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interesting that you mention the low speed aspect of it. the only time the gps comes even close to its specified accuracy is when the boat isnt moving. at all. Not even rocking. Its stated accuracy is a CEP based upon RMS figures. The faster you go, the further off it is.
Why does everyone think their pitots or paddle wheels are so inaccurate? Maybe its because they are comparing them to the display of their GPS plotter, which is the result of a whole lot of filtering and averaging. Pitots and paddlewheels take measurements. GPS only makes calculations. Its not actually 'measuring' anything, other than how long the sat signals take to get to it. Thats it.
And pitots are so notoriously inaccurate that every aircraft in the world uses them for speed measurement. Go figure.
I am wondering if people are getting 'accuracy' and 'repeatability' mixed up. If you took someone on your boat who asked you to demonstrate how accurate your GPS was, would you put the bow against the corner of a fixed, surveyed marker and then measure the offsets from the surveyed marker to your antenna and compare them? I doubt that you would, but that would be demonstrating 'accuracy'.
I think you would be more likely to mark something in the water with your GPS, using a waypoint, and then return to it to show them how it worked. Thats not accuracy. Thats repeatability.
So how do you know how inaccurate your pitot tube is? Because your GPS gives you a steadier, averaged reading? Even though its purely calculated mph is basically wrong, you'll believe it over a direct measurement. Its only better for calculating fuel efficiency if you can pin the ocean down to the earth like a big sheet of plexiglass.
gph is a better gauge anyway which is the actual fuel burn and doesn't matter how much current or whatever else the pitot is reading.
You're right and that's mostly the point............. How many gallons per hour that are burned is an accurate fuel consumption reading, BUT..............you will burn an entirely different amount of gallons per hour at a given SOG, in varying conditions. Those varying conditions will cause you to burn more or less fuel to maintain the same SOG.
I was using current as an example because you will burn more GPH to maintain the GPS's SOG while "pushing" into a current or even headwind than you will "down stream" or "down wind".
Why is that so hard to understand?
__________________
2006 Pursuit 3480 Drummond Island Sportfish
F250 Yamaha's
no, was product manager for underwater tracking and navigations systems used for DP and ROVs, inertial military packages, mine countermeasures...stuff like that.
and Hey, Joe...I'll stop if you will.... besides, not talking about you personally, but sometimes it feels like trying to explain physics to a chimp who's trying to teach me how to peel a banana.
I've never seen a better example of being "blinded by the science"
Every statement you've made about current flow, gps readings taken "in average" etc etc etc are all completely true. But you are without doubt attempting to compare apples and monkeys. Are we talking about Miles per gallon, or all the factors that determine fuel efficiency? They have very little to do with each other as you have stated.
If a boater wants to know MILES per GALLON of fuel, he is 99.9% of the time talking about Point A to Point B measurements over the earth. In the real world that computation takes into account windspeed, current flow, wave height, etc. I'm fairly certain that most people understand that if the currents raging, the wind's blowing and the waves are cresting, it's going to take more fuel to accomplish that. Even people below your level of cognitive thought tend to grasp the idea.
So, let us suppose that our lowly brain cell challenged boater is running home in a storm. The wind is blowing 35+, waves are crest/breaking, and he's desperately trying to get home. He's scared to death. He knows that his GPS says he's 10 miles from port. He knows he's got "X" amount of fuel left according to his flowscan calculations.
He does not care one iota about anything else at that moment except to calculate if he has enough fuel in the tank to get his butt home. He would actually figure that out by taking the real distance traveled over ground, by the gallons per hour he's using to do it at the speed he's presently traveling.
If in the current conditions, he's been able to travel 10 true distance miles in one hour, using 10 gallons per hour of fuel to do it, he can effectively surmise that to get his arse home, he's going to need a minimum of 10 gallons of fuel.
Taking a GPS reading of miles between your departure point ( lets call it A) and your present position (lets call it B) and dividing that by the amount of time that's elapsed, will indeed give you a real world reading of MILES per hour, and it will INCLUDE every variable you've mentioned such as tide, wind, etc.
Gallons per hour used, can be measured quite reliably with a paddlewheel device , or a turbine device installed in the fuel line, such as a flowscan utilizes. They are quite accurate if you adjust them properly.
If you use the gallons per hour of fuel burn the flow scan displays, and the distance you've traveled from A to B in that same hour, you can effectively determine Gallons per mile. Variables such as Tide flow, direction, strength, wind speed, wave patterns ect was ALREADY included in the equation. It was all determined by the Miles Per Hour. You either went somewhere in that hour or you didn't. all the variables that make up as to how far, etc were determined.
So, in a laboratory, There are certainly times where I'd want to know via use of a paddle wheel in the medium I'm measuring what the flow past a certain point is ( think hydro electric dam) But in the real world of computing miles per hour, gallons per hour, and gallons per mile, I believe using fixed point reference make tremendously more sense.
__________________ "Had I known I was going to live this long, I would have taken better care of my body"....Mark Twain
I am pretty sure I stated repeatedly that my comments were in reference to ( page 1 post)
(quote)
"Mines connected this way and I've still got MPG. It's just more accurate when it gets GPS data than when based on data from the Speedometer," (unquote)
you are talking about navigation, I was talking about sensor accuracy and propulsion efficiency. GPS is not a sensor. it doesnt measure anything. How could it be better than a sensor that DOES measure speed thru water? People believe their GPS data because of the way its presented. A lot of them dont have a clue about 'accuracy' or where that display comes from. Statements were made that the pitot or paddlewheel were inaccurate, by people who dont understand what accuracy is. Please forgive me for thinking at least some people would be interested in the difference, and that it couldnt hurt anyone to know more about it.
So, how bout them Colts, eh? Pass the Redman. Anybody got a Shiner Bock?
Gringo - 2/2/2007 5:36 AM
And pitots are so notoriously inaccurate that every aircraft in the world uses them for speed measurement. Go figure.
In fact aircraft Pitots ARE innaccurate when measuring actual speed over the ground. Read up on the difference between Indicated airspeed, calibrated airspeed, true airspeed, and ground speed.
That's not to say that aircraft Pitots aren't useful to the pilot, however.
Gringo - this is my LAST attempt to help as this thread is AMAZING to the point I wonder if you really understand it all and are just 'egging' people on.
To clarify (as this thread was hijacked in the first place) your question was "but have one question...How could MPG be more accurate using GPS? " - my response is still to that specific question.
First Term Definitions:
MPG - miles travelled per gallon of fuel used (for me a mile is 5280 feet) - miles represent distance
2nd item - gallons burned, as determined by a fuel flow meter seem to be agreed as a 'constant' and not an issue, so the only variable in discussion here is 'distance traveled' and more importantly the most accurate measurement of that distance with the debate being pitot tube versus GPS.
So, distance measurement is all based on one thing, a point of reference (you do agree on that right?) and I said in my first reply, point of reference is key here:
Most of us (and I'll post a poll question in another thread for fun) belive that in measureing distance on the water it is the distance travelled as if the water were a pure flat horizontal surface (like you were walking on perfectaly calm water). This is the same distance that if one uses a nautical chart and ruler you will determine that it is 5.2 miles from point a to point b. Nautical charts have a 'fixed point of reference' that being a fixed horizontal plane that we call the 'surface' and on the water this is 'as the crow flies' at this has an elevation of zero - sea level.
So if your definition of distance is different, which I believe it is as you are concerned about 'water moving under the hull, etc.) then that is your confusion and why people such as myself are essentially wasting our time discussing this with you. Me personally I could care less about how much 'water moves under my hull' as this will vary so much and no chart will ever tell me how much water will move under my hull to get from point A to point B and that is what I really care about and why I want to know my MPG, to make sure I CAN GET HOME! Clearly your point of reference is "the surface of the water itself" such that if I go up and down a large swell I may have travelled 100' 'over the water' with your point of reference but only 75' with my point of reference. Similar to if I drive over a mountain and have to drive 1000 'UP at 60 degrees then Down 1000' at 60 degrees I will travel 2000', and this is farther than if I tunneled through the mountain at it base which would only be 1000' (Yea I know 60 degrees is stupid, but I did not feel like doing much math this early so picked an equalaterial triangle to use as my mountain). DO YOU SEE THIS?
So, I do think we all agree that devices that calcuate distance travelled via constantly changing posistion (lat/lon), such as our GPS units, are MOST accurate for measureing distance give 'my point of reference' and will most accurately measure the distance one gets when one looks at a nautical chart to determine the distance from point a to point b.
I also think that most will agree that a GPS can NOT measure water 'flowing under the hull', but pitot tube can, and if this is important to you, then you need a pitot tube.
So if you really want to measure your MPG based on the water flowing under your hull with the instantenous surface of the water as your point of reference, go for it and a GPS WILL NOT be most accurate.
BUT if you end up being like 99.9% of people on here (we will let the poll decide) and want to know how much fuel you are burning realted to the distance you are traveling the point of reference 99.9% of the population uses, the GPS WILL BE the most accurate.
For those that continue to help Gringo....."I salute you for you are about to be driven crazy!"
Oh dont get your panties in a twist. Do you really think you are 'helping' me understand navigation? That made me really smile.
For the last time..really, what I just wrote a few back, was:
"you are talking about navigation, I was talking about sensor accuracy and propulsion efficiency"
So your argument to me now is to tell me the same thing in your own worlds? Thats not accuracy, thats repeatability...
obviously, this is wasting our respective time. You think you understand what you are talking about, and you dont. You treat boating like you were driving a car across a frozen ocean, because thats exactly your understanding of the world you are operating in, and seamanship, to you, means being able to start an outboard. Navigation to you means "Which button do I push to make a waypoint?".
Good luck with it. And its a shame you dont have to know as much about boating to drive a boat, as you do about aircraft to fly.
Oh dont get your panties in a twist. Do you really think you are 'helping' me understand navigation? That made me really smile.
For the last time..really, what I just wrote a few back, was:
"you are talking about navigation, I was talking about sensor accuracy and propulsion efficiency"
So your argument to me now is to tell me the same thing in your own worlds? Thats not accuracy, thats repeatability...
obviously, this is wasting our respective time. You think you understand what you are talking about, and you dont. You treat boating like you were driving a car across a frozen ocean, because thats exactly your understanding of the world you are operating in, and seamanship, to you, means being able to start an outboard. Navigation to you means "Which button do I push to make a waypoint?".
Good luck with it. And its a shame you dont have to know as much about boating to drive a boat, as you do about aircraft to fly.
Please point out where exactly i was talking about navigation.
I'm not the one who made the moronic claim that goundspeed doesn't matter in aviation.
obviously, this is wasting our respective time. You think you understand what you are talking about, and you dont. You treat boating like you were driving a car across a frozen ocean, because thats exactly your understanding of the world you are operating in, and seamanship, to you, means being able to start an outboard. Navigation to you means "Which button do I push to make a waypoint?".
your idiotic assumptions are making you look like an.......
you cant get it right at all. If you are just interested in getting from one point to another....thats navigation. Good thing you dont need seamanship to do it, if everything goes right.
Second half of your post: I didnt write that" groundspeed does'nt matter in aviation."
Your idea of an intelligent debate is to start by twisting something thats plainly written? How can you dispute what I wrote? Go read it, again, slowly.
Again, your reply is to try to re-explain something I just told you. But you can't get it straight what I did just tell you. Hopeless.
We dont have enough years left to discuss all the things you dont completely understand. We agree on that, dont we?
Do you perhaps have a reading disorder? That could explain a lot.
read up on it? I understood it well enough to get the license to use it.
an airplane doesnt really care about its speed over the ground, which has nothing to do with how well its flying.
You asked for it again, here you go.
By applying this logic to the current discussion you are either one of two things:
1. A troll
2. An idiot who has no idea what he is talking about.
The discussion is about best MPG. What part of the MILES portion of miles per gallon do you not understand? As others have stated most don't care about how effiecient thier boat is moving through the water, only how efficiently a boat can move from point A to point B. Your logic will only work if you leave the dock and do circles all day, within feet of the dock.
Do I understand what you are talking about. Yep. And you are wrong. Unfortunatly, YOU don't know what you are talking about and therefore you resort to claiming that others don't understand.....when you don't understand yourself. A classic strawman arguement.
I just dont know how to communicate with someone who sounds reasonable, but who repeatedly demonstrates the inability to read a simple sentence and even repeat it accurately without using copy and paste. Then you copy it to quote, but still dont even understand it.
You read that, in plain english, and then claim that I made the "moronic claim that groundspeed doesnt matter in aviation:" I didnt say that. I said the airplane doesnt care about groundspeed to fly. Groundspeed is not a factor in the physics of flight.
Can you focus on one sentence long enough to understand it? If you cant even get that far, frankly the rest of what you say you understand is pretty questionable.
Gringo Is that why my autopilot tracks at a differnt heading than my GPS tracks but gets me to the same spot? I was always impressed with how good good my yamaha speedo work.
Re: Connecting Yamaha Gauges to GPS 
besides, not talking about you personally, but sometimes it feels like trying to explain physics to a chimp who's trying to teach me how to peel a banana. 
