Speed Through GPS Versus Old Fashioned Paddle Log

[Sailor Doug]

I am just having fun whaling for the ice to melt.


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[goboatingnow]

Originally Posted by Sailor Doug

Wow goboatingnow are you a sailboater and have you ever raced? No instance speed with that gps, do you new know the scan time? If you sailed a lot would notice false gps speed when the boat is not moving and latent acceleration. What credentials do you have to cling to your sifi statements.


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There's are a series of reasons why sometimes you get false speeds at zero etc

And yes I race ( beneteau 8metre, & J 24 )

On what basis do you accuse me of si-fi. Would you like to describe how GPS works or advance an alternative theory as to the basis of inferometry as it applies to GPS position determination.


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[Sailor Doug]

It appears you may know that a knot log is better if you are looking for small changes of speed in a short period of time, GPS is not so good at this but will give a very accurate time over bottom. I realize GPS uses complex information based on time differential and Doppler shift to calculate position and velocity on a complex plane. This is no where in the same accuracy of a doppler knot log.


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[goboatingnow]

Originally Posted by Sailor Doug

It appears you may know that a knot log is better if you are looking for small changes of speed in a short period of time, GPS is not so good at this but will give a very accurate time over bottom. I realize GPS uses complex information based on time differential and Doppler shift to calculate position and velocity on a complex plane. This is no where in the same accuracy of a doppler knot log.


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I would argue GPS speedo in the absense of significant tide is much more accurate then a paddle wheel.


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[sailorchic34]

A doppler speed log has an accuracy of 1% of range according to manufacturers data. http://furunousa.com/ProductDocument...20Brochure.pdf

With a 40 knot range, the accuracy would be 0.4 knots.

A non-differential GPS has a speed accuracy or .2m/s or 0.44mph as tested by NCBI. Accuracy of non-differential GPS for the determination of speed ove... - PubMed - NCBI

Pretty close to the same in my book.

[Sailor Doug]

I said change of speed not stable speed. The length of the algorithm is very complex and takes time for gps to process so changes in speed are slower than a simple counter. This has been my point of pointing out why a knot log is very usefully. All so some of the disinformation on how a gps functions.


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[sailorchic34]

Originally Posted by Sailor Doug

I said change of speed not stable speed. The length of the algorithm is very complex and takes time for gps to process so changes in speed are slower than a simple counter. This has been my point of pointing out why a knot log is very usefully. All so some of the disinformation on how a gps functions.


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Honey, Accuracy is for all intents the same between GPS and a Doppler log, NCBI testing was not at static speed but in real world testing against a calibrated wheel speedo.

At typical cruising sailboat speeds, there is no appreciable difference in accuracy. Yes a GPS is doing calculations, so is a Doppler log and the doppler log is per the manufacture information, accurate to 1% of range, which on the unit I listed is 40 knots. Distance measured is also to within 1 percent.

The log may look more accurate with the 0.1 knot resolution, but manufacturer only lists a 1% accuracy so it could be off by .4 knots. If it was better I'm sure the marketing folks would have put it in the data.

[sailorchic34]

Signet also lists a +/-1% for their speed log, after calibration.

http://www.signetmarine.com/products...7A/SL267A.html

[Sailor Doug]

Please let's not get into Doppler log. This post has been into the function of a simple knot log verses gps. My reference to a doppler log was that it uses only Doppler shift to calculate speed. The argument being that the latent speed change of a gps is a disadvantage and it's accuracy at slow locale speed is often wrong. Also long argument on how a gps calculates speeds.
Thanks sweetie I am only having fun.



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[goboatingnow]

Originally Posted by Sailor Doug

I said change of speed not stable speed. The length of the algorithm is very complex and takes time for gps to process so changes in speed are slower than a simple counter. This has been my point of pointing out why a knot log is very usefully. All so some of the disinformation on how a gps functions.


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I'd have to disagree with this, in practice gps instantaneous speed is one thing and is not a metric we use much on a boat from navigation. Distance travelled per time sequent is what most people use.

If you did want to mark up EPs using a gps calculated log , it will be in the most part more accurate then a paddle wheel.

Personally I have a beautiful walker log I use from time to time !! , I bought it some years ago for £ 50 from a friend.

Dave


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[transmitterdan]

Almost any method of measuring speed using Doppler is more accurate than a paddle wheel. And Doppler speed measurement is very fast. So if you want accuracy and fast updating then Doppler is the way to go.

[Sailor Doug]

I once got stuck on a sand bar, I revved the diesel to try to free my boat. Garmin gps read 0.3 knots so I kept trying. Finally I spat over the side and found I was not moving.
Instance reading does not equal real change in velocity, all of our computers take time to calculate and display value.


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[Robin3]

Originally Posted by goboatingnow

I'd have to disagree with this, in practice gps instantaneous speed is one thing and is not a metric we use much on a boat from navigation. Distance travelled per time sequent is what most people use.

If you did want to mark up EPs using a gps calculated log , it will be in the most part more accurate then a paddle wheel.

Personally I have a beautiful walker log I use from time to time !! , I bought it some years ago for £ 50 from a friend.

Dave


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Funny that is what I sold my last one form Maybe we knew each other ??. In later years I mainly used mine to calibrate or check the paddlewheel log but suitably mounted on a nice piece of varnished teak it would make a nice table ornament too.

[Robin3]

Originally Posted by goboatingnow

I would argue GPS speedo in the absense of significant tide is much more accurate then a paddle wheel.


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Indeed, and If We all only ever sailed on ponds or lakes where there are no currents then there would be no discussion needed but then we would have missed over 150 contentious replies

[MarkSF]

My paddle wheel is actually quite a decent measure of whether I need to get the bottom cleaned, it is so sensitive to water flow over it. Immediately after cleaning it over-reads by 10% or so. It is spot on after a month, and 10% low after a month or so more. Presumably this is because it is very close to the hull, in the laminar flow region.