PROPELLERS

Evening Post, Volume CVII, Issue 140, 14 June 1924, Page 16

PROPELLERS

LATEST METHOD OF SHROUDING.

The open type of screw propeller is an instrument whose principles of design have long been known by engineers, says the ■''' Scientific American.'' To-day it is possible, to predict the performance of a correctly-designed open screw with remarkable accuracy, provided the performance is made under ideal or laboratory conditions. Such a propeller, when mounted on a ship and sent to sea meets with. varying conditions of .work, constantly varying draft, etc., so that it usually . becomes necessary at some loss in efficiency to use a type of propeller which is the most: economical compromise between the dictates of theory and of practice. . ' The shrouded propeller has been designed, according to its English manufacturer, ,to make its own conditions with the water. The shroud consists of , a band fixed to the outer end of the blades', and this band has a foro-and-af f taper; so that the stream produced by the propeller "is confined. In addition, the working pitch of the blades increases axinlly. Owing to these two facts there is a regular acceleration in liic stream acted upon. A typical open propeller, with a- shroud fitted to it-, v.oukl not constitute a satisfactory propeller. It would be so large that it would overload the engine, the shrouded propellor usually being made '10 to 15 per cent, smaller in, its determining proportions than an open screw for the same installation. Again, such a propeller would lose much of its efficiency, it is stated that the shroud of the'new propellor takes the place of the- surface of tearing and eddies which in the usual screw surrounds the stream of water directly acted upon. The distinguishing feature of-this .shrouded screw is that it causes allvthe water acted upon lo be thrust astern instead of in a diversrei:i direction, £he latter effect being due to -a-, centrifugal force. This 'introduces the subject of " slip " and it is etated that a propeller whoso advance through a solid nut would be about one and a quarter times that of tho actual forward advance of the sliip i 3, generally speaking, more efficient than one with less "slip." In this sense the shrouded propeller is less wasteful in attaining the requisite value of slip than are open screws. Another advantage of the- shouded propeller is evident when working under very shallow conditions, for it is very much less likely to "break -water" than the open screw. It also develops the merits of tho broad tipped blade without incurring vibration. "• It obviates " squatting " at the stern and leaves a relatively smooth propeller wake, the latter being a very valuable attribute in the case of vessels operating in "canals and other waterways of restricted area..

The commonest objection to the shrouded propeller 'is . that the shroud occasions increased friction due -to its drag. However, it is claimed' that considerably more useful effect is produced by the shroud than is lost by its surface friction. Another objection frequently raised is that the shroud should be fixed with the propeller rotating within it. But such an arrangement would be unsound hydraulically on account of excessive eddy losses. In .practice, it Has been found that if any solid objectsit meets are too large to go through the opening, they are thrown off. In short, in meeting these objections, the p.oof of the propeller i 3 in the propulsion, and the shrouded propeller is stated to have produced towing results o per. cent, better than those attained with the best open propeller available.