It is not too much to say that the general use of the speed launch will revolutionize yachting as a pastime, if not as a sport. Of course, old salts and sailing yachtsmen do not consider "perfume boating" as a sport, but driving, for instance, is called sport, and there is much in common for both. Be this as it may, the speed launch has come to stay, and new ones are being built all over the country.
No doubt many of these launches will fail to attain their estimated speeds by a wide margin; of course, all the blame will be laid on the engine, when, in fact, the hull itself is the real cause of the failure, in nine cases out of ten. For there is such a thing as the form of least resistance, and unless a hull is properly designed, and its speed has a certain relation to its length, it cannot be driven fast, no matter how much power you put in it. The more power, the more weight, and the more resistance, yes, and there you stick. But given hull of fine form, the more power you put into it the faster it runs; there is practically no limit to speed in this case, except strength of hull and space for the machinery. But the higher the speed, the less the increase for a certain extra power, because the power to drive a vessel runs up much faster than the speed, although the rate of increase is not regular by any means. That is, at certain speeds depending on the length of the launch, a slight increase in power makes the boat go perceptibly faster, while at other speeds, the proportionate addition to power does not seem to make any change in speed at all, and the launch seems to just stick in the water. This phenomenon is caused by the interference of the bow and stern waves, and without now going into further details, the writer would here state that for a slight difference in length, other things being equal, the power necessary to drive a vessel at a certain speed, may vary as much as fifty per cent, or more.
Hence the imperative necessary of giving a high speed launch a length suited to her speed, in order to make the resistance as small as possible. Only so can the best results be obtained. Yet hitherto launches as well as ships have been designed mostly without reference to speed, and it is safe to say that millions of dollars are lost yearly in attempts to force ships to speeds where there can be very little economy of propulsion.
But there are other qualities besides a fine form that a successful high speed launch must be given. In the first case, everything must be sacrificed to speed, the hull and the machinery made as light as possible, consistent with strength, and further, the hull must have the form of least resistance at the particular speed and displacement required. All this sounds formidable, and indeed, the highest skill of the naval architect and engineer is necessary to bring about a real high-speed vessel. The difficulties to surmount are tremendous, for what you try to gain in one part of the design, you generally lose in another, and in fact, the more you get acquainted with the subject, the more intricate it becomes. After years of hard study and work only will you begin to see the principles clearly. But it is an interesting study, the designing of high speed launches, and carries its own reward.
In the design of a high speed launch, the main desideratum consists in reducing the resistance, to the lowest possible amount of a displacement large enough to carry the machinery necessary to drive the launch at the required speed. Now at low speed, (relative to the length of the vessel) the resistance is almost entirely caused by the adhesion of the water to the surface of the launch, skin resistance, as it has been called. A full, round form has very little surface in proportion to its displacement, hence such a form gives the least resistance at low speed. But as the speed increases, the waves created by the launch begin to take a very active part in the resistance, until at last the skin resistance would sink into insignificance, if we were to retain the full round form. To put it in figures, we will say that the skin resistance increases as the square of the speed, which is very nearly true, and the wave resistance as the fourth power, on an average. For instance, a 30-foot launch may have at 10 miles a skin resistance of 110 pounds, and a wave resistance of 75 pounds. At 30 miles, if such a speed were possible, the former resistance would be 4 times 110 equals 440 pounds, while the latter has grown to 81 times 75 equals 6,100 pounds about.
The wave resistance, however, is dependent on the fineness of the hull, hence in order to keep it within reasonable limits, we must make the launch long and narrow. But hereby we increase the surface and the skin resistance, therefore, we cannot keep on increasing the fineness of the hull beyond a certain point, depending on the speed and the size of the vessel. It is possible to determine this point by calculation.
When a launch is speeding through the water at a good rate, the bow pushed the water aside, largely in a horizontal direction, said water being replaced at the stern by other coming up from under the hull, not form the sides; this phenomenon can be seen in brackish water, when the wake of the ship is always clear, salt water, but along the sides is the lighter fresh, mostly discolored water.
To assist the water particles in their natural motion caused by the force of gravity, the forebody must be made to extend vertically, and the afterbody horizontally. In other words, the forebody must be designed deep and narrow in sections, the afterbody shallow and wide. Incidentally this form also keeps the bow of the launch from rising, and the stern from sinking in the water, or squatting, at high speeds. This change of a vessel's trim at high speeds does not, however, change the displacement, which always remains the same, contrary to the popular notion.
The considerations now mentioned, pushed to their utmost limits, produce a form of hull as shown in the accompanying design of a speed launch, which is the logical product of the theory of least resistance, as developed by the writer after years of study and experiments.
The widest part of the under water body is at the stern, from where it gradually tapers to the bow; and the deepest part is at the bow, rising in a fair curve to the stern. In each case the longitudinal form is such as to displace and replace the water with the least possible disturbance, and consequently, with the least possible resistance at high speed.
Thus, to all intents and purposes, the forebody of this design has a length equal to the length of the hull; and the afterbody extends also through the entire length. Hence, the lines are as fine as if the hull really were twice as long as shown, with the wave resistance reduced in proportion. But the skin resistance is little, if any, larger than that of a common hull having the same general dimensions, and the displacement remains unchanged, no matter where the midship section is placed, as long as the general form of the longitudinal lines remain the same.
In this manner the resistance, for a given speed and displacement, can be reduced to a minimum, hence we may properly call the design here shown the form of least resistance. Of course, the dimensions and the proportions will change with the speed, but the form, taken as a whole, remains the same. It is not a form to be recommended in open waters, or in a seaway, as its razorlike bow would dive right through the waves; but for smooth water it would be just the thing for a speed launch of any size, which, furthermore, is not required to be driven fast in the open sea. Still, even in the case of a sea-going launch, an approximation to this form should give very good results, as shown by the modern torpedo-boats, for instance.
(Transcribed from The Rudder, March, 1903, pp. 198-199. )
[In several of the early launch races on the salt water surrounding New York, Connecticut and Massachusetts many of the contestants would become painfully aware of Mr. Liljegren's warnings of the inadequacies of the speed launch design in an open seaway. GWC]
[Thanks to Greg Calkins for help in preparing this page. LF]
Hydroplane History Home
This page was last revised Thursday, April 01, 2010.
Your comments and suggestions are appreciated. Email us at firstname.lastname@example.org
© Leslie Field, 1999