The Speed Of Motor Boats
The Speed Of Motor Boats
And Their Rating For Racing Purposes
Read at the Spring Meeting of the Institute of Naval Architects, 1906
It is with considerable diffidence that I have ventured to bring before this institution a few details in connection with motorboats and their rating rules. I feel that it is at best an unimportant matter, which can only interest a few naval architects, who, like myself, have taken up this particular branch of the subject. I trust, however, in view if the great possibilities in the direction of high speeds which the use of the internal combustion engine has opened up, that I shall be pardoned for attaching some value to the data obtained from the racing of motorboats, as the results of this racing may have some influence on the design of vessels of the torpedo-boat type in the future. That motorboats are likely to be of service for naval purposes is, I think, proved by the keen interest taken in them by our most noted torpedo-boat builders, Messrs. Thornycroft & Co., and Messrs. Yarrow & Co. Both of these firms have expended an enormous amount of time and money in experimenting with these craft, and have obtained highly successful results.
To show that the internal combustion engine has made it possible to obtain a great increase of speed as compared with the best results obtained from steam, I will take as an example the reputed fastest 30-foot steam launch on record. Her power is stated to be about 110 B.H.P., her displacement 2.75 tons, and her best speed 19 knots, which is no doubt an excellent performance for a boat with lean ends and a large mid-section. On the other hand, we have a 26-foot motorboat, Rapiere, of approximately 1.2 tons displacement and 110 B.H.P., with a speed of 25.5 knots. There is no doubt that the latter boat was a far superior design for high speeds, and she obtained an increase of 34 per cent in speed for the same power on 4 feet less length, with only 44 per cent of the others displacement. Rapiere proved herself to be an able sea boat, and quite strong enough; while she was run by two men only, as against the three on the steam launch. She was, therefore, quite as useful as the other, and had the advantage of being much faster and lighter with a smaller crew; added to which, she was noted for her reliability and regular running. This will, I think, show that at any rate, for picket boats and such small fry, the motorboat can show a marked superiority over the steamboat in speed, weight, and ease of handling, to say nothing of the reduction in machinery space.
I will now endeavor to show that motorboat racing has some definite use, and that it is not a mere craze of the moment. The reason why motorboat races are useful is that, unless a new type of engine, such as the internal combustion motor, is tried to the breaking point in races, its development would be a matter of years, and, even then, it is a question whether anything would find out the weak spots of design and construction so thoroughly as the strain and keen competition of racing. It may be said that a racing motorboat is only a flimsy affair, just strong enough to hold together for a race ot two; but this paring away of every superfluous ounce of weight, while it may make that particular boat too short-lived for commercial purposes, does more to teach the engineer the exact stresses and strains which are to be reckoned with than years of ordinary running with a big margin of safety. The same arguments apply to the design and construction of the hulls. The following examples may be of interest.
"Napier I" (1903)
This boat was one of the pioneers of the present racing motorboats in this country, for she was the first 40-foot racer ever built in England. The year 1903 saw the inauguration of motorboat races in Great Britain, with the first race for the Harmsworth Cup at Queenstown, and it was to defend this cup that I was commissioned to design Napier I. The cup race itself lost a great deal of its interest owing to the breakdown of the only foreign competitor just before the start, and to the fact that the other two British boats, Scolopendra and Durandel, were only 30 footers, while Napier I was a 40 footer, with more power than either of the others, which enabled her to win easily.
On referring to the design of Napier I, it will be seen that she is a canoe-shaped boat so far as her deck plan and water-line are concerned, and that the latter is rather full forward compared with the later boats. She is extremely flat-bottomed and wall-sided, he mid-section being composed of three straight lines for the bottom and sides connected by quarter circles at the bilges. This form of section is carried right aft, and for some considerable distance forward, where it gradually merges into a compromise between a flattened U and a blunt V. She was designed for a displacement of 30 cwt., but the weights of machinery given me when designing the boat proved to be considerably under the actual weight of the complete engine and accessories, which exceeded the estimated weights by some 5 cwt. The weight of the steel hull also exceeded the estimated weights by over 1 cwt, as it was found impossible to obtain ready-made angle bars of the required size for the frames; consequently we had to do the best we could, and use frames of a total weight of about 50 per cent more than we had intended. The frames are spaced 1 foot centre to centre, instead of 6 inches centre to centre, as I had specified. The plating is 20 gauge steel, with the exception of the keel plate, which is 14 gauge. No doubt, if we had succeeded in obtaining the lighter frames, and had been able to space them half the distance apart, she would have been a better boat; but, in any case, I do not think that steel can compete for a moment with wood, on the score of strength for weight, in these small boats. The main strength of the boat is in the pair of deep fore-and-aft girders which extend from end to end of the hull, carrying the motor, clutch gear, and the thrust, and I believe she was one of the first boats to be built with the idea of utilizing the engine bearers as the main longitudinal strength of the hull, in place of the ordinary keel construction. The motor is a four-cylinder Napier of about 66-b.h.p. It ran fairly well, and considering its very moderate power, drove the boat at a reasonably good speed. She had, I believe, a two-blade propeller, 28 inches in diameter, during her first year's racing; but, as the propeller was fitted while I was abroad, I was never able to examine it, or to obtain any accurate details as to pitch and blade area. Her best race record is 18.88 knots over a course off Ryde which included several sharp turns, so no doubt she actually touched a mean speed of 19 knots. probably her best speed on a measured mile under favorable weather conditions would be about 19 1/2 knots.
This boat was originally known as Gardner-Serpollet, and was designed by me at the end of 1903 for M. Legru of Paris, being built in his own workshops by men specially sent over by Mr. Saunders, of Goring. She was built of four thicknesses of mahogany and cedar on the well-known Saunders' sewn system, and has, I believe, shown no signs of wear after two years of very rough treatment. Like Napier I, she has the long and deep engine bearers forming her chief longitudinal strength; ut in design she varies considerably from the earlier boat, having a much finer entrance, rounder bottom, with broader and flatter sections right aft, the greatest breadth on the water line being about 0.63 of the length from the stem, instead of 0.57 as in Napier I. Her curve of areas as originally designed was also different. She was designed for a displacement of 2.5 tons, and was originally fitted with a Gardner-Serpollet steam engine; this, however, proved unsatisfactory, her best speed under steam being only about 16 knots, although the engine was stated by the makers to be capable of developing anything up to three or four hundred horse-power. The steam engine was then removed with all its cumbrous accessories of generator, condenser, etc., and a pair of four-cylinder petrol motors were installed in its place. During the early part of 1905 she did nothing much, although she showed on several occasions that she would have a good turn of speed when tuned up. However, by August last, she was in good running order, and in the Rondet Saint Trophy race, from Trouville to Honfleur and back, she attained a mean speed of 26.58 knots, the mean revolutions of the motor being 780 per minutes. The maximum brake horse-power of the two motors is 170.
In a subsequent trial on the Seine, Legru-Hotchkiss attained a speed of 29.68 knots over a distance of 5 nautical miles, the speed of the motor being 915 revolutions per minutes. This is considerably in excess of any speed hitherto attained by motorboats, the best previous record being, I believe, 28.19 knots over one nautical mile, by the 50-foot Dubonnet with 300 h.p..
In comparing these two performances of 26.58 and 29.68 knots, it should be noted that the propeller is 27 1/2 inches in diameter and 49 inches pitch; the slip at 26.58 knots with 780 revolutions, is just under 16 per cent, while at 29.68 knots and 915 revolutions the slip is 19 1/2 per cent, showing an increase of over 3 1/2 per cent slip in three knots; whereas, in the case of a similar boat, the slip hardly varies between 22 and 25 knots. The two motors are set in tandem, and consist of 8 cylinders of 180 mm. bore, or 39.4 square inches in piston area, and 0.56 feet stroke, the power under the Marine Motor Association's rule being 137.7 m.p. at 780 revolutions, and 161.5 m.p. at 915 revolutions.
In the autumn of 1903, I designed this boat for J. E. Hutton. She was designed for a pair of 140 B.H.P. motors and twin screws; but, owing to the machinery weights having exceeded the maker's estimates, the motors were found to be much too heavy for a pair to be used, and it was therefore decided to alter the internal framing of the boat, and fit a single motor in her. She was a very strongly built boat, with a single skin ribband carvel cedar planking 5/8 inch thick at the bottom and 1/2 inch at the sides, with steamed timbers of rock elm 5/8 inch by 3/4 inch, spaced 4 inches centre to centre, bent right around under the keel and scarphed along the centre line of the deck. She had originally three lines of deep girder engine bearers extending throughout her length, of 2-inch Oregon pine, about 16 inches deep at the fore end of the motors, one of them forming the keel. When it was decided to abandon the twin motors, the deep internal keel or central engine bearer had to be cut away to within 3 inches of the skin for some 8 feet or so, to make room for the single motor amidships. Two short supplementary engine beds were then fitted on the oak floor frames, and connected with the main fore-and-aft girders, which were now too far apart to carry the single motor. The engine was placed 3 inches to port of the centre line of the boat, giving her a slight list, which just counterbalanced the effect of the propeller, and prevented any list to starboard when the clutch was thrown in suddenly. She has proved herself an excellent sea boat, and runs remarkably clean. Hutton II has a 6-cylinder Hutton motor of 140 B.H.P. at 1,150 revolutions.
I think the proportions of power to displacement in these three boats show that internal- combustion motors have great advantages over even the most modern forms of steam engine, when high speed and lightness are required in small craft; and, as all the fastest motorboats have been produced entirely through the keen competition of international racing, I think that it is only reasonable to admit that this racing is doing useful work towards the evolution of high-speed boats.
(Excerpts transcribed from The Motor Boat, June 10, 1906, pp.11-17. )
[Thanks to Greg Calkins for help in preparing this page LF]