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Seamanship Question #15 Props
5 points (Impress me with your genius)
You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
Remove it, turn it around, put it back on. CN "Bart Senior" wrote in message . net... 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
Close Neal. Because of the taper fit, that won't work. Care to elaborate on your answer? "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in message . net... 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
If the shaft has a taper there is no way to put it on
backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Bart Senior" wrote in message . net... Close Neal. Because of the taper fit, that won't work. Care to elaborate on your answer? "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in message . net... 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
You replaced it with one that was identical. Try
replacing it with one that isn't old and brittle. S. "Bart Senior" wrote in message . net... : 5 points (Impress me with your genius) : : You just bought a new boat and notice the prop is old and brittle. : : You replace it with an identical prop. After motoring around for a : while you find the top speed of the boat is much lower, estimated to : be 2.5-3 knots and the prop seems to be slipping. : : What is the most likely cause, explaination, and cure? : : |
Putting a LH prop on a RH setup engine would require running in reverse.
This will work on some engines/trannies, others will be destroyed by constant running in reverse, and many will have a different gear ratio. In any case, I think it would be noticeable on any modern setup. Capt. Neal® wrote: If the shaft has a taper there is no way to put it on backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Bart Senior" wrote in message . net... Close Neal. Because of the taper fit, that won't work. Care to elaborate on your answer? "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in message . cv.net... 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
The brittleness is an indication of surface hardening and roughening, either
by mechanical or electrochemical means. Since the neutral axis of the chord of the blade lies in the concave region of the cusp, the entire blade acts intension and the surface hardening has caused the cusp to increase (shorter surface length on the inside of the cusp) and the water displaced with each turn of the propeller is more. Now to your anticipated answer, which is wrong. The new prop does not slip because it has less surface friction. The surface friction only drags the water in the direction of the propeller rotation, it cannot create a water trajectory normal to the propeller surface. In fact, propellers with less surface friction are faster. Amen! Bob Crantz "Bart Senior" wrote in message . net... 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
On Sat, 20 Nov 2004 04:29:47 GMT, "Bart Senior"
wrote this crap: 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? Outboard or inboard, dumbass? I've never heard of an inboard with a prop that slips. But on my Evinrude 9.9, it didn't have a sheer pin, and the rubber ring caused it to slip. I had to buy a new prop. BTW, how can a prop be old and brittle? What's it made of? plastic? Pathetic Earthlings! No one can save you now! |
Bart,
The First thing that came to mind was that the old prop had been "re-pitched" and wasn't truly what the stamping indicated. In the old days (Depression) this was a cheaper solution than a replacement. ???? Ole Thom P/S remove the new prop and go with the Old. |
On Sat, 20 Nov 2004 14:45:22 GMT, "Bob Crantz"
wrote this crap: The brittleness is an indication of surface hardening and roughening, either by mechanical or electrochemical means. That's the stupidest thing I ever heard. Since the neutral axis of the chord of the blade lies in the concave region of the cusp, the entire blade acts intension and the surface hardening has caused the cusp to increase (shorter surface length on the inside of the cusp) and the water displaced with each turn of the propeller is more. That's not likely. I think you're making up this bull****. Now to your anticipated answer, which is wrong. The new prop does not slip because it has less surface friction. The surface friction only drags the water in the direction of the propeller rotation, it cannot create a water trajectory normal to the propeller surface. In fact, propellers with less surface friction are faster. This is bull****. Pathetic Earthlings! No one can save you now! |
"Bart Senior" wrote in message .net...
5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? Put the woodruff key back on/in. Joe |
"Horvath" wrote in message ... That's the stupidest thing I ever heard. That's not likely. I think you're making up this bull****. This is bull****. http://www.armadahull.com/proparticle.htm Amen! Bob Crantz |
5 points to Neal.
Inboard diesel powered boats commonly have different gearing in reverse. Incorrectly installing a RH or LH pitched prop when the opposite was called for could cause a boat to run at slower than expected speeds, albeit it would move forward in the reverse position and vice versa. Neal immediately grasped that the gear ratio might be wrong and this could be explained by the boat running in reverse, and at a lower gear ratio. Capt. Neal® wrote If the shaft has a taper there is no way to put it on backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
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Why don't you come right out and say you're an idiot, instead of giving us
the numbers. -- "j" ganz @@ www.sailnow.com "Capt. Neal®" wrote in message ... You're just upset because I'm so brilliant and you're not. Even though I am a sailor, I know more about motors and gears than the average bear. When one has an IQ, measured by several different professionally administered tests, in the 148 to 155 range, things that most people have to think long and hard about just seem to be immediately available in my brain without even giving them much thought. This is perhaps why I have so little patience with liberals. To be a liberal is to be stupid. There is no other explanation. CN "Joe" wrote in message om... (N1EE) wrote in message . com... 5 points to Neal. Inboard diesel powered boats commonly have different gearing in reverse. Incorrectly installing a RH or LH pitched prop when the opposite was called for could cause a boat to run at slower than expected speeds, albeit it would move forward in the reverse position and vice versa. Neal immediately grasped that the gear ratio might be wrong and this could be explained by the boat running in reverse, and at a lower gear ratio. And you said identical. Identical is not a diffrent rotating prop. To much rum Bart? WTF.... you usually do much better than this. Sheeeeeze Joe Capt. Neal® wrote If the shaft has a taper there is no way to put it on backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
Capt. Neal® wrote in message ...
You're just upset because I'm so brilliant and you're not. Even though I am a sailor, I know more about motors and gears than the average bear. I have to dis-agree. Idential is not a counter rotating prop. If he said identical pitch then perhaps. But he did not. Can you not understand a question? RH RH = identical LH LH = identical LH RH is not RH LH is not Please look the word "Identical" in the dictionary. The only one who had, the only proper answer, was me. A missing woodruff key is the only proper answer to Barts question, besides the transmission bands slipping. THATS IT. Joe When one has an IQ, measured by several different professionally administered tests, in the 148 to 155 range, things that most people have to think long and hard about just seem to be immediately available in my brain without even giving them much thought. This is perhaps why I have so little patience with liberals. To be a liberal is to be stupid. There is no other explanation. CN "Joe" wrote in message om... (N1EE) wrote in message . com... 5 points to Neal. Inboard diesel powered boats commonly have different gearing in reverse. Incorrectly installing a RH or LH pitched prop when the opposite was called for could cause a boat to run at slower than expected speeds, albeit it would move forward in the reverse position and vice versa. Neal immediately grasped that the gear ratio might be wrong and this could be explained by the boat running in reverse, and at a lower gear ratio. And you said identical. Identical is not a diffrent rotating prop. To much rum Bart? WTF.... you usually do much better than this. Sheeeeeze Joe Capt. Neal® wrote If the shaft has a taper there is no way to put it on backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
One has to live in the real world. Especially these
days. When somebody uses the word 'identical' a smart person realizes an identical in every way prop would behave the same as the original. Therefore one must conclude the prop was not identical provided it was not installed backwards. Backwards installation was my original answer which was made impossible with the inclusion of the information on a tapered shaft. Therefore, using logic, the only answer was 'identical' was not identical. Brilliant minds always consider real world situations and do not rely on strict definitions of words most people use very loosely. Savvy? CN "Joe" wrote in message m... Capt. Neal® wrote in message ... You're just upset because I'm so brilliant and you're not. Even though I am a sailor, I know more about motors and gears than the average bear. I have to dis-agree. Idential is not a counter rotating prop. If he said identical pitch then perhaps. But he did not. Can you not understand a question? RH RH = identical LH LH = identical LH RH is not RH LH is not Please look the word "Identical" in the dictionary. The only one who had, the only proper answer, was me. A missing woodruff key is the only proper answer to Barts question, besides the transmission bands slipping. THATS IT. Joe When one has an IQ, measured by several different professionally administered tests, in the 148 to 155 range, things that most people have to think long and hard about just seem to be immediately available in my brain without even giving them much thought. This is perhaps why I have so little patience with liberals. To be a liberal is to be stupid. There is no other explanation. CN "Joe" wrote in message om... (N1EE) wrote in message . com... 5 points to Neal. Inboard diesel powered boats commonly have different gearing in reverse. Incorrectly installing a RH or LH pitched prop when the opposite was called for could cause a boat to run at slower than expected speeds, albeit it would move forward in the reverse position and vice versa. Neal immediately grasped that the gear ratio might be wrong and this could be explained by the boat running in reverse, and at a lower gear ratio. And you said identical. Identical is not a diffrent rotating prop. To much rum Bart? WTF.... you usually do much better than this. Sheeeeeze Joe Capt. Neal® wrote If the shaft has a taper there is no way to put it on backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
Joe,
Have you considered that the previous owner of a boat might bave screwed up and put the wrong prop on? Then you as the new owner might make the false assumption that replacing the prop, with an indentical one, would be a solution. Bart (Joe) wrote in message . com... (N1EE) wrote in message . com... 5 points to Neal. Inboard diesel powered boats commonly have different gearing in reverse. Incorrectly installing a RH or LH pitched prop when the opposite was called for could cause a boat to run at slower than expected speeds, albeit it would move forward in the reverse position and vice versa. Neal immediately grasped that the gear ratio might be wrong and this could be explained by the boat running in reverse, and at a lower gear ratio. And you said identical. Identical is not a diffrent rotating prop. To much rum Bart? WTF.... you usually do much better than this. Sheeeeeze Joe Capt. Neal® wrote If the shaft has a taper there is no way to put it on backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
Inboard diesel powered boats commonly have different gearing in reverse. Interesting ..... says who? |
I see you and otnmbrd have come crawling back because
of the intellectual nature of the threads I start and reply to. Some people never learn that taking a beating is not really necessary. Try starting out on a new foot by agreeing with me and things might go well for you two pretenders. CN "Shen44" wrote in message ... Inboard diesel powered boats commonly have different gearing in reverse. Interesting ..... says who? |
Subject: Seamanship Question #15 Props
From: I see you and otnmbrd have come crawling back because of the intellectual nature of the threads I start and reply to. To crawl back, one must first leave. Other than some short trips, been here all along. Some people never learn that taking a beating is not really necessary. Try starting out on a new foot by agreeing with me and things might go well for you two pretenders. G I'm neither agreeing or disagreeing, merely asking for clarification. Seems your sabbatical did nothing to improve your reading comprehension. Shen CN "Shen44" wrote in message ... Inboard diesel powered boats commonly have different gearing in reverse. Interesting ..... says who? |
Hey, Capt. Shen,
I just remembered something you said last year when the discussion about the Pilot of the ferry that crashed full speed into the dock up on Staten Island which resulted in several deaths and numerous injuries, was being held here. At the time I said it was obviously pilot error and the pilot should be relieved of his Captain's license, fined, imprisoned, etc. You maintained that until all the evidence was in no valid conclusions could be drawn. It seems you were wrong yet again. I used the evidence found only in the newspaper articles, considered the evidence in light of my knowledge of all things Captain and came to the conclusion that turned out to be the very same the courts found. I think you owe me an apology for doubting my wisdom and powers of reasoning. Are you man enough to apologize? CN "Shen44" wrote in message ... Subject: Seamanship Question #15 Props From: I see you and otnmbrd have come crawling back because of the intellectual nature of the threads I start and reply to. To crawl back, one must first leave. Other than some short trips, been here all along. Some people never learn that taking a beating is not really necessary. Try starting out on a new foot by agreeing with me and things might go well for you two pretenders. G I'm neither agreeing or disagreeing, merely asking for clarification. Seems your sabbatical did nothing to improve your reading comprehension. Shen CN "Shen44" wrote in message ... Inboard diesel powered boats commonly have different gearing in reverse. Interesting ..... says who? |
And, you were, as usual, wrong. The fact is that when all the evidence was
brought to light (the key phrase), then and only then was the guy relieved of his license, etc. -- "j" ganz @@ www.sailnow.com "Capt. Neal®" wrote in message ... Hey, Capt. Shen, I just remembered something you said last year when the discussion about the Pilot of the ferry that crashed full speed into the dock up on Staten Island which resulted in several deaths and numerous injuries, was being held here. At the time I said it was obviously pilot error and the pilot should be relieved of his Captain's license, fined, imprisoned, etc. You maintained that until all the evidence was in no valid conclusions could be drawn. It seems you were wrong yet again. I used the evidence found only in the newspaper articles, considered the evidence in light of my knowledge of all things Captain and came to the conclusion that turned out to be the very same the courts found. I think you owe me an apology for doubting my wisdom and powers of reasoning. Are you man enough to apologize? CN "Shen44" wrote in message ... Subject: Seamanship Question #15 Props From: I see you and otnmbrd have come crawling back because of the intellectual nature of the threads I start and reply to. To crawl back, one must first leave. Other than some short trips, been here all along. Some people never learn that taking a beating is not really necessary. Try starting out on a new foot by agreeing with me and things might go well for you two pretenders. G I'm neither agreeing or disagreeing, merely asking for clarification. Seems your sabbatical did nothing to improve your reading comprehension. Shen CN "Shen44" wrote in message ... Inboard diesel powered boats commonly have different gearing in reverse. Interesting ..... says who? |
I'll take some of those points - It was me, not Neal, that pointed out
that the gearing could well be different in reverse. I also pointed out that it would be easily noticeable. Curiously, his first answer, that the prop was mounted backwards, may have been a better answer. That doesn't change the handedness, but if the blade is not symmetrical it will affect the efficiency. N1EE wrote: 5 points to Neal. Inboard diesel powered boats commonly have different gearing in reverse. Incorrectly installing a RH or LH pitched prop when the opposite was called for could cause a boat to run at slower than expected speeds, albeit it would move forward in the reverse position and vice versa. Neal immediately grasped that the gear ratio might be wrong and this could be explained by the boat running in reverse, and at a lower gear ratio. Capt. Neal® wrote If the shaft has a taper there is no way to put it on backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
(N1EE) wrote in message . com...
Joe, Have you considered that the previous owner of a boat might bave screwed up and put the wrong prop on? Yes Then you as the new owner might make the false assumption that replacing the prop, with an indentical one, would be a solution. No, if it slipping as defined by cavitating then its pitch is to great. And if I put another opposite rotating prop on it would not be IDENTICAL. And I can not think of any marine transmission that does not have the same ratio forward and reverse. So all around your question is screwed up. The only way a identical prop would be slipping and losing speed is A: The transmission bands are slipping B: You did not put the woodruff key in and the prop slips underload. C: The woodruff key is missing in the shaft coupling. No shame in it Bart, Just admit your wrong. Joe Bart (Joe) wrote in message . com... (N1EE) wrote in message . com... 5 points to Neal. Inboard diesel powered boats commonly have different gearing in reverse. Incorrectly installing a RH or LH pitched prop when the opposite was called for could cause a boat to run at slower than expected speeds, albeit it would move forward in the reverse position and vice versa. Neal immediately grasped that the gear ratio might be wrong and this could be explained by the boat running in reverse, and at a lower gear ratio. And you said identical. Identical is not a diffrent rotating prop. To much rum Bart? WTF.... you usually do much better than this. Sheeeeeze Joe Capt. Neal® wrote If the shaft has a taper there is no way to put it on backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
"Shen44" wrote in message ... I think you owe me an apology for doubting my wisdom and powers of reasoning. Are you man enough to apologize? CN LOL don't hold your breath waiting. To date, I haven't seen the final NTSB report of this incident (Jeff seems to find these more easily than I [hint]), so I won't comment as to the validity of a Federal courts findings. Shen Well you're _not_ Neal! Neal is omniscient. He KNOWS what happened ;) Be a man and apologize! John Cairns |
The issue of whether the city's liability can be drastically limited for the crash turns on whether the ferry management had "privity or knowledge" of the negligence that led to crash. The ferry pilot, Richard Smith, pleaded guilty in August to seaman's manslaughter charges, admitting he passed out at the wheel from fatigue and the effects of medication. Like I said, the man was not fit to pilot that ferry . . . Damned druggie! The above is from: http://www.newsday.com/news/local/ne...news-headlines Now do I get that apology? CN "Shen44" wrote in message ... I think you owe me an apology for doubting my wisdom and powers of reasoning. Are you man enough to apologize? CN LOL don't hold your breath waiting. To date, I haven't seen the final NTSB report of this incident (Jeff seems to find these more easily than I [hint]), so I won't comment as to the validity of a Federal courts findings. Shen |
Shen44 wrote:
LOL don't hold your breath waiting. To date, I haven't seen the final NTSB report of this incident (Jeff seems to find these more easily than I [hint]), so I won't comment as to the validity of a Federal courts findings. It takes a few years for the NTSB reports to be published. Here's the current list: http://www.ntsb.gov/Publictn/M_Acc.htm The CG reports are harder to track down. And the court findings are buried in the various appeals courts. |
"Jeff Morris" wrote in message ... Shen44 wrote: LOL don't hold your breath waiting. To date, I haven't seen the final NTSB report of this incident (Jeff seems to find these more easily than I [hint]), so I won't comment as to the validity of a Federal courts findings. It takes a few years for the NTSB reports to be published. Here's the current list: http://www.ntsb.gov/Publictn/M_Acc.htm The CG reports are harder to track down. And the court findings are buried in the various appeals courts. Great link. Didn't realize there were so many vessel/bridge allisions resulting in fatalities. John Cairns |
Subject: Seamanship Question #15 Props
From: The issue of whether the city's liability can be drastically limited for the crash turns on whether the ferry management had "privity or knowledge" of the negligence that led to crash. The ferry pilot, Richard Smith, pleaded guilty in August to seaman's manslaughter charges, admitting he passed out at the wheel from fatigue and the effects of medication. Like I said, the man was not fit to pilot that ferry . . . Damned druggie! The above is from: http://www.newsday.com/news/local/ne...58041.story?co ll=ny-nynews-headlines Now do I get that apology? CN ROFL NOT a chance! When the NTSB report comes out ( I believe Jeff's right .... too early for this one) I'll decide whether you made a lucky guess or not. Till then, take any news reports and politically motivated court decisions and stick them where the sun don't shine. Cheer up, though, you could be partially correct ... if so, not from any first hand knowledge on your part. Shen |
Subject: Seamanship Question #15 Props
From: Jeff Morris Date: 11/26/2004 10:29 AM Pacific Standard Time Message-id: I'll take some of those points - It was me, not Neal, that pointed out that the gearing could well be different in reverse. I also pointed out that it would be easily noticeable. Curiously, his first answer, that the prop was mounted backwards, may have been a better answer. That doesn't change the handedness, but if the blade is not symmetrical it will affect the efficiency. Wish I had a prop handy. My own feeling is that different gearing is a rarity. However, props are designed to be most efficient when ahead, which means less efficient when astern, so basically I agree. Shen |
Shen44 wrote:
Subject: Seamanship Question #15 Props From: Jeff Morris Date: 11/26/2004 10:29 AM Pacific Standard Time Message-id: I'll take some of those points - It was me, not Neal, that pointed out that the gearing could well be different in reverse. I also pointed out that it would be easily noticeable. Curiously, his first answer, that the prop was mounted backwards, may have been a better answer. That doesn't change the handedness, but if the blade is not symmetrical it will affect the efficiency. Wish I had a prop handy. My own feeling is that different gearing is a rarity. However, props are designed to be most efficient when ahead, which means less efficient when astern, so basically I agree. I can't say about ALL small diesels, but Yanmar is one of the most common. Here's the spec sheet for the new 30HP, which, with its siblings, may become the most common engine of its size fitted in the US. http://www.yanmarmarine.com/products...0_TechData.pdf While the saildrive gearing is symmetrical, the normal transmission has three gearing options, none of which are symmetrical. I remember an article (by Pascoe, I think) about a large power boat that used reverse gear to have a counter-rotating prop. The shift linkage was reversed, of course. The owner couldn't figure out why the transmission needed service every 50 hours. |
While the saildrive gearing is symmetrical, the normal transmission has
three gearing options, none of which are symmetrical. I remember an article (by Pascoe, I think) about a large power boat that used reverse gear to have a counter-rotating prop. The shift linkage was reversed, of course. The owner couldn't figure out why the transmission needed service every 50 hours. Interestin. G we learn something new everyday. I do note the reduction difference is not all that great between ahead and astern and would have to wonder how noticeable the difference would be, versus putting the prop on backwards. I also have a problem seeing how someone could put the prop on backwards, other than in a twin screw application. Shen |
True. 2 points to you Jeff.
Would you know if tapering the shaft is the standard? I haven't seen any otherwise, but my experiece is limited to puling props on only a few boats. I was wondering if some shafts come straight with just a key way to lock them in position? Bart Senior Jeff Morris wrote I'll take some of those points - It was me, not Neal, that pointed out that the gearing could well be different in reverse. I also pointed out that it would be easily noticeable. Curiously, his first answer, that the prop was mounted backwards, may have been a better answer. That doesn't change the handedness, but if the blade is not symmetrical it will affect the efficiency. |
(Joe) wrote
(N1EE) wrote Joe, Have you considered that the previous owner of a boat might bave screwed up and put the wrong prop on? Yes Then you as the new owner might make the false assumption that replacing the prop, with an indentical one, would be a solution. No, if it slipping as defined by cavitating then its pitch is to great. And if I put another opposite rotating prop on it would not be IDENTICAL. Joe, you've been drinking TOO, not "to" much. And I can not think of any marine transmission that does not have the same ratio forward and reverse. It shows how limited your experience is then doesnt' it? I'd guess the reason for reduced gearing in reverse is to limit speed and prevent rudder pressure from ripping the helm from your grasp. I will admit that differnt gearing may not be as common as I thought. Bart So all around your question is screwed up. The only way a identical prop would be slipping and losing speed is A: The transmission bands are slipping B: You did not put the woodruff key in and the prop slips underload. C: The woodruff key is missing in the shaft coupling. No shame in it Bart, Just admit your wrong. Joe Bart (Joe) wrote (N1EE) wrote 5 points to Neal. Inboard diesel powered boats commonly have different gearing in reverse. Incorrectly installing a RH or LH pitched prop when the opposite was called for could cause a boat to run at slower than expected speeds, albeit it would move forward in the reverse position and vice versa. Neal immediately grasped that the gear ratio might be wrong and this could be explained by the boat running in reverse, and at a lower gear ratio. And you said identical. Identical is not a diffrent rotating prop. To much rum Bart? WTF.... you usually do much better than this. Sheeeeeze Joe Capt. Neal® wrote If the shaft has a taper there is no way to put it on backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
(N1EE) wrote in message . com...
(Joe) wrote (N1EE) wrote Joe, Have you considered that the previous owner of a boat might bave screwed up and put the wrong prop on? Yes Then you as the new owner might make the false assumption that replacing the prop, with an indentical one, would be a solution. No, if it slipping as defined by cavitating then its pitch is to great. And if I put another opposite rotating prop on it would not be IDENTICAL. Joe, you've been drinking TOO, not "to" much. And I can not think of any marine transmission that does not have the same ratio forward and reverse. It shows how limited your experience is then doesnt' it? Name one marine transmission that has a diffrent ratio in recerse than it has in forward. That would be a big mistake by the mfg. Can you guess why? I'd guess the reason for reduced gearing in reverse is to limit speed and prevent rudder pressure from ripping the helm from your grasp. Sheeeze....... I will admit that differnt gearing may not be as common as I thought. Bart So all around your question is screwed up. The only way a identical prop would be slipping and losing speed is A: The transmission bands are slipping B: You did not put the woodruff key in and the prop slips underload. C: The woodruff key is missing in the shaft coupling. No shame in it Bart, Just admit your wrong. Joe Bart (Joe) wrote (N1EE) wrote 5 points to Neal. Inboard diesel powered boats commonly have different gearing in reverse. Incorrectly installing a RH or LH pitched prop when the opposite was called for could cause a boat to run at slower than expected speeds, albeit it would move forward in the reverse position and vice versa. Neal immediately grasped that the gear ratio might be wrong and this could be explained by the boat running in reverse, and at a lower gear ratio. And you said identical. Identical is not a diffrent rotating prop. To much rum Bart? WTF.... you usually do much better than this. Sheeeeeze Joe Capt. Neal® wrote If the shaft has a taper there is no way to put it on backwards so, perhaps, they shipped a LH prop instead of a RH prop or vice versa? Diameter and pitch could be identical but it would be so slow if turning direction were wrong. CN "Capt. Neal®" wrote Remove it, turn it around, put it back on. CN "Bart Senior" wrote in 5 points (Impress me with your genius) You just bought a new boat and notice the prop is old and brittle. You replace it with an identical prop. After motoring around for a while you find the top speed of the boat is much lower, estimated to be 2.5-3 knots and the prop seems to be slipping. What is the most likely cause, explaination, and cure? |
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Joe wrote:
(N1EE) wrote in message . com... (Joe) wrote (N1EE) wrote Joe, Have you considered that the previous owner of a boat might bave screwed up and put the wrong prop on? Yes Then you as the new owner might make the false assumption that replacing the prop, with an indentical one, would be a solution. No, if it slipping as defined by cavitating then its pitch is to great. And if I put another opposite rotating prop on it would not be IDENTICAL. Joe, you've been drinking TOO, not "to" much. And I can not think of any marine transmission that does not have the same ratio forward and reverse. It shows how limited your experience is then doesnt' it? Name one marine transmission that has a diffrent ratio in recerse than it has in forward. I already posted a link to one of the most common small marine engines. Their transmission uses different ratios for all options, except the saildrive. http://www.yanmarmarine.com/products...0_TechData.pdf That would be a big mistake by the mfg. Can you guess why? So I checked the bigger Yanmar, such as the one used by the PDQ Powercat. This is offered with the Hurth ZF30 which uses different ratios for fwd/asn, but is clockwise only. If you want a counter-rotating pair, there are several with the same ratios, including the ZF25, which is what PDQ uses. Of course, this is of little interest to most sailors - even catamarans don't usually use counter-rotating props. http://www.yanmarmarine.com/products...E_TechData.pdf |
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