In a variable‐pitch screw propeller each blade has a hollow root portion rotatably mounted over a spigot on the boss and a threaded sleeve member is located between them, one end of it terminating in a ram, the blades and spigots having relatively inclined threads co‐operating with threads on the sleeve member whereby, when pressure is applied to the ram, the parts are displaced so as to rotate the blades for pitch variation. The blade 4 has an inner bush 8, which is secured thereto by screws 10 and carries a bevel wheel 25. This sleeve is splined at 9 to engage splines on a ram sleeve 11, the upper part of which projects into a chamber 22 which is in communication by ducts 21 with passages in the propeller hub and shaft leading to a fixed pipe supply 13. The inner part of the ram member 11 has helical splines which engage corresponding helical keys on the boss spigot 9. With increase in revolutions, the sleeve block 4, under centrifugal force, tends to move outwardly and restraint thereto is applied by the fluid pressure in the space 22. A second set of supply ports 24 may be formed at a different position relative to the travel of the sleeve whereby the degree of movement of the sleeve can be controlled by using different pressures of oil supply. Where the weight of the ram members 11 is not sufficient, the gear‐wheels 25 may gear with others on axes at right‐angles which are secured to similar movable sleeves, but these do not directly operate other propeller blades. Alternatively, there may be fitted at the base of the sleeve a spring supplementing the centrifugal force and tending to cause it to move outwards, in which case the member 11 can be lighter in weight.
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