As typical of the engines adopted in the yachts built by the Scotts, we give an illustration on [Plate XXVI]., facing page 72, of the engines of the Grianaig. In the thirty years that have elapsed since the first Greta was built, the ratio of horse-power to tonnage has increased from 1 to 1 to 2 to 1, the steam pressure from 74 lb. to 200 lb.; and the piston speed from about 300 ft. to 675 ft. per minute. The aim has been to ensure reliability by a steady- and easy-running engine.
An effective appearance has always been aimed at, and the result has invariably been a highly-finish design. Yachts' engines are invariably balanced, whether so specified or not, as the gain in comfort to all on board, owing to the absence of vibration, is so marked as to more than compensate for the extra cost involved. Forced lubrication has also been applied, although the engines may be of the ordinary open type: the main bearings, crank-pins, cross-heads, eccentrics, valve gear, pump gear, etc., are all included in the system, which has given every satisfaction.
The Grianaig's engines developed on trial 740 indicated horse-power at 148 revolutions per minute, with a boiler pressure of 190 lb. per square foot, and a condenser vacuum of 26.5 in. Some of the details, being typical of the practice of the firm in respect of yacht machinery, are quoted from the specification on the next page.
The arrangement of cylinders is as follows: H.P. 14 in. in diameter, I.P. 22 in. in diameter, L.P. 35 in. in diameter, Stroke 24 in. The piston and connecting-rods are of steel; the guide-shoes for the crossheads are of cast iron, the ahead face having white metal, and the astern face being left plain. The back columns are of the usual cast-iron box type, the front columns, being steel, are turned. The high-pressure cylinder has a piston valve, and the intermediate- and low-pressure cylinders flat slide-valves. None of the cylinders is provided with liners. A single-stroke reversing engine is situated at the back of the main engine, but is operated from the starting platform. The condenser is of the surface type with a circular cast-iron shell; the total cooling surface is 1300 square feet.
Steam is supplied to the main engine by one single-ended cylindrical boiler 13 ft. 9 in. in diameter by 10 ft. long, working at a pressure of 190 lb. per square inch. There are three furnaces, the mean internal diameter being 3 ft. 5-3/4 in. and the length 6 ft. 10 in. The grates are 6 ft. long, giving an aggregate area of 61.5 square feet. The boiler tubes are 3-1/4 in. in diameter and 6 ft. 10-3/4 in. long, the total heating surface being 1899 square feet.
Plate XXVI.
