For more than twenty-two years the side wheel steamboat Mary Powell has been recognized as the fastest boat on the Hudson River : she makes an average , of twenty miles an h o u r. and according to a pamphlet issued by the owners, in the year 1882, she ran at the very fast rate of 26 miles an hour between Milton and P-ou g l k e e psie, making the four miles in nine minutes. Boats of all sorts of shapes, big and little, side wheels and propellers, have un-succes s full y attempted to wrest from her the well earned title of Queen ofthe Hudson. But on the 10th inst. she was badly beaten in a long run by a small steam yacht of very insignificant appearance. TheruUILwa& fem. this city to Sing Sing, a distance of thirty miles, and was made by the steam yacht Stiletto in one hour and fifteen minutes, the Mary Powell, on her regular trip to Rondout, being beaten about two miles. The Stiletto was designed and built by the Herres-hoff Manufacturing Co., of Bristol, R. 1. She is 94 feet long over all, 90 feet on the water line, 11 feet beam, and weighs 28 tons. The hull is double planked, and sharp at both ends, the curves extending far toward the center. A slightly arched deck covers the whole boat. Forward is a pilot house sufficiently large to serve as a commodious cabin. Owing to the extremely small space taken up by the engine and boiler r o o II S, there is ample room for comfortable quarters for the crew and staterooms for the owner, guests,and officers. Power is furnished by a compound co n-densing engine of 12 inch stroke and cylinders 12 and 21 inches in diameter; the engine is supplied by a sectional w ate r tube b o i 1 er in which steam can be got up quickly and which is calculated at 450 horse power. A1-though this boiler is similar in principle and operation to those of the regular Her- reshoff type it varies greatly in construction, the tubes being arranged horizontally in .sets immediately over the fire—each set being at right angles to those just above it. Exhaust steam is led to a surface condenser. An ordinary pump takes the water from the condenser, forces it into the upper set of boiler tubes, through the boiler to a separator located in front of the boiler, and to which the steam pipe is connected. The boiler will work safely with 160 pounds of steam, but in the race with the Mary Powell it was only found necessary to use from 120 to 125 pounds. The fire box is 6)3 feet square. The screw is four-bladed, 4 feet in diameter, and 6 feet pitch. At the stern the boat draws 4 feet and at the bow 3 feet. We may notice that there are now building at the yards of Yarrow & Co., England, two torpedo boats which are expected to run, when light, at the rate of 24 knots an hour, or nearly 28 miles. The Stiletto must do better than 25 miles an hour before she can claim the broad title of the fastest boat in the world. In our issue of next week we will illustrate and describe in detail the construction of the boiler and engine and the method of forcing the circulation.