Speed Through GPS Versus Old Fashioned Paddle Log

[Tayana42]

I agree there is an advantage to knowing STW and SOG. Knowing the effect of current is useful. But I'm lazy and I suspect many other cruisers are too. The last time I pulled the paddle wheel to clean it (which I had to do every week or so in the summer) I just didn't put it back. That was years ago. So, any bright new ideas to keep the paddle wheel cleaner longer particularly to keep the crusty white worms off?


S/V B'Shert

[transmitterdan]

Originally Posted by goboatingnow

I dont follow, the speed algorithms I've seen in the devices, merely ( well as a simple level - merely ) compute position and average over time, deriving speed over the ground, in that regard since position errors are randomly distributed , this also transfer into speed errors



Dave

Speed is available "instantly" in almost all GPS chipsets and much faster than waiting for multiple position fixes. Nearly all GPS chipsets output speed which is not based on a change in computed position. It is a continuous derivative function. If speed relied solely on a change in position divided by time it would be a very "noisy" and slow process. For example, if speed derived as you imagine it could never read zero without some software trickery. About the best a consumer GPS receiver could do would be 1kM/Hr.

Edit: I should have added the measurement the GPS chipset uses is the doppler shift frequency of the various satellites. This can be used to compute speed "instantly". I think 99.9% of all GPS chipsets use doppler shift to compute real time speed. But speed displayed on a recorded track is done as you say by taking distance between position fixes and dividing by time between the fixes. Not many track recorders record the real-time speed at each point probably because it would just confuse people.

[valhalla360]

Originally Posted by Sailor Doug

I will try again. Have you ever sat tied to a dock and left your gps track screen on. Usually you will see points 100 feet or more around your true position. When you sail the same thing happens over time and are not apparent. When gps measures distance travelled it calculates distance between all these untrue readings. The result is distance travelled will read longer travelled than actual. GPS is great I don't sail without two of them running paddle wheel transducer is used to trim sails. If you can see a use for it not drill another hole in the bottom of your boat to install one.
For the HU! Raymarine supplied transducers have a thermocouple imbedded in it that display water temperature. I know of a sailor who replaced transducer when thermocouple stop functioning.


Sent from my iPad using Cruisers Sailing Forum

I've watched while sitting at the dock or at anchor. It rarely exceeds 0.2kts and when it does, it's usually jumps up for a couple seconds and then drops back to hang out near zero for extended periods of time.

Are you possibly thinking of before they turned selective availability off? Back then it would jump around quite a bit. With current GPS, it typically shows us in the exact slip that the boat is in. Even during the blips, it rarely is more than 10-20ft from our true position.

Again, those couple second blips can be an issue for a racer who is ready to tweak at a moments notice. For a cruiser just loafing along, if they see a sudden 1/2kt difference, they will wait a couple seconds to see if it returns to normal, shrug and go about having a nice day.

[boatman61]

Originally Posted by transmitterdan

* A man with two watches never knows what time it is.

Whereas man with broken watch is always correct twice a day.

[transmitterdan]

Originally Posted by Sailor Doug

I will try again. Have you ever sat tied to a dock and left your gps track screen on. Usually you will see points 100 feet or more around your true position. When you sail the same thing happens over time and are not apparent. When gps measures distance travelled it calculates distance between all these untrue readings.

Doug,

That is not how 99.9% of GPS units work. They are able to compute speed without regard to position. The speed displayed by most GPS units is pretty accurate within about 0.1knots. The main cause of speed errors is the tiny fluctuation in the speed of light as the radio waves travel through the atmosphere. The location of the antenna also contributes. If it is on a long pole the rocking motion of the antenna will result in a small speed indication.

The "noise" you see in position wandering around while not moving has nothing to do with the accuracy of GPS speed. The speed calculation doesn't use position.

[valhalla360]

Originally Posted by transmitterdan

Doug,

That is not how 99.9% of GPS units work. They are able to compute speed without regard to position. The speed displayed by most GPS units is pretty accurate within about 0.1knots. The main cause of speed errors is the tiny fluctuation in the speed of light as the radio waves travel through the atmosphere. The location of the antenna also contributes. If it is on a long pole the rocking motion of the antenna will result in a small speed indication.

The "noise" you see in position wandering around while not moving has nothing to do with the accuracy of GPS speed. The speed calculation doesn't use position.

Ummm...they use smoothing functions but it is very much about taking the distance and time between readings. When you get right down to it, all gps can do is note the current time and position. Everything else is derived from that.

I do agree that speed is extremely accurate though.

[transmitterdan]

Originally Posted by valhalla360

Ummm...they use smoothing functions but it is very much about taking the distance and time between readings. When you get right down to it, all gps can do is note the current time and position. Everything else is derived from that.

I do agree that speed is extremely accurate though.

That is just not correct. GPS can and does easily determine speed without knowing position. There are low cost devices available for special applications that have only speed output and no way to output position.

[lordgeoff]

Originally Posted by goboatingnow

true


.

Maybe,both right and wrong as considerable filtering and running averaging is performed to remove the effects of the circle of Error probability arising from the makeup of GPS ( nothing to do with SA). The nature of GPS reception is that some position readings can actually have considerable error in them, especially in compromised H and VDOP situations




No, not correct car speedo are actually be to a typically 10% accuracy, but that inaccuracy is all positioned as over- reading as laws typically require that speedos must never underread. Hence speed's are intentionally constructed to over read and display a greater speed then actual.


Not correct , historically harbours speeds are clearly taken as speed through the water, in fact it has historically been the case that all speeds expressed in Kn are through the water speeds, since nothing existed to measure " ground speed"

Re harbour speed limits. Use this example.
1 I am going west to east on the C&D Canal from Chesapeake to Delaware.
2 I know the speed limit is 5 Kn along the length of the C&D Canal.
3 I also know there is a speed camera half way through the C&D Canal to photograph and record speed infractions.
4 I am a good skipper and throttle back so my paddle log says 5 Kn......

5 Unfortunately I did not research the trip and failed to realise there is a west to east tide running at 4 Kn.
6 As I pass the speed camera I am clocked by the (radar) speed camera and photographed at 9Kn. (5kn + 4 Kn tide current)
7 A month later I get the fine in the mail for motoring at 9 Kn in a 5Kn zone.
The Moral of the Example is that if I had not had or used my paddle Log and set my speed by my GPS I would have $100 more in my pocket for Beer and Pizza.

[Stu Jackson]

Originally Posted by valhalla360

(how do you know your paddle wheel is properly calibrated?)

Thanks for your answer, I now understand.

Answer: we have a measured nautical mile on the Oakland Alameda Estuary. Always fun to check. And yes, there are currents there.

[Stu Jackson]

Originally Posted by Reefmagnet

Well, you know technically it can more accurate for measuring the distance a boat actually travels. It's all in the context. Imagine a car driving between two points over a mountain range and winding roads. Which measurement would be the most accurate for distance travelled: 1) The trip odometer or 2) the distance of the GPS route?

I think you're close.

I understand the concept, BUT...

It would be true if you used the GPS TRACK. Perhpas we're saying the same thing when you say ROUTE, but I think of route as a planning tool that has straight lines between waypoints, whereas the TRACK is the record of the distance sailed. On my GPS the only way to see that distance is to download the track to the software on the computer, pull up the track and read the output. I am guessing that later GPSs have the ability to see this data right on the GPS. Mine is a hh Garmin GPSMap 76Cx.

Do we agree?

PS - Real story: In the Alps two years ago, windy mountain road. GPS kept saying "Turn left in 100 meters." Woulda been off a bloody cliff!!!

[valhalla360]

Originally Posted by Stu Jackson

Thanks for your answer, I now understand.

Answer: we have a measured nautical mile on the Oakland Alameda Estuary. Always fun to check. And yes, there are currents there.

While interesting, do you run a measured mile before each days sailing? A few days growth can impact the results of the paddlewheel, so if you calibrated it 2 wks ago, it may be totaly off today.

How do you calibrate the wheel to account for cross current of unknown velocity?

[lordgeoff]

Originally Posted by Stu Jackson

I think you're close.

I understand the concept, BUT...

It would be true if you used the GPS TRACK. Perhpas we're saying the same thing when you say ROUTE, but I think of route as a planning tool that has straight lines between waypoints, whereas the TRACK is the record of the distance sailed. On my GPS the only way to see that distance is to download the track to the software on the computer, pull up the track and read the output. I am guessing that later GPSs have the ability to see this data right on the GPS. Mine is a hh Garmin GPSMap 76Cx.

Do we agree?

PS - Real story: In the Alps two years ago, windy mountain road. GPS kept saying "Turn left in 100 meters." Woulda been off a bloody cliff!!!

Ahhhhh. Now we can talk about the accuracy of electronic charts.....goody.

[valhalla360]

Originally Posted by transmitterdan

That is just not correct. GPS can and does easily determine speed without knowing position. There are low cost devices available for special applications that have only speed output and no way to output position.

Just because a device doesn't output position doesn't mean it isn't using position to calculate speed. I could see where such a device might be able to get a "speed fix" faster on start up as it doesn't care about positional accuracy only accuracy relative to the last position (there might be a stumbling block in there though).

I find this interesting so I poked around a bit and could find nothing online to conflict with my undertanding. If you have a link to documentation that GPS can measure speed directly, I would be interested.

[Stu Jackson]

Originally Posted by valhalla360

While interesting, do you run a measured mile before each days sailing? A few days growth can impact the results of the paddlewheel, so if you calibrated it 2 wks ago, it may be totaly off today.

How do you calibrate the wheel to account for cross current of unknown velocity?

No. I pass by it every time I go out and return.

Not wanting to discuss the "accuracy" of electronic plotting/charting, etc., BUT...

It's all relative. Just because the instruments have 00.XY or 00.A decimal places means bupkis. Whether I'm going 5.4 knots or 5.6 knots is NOT what I need to know. I need to know my paddlewheel is somewhere close to reality or that it is fast or slow compared to the measured mile. Then I compare it to the GPS. I use it for finding out what the current is doing.

The decimal places make it seem to perhaps some people that they are accurate, but that would be incorrect. The decimal places are there because that's how the chips work.

Remember when some cars had digital speedometers? My f-i-l had one. I loved it. Most folks hated it and you don't see them anymore. I always wonder why. Anybody know?

Fooling yourself that you're going 5.9 knots vs 6.1 is foolish. In the old analog (sundial ) days, you'd say I'm going 6. Right? However, I'm sure some skippers have invested time & money into getting their gear as close as possible to actuality.

I guess I'm in the "Middle Camp" between hard core racers who NEED this information for all kinds of reasons, and those who are "cruisers" who don't care, also with good reason.

Look, there's merit in ALL of these approaches. There is no "right" answer. It's kinda like the "What's the BEST...?" questions that keep popping up. :smile :

[Cowboy Sailer]

you would be interested in: High accuracy speed measurement using GPS (Global Positioning System)
by Tom J. Chalko, MSc, PhD*
©Tom Chalko 2007, all rights reserved. This document may be distributed freely, providing that no content is modified