steve@uunet.UU.NET (Steve Nuchia) (06/06/90)
From: nuchat!steve@uunet.UU.NET (Steve Nuchia) In article <16131@cbnews.ATT.COM> carroll1.cc.edu!tkopp (Tom Kopp) writes: >In article <15934@cbnews.ATT.COM> nuchat!steve (Steve Nuchia) writes: >>Cavitation occurs on the *forward* side of the screw blades, so .... >Maybe I need an explanation of cavitation. When I first heard about it, >and that it was caused by propeller-induced vacuum, the following is what >I reasoned out: >The propeller, being canted to provide thrust, "compresses" water ahead of [absolutely correct description deleted] >Where did I go wrong with this? A simple matter of semantics. My "forward" referred to the overall direction of travel, Tom's "ahead" refers to the motion of a blade transverse to the direction of motion. We said the same things in different reference frames. The actual motion of a point on a propeller blade is a helix, with the water passing more-or-less cleanly across the broad surfaces of the blades. If you think of the entire propellor as a disk, it is the pressure difference across it that provides thrust, the rear surface having higher pressure than the forward surface. If you try to go faster you have to increase the pressure difference, some of which shows up as a reduction in the pressure ahead of the screw. When the absolute pressure drops low enough, cavitation occurs: first at localized imperfections and then generally. A submarine at depth is able to use more power for a given screw size because the absolute pressure ahead of the screw starts out greater. The design of the propeller is tricky, since the pitch has to vary with the radius. (linear velocity constant, angular velocity constant, circumferential velocity changes with radius.) Airplane propellers have a "twist" in the blades for this reason. The blades of a marine propeller are much broader, which make the curves more complex. It is not easy to build a large screw with such complex compound curves, and imperfections lead to early cavitation, not to mention ordinary noisy turbulence. That is why there was such a flap about Toshiba selling those computer controlled milling machines to the Russians a few years ago. Supposedly they were ideal for making submarine propellers, and our government wasn't real happy about the sale. -- Steve Nuchia South Coast Computing Services (713) 964-2462 "The study of the art of motorcycle maintenance is really a miniature study of the art of rationality itself. Working on a motorcycle, working well, caring, is to become part of a process, to achieve an inner peace of mind. The motorcycle is primarily a mental phenomenon." -- Robert M. Pirsig
dick@zombie.dtc.hp.com (Dick Lucas) (06/06/90)
From: dick@zombie.dtc.hp.com (Dick Lucas) If you think about it, your explanation where the pressure is higher on the forward part of the propellor will cause the thrust to be reversed from what normally happens. A propellor works much the same way as an aircraft wing, with the low pressure on the forward (upper surface in the case of a wing) surface. If the pressure gets too low, the water will boil (cavitate) and the forces generated when the small bubbles collapse can be high enough to erode metal.