A Paper by Clinton Crane 
Problems In Connection With High-Speed Launches
by Clinton H. Crane
(This paper was read before the Society of Naval Architects and Marine Engineers)
The very recent development of the so-called "gasolene engine," of extremely light weight in proportion to its power, has given the opportunity of making another step forward in the direction of great speed upon the water.
Up to the present time gasolene engines of extremely light weight have only been available in powers up to 300 h.p., and in the majority of cases of successful operation in powers not exceeding 150 h.p.
In utilizing this form of power, up to the present time the size of the boats has necessarily been small. In choosing a type of hull naval architects had to avail themselves of what had already been done in steam.
A glance at the four torpedo-boats, which are illustrated in the plate appended to this paper, will show the very great differences in the shapes of water lines, propellers and sections adopted by these four successful designers, to produce an easily driven hull. Such differences would seem to show that, with a given displacement and power, the shape of the under-water form, provided it is reasonably fair, was not of the greatest moment, or whether, that there might be objections to the more easily driven form which made some compromise advisable.
This statement must be modified, as I do not wish to be understood that the services of a skilled naval architect are unnecessary in the building of the fastest possible launch. The saving of weight to the last ounce is a matter of great engineering skill. The choice of a proper propeller, the adaptation of a form which can maintain its speed in rough as well as smooth water -- all these are matters calling for the greatest possible training, experience and attention.
The simile of the skipping stone has been used by a number of engineers in describing the action of these light, high-speed vessels. The idea that a boat should skim over the top of the water and not go through it is by no means a new one.
Over thirty years ago a clergyman, the Rev. C. Ramus, was so firmly convinced that all vessels should proceed in this manner that he succeeded in prevailing on the English Admiralty to give his ideas a trial in the experimental tank at Torquay. The result of these trials seemed to show the absolute fallacy of the reverend gentleman's ideas; but the experiments were based, on what was then possible in the direction of displacement and power, and with what then seemed a most liberal prediction of future possibilities. We can only feel that Mr. Ramus lived too early.
Two years ago Mr. Yarrow conducted a number of experiments with full-size models in a 40-foot launch towed by one of his 25-knot torpedo-boats. The model which gave the least resistance at this speed was a flat-bottomed scow, which appeared to glide along the surface of the water. Based on this experiment Napier II was designed for such modification of bow as seemed essential for a boat which would be called upon to perform in a seaway.
The final successful trial of this boat was made in March last year on the Tames at Greenwich, and a speed of practically 26 knots was attained. The boat was driven by two four-cylinder Napier engines, and the displacement on trial was three tons four hundredweight.
This gives us a very interesting comparison with Dixie, which was driven by a single engine of eight cylinders of the same diameter and slightly greater stroke. It may be reasonably assumes that the powers developed in the two boats are practically the same. The speed attained by the Dixie was practically the same, although her displacement is nearly 2,000 pounds than that of the Yarrow boat.
This seems to confirm Mr. Yarrow's conclusion that in boats of this size and going at this speed the skimming idea gives the lesser resistance. It is, however, associated with the form of hull, which is very badly adapted to maintaining high speed in a seaway.
As a matter of fact, in spite of her extra weight of hull, Napier II was obliged to withdraw from a number of her races, owing to leaks caused by the pounding of her flat bottom on the waves.
I think no one will question the fact that sharp ends and sharp floors go through a sea with far less shock and trouble than extremely full ends and flat floors. It is not so much that the flat floors stop a boat's progress as that they put a shock and jar on the whole fabric, which necessitates heavier construction. and does away with the possible advantages gained by this particular form of hull.
The most serious objection to sharp bows and "toothpick" sterns, as they used to be called, is the loss of stability which this form involves. Apparently this loss of stability is much more serious and more noticeable at high speeds than at low. Whether this is entirely due to the increased torque of the propeller, is a matter about which the writer is not yet entirely clear. This torque of the propeller in a fast single-screw launch is a matter of serious consequence. In the case of Dixie, at full power, this torque amounts to placing a weight of 350 pounds at the gunwhale, or about 8 per cent of the boat's weight.
This can, of course, be partially compensated for by placing the machinery off the center. Unfortunately, however, for the comfort of those on board, this torque is by no means a constant quantity. If the boat throws its wheel out of the water, even to a small extent, in the mass of a wave, this torque is suddenly relieved and the recoil in the opposite direction is sudden and disconcerting.
It is possible that this torque may alone account for the trouble which a number of the high-speed boats have experienced this Summer. In designing Dixie this matter was carefully considered, and she was of practically the same metacentric height as the Vingt-et-Un II's G.M. being .725 and Dixie's G.M. .675. it was expected that the increase in the value could be compensated for by the placing of the crew. The actual result is that Dixie behaves very much as Vingt-et-Un II did up to 21 or 22 knots. If driven faster than that in a seaway she becomes very cranky, rolling so much as to shake the nerve of the uninitiated.
It has been found necessary on all these boats to use a comparatively large size rudder. The uneven thrust of the screw gives a very strong pressure to deviate from the course. Right-handed screws throw the stern to the starboard, left-handed to the port; this is no doubt being partly due to the fact that the bottom of the screw is working in more solid water, and partly to the rather rank inclination of the shaft necessary by the large power and extremely light displacement, which increases the slip of the descending blade and decreases it on the ascending. This difference of slip is a s much as 4 or 5 per cent. That is, if the slip of the descending blade is 25 per cent, the slip of the ascending blade would be 20 per cent.
In Dixie we have been using a propeller to obtain a maximum speed of 23 1/2 inches diameter, 32 inches which, with a surface ration of 55 per cent, this wheel turning 1,120 revolutions. The boat has not been raced with this wheel, as the piston speed is a little excessive for long continued running.
This wheel should be perilously near the point of cavitation. As a matter of fact I have encountered cavitation this year on a smaller wheel, running at fewer revolutions in a slower hull.
There is one more point about the shape of the model which is worth consideration. The sharp-bowed boats throw a much smaller bow wave than the full-bowed ones.
This mass of water, although it is usually a thin, transparent film, adds greatly to the discomfort of those on board. The boat driving along at nearly 30 miles an hour against a wind and sea, this water strikes the face as from a fire hydrant.
The table which is given below will be of interest. The lengths, weights, speeds and sizes of cylinders, I can vouch for as being correct. The horse-powers of the gasolene engines are, with the exception of Vingt-et-Un II, subject to a general calculation, and are based on the sizes of the cylinders and an assumed piston speed. These are certainly accurate within 10 per cent. The column "Weight in Pounds/h.p." is one of extreme interest. As all these boats were actually weighed by disinterested parties, the results can be taken as accurate.
In my opinion, for the best use we need a compromise between the extremely sharp and the extremely flat types. it is in arriving at this compromise that the naval architect can best show his skill.
|Designer||lgth W.L.||full displ. lb||h.p.||cylinders||engine maker||speed kts||weight lb/ h.p.|
|XPDNC||Herreshoff||42.2||3250||75||4||6 1/2||5 1/2||Mercedes||23.0||43.4|
|Napier||Yarrow||40||7170||150||8||6 1/2||6||Napier||25.98||47.8 (2-screw)|
|Dixie||Crane||40||5160||150||8||6 1/2||6 3/4||Smith & Mabley||26||34.4|
|Vingt-et- UN II||Crane||38.75||3850||75||4||6 1/2||6 3/4||Smith & Mabley||22||51.4|
|Designer||Length W.L.||Full displacement||H.P.||Speed kts||Weight lb/hp|
(Transcribed from The Rudder, December 1905, pp. 647-650.)
[Thanks to Greg Calkins for help in preparing this page]
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