Delta dawn
Member
- Joined
- Oct 9, 2004
- RO Number
- 15495
- Messages
- 44
What is the difference in speed over ground (GPS) and speed through water other than one is in the air and one is in the water? How different are the readings? if any. Thanks!
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Speed Readings
- Thread starter Delta dawn
- Start date
A very simple question with a not so simple answer. If you are in a lake with no current or wind, they are the same. If you are proceeding north at 15 kts, by GPS, and you are fighting a 5 kt current flowing south, your boat is actually doing 20 kts thru the water. If the current was flowing north at 5 kts, your boat would have been only going 10 kts thru the water.
A GPS derived speed is calculated by measuring how far you will travel between two geographical coordinates in a given amount of time. This is a "true" speed in respect to the surface of the earth.
Air speed or water speed are measurments that exclude the speed of air or water relative to the surface of the earth. If your boat is traveling through still water at 10 knots, then your water speed will be 10 knots. If your water speed is 10 knots through water that is moving against you at 3 knots of current, then your true speed (like GPS speed) is 7 knots.
Air speed or water speed are measurments that exclude the speed of air or water relative to the surface of the earth. If your boat is traveling through still water at 10 knots, then your water speed will be 10 knots. If your water speed is 10 knots through water that is moving against you at 3 knots of current, then your true speed (like GPS speed) is 7 knots.
GPS indicated speed is based on establishing where you were at time "A", where you are at time "B", and assuming you went in a straight line between those two points.
GPS can also have a significant error. With "selective availability" turned off (which it has been for a number of years now but it used to be turned on) the error isn't too bad but with SA on you could easily have a reading of 5 MPH while tied to the dock. With SA off the GPS error is a function of satelite position. If the satelites are in a poor position for you and then move into a better position (note that they move pretty fast!) the improvement in position accuracy can translate into a false speed.
Like others have said, the GPS will give you the speed with respect to the chart, but not with respect to the water.
Rod
GPS can also have a significant error. With "selective availability" turned off (which it has been for a number of years now but it used to be turned on) the error isn't too bad but with SA on you could easily have a reading of 5 MPH while tied to the dock. With SA off the GPS error is a function of satelite position. If the satelites are in a poor position for you and then move into a better position (note that they move pretty fast!) the improvement in position accuracy can translate into a false speed.
Like others have said, the GPS will give you the speed with respect to the chart, but not with respect to the water.
Rod
Delta dawn
Member
- Joined
- Oct 9, 2004
- RO Number
- 15495
- Messages
- 44
Thanks all! Seems as if it's better to use the GPS readings since they are true?
Yeah, GPS readings are pretty accurate these days since they turned "selective ability" off. I don't think they'll turn it back on, what with all these consumer products based on it, all of a sudden these in-car navigation things would show you on the wrong road.
Back before mine got stolen from my truck, we could check our speed on the boats. We'd go one direction at full throttle, then turn around and go back, and it would show a 2 mph difference: because of the 1 mph current.
Back before mine got stolen from my truck, we could check our speed on the boats. We'd go one direction at full throttle, then turn around and go back, and it would show a 2 mph difference: because of the 1 mph current.
One time when I was going to the yacht club by boat, the current was really strong, so I put the boat into neutral and let 'er fly. I checked the GPS speed: 5.5kts over the bottom. Wow! All I did was slip it into gear occasionally for steering corrections. Quite the ride!
GPS is close enough for me.
How about this idea---sitting in my slip, I wonder how fast the current is flowing under the boat? I have heard various numbers, from 5mph up to about 7 knots. I don't know how fast it is moving, but I can feel it. With an oversized Bow Thruster I can correct easily enough.
Regardless, I have a Datamarine unit. It has a "wheel" under the boat. I am told as we move through the water, this wheel is forced to spin by the water passing through its blades. Sooooooo if the water is coming straight at me, and I turn the DataMarine unit on, shouldn't it give me the speed of the water?
How about this idea---sitting in my slip, I wonder how fast the current is flowing under the boat? I have heard various numbers, from 5mph up to about 7 knots. I don't know how fast it is moving, but I can feel it. With an oversized Bow Thruster I can correct easily enough.
Regardless, I have a Datamarine unit. It has a "wheel" under the boat. I am told as we move through the water, this wheel is forced to spin by the water passing through its blades. Sooooooo if the water is coming straight at me, and I turn the DataMarine unit on, shouldn't it give me the speed of the water?