#22
|
|||
|
|||
O/T: knots
On Tue, 22 Dec 2015 02:51:04 +0000, Phil W Lee
wrote: John B. considered Mon, 21 Dec 2015 08:01:54 +0700 the perfect time to write: On Sun, 20 Dec 2015 20:04:43 +0000, Phil W Lee wrote: John B. considered Sun, 20 Dec 2015 17:50:33 +0700 the perfect time to write: On Sun, 20 Dec 2015 02:25:09 +0100, "Jakob Krieger" wrote: - John B. / Tue, 15 Dec 2015 11:49:53 +0100 Of course with plastic ropes and fixtures, you don't need a knot any more for many things. But for rescuing »man overboard« or joinig ropes for more length, classical knots are still used. What ropes would that be? The main halyard? About 80% wire rope? The main sheet? Wire again, or the jib sheet... wire once again. Well, it used to be considered as a good precaution to have a roll of rope lying somewhere in the boat. Even in cars, towing-ropes can be found. May be except for GPS sailors, they don't know what a knot or even a rope is. Sort of snarky remark isn't it? After all big ships navigate with GPS, airplanes navigate with GPS. It has been quite a number of years now since anything commercial used the stars. Nobody with a little bit of experience (bike or car drivers included) relies on GPS only. I see. Do you really think that the 1st officer on, say the Emma Maersk" is out on the bridge wing every day taking his noon sight? Or that a B-52 comes equipped with a sextant? Or that any modern commercial or military vehicle comes with a copy of the six H.O. tables? The U.S. navel Academy stopped teaching celestial navigation nearly 20 years ago stating that while celestial was accurate to a 3 mile radius that GPS was accurate to a 60 ft. radius. Almost everything of any size used inertial navigation in between traditional and GPS, and certainly until VERY recently, it was not legal to use GPS as the primary means of navigation in an aircraft of UK registry. That's what radio beacons are for, in all their various types - VOR, DME, NDB, and ILS (and yes, I know how to use them all, although not for all the various functions required for a full UK instrument rating - the NDB approach is notoriously difficult). I'm fairly sure that in coastal waters, ships use a similar system of beacons, mostly housed in what used to be primarily lighthouses (and sometimes still perform that function as well). So, do you navigate across the Atlantic, or even worse, across the Pacific with radio beacons? Actually, you can. DR gets you close enough to pick up beacons in plenty of time to avoid bumping into anything and correct your course to make landfall where you need to. That isn't the way that you described navigation in the U.K. :-) Re costal radio beacons. It might be an anomaly but I certainly have never seen them anywhere in Asia, nor have I seen a reference to them and certainly if they were in common use navigation charts and sailing directions would have reference to them. Any active light house is marked on the chart and it's light signal described. Bubble sextants were certainly part of the standard equipment on the Vulcan, so why not the B-52? The last (I believe) U.S. made aircraft that had the ability to use celestial navigation were early models of the Boeing 747, which were phased out in the 1960's. The SR071 had a automated celestial and inertial navigation. The last SR-71 left service in 1989. The last Vulcan was delivered in 1965. You are talking about old technology. Which you certainly CAN use in anything which has outside windows big enough to give horizon and solar/star sighting at the same time. There are even specialist gyro stabilised bubble sextants designed for the purpose, and tables to offset horizon angle by altitude. Battery life isn't really much of a problem in the duration of any normal aircraft flight, and on water you can pick your moment over a much larger timescale. Yup, certainly. But no windows :-) And, without a bubble sextant it is awful hard, at 30,000 ft to get a good horizon. And if a fancy electronically stabilized sextant then why not just use GPS. By the way, to accurately locate the seismic lines used in oil field exploitation they haven't used a sextant in years and years. All GPS these days. But why all the excitement about sextants? Just use your D.R. and when you get close look for a landmark. All sailing directions have a wealth of descriptions of landmarks. And, there are days, sometimes weeks, that you can't see well enough to get a sight, and other days when it is too rough to get a good sight and even when you do your "cocked hat" is probably a mile on more on a side... if you are really good and more then likely, in a small boat, a lot bigger. Even in rough weather and a solid overcast a GPS will give you a location within feet :-) Note that INS units had to be "borrowed" from some old airliners in museums to strap down in the crew compartment of the Vulcans used in the "Black Buck" operations of the (pre-GPS) Falklands war. In their original (nuclear) role, they were expected to navigate by DR and celestial, as it was assumed that most beacons would be off air or out of range, and the DR part is kind of difficult over a featureless ocean like the South Atlantic, as it relies on position checks (which were carefully surveyed for the routes the Vulcan force was assigned to on their nuclear role). All INS units are large, as they have big gyros in them, so aren't suitable for small craft, and have to be set up with accurate starting positions - in aircraft this is done by positioning the craft on a carefully plotted navigation marker painted on the hardstanding and hand entering the exact latitude and longitude. The aircraft has to be kept completely still for several minutes while the internal gyros spin up to speed and self-tests are performed. They also accumulate errors over time, as they have to make a "best guess" approach to precession. I had no idea that US naval navigators were so badly trained - aren't naval forces supposed to be able to operate in time of war, when EMP could have killed the GPS constellation, along with most other outside reference signals? Or maybe they rely on being able to scavenge some old kit out of obsolete vessels in that situation? Of course, on smaller craft, the bigger problem is likely to be reliance on power - particularly on sailing vessels. One of the major certification requirements for aero engines is that they be electrically self-contained, usually achieved by using magneto ignition. I hate to be the one to disillusion you but I don't believe that any operational U.S.A.F. engine today uses magneto ignition :-) I think you'll find that some of the smaller trainer aircraft do use piston engines, complete with magnetos. I made a very quick check and I don't believe that the USAF has any reciprocating engine aircraft in use. At least every thing I see with a propeller is a turbo-prop. I did, how ever, work on what may have been the last of the reciprocating powered bombers that the USAF had - the B-50 which did have magnetos.... however to start the engines the system used "voltage boosters" that served to feed a higher voltage to the ignition system than the magnetos could produce at starting RPM. The "voltage boosters" were operated by the air craft's electrical system, that for starting was powered by an external power supply. Nope. A voltage booster. the P&W 4360 had a low voltage magneto system to avoid voltage leaks at high altitude and they definitely had a voltage booster for each of the two mags. The old tried and true of making a rough check of whether the mags are putting out was to grab a spark plug lead and tell the guy to "hit the starter". The shock was enough to jolt you but it did make for a quick check. I once watched a bloke try the same thing with the R-4360, which had voltage boosters, and it almost knocked him off the stand :-) IIRC, magnetos don't lose much in the way of sparks at low revs - it's more to compensate for the voltage sag that engaging the starter causes. Nope, magnetos produce a lower voltage at starting revs. Way back they used what was refereed to as an "impulse coupling" to sort of kick start the mag to get enough voltage to fire the plugs at starting. Magnetos, you may recall, work perfectly well on motorcycles with kickstarters! But, you get more revs from the kick starter on a bike than the starter on a large aircraft engine. That goes pear-shaped pretty quickly if you then can't find a runway to land on as soon as the power goes off! This is also why at least a basic set of primary instruments are vacuum powered, so you can at least keep the aircraft flying the right way up and in the right direction if all the smoke comes out of the electrical system. Come now. I flew with my father when I was just a lad, in a Piper J-3 and it didn't have any vacuum instruments in it. The snarky remark was meant about people who have no orientation at all without GPS (can't even find their own bathroom) Yes, a good sailor should be able to maintain and repair the boat, as well as drive it. Anything can break if sufficiently abused over a long enough period, and extended abuse is not a bad working diefinition of a trans-oceanic voyage in a small boat! Of course, if you creep around the coastal areas, you can generally find a port to put into if (when) something goes wrong or the weather turns nasty. That sounds like a very logical argument... Until one discovers that few if any commercial shipping carry sufficient personal and/or equipment to "to maintain and repair the boat". The Emma Maersk, for example carries a crew of 13, 4 Deck Officers, 2 Engine Officers, 3 ABs, 2 OS, 2 Oilers. Who do you think repairs the navigation gear? Or the bow/stern thrusters? If a navigation antenna gets blown off, who do you think replaces it? Or do you think they just keep going and play blind-mans-buff? Who replaces the lamps in navigation lights if (when) they fail? Which navigation antenna is that? The GPS antenna? Or the big radome? Of course, a yacht is a rather different proposition. Not many yachts (and even less of their normal equipment) are really designed with genuinely extreme weather in mind, so if you're going to use one for trans-oceanic voyages (or anything too far out to be able to run for the nearest port at the first hint of a bad forecast), you do need to be able to make-do-and-mend, unless you only sail in events with safety rescue boats available. If you are crossing oceans there is no running for harbour so you try to make your crossing in the benign seasons :-) Talk to anyone who is doing a sailing circumnavigation and you find that they seem to spend lots of time in harbour. You get to Thailand and you have to wait for the monsoons to change before you can set out for the Indian Ocean. To a large extent, similar constraints apply to those of bicycle components, as weight and size matter. And the biggest market for equipment is the weekend user who spends as much time looking after their equipment as they do actually using it. -- cheers, John B. |
Thread Tools | |
Display Modes | |
|
|