HIGH-PRESSURE MACHINERY IN THE "THETIS."

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The three vessels which inaugurated the very successful Holt line were named Agamemnon, Ajax, and Achilles, and were built of iron by the Scotts in 1865-6. They were each 309 ft. in length between perpendiculars, 38 ft. 6 in. beam, and 29 ft. 8 in. in depth, with a gross tonnage of 2347 tons—dimensions which were then deemed too great for the China trade, but which experience soon proved to be most satisfactory. Sails were fitted to the vessels, as shown in the engraving on the [Plate] facing page 40.

Alfred Holt was the first to apply the compound engine to long voyages, and his vessels were the earliest of the type built for the merchant service by the Scotts. It is true the Pacific Company had compound engines fitted to one or two ships prior to this, but these were only used in the coasting trade. The engines of these Holt liners are therefore of historical interest, and general drawings are reproduced on the next page and on [Plate XII]. A feature in these liners was that the propeller was abaft the rudder, which worked in an aperture in the deadwood corresponding to that for the propeller in single-screw modern ships.

A detailed description from the specification of the machinery may be reproduced, as it indicates the practice of the Scotts for a considerable time. Indeed, this type of compound engine, with slight modifications, was the standard engine for Holt liners until the advent of the triple-expansion engine. The details follow:—

The cylinders were: high-pressure, 30 in. in diameter; low-pressure, 62 in. in diameter, with 4 ft. 4 in. stroke, arranged vertically in tandem fashion, with the low-pressure cylinder on the top. There were two connecting-rods, but a common crosshead for the tandem cylinders, and a common crankpin.

The crankshaft was 13-1/2 in. in diameter, with a bearing 30 in. long at the aft end of the bedplate, which took the propeller thrust. The propeller was three-bladed, 17 ft. in diameter, with 26 ft. 6 in. pitch; with 46 revolutions per minute the piston speed was 400 ft. per minute. To ensure smooth working with the single crank, a heavy flywheel was fitted, and the pump levers carried a massive weight to help to balance the weight of pistons and rods.