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Put a dead boat in the water, turn on the GPS, and eventually it will travel a thousand miles burning NO fuel at all.
That's right, and when this happens, you're getting GREAT mileage.
Gringo, if you want a ROUGH approximation of speed through the water and a ROUGH approximation of miles of water under the keel per gallons of fuel burned (NOT MPG), stick with the pitot input. Not sure why anyone would want this. As noted previously, if you're interested in calculating fuel burn numbers, you can still get more accurate data from the GPS by averaging the results of out and back runs. This is not enormously complicated.
If you want a relatively exact indication of speed over the ground and MPG, go with the GPS input.
It's MPG that we're mostly interested in, isn't it? I, for example, would be kind of interested in knowing if I have enough fuel to get to my destination.
Yes, and that depends upon the water flow direction. Been thinking about this, and I see that maybe we didnt define the circumstances very well.
In a calm lake, (forget the wind for now) with no water movement, where if you dropped a beer can over the side it would still be exactly there an hour later, GPS position for a spot over the bottom is right. So, that makes one set of arguments right. In a lake. Water is just a thick coat of paint on the land. Your distance from your destination is not being adjusted by the movement of that coat of paint.
However, what I was focusing on was real world, and to me thats the ocean. thats where i live. I didnt take into consideration that some people never take tides and currents as a factor. I apologize. in that case, accurate flow measurement would be better than GPS. GPS doesnt know how fast the water is flowing past the hull. It just knows where the hull is located. And thats all.
So if you want to know how much fuel you need to get home...in a lake, GPS is more accurate.
Again, it doesn't matter how much water goes past the hull. The distance you travel is the same, wind, waves or not. The fuel meter will tell you how much gas the motor is using, and that is what is important.
Lets say on a rough sea you use 2 gallons to go 10 miles. Your GPS records you have moved 10 miles; your fuel use measuring device says you used 2 gallons: So your fuel mgt gauge would say you got 5 miles per gallon.
Lets say on a calm sea, you do the same trip, but only use 1 gallon. You still traveled 10 miles, and according to your fuel use measuring device, used 1 gallon of gas. So your MPG doubled--and your Fuel Mgt gauge would say you got 10 miles per gallon.
If you used a pitot tube or wheel on the rough day rather than the GPS, due to currents, waves, etc, it might say you traveled 15 miles--which is incorrect. You really went, in a straight distance, only 10 miles.
"Again, it doesn't matter how much water goes past the hull. The distance you travel is the same, wind, waves or not". (unquote)
This is simply not true. You burn more gas going against the water flow to get from one fixed geographic point to another. GPS can tell you how far apart the points are, but without factoring in the movement of the water, the MPG is not accurate.
I'm with you Gringo......................simple really.
For example:
When I high speed troll, for Wahoo, I try to maintain 14 MPH. When trolling North, with the Gulfstream, I can actually slow the engine speed to maintain 14 MPH, SOG.
When trolling South, into the Stream, I must increase engine speed to get to 14 MPH, SOG.
I am trolling 14 MPH, SOG, in both directions, BUT, use entirely different engine RPM's to do so..................probably a difference of 400 RPM.
What is the correct and accurate fuel economy reading?
__________________
2006 Pursuit 3480 Drummond Island Sportfish
F250 Yamaha's
Ah, thanks! I just feel like I am doing a lousy job explaining it.
But, if your purpose is to move your bait the same speed through the water in both directions, I would think a constant RPM would be the answer to that, no matter which way or how fast the block of water you are sitting on top of is moving.
To get good fuel efficiency numbers using GPS you would have to run one direction for a fixed GPS distance, and then turn around and run it the other way and average the two. But you are assuming the water movement didnt change while you are doing that, and it DOES change. Keep the runs short and dont waste time between them?
The connection to GPS for mpg is what most boaters want. The paddle wheel indicates water speed for trolling use that. For navigating from waypoint to waypoint, gps which indicates physical boat movement over ground, will be the way to go for figuring out how far you can go at that given time with those conditions. If conditions change, you'll either need more or less fuel to continue at that same ground speed, in order to maintain the same speed. GPS gives instantaeneous changes in speed in conjuction with the fuel flow to motor(s) to give updated info that is more accurate than the pitot tube (like if seaweed blocks have the flow). The end.
"GPS gives instantaeneous changes in speed in conjuction with the fuel flow to motor(s) "
shut the motor off. Does the GPS speed stop and read zero movement? If it doesnt, its not accurate. With the boat dead in the water, the pitot or paddlewheel will read zero. GPS tells you what the other forces are doing to your boat in that case, but its not giving you miles per gallon of fuel used.
As far as the accuracy of the flow sensor being used, its not important that it be accurate, just that its repeatable. You are interested in relative numbers..which throttle setting, prop, trim parameters move the boat through the water the best. SOG is only accurate if the water isnt moving. I think pilots understand the concept a bit better, where dealing with headwinds and crosswind components is an every-trip exercise.
I have had similar conversations trying to explain to some people why they cannot get ships heading from their GPS. They either understand the concept, or they dont.
If you want a TRUE MPG (the measurement "Mile" being the key here) you would us GPS. Aircraft do make use of GPS and it tells them an accurate range for the existing conditions. (same with boats) If you want to know accurate speed versus GPH then Gringo you are correct the pitot tube would take into consideration the movement of the current. (Assuming that your pitot is as accurate as the prevailing speed of the current)
sigh. Ok..tell me this. If you point your boat exactly balanced into the wind and current vectors, and turn just enough rpm to hold that position, (imagine holding the bow just off a buoy or platform when its too deep to anchor) what will your GPS tell you?
It will tell you that you are not moving. Yet you are burning fuel and turning the prop and water is flowing by the hull. So, somethings wrong with your calculations, isnt it.
So, you are getting zero MPG as far as GPS is concerned. Actually, it would be a negative number. Zero miles moved divided by the number of gallons you are burning NOT to move.
Now shut the engine off and drift. GPS will tell you what? that you are moving. And you are. But you are burning no fuel. So now you have the boat doing something like .5 miles per hour burning zero fuel. Now your mpg is unlimited. Again, not right.
The theoretical accuracy of the GPS over flow measurement past the hull is not an advantage, because its not what you want to measure. You want to know how much water you pumped through your props, and in a moving body of water, thats not directly related to two fixed points.
I give up. Somebody else who understands it should try explaining it. I was even thinking of an analogy with a plate of glass being slid over a pool table. GPS will tell you the location of two chalk marks on the pool table. But if you are driving on the top of the sliding sheet of glass....
Forget I said anything. Please. Lets talk about triumph boats, or global warming, or why E-tec engines are fostering their own religion...or how to catch mahi, or why pangas are better boats at half the cost...
Guys, I think everybody here recognizes that GPS input gives you accurate speed/distance traveled over ground, whereas a pitot/paddle input gives a rough measurement of speed/distance through the water. I think this discussion boils down to a question of what you want to do with the data. For just about any purpose I can think of, the GPS data is more useful.
Distance Traveled Over Ground: GPS wins. The pitot/paddlewheel measurement, inaccurate to begin with, will be made even more inaccurate by any current effect.
Speed Over Ground: GPS wins for the same reason.
MPG: GPS wins. If you're drifting in a current with the engine idling, GPS, in conjunction with your fuel management system data, will tell you your MPG, and it will also tell your direction and speed. A pitot/paddlewheel will tell you that you are not moving (wrong) and that you are getting zero MPG (also wrong).
If you're idling in gear into a current such that you are making no headway, i.e. you are fixed in relation to a spot on the earth, GPS will correctly tell you that you have zero speed and that you are getting zero MPG. (You cannot get negative MPG. The worst you can do is zero MPG. Mileage is not a vector.) A pitot/paddlewheel will tell you that you are moving at about the speed of the current (wrong) and that you are getting some non-zero MPG (also wrong).
Speed through the water: Edge still goes to the GPS! For all but the lowest boat speeds, like below 10 knots, the inaccuracy inherent the pitot/paddle speed reading will be greater than the inaccuracy introduced into the GPS reading by any current effect.
So why would anyone ever put a pitot/paddlewheel on a boat? I can think of three reasons. Many boats do not have or need a GPS, and a pitot/paddlewheel based speed reading is better than nothing. A pitot/paddlewheel reading could come in handy if your GPS craps out and you don't have a backup. Finally, paddlewheels are OK at measuring low speeds through the water, like trolling speed, but many prefer to use other methods like the angle of the dangle of their fishing lines or the wake of their hull.
I think it all comes down to your frame of reference. If you want to define your frame of reference as a single water molecule drifting in a body of water, fine, you can do that. Most people, however, choose to navigate and describe their movements in relation to points on the earth.
"A pitot/paddlewheel will tell you that you are moving at about the speed of the current (wrong) and that you are getting some non-zero MPG (also wrong). " is not accurate.
you ARE moving at the speed of the current. If the current suddenly stopped dead still, and you didnt change a thing on your throttle settings.. you would start moving over ground. But in order to maintain position you DO have to move at the speed of the current.
And GPS is great for navigating. Its just about perfect. But the original idea of this was that it was more accurate to determine engine efficiency, which is not right. Thats how much fuel you use to propel you through the water. If you are tweaking tilt and tabs and rpm to see what combination of things moves you through the water most efficiently, you need to know your boat speed relative to the water. Not to some fixed point on the earth.
And its not important that paddle wheels be accurate, only that they be repeatable and linear.
"A pitot/paddlewheel will tell you that you are moving at about the speed of the current (wrong) and that you are getting some non-zero MPG (also wrong). " is not accurate.
you ARE moving at the speed of the current. If the current suddenly stopped dead still, and you didnt change a thing on your throttle settings.. you would start moving over ground. But in order to maintain position you DO have to move at the speed of the current.
And its not important that paddle wheels be accurate, only that they be repeatable.
If a boat is fixed in relation to a spot on the earth, most would say that it is not moving, yet you insist that it is. This is because you have adopted a rather unconventional frame of reference.
I would prefer that speed, distance, and MPG numbers be accurate AND repeatable. You get that with a GPS, not with a pitot/paddlewheel. If I post that my 23 footer is getting 15 MPG at 35 knots, folks are gonna call BS, even if I can repeat the inaccurate measurement.
I'm with you Gringo......................simple really.
For example:
When I high speed troll, for Wahoo, I try to maintain 14 MPH. When trolling North, with the Gulfstream, I can actually slow the engine speed to maintain 14 MPH, SOG.
When trolling South, into the Stream, I must increase engine speed to get to 14 MPH, SOG.
I am trolling 14 MPH, SOG, in both directions, BUT, use entirely different engine RPM's to do so..................probably a difference of 400 RPM.
What is the correct and accurate fuel economy reading?
Your speed hasn't changed, but your gasoline usage has. This is why it isn't important (at least as I se it) what the speed of the water, the wind or the waves are; as long as your speed is constant, your "variable" is your fuel flow, and what it outputs to your FM gauge--whether that be a wheel in the line or fuel injector pulse rates.
sigh. Ok..tell me this. If you point your boat exactly balanced into the wind and current vectors, and turn just enough rpm to hold that position, (imagine holding the bow just off a buoy or platform when its too deep to anchor) what will your GPS tell you?
It will tell you that you are not moving. Yet you are burning fuel and turning the prop and water is flowing by the hull. So, somethings wrong with your calculations, isnt it.
So, you are getting zero MPG as far as GPS is concerned. Actually, it would be a negative number. Zero miles moved divided by the number of gallons you are burning NOT to move.
Well since you are not moving, you would be getting zero mpg. I would still have fuel usage, but since there is no forward speed, there is technically no "miles per gallon". The FM gauge would register total fuel used and when I did start positve forward movement, read out how many miles I left on that tank (as an average).
If this is the "error" you are trying to explain? I would say given the built in inefficiency of a pitot tube or water wheel, and their proclivity for returning huge differences in real v actual speed, that this "error" is*completely outweighed by the very few times that you would be at a position for it to actually be more "efficient" than a GPS.* And I'm not sure "efficient" is even correct here, as technically, you would be getting zero mpg.
Quote:
Gringo - 1/30/2007 10:55 AM
Now shut the engine off and drift. GPS will tell you what? that you are moving. And you are. But you are burning no fuel. So now you have the boat doing something like .5 miles per hour burning zero fuel. Now your mpg is unlimited. Again, not right.
My FM guage won't read anything; I think it shows "--" when the motor is turned off. Not sure I can argue that it's not using any gas when the motor is off. Were I pulling it on a trailer, would you think I should I see some outrageous MPG number, even though the boat isn't moving under it's own power?
Quote:
Gringo - 1/30/2007 10:55 AM
The theoretical accuracy of the GPS over flow measurement past the hull is not an advantage, because its not what you want to measure. You want to know how much water you pumped through your props, and in a moving body of water, thats not directly related to two fixed points.
I don't care how much water is moving past my prop; I care about fuel usage over a distance. Given the same forward movement of the boat, the fuel usage (and water through my prop) would necessarily change with differing sea conditions, hence affecting my "miles-per-gallon".
I agree with what was stated earlier; you frame of reference is incorrect.
Re: Connecting Yamaha Gauges to GPS 
