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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #51  
Old 10-09-2006
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I would really hate to have you as a navigator. You go around the world to avoid the simplest thing. Air density decreases with altitude. Period. Going fast. Going slow. Doesn't matter. There is still less air up there. Drag decreases with altitude. Going 1 knot. Going 1,000 knots. Makes no difference. Mach number does vary due to the true vs indicated airspeed. Bird strike? Do you know how high migrating geese have been seen at? Way the hell higher than 10,000ft, even 15,000ft! Are the planes restricted up there due to the possibility of a mid-air with a goose? No, they're not? Why? It's not because of a bird strike. Get real. Now, once again, get back to the subject at hand.....WATER, which, while being a fluid, has constant density, does not compress, and deals with a different type of propeller, at very different speeds. Like I said, you cannot simply draw comparisons, conclude they're both fluids, and decide they must produce the same results.
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  #52  
Old 10-09-2006
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Quote:
Originally Posted by seabreeze_97
I would really hate to have you as a navigator..
Good thing we are not talking navigation here huh?



Quote:
Originally Posted by seabreeze_97
You go around the world to avoid the simplest thing. Air density decreases with altitude. Period. Going fast. Going slow. Doesn't matter. There is still less air up there. Drag decreases with altitude. Going 1 knot. Going 1,000 knots.
All true but I was clarifying the difference between true and indicated since that was neglected the 1st time and very misleading. Water density does change with salinity I believe and not constant at all. But that is irrelevant in this discussion
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Originally Posted by seabreeze_97
Makes no difference. Mach number does vary due to the true vs indicated airspeed.
Not true, the mach number is another measurement of true airspeed and indicated is only reading the dynamic pressure exerted on a diaphragm when air is shoved in a tube (pitot). The airplane still responds and performs with indicated. For example we both know that stall is a function of angle of attack right and not airspeed. But for arguments sake we will say that in un accelerated flight and at a constant weight an airplane will stall at 100 kts indicated. At 40,000 ft or sea level it will stall at 100kts indicated but obviously as far as true airspeed is concerned it will be traveling faster at altitude. Actually temperature has more of an effect on mach then altitude.
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Originally Posted by seabreeze_97
Bird strike? Do you know how high migrating geese have been seen at? Way the hell higher than 10,000ft, even 15,000ft! Are the planes restricted up there due to the possibility of a mid-air with a goose? ..
Actually larger birds go even higher then that. The birdstrike situation is just one of many considerations when designing aircraft for commercial passenger use. I have been flying proffesionally since 1982 and I am currently a pilot for a major carrier and I am passing on what is passed on to us so I do know what I am talking about. I am not much of a sailor but I am a commercial pilot. I don't just play one TV nor do I stay in a Holiday Inn Express.

We can talk navigation also if you would like.

Jerry
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Last edited by jerryrlitton; 10-09-2006 at 11:47 AM.
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  #53  
Old 10-09-2006
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Water is compressible.... much less than air, but it is compressible.

The compressibility of water does vary a very little with temperature and pressure, but it does vary.

We have to work with the figures in oil wells.... big volumes, big pressures..... and the compressibility will talk loud then.
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  #54  
Old 10-09-2006
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Quote:
Originally Posted by jerryrlitton
..Actually larger birds go even higher then that...
I've flown gliders in lee wave at FL250 and above with birds on more than one occasion, and have been less worried about a strike {even when flying multi's} than I am of large hail popping out of some CuNim 10 miles away! The hail can do a lot more damage.

Since this is not an aviation forum let us not confuse the non-flyers with indicated vs. true airspeed and mach number {thankfully we haven't gone so far down that road as to discuss Re numbes}.

The initial trigger for this was that there is a difference between propellors in water and propellors in air - and I think that all of us agree that there are differences stemming from water's non-compressability.

I still haven't really gotten an answer to the question of a freewheeling propellor at 7 knots water speed vs. having it locked in place; all other factors being equal. I think it is time to hit the library an check out a fluid dynamics book and figuring it out for myself.

-Zanshin (no ATPL, but CFI)
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  #55  
Old 10-09-2006
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You are correct about doing more research on how water behaves and I need to quit assuming. Or we can rig up a device we tow behind a boat and measure the tension on the line with a spinning and fixed prop. I apologize for getting this conversation way off track.

Jerry.
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  #56  
Old 10-09-2006
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Over the years (more years than I care to admit to) I have seen this argument reprised many times. In person, in the letters to the editor of sailing publications in the old days, and in on-line forums. The discussions all have something in common. There is a total lack of documented hard evidence and an awful lot of (amateur) theory and "common sense (sic)".
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