The Thornycroft Racers 
Miranda IV has been on trial and has attained a mean speed of just under 35 knots on six measured runs with and against the tide. This accomplishment on the second day that she was afloat gives promise that she will obtain the 38 knots which I predicted that she was designed for. The engine is not yet running at its full speed, and I believe that she has nearly another couple of hundred revolutions per minute in hand. Miranda IV is the boat which Sir John Thornycroft has constructed for his own use and is a sister boat to Zigarella, which Mr. Hanbury will start in the race for the British International Trophy.
While Miranda IV has been fitted out under the supervision of Thornycroft’s, Zigarella has been in the hands of F. R. S. Bircham. Both hulls have been laid down to the same design, that for Miranda IV having been built on the Thames by Hart-Harden, while Zigarella was turned out by Messrs. Luke of Hamble, Southampton. While Zigarella had some trouble with her supply of water for the circulation system, Miranda IV has proved an unqualified success from the moment she was put afloat. This difference is due to the fact that the installations vary slightly, and the water intake was originally put further forward in Zigarella than it is in Miranda IV.
Zigarella went to Kiel, where she had to meet Ricochet XXII, the latest French hydroplane of the Lelas type. At the first meeting both boats failed to complete the course. When they came together the second time Zigarella won at the very moderate speed of 25 knots. The third race between them was off, owing to Zigarella’s early return to England.
The leading dimensions of this boat are: length, 26 feet; beam, 6 feet.. When at rest the boat gives the impression of a good beamy craft of the ordinary type. It is only when a fairly high speed is attained that any peculiarity is seen. As soon, however, as the engine is let out, the bow rises and one notes a curious section given to the bottom of the fore-body. There appears to be a very hard turn starting some twelve inches above the keel, and I am told that this section dies away amidships, where there is a sudden change, the lines of the after part of the body being quite hollow and terminating in a flat form of stern which is slightly wider than the water bearing section just forward of it. The effect of this form ids that the boat rises quite easily till it runs on the surface of the water, touching only on a small area amidships, and again just at the stern. One of the most extraordinary features is that in place of the broad, flat stern to which racing boats have accustomed us, the lines run away to a plumb stern post, as in the type known as a "double-ended" boat.
There are no flat places in the bottom of the hull which would be liable to damage in rough water were the boat going at high speed from wave to wave. A boat has been obtained which is sufficiently like the ordinary type to run without undue resistance at moderate speed, and yet, though very fast, quite as seaworthy as an ordinary boat in rough water. Sir John Thornycroft has departed entirely from the type of hydroplane which depends upon power and light weight for its high speed. What he has evolved is a form of hull which, while giving the high speed of the light hydroplane with just as moderate power, appears so little influenced by weight that no paring is necessary in either engine or hull. Miranda IV weighs nearly a ton, so that her ratio of weight to power is about 20 pounds per horsepower. It is remarkable that with this a speed of 35 knots can be obtained almost at the very first trial and that a higher speed is sure to be reached.
An eight-cylinder engine of about 110-hp. is installed. This engine alone weighs just short of nine cwt. The cylinders are cast in pairs, 4 inches bore by 7 inches stroke, and with valve 2¼ inches diameter. The inlet valves are overhead while the exhaust valves are at the bottom of valve pockets which face the center line. The exhaust is taken away through two oxy-acetylene welded pipes, into which the water from the jackets is discharged, so that from the engine to each side of the boat the exhaust pipes are cool. They are not bent round at a sharp angle but are taken away by a very easy curve to the planking. Instead of having a carburetor between the two sets of cylinders, which are mounted four aside in V fashion, so that a very short induction pipe system can be obtained, the carburetor is to be found high up abaft the engine and delivering the mixture into two long branches which run round to the overhanging cylinders and there feed endless induction pipes formed in the shape of a figure "8." The idea of this design is that instead of having a mixture kicking backwards and forwards in a straight pipe, there is a continuous flow of gas round the figure "8," and the inertia of this column of mixture causes the supply to the cylinders to commence immediately the respective inlet valves open. The carburetor itself is interesting as having a perforated tube in place of the more usual jet, and it is controlled by a rod entering the tube from above and covering or uncovering a number of holes as the auxiliary air inlet ports are regulated by the piston throttle. High-tension magneto ignition is employed, the magneto having been specially made by the Bosch Company to give four sparks per revolution, as otherwise a very high speed would have been necessary. All the gear wheels are at the forward end of the engine with the magneto, besides a skew- geared cross shaft for the oil and water pumps. The oil is cooled by water, and is thus supplied cold to the main bearings and through the hollow crankshaft and crank webs to the big ends of the connecting rods.
A whale-back extends over the engine and affords shelter to the helmsman. The boat is extraordinarily dry and oilskins are not required. Though not pretty, the whale-back is useful, because it has sufficient headroom to enable one to get at the engine. A space is cut in it just forward of the engine, to enable the mechanic to get as much fresh air as he likes. There is no sliding hatch over this space, because no water can be thrown over it, it being really forward of the section which supports the boat at high speed. It is quite a novelty to be on a racing boat which is so dry.
At full speed there is none of the hard spanking which one would expect . During a run in the boat on the Thames, seated on hard wood, I found not the slightest discomfort even when we bounced form wave to wave in the wash of a tug. The impression of speed is not in the least startling when one is aboard the boat. It is only by perceiving objects flying by that one gathers how fast the boat is traveling. Wings of light spray are thrown up on each side, but not a drop over the boat except when she is jogging along at about half speed. Apparently she cares little for the number of passengers aboard because she flits over the water in her best manner even with a load of five.
With planking about half an inch thick at the bottom, and ribs spaced only three inches apart, the hull is so rigid that one does not feel the slightest tremor in the boat even when she spanks resoundingly from wave to wave. The bottom planking is of mahogany and the top sides of pine, painted in the English racing color, green, and white underbody and white deck.
(Transcribed from MotorBoat, July 25, 1910, pp. 39-41.)
[Thanks to Greg Calkins for help in preparing this page --LF]