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There seems to be a bit of confusion here. There are basically 4
different types of props with movable blades. All can improve sailing performance to one degree or another. Folding props: Blades with a fixed pitch are pivoted along the axis of the shaft so that they fold back in the fore and aft direction. Blades may be geared together or independent. In forward they are held open by the forward thrust. Most have some camber so they are close to the efficiency of fixed blades in forward but in reverse they are held open by centrifugal force which means that you have to apply more power to get them to perform in reverse. Folding props are preferred when sail performance take preference over powered performance. Feathering props: Blades are pivoted (more or less) perpendicular to the shaft. They remain extended when idle but align themselves with the flow to present the smallest cross section. The blades are geared to the shaft so that they are held open by the torque. Pitch can be adjusted by modifying the stops. In reverse the torque flips the blade over so that you get the same pitch (and performance) in forward and reverse. However, to achieve the lowest drag the blades usually do not have any camber making them slightly less efficient. Feathering props are preferred where a balance must be struck between sail and powered performance. Variable Pitch Props: Blades are assembled similar to feathering props but are geared to a control shaft concentric to the drive shaft. By adjusting the position of the control shaft relative to the drive shaft from inside the hull the pitch can be varied to meet current conditions. Usually the blades are cambered to optimize forward performance. Variable pitch props are preferred where maximum performance under power in all conditions is desired and cost is not a limiting factor. Auto-Prop: Blades are pivoted similar to feathering props but are independent of each other and can rotate a full 360º. The offset geometry of each blade is designed to find its own most efficient pitch by balancing torque against water pressure. Auto-Props can give close to optimum performance in most conditions in forward or reverse. They are not truly feathering however. With no torque water pressure forces the blades back slightly which results in considerably more drag than normal feathering props. Also they have considerably more mass which puts a lot of strain on the drive train when shifting from forward to reverse and back. If left idle for any length of time they require considerably more maintenance than the others to keep the blades rotating freely. Auto-Props are best where powered performance takes precedence over sailing performance. -- Glenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com |
On 30 Aug 2004 03:34:57 GMT, (JAXAshby) wrote:
rhys, hate to tell you this, but an Auto-Prop doesn't feather. From www.autoprop.com "In forward the propellor rotates to the correct pitch automatically. This results in increased propulsion efficiency whcih reduces fuel consumption, and extends cruising range. "While under sail the propellor feathers itself to reduce drag by 85% compared to conventional 3-blade propellors. "The Autoprop also self pitches in reverse to give you the same thrust in reverse as you would get in forward. This greatly improves stopping power, backing down, and overall maneuverability." Note the use of the word "feather" as in "feathers itself", a concept with which I believe you would be familiar, JAX, particularly on those lonely Saturday nights. Still, the clown spent $3,500 Cdn, so you gotta expect him to tell you *something* That "clown" has seen more salt water than Mr. Morton during a flash flood, JAX, including the first private yacht transit of Hudson's Bay since...well, Hudson, so let's just say he doesn't quite have the credibility gap you exhibit with most of your bilious, ill-informed, poorly argued and borderline dyslexic posts. Now, punk, go measure a footwell or something. Sailors are speaking here. Really, could the barrel get any smaller or the fish and the cannon any bigger? R. |
On Mon, 30 Aug 2004 09:39:46 -0400, Glenn Ashmore
wrote: Auto-Prop: Blades are pivoted similar to feathering props but are independent of each other and can rotate a full 360º. The offset geometry of each blade is designed to find its own most efficient pitch by balancing torque against water pressure. Auto-Props can give close to optimum performance in most conditions in forward or reverse. They are not truly feathering however. They are close enough in function to use the term constructively. I don't think Autoprop's self-description of their product as a "feathering prop" constitutes trade fraud in this instance. With no torque water pressure forces the blades back slightly which results in considerably more drag than normal feathering props. Also they have considerably more mass which puts a lot of strain on the drive train when shifting from forward to reverse and back. Agreed. As noted, my friend accepts the wear as adequate pay-off for the motoring performance enhancements he was seeking. I wouldn't put an Autoprop on a J-Boat, for instance, or any racer-cruiser. It's a good compromise if you understand the pros and cons, not a universal panacea for prop drag. If left idle for any length of time they require considerably more maintenance than the others to keep the blades rotating freely. He hauls in a TraveLift once a year (luckily his club possesses one) and inspects and adjusts then as part of his general yearly hull maintenance/cleaning/repainting. He says it's pretty straightforward so far, but he acknowledges that they are complex pieces of machinery for props. Auto-Props are best where powered performance takes precedence over sailing performance. Debatable, if you consider the alternative as being a fixed prop or a folding prop. I think you have to consider hull type, displacement and engine output along with intended use. My friend takes his large steel ketch out alone a great deal, and while he is fine sailing it solo, he appreciates the degree of control his Autoprop gives him in tight situations and in solo docking. Certainly that aspect--the degree of control of a 15 ton boat-- is quite noticeable and is obviously worth it to him in his use of a heavy displacement cruiser. That's why I tried to give both pros and cons, as the Autoprop isn't particularly well-known, being British. Getting one personally would be senseless for my current boat, but seems a good compromise for him and has bought him a few more years out of his 35 HP Volvo, even if he has to rethink transmission isolation and so on. But it's not for everyone. No "marine solution" is, except maybe for those wooden tapered plugs people hang off seacocks. G R. |
On 30 Aug 2004 11:58:56 GMT,
JAXAshby wrote: The blades rotate until they are inline with the shaft axis, how is this not feathering? take a look at the blades. they rotate and thus *reduce* drag, but they certainly don't "feather". I have, the blades rotate to line up with the axis of the shaft, how is that *not* feathering? -- Jim Richardson http://www.eskimo.com/~warlock If Bill Gates had a nickel for every time Windows crashed... Oh wait, he does |
the blades are curved, and rather dramatically so.
The blades rotate until they are inline with the shaft axis, how is this not feathering? take a look at the blades. they rotate and thus *reduce* drag, but they certainly don't "feather". I have, the blades rotate to line up with the axis of the shaft, how is that *not* feathering? -- Jim Richardson http://www.eskimo.com/~warlock If Bill Gates had a nickel for every time Windows crashed... Oh wait, he does |
So,Doodles, what you are saying, is that unless the blades are totally
flat, the term "feathering" can not be used? otn JAXAshby wrote: the blades are curved, and rather dramatically so. The blades rotate until they are inline with the shaft axis, how is this not feathering? take a look at the blades. they rotate and thus *reduce* drag, but they certainly don't "feather". I have, the blades rotate to line up with the axis of the shaft, how is that *not* feathering? |
of course, or very nearly so. That is what the word means. At least to the
professionals. rubber ducky sailors are a different breed, of course. Subject: What are the pros and cons of a folding prop? From: otnmbrd Date: 8/31/2004 10:20 PM Eastern Daylight Time Message-id: . net So,Doodles, what you are saying, is that unless the blades are totally flat, the term "feathering" can not be used? otn JAXAshby wrote: the blades are curved, and rather dramatically so. The blades rotate until they are inline with the shaft axis, how is this not feathering? take a look at the blades. they rotate and thus *reduce* drag, but they certainly don't "feather". I have, the blades rotate to line up with the axis of the shaft, how is that *not* feathering? |
So,Doodles, what you are saying, is that unless the blades are totally
flat, the term "feathering" can not be used? otn let me help improve, over the knee, with your understanding of English words. v. feath·ered, feath·er·ing, feath·ers v. tr. To turn (an oar blade) almost horizontal as it is carried back after each stroke. To alter the pitch of (a propeller) so that the chords of the blades are parallel with the line of flight. |
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