Knots vs MPH
- Thread starter Baker
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If you have answers, please help by responding to the unanswered posts.
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78puget-trawler
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Its speed made good, not velocity. CG will not ask for velocity.
markpierce
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Hope everyone speaks plain English in "my" territory.
dhmeissner
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Wifey B: You mean in olden days?
I've sailed but nothing serious so haven't heard or used that term. Still in ordinary conversation it just doesn't come up often. I wonder what percent of the people even know it's meaning. I think most probably think it is just another word for speed...Wrong. Oh well. Climbing back under my chair. Not really.
Just waiting for lunch. Darn this lake is calm and beeeeeuuuuutiful. Almost glassy. What a change from a few days ago.
WifeyB, what Baker was describing there as velocity made good, is now referred to on GPS s as speed over the ground. (SOG) In other words the actual speed you are doing, as the satellite sees you, in the direction you are heading, and it automatically is compensating for the effects of currents and drift.
Tony B
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Velosity made good and speed over ground are two different things.
Speed over ground is as you stated. It is the actual speed you are moving over the ground as opposed to speed over water, which is affected by currents. For example, if you are heading up river and you had a paddle wheel speedometer which was common many years ago, it might read 10MPH. but the river is flowing downstream at 3 MPH. Your real speed over the ground would only be 7 MPH. The apparent speed of 10MPH (the speedometer) minus the river currrent of 3mph.
The velocity made good is the speed you are making towards your target (waypoint).
Say for instance, You are on a river and your waypoint is due north. When you are heading due north, your speed, velocity made good, is the same as your speed over ground. When there is a bend in the river and you turn 90* your speed Velocity made good (VMG) would be zero (possibly a minus also). If the bend then turned south, your VMG would be a minus number. If a bend took you NNW, you would show most of your speed over ground as a positive number on your VMG, but not all of it because some og your speed is still going in somewhat the wrong dircction. This is why VMG is not very practical on rivers. It is handiest in open water on a sailboat when you are tacking because you are rarely heading directly toward toward your target. In a powerboat, you will be most likely always heading toward your target in open water and so again, VMG not all that useful because your speed will be the same as the speed over ground which will also be your VMG.
I hope that made some sense to someone. If it did, please explain it back to me. LOL
Speed over ground is as you stated. It is the actual speed you are moving over the ground as opposed to speed over water, which is affected by currents. For example, if you are heading up river and you had a paddle wheel speedometer which was common many years ago, it might read 10MPH. but the river is flowing downstream at 3 MPH. Your real speed over the ground would only be 7 MPH. The apparent speed of 10MPH (the speedometer) minus the river currrent of 3mph.
The velocity made good is the speed you are making towards your target (waypoint).
Say for instance, You are on a river and your waypoint is due north. When you are heading due north, your speed, velocity made good, is the same as your speed over ground. When there is a bend in the river and you turn 90* your speed Velocity made good (VMG) would be zero (possibly a minus also). If the bend then turned south, your VMG would be a minus number. If a bend took you NNW, you would show most of your speed over ground as a positive number on your VMG, but not all of it because some og your speed is still going in somewhat the wrong dircction. This is why VMG is not very practical on rivers. It is handiest in open water on a sailboat when you are tacking because you are rarely heading directly toward toward your target. In a powerboat, you will be most likely always heading toward your target in open water and so again, VMG not all that useful because your speed will be the same as the speed over ground which will also be your VMG.
I hope that made some sense to someone. If it did, please explain it back to me. LOL
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Tony B
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I think the confusion stems from the common usage we have with speed expressed in MPH. While a knot is 1 nautical mile per hour and 6 knots would be 6 nautical miles per hour, the 'nautical miles per hour' is inferred and just left out and expressed simply as 'knots'.
So when on a boat in the USA, if you say MPH, you are implying statute miles per hour and if you say "Knots", you are implying 'nautical miles per hour'.
Where the metric system is used, they would say "KPH" instead of "MPH", but when they say "Knots", we are all on the same page.
oscar
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There was a time when it mattered, and you actually had to know all this stuff as tools of the navigation trade. Now all you need is a few hundred bucks and you can buy yourself a chart plotter and with a little reading of the instructions get to where you want to go, hopefully without hitting anything, straying into a restricted area, getting run over in a traffic management scheme etc. etc.
Not the same as navigating.
Like so many other things, finding your way around the water is changing.
Those of us that learned the trade, whether for money or for pleasure, care about terminology, and I believe that we have a better understanding of what went into the little magic box.
Remember, the regulation says that "no mariner shall rely on a single source of navigation".
I still keep somewhat of a log, a complete one on longer and/or off shore missions, and I keep "paper charts" out even though this may be in the form of a raster chart on a different electronic device with it's own power source. (We DO all know the difference between a raster and a vector chart and their respective inherent limitations right?)
Working in nautical miles when looking at a paper chart is only logical, as one minute of latitude is one nautical mile.
Someone also correctly brought up the fact that using NM takes care of differences between standard and metric countries.
All aviation is still done in NM.
Not the same as navigating.
Like so many other things, finding your way around the water is changing.
Those of us that learned the trade, whether for money or for pleasure, care about terminology, and I believe that we have a better understanding of what went into the little magic box.
Remember, the regulation says that "no mariner shall rely on a single source of navigation".
I still keep somewhat of a log, a complete one on longer and/or off shore missions, and I keep "paper charts" out even though this may be in the form of a raster chart on a different electronic device with it's own power source. (We DO all know the difference between a raster and a vector chart and their respective inherent limitations right?)
Working in nautical miles when looking at a paper chart is only logical, as one minute of latitude is one nautical mile.
Someone also correctly brought up the fact that using NM takes care of differences between standard and metric countries.
All aviation is still done in NM.
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dhays
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Oscar, you sound like a cranky Old Fart. My kids say I sound that way all the time.
I actually have paper (real paper) charts on board in three forms. I have single sheet charts (2/3 scale) from Bellingham Chart Printers for US San Juans and Puget Sound. The second form are chart books that I have for my cruising grounds (Puget Sound and BC), and the last are Canadian charts for BC. I do use them on occasion, but most of the time I use my chart plotter because it is convenient.
And no, I have no idea what the difference is between Raster and Vector charts. I am interested however.
psneeld
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Holy cow what's the big deal?
Except for a few...most are traveling so slow..you can use a slide rule to navigate.
Some here are very smart and the vast majority smart enough to do the rough cakculation in your head.
Not sure why it is an issue which numbers you use except as BzndB once stated...just pass down the right format to the oncoming watch.
Except for a few...most are traveling so slow..you can use a slide rule to navigate.
Some here are very smart and the vast majority smart enough to do the rough cakculation in your head.
Not sure why it is an issue which numbers you use except as BzndB once stated...just pass down the right format to the oncoming watch.
Differences Between RNCs and ENCs
To oversimplify, just think of the fact that Raster charts are just images, just photos. It looks like a paper chart with none of the advantages that electronic charts have. No layers, no zooming, no rotating, but very good for printing.
psneeld
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I prefer raster over vector....they are really the backbone of old school nav....
Most of the advantages I see people tout never have been an advantage to me.
Now all the whiz bang overlays may be OK for a few things...but never needed them for my cruising. Plus they are more of a function of the plotter than chart type...like all the guides where you can quickly switch back and forth to view what you want.
For the ACIW...I have found the raster more accurate than the vectors for keeping me in deep water.
Most of the advantages I see people tout never have been an advantage to me.
Now all the whiz bang overlays may be OK for a few things...but never needed them for my cruising. Plus they are more of a function of the plotter than chart type...like all the guides where you can quickly switch back and forth to view what you want.
For the ACIW...I have found the raster more accurate than the vectors for keeping me in deep water.
ranger58sb
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Although there's an exception (of sort) to that.
Many plotters will seamlessly switch between "different scale charts of the same area" as the display zooms in and out.
Within any one given chart, zooming in is simply like blowing up a photo... and resolution eventually suffers. But zooming in on a given chart, and then having it automatically replaced with a close-focus scale as the plotter continues to zoom... often improves the detail available...
-Chris
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I agree. There can be issues with Raster charts. Most of it comes down to whether the "positions" on a raster chart are an accurate representation of the truth, whether it's a point of land or a specific depth sounding. But there can be information on a raster chart that may not shown be shown on a stand-alone ENC. Current vectors come to mind. The Navionics charts don't show direction of current, while the CHS raster chart does. (Yes there are addons that provide this, but not the bare bones version). In addition, there are no land contours, and I use these all the time when I am piloting through areas that are new to me. CHS ENCs do not yet have land contours, while the Rasters do, and it has to do with file size. ENCs with contours would be a lot larger.
Jim
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ranger58sb
Guru
I've never seen current vectors on either raster or vector charts.
Our plotter displays current vectors in either mode, but it appears to be a plotter overlay function, extracted from a separate tides and currents prediction file), not something on the charts themselves.
???
-Chris
MYTraveler
Guru
I am surprised this hasn't evolved into a course up vs. north-up presentation. Personally, I measure speed in knots and display my course (but not radar) north-up. I suspect that the mph crowd prefers course / heading -up.
That sounds like it was intended as an insult.
We all have options and we get to choose the options that work best for us. Nobody is superior or inferior based on their choice.
I didn't see any part of that as insulting to anyone. What made you feel insulted?
MYTraveler
Guru
Not at all. I apologize for any offense (and don't even know what I did wrong).
ulysses
Guru
The question arises is if I run course up, what bathroom am I supposed to use ?
Seems like someone is offended by most anything these days.
No apology was required or expected from me, I did not find anything offensive or insulting in the previous comments.
Seems like someone is offended by most anything these days.
No apology was required or expected from me, I did not find anything offensive or insulting in the previous comments.
pearlwindham
Veteran Member
Yes
CPseudonym
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If someone gets offended by what has been posted in this thread they need to pull the panties out of their crack, build a bridge and get over it IMO.
psneeld
Guru
Obviously there are different "languages" in navigation in terms of speed, depth, type of charts.....etc...etc...
Like in the real world of doing anything....being multi-lingual is your best bet....
Being able to flip charts and speed and meters to fathoms to feet in your mind......is almost required....and pretty danged easy if you do it a lot....so again.....what is the big deal?
I think anyone smart enough to be posting here can use what they like but not be limited to only dealing with one language of navigation.
Like in the real world of doing anything....being multi-lingual is your best bet....
Being able to flip charts and speed and meters to fathoms to feet in your mind......is almost required....and pretty danged easy if you do it a lot....so again.....what is the big deal?
I think anyone smart enough to be posting here can use what they like but not be limited to only dealing with one language of navigation.
