Speed Through GPS Versus Old Fashioned Paddle Log

[Sailor Doug]

Most electronics have a refresh time and speed averaging. Gps is slower to update speed than paddle wheel so much harder to trim sailboat to maximum speed. If you use distance traveled by a calibrated paddle wheel it is more accurate than a gps distance traveled. GPS still gives you a exact position.
My paddle wheel also tells me sea temperature good to know.


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[Stu Jackson]

Originally Posted by Sailor Doug

Most electronics have a refresh time and speed averaging. Gps is slower to update speed than paddle wheel so much harder to trim sailboat to maximum speed. If you use distance traveled by a calibrated paddle wheel it is more accurate than a gps distance traveled. GPS still gives you a exact position.
My paddle wheel also tells me sea temperature good to know.


Sent from my iPad using Cruisers Sailing Forum

Huh?

You might want to go back to Navigation 101.

[MarkSF]

A GPS measure miles digitally. A knotmeter is analog. As everyone knows, analog is better.

[Stu Jackson]

Originally Posted by MarkSF

........As everyone knows, analog is better.

SUNDIAL

[MarkSF]

Well that explains a lot, I thought it was a compass. Turns out its a sundial. Explains why i keep getting lost, and never know what time it is.

[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?

[valhalla360]

Originally Posted by Stu Jackson

Yes it is, since someone made the point that faster boats are less affected by current than slower ones. That rig could easily do twice or three times the speed of a sailboat.

I "navigated" in The Gulf Islands with my f-i-l's outboard powered boat with no instruments, too. But if I was in a sailboat, I'd sure like to have had STW and SOG.

But back to the question: How did you learn boatspeed if not with instruments?

Didn't back then.

On the current boat, I've watched the GPS speed under a wide variety of conditions and throttle settings. On waters without current and making round trips, that's far more accurate than a paddle wheel (how do you know your paddle wheel is properly calibrated?)

[lordgeoff]

Originally Posted by Sailor Doug

Most electronics have a refresh time and speed averaging. Gps is slower to update speed than paddle wheel so much harder to trim sailboat to maximum speed. If you use distance traveled by a calibrated paddle wheel it is more accurate than a gps distance traveled. GPS still gives you a exact position.
My paddle wheel also tells me sea temperature good to know.


Sent from my iPad using Cruisers Sailing Forum

A paddle wheel is like a badly made clock. It will only ever give you an approximate speed. You will never know how accurate it is or isn't.
A GPS, if talking to satellites, and is not faulty will always give you an accurate speed over the earth. As an example a car travelling at GPS speed of 100 kph will match a Police radar reading of 100 kph. The cars analogue speedo will show any number but usually a few kph faster. An analogue paddle log is greatly less accurate than a cars speedo and will show any speed depending on weed growth, shaft friction or how well it was positioned/fitted. It can be calibrated..... If you compare it to a GPS speed. If you want to stick to the harbour speed limit use your GPS......

[sailorboy1]

Paddlewheel knotmeters are for people who just need to have something to clean. They probably have other boat items they are slaves to out of some misplaced "must to" school of boating.

[goboatingnow]

Originally Posted by lordgeoff

A paddle wheel is like a badly made clock. It will only ever give you an approximate speed. You will never know how accurate it is or isn't.

true

A GPS, if talking to satellites, and is not faulty will always give you an accurate speed over the earth

.

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

As an example a car travelling at GPS speed of 100 kph will match a Police radar reading of 100 kph. The cars analogue speedo will show any number but usually a few kph faster.

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.

An analogue paddle log is greatly less accurate than a cars speedo and will show any speed depending on weed growth, shaft friction or how well it was positioned/fitted. It can be calibrated..... If you compare it to a GPS speed. If you want to stick to the harbour speed limit use your GPS......

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"

[transmitterdan]

GPS speed accuracy is not much dependent on VDOP or H circle of uncertainty. It is not merely the distance traveled divided by time but a real time derivative of displacement and time. But log distance is typically based on position readings and that's why GPS logs tend to be averaged and more granular. GPS speed integrated over time is a pretty accurate measure of distance traveled. But it won't match closely with a GPS log updated every 0.1nM and so it would create the "man with two watches"* syndrome.

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

[DrivinSteve]

Originally Posted by Jonathancpwalsh

Can anyone tell me why so many boats still use the "old fashioned" impeller log to record their speed? GPS obviously gives much more accurate speed over the ground which is, ultimately all you really want, isn't it? Or am I missing something? I'm not a racer - just cruising! Would appreciate your thoughts!

Don't ignore the usefulness of a knot/log to determine natural boat speed/distance in a MOB recovery situation. Pushing the MOB button on a GPS will mark the spot on the ground but the MOB will be subject to the same forces (wind drift/currect) as the the water, not the spot on earth.

[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.


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[goboatingnow]

Originally Posted by transmitterdan

GPS speed accuracy is not much dependent on VDOP or H circle of uncertainty. It is not merely the distance traveled divided by time but a real time derivative of displacement and time. But log distance is typically based on position readings and that's why GPS logs tend to be averaged and more granular. GPS speed integrated over time is a pretty accurate measure of distance traveled. But it won't match closely with a GPS log updated every 0.1nM and so it would create the "man with two watches"* syndrome.

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

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

[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"

Yes. They are set to over read so any error will show the car going slower than speed limit. The actual accuracy of a car speedo is the sum of its manufacturing and analogue parts. You don't know your actual speed by this device but the errors will most likely not end up having you over the speed limit. Travelling on roads it is safer to use your Tom Tom as your speed reference.