Speed Through GPS Versus Old Fashioned Paddle Log

[goboatingnow]

Originally Posted by Robin3

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

Yes but in most places the tidal stream is not much and anyway your goal is an ep , which the gps gives you.

All I'm saying is that there is actually little use for a log on a modern day boat, perhaps justified as a backup, but since it's now electronic too, I don't think it's much of a backup. Occasionally as a STW speedo it's useful for sail trim, that's about it.


It's worth remembering that mariners actually always wanted distance over the ground. It's just till recently they had no practical way of determining it. It's the same way we call true wind , even though in reality true wind is ground wind.


Dave


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[valhalla360]

Originally Posted by 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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So if your paddle wheel was clear of the bottom and there was current flowing over it in the same direction, you are trying to go, you have the same issue where your instruments say you are moving when you aren't.

Spitting over the side really doesn't tell you anything by itself.

I would still go with the GPS in your scenario. It's unlikley the GPS will continue to read 0.3kts for any length of time while not moving but if the current is giving you a false reading, it likely will do so for a very long time.

I've found the How GPS Works discussion interesting and I've learned some stuff but the original question still has the same answer. A paddle wheel is nice if you are a racer/performance oriented so you can tweak the last little bit of speed out of the boat but for a typical cruiser, it really isn't needed or of much benefit.

[Robin3]

Originally Posted by goboatingnow

Yes but in most places the tidal stream is not much and anyway your goal is an ep , which the gps gives you.

All I'm saying is that there is actually little use for a log on a modern day boat, perhaps justified as a backup, but since it's now electronic too, I don't think it's much of a backup. Occasionally as a STW speedo it's useful for sail trim, that's about it.


It's worth remembering that mariners actually always wanted distance over the ground. It's just till recently they had no practical way of determining it. It's the same way we call true wind , even though in reality true wind is ground wind.


Dave


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In the inlets to/from the ICW, in the Gulf Stream?? In places in the ICW itself it is circa 2kts. Best not sail in the English Channel ( my home stomping grounds) where it is +/-4kts and in some locations double that Mind you it explains why the very nice US couple we once met in a French restaurant had just left their boat parked on the rocks east of Cherbourg Harbour's eastern entrance where the cross tides run at around 4 or 5 kts. They had safely crossed the Atlantic but then spoiled their dream European cruise by not allowing for tidal streams being something more than ignorable.

[goboatingnow]

Originally Posted by Robin3

In the inlets to/from the ICW, in the Gulf Stream?? In places in the ICW itself it is circa 2kts. Best not sail in the English Channel ( my home stomping grounds) where it is +/-4kts and in some locations double that Mind you it explains why the very nice US couple we once met in a French restaurant had just left their boat parked on the rocks east of Cherbourg Harbour's eastern entrance where the cross tides run at around 4 or 5 kts. They had safely crossed the Atlantic but then spoiled their dream European cruise by not allowing for tidal streams being something more than ignorable.

Amazing how people produce these arguments , thereby explaining how they don't understand the basics.

A paddle wheel log in fact conveys NO tidal stream information, whereas, at least a gps " log" would convey the fact that there was a stream. At of course a Position plot using GPS will of course convey clearly the effect of tidal streams

you might examine the mechanism for working up an EP.

A log , using the passage of the boat through the water as a measurement , simply exists because the ability to directly measure ground related measurement did not historically exist. But in fact for all navigation the goal is establish the ground track and as a result the actual position relative to the ground , and not the water.

So speed through the water has no real use in NAVIGATION , even if it has some minor use in sail trim.

Your French couple , had they used their gps correctly, would have navigated into Cherbourg with no issue had they understand the application of tidal streams to navigation, this has nothing to do with the availability of STW.

Ones granny knows how to suck eggs -thanks


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[Robin3]

Originally Posted by goboatingnow

Amazing how people produce these arguments , thereby explaining how they don't understand the basics.

A paddle wheel log in fact conveys NO tidal stream information, whereas, at least a gps " log" would convey the fact that there was a stream. At of course a Position plot using GPS will of course convey clearly the effect of tidal streams

you might examine the mechanism for working up an EP.

A log , using the passage of the boat through the water as a measurement , simply exists because the ability to directly measure ground related measurement did not historically exist. But in fact for all navigation the goal is establish the ground track and as a result the actual position relative to the ground , and not the water.

So speed through the water has no real use in NAVIGATION , even if it has some minor use in sail trim.

Your French couple , had they used their gps correctly, would have navigated into Cherbourg with no issue had they understand the application of tidal streams to navigation, this has nothing to do with the availability of STW.

Ones granny knows how to suck eggs -thanks



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But distance does.



Happy St Patrick's Day! I'm off out, need some more eggs!

[goboatingnow]

Originally Posted by Robin3

But distance does.



Happy St Patrick's Day! I'm off out, need some more eggs!

Nope ground distance does. Distance through the water has to be corrected to establish distance over the ground. If you can measure distance over the ground you have a better estimate.

As I said , too many people argue the status quo, without thinking exactly what the purpose of all this navigation metrics are for.

In reality all navigation is either establishing where we are now ( on the planets surface, not the water ) or attempting to determine where we will be on the planets surface at some point in the future.

Hence distance through the water in itself is a fairly useless metric, since harbours ,rocks etc are not fixed to the water but to the planets surface.

So a DR using un-corrected water based distance is the most inaccurate.

An EP, where we attempt to correct water based metrics for tide effects ( a d others ) is more accurate

An GPS fix, where all these are effective computed out is the most accurate form of position determination we have at present.

Again water based metrics other then sail trim, have no use on board in reality, it's why tall ships never had speedometers !!!

That's not to say , one shouldn't have a log as a backup, that's an entirely different argument

Ps if you are looking for eggs on paddys day, I think you've missed the point ( or pint )


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[Sailor Doug]

GPS=accurate speed and position all boats should have two
Speed log=great tool for sail trim only need one.

Happy St.Patricksday


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[Robin3]

I'm beginning to realisen ow why I only won just a few offshore sail races back in the days before full blown electronic wizardry was permitted on all boats and the navigator was 'king'. I had a B&G log back then which also had a performance indicator whereby you set the initial speed as a centre needle spot on another analog dial and thenwatched to see if sail trimming adjustments moved the needle left (slower) or right ( faster)


Found some eggs, but forgot the pins or needles to prick them with. Have to stick to choccy eggs this Easter.

[conachair]

Originally Posted by Sailor Doug

Speed log=great tool for sail trim only need one.

Very handy for making the most of ocean currents as well. And worth it just to feel good with all those free miles of log against gps distance every days run

[Dockhead]

Originally Posted by leftbrainstuff

Gps is a crude speed readout while underway but without differential gps at slow speeds its not very accurate over short distances. Over a full day its very accurate.
. . .

I disagree with this. As long as you have a decent 3D position lock, a modern GPS receiver will give you SOG accurate to less than a 10th of knot updated at 10hz or better. It is far more accurate, like orders of magnitude more accurate, than any normal log, or indeed any other method of speed calculation available to consumers. They use Doppler shift of the signals from the satellites to make this calculation (which does not need differential correction), which are made internally at a far higher rate than what is outputted to your network (10hz in case of the Simrad GS25). SOG is outputted as a separate NMEA sentence (or PNG); it is calculated inside the GPS receiver.

Unfortunately, as others have said, SOG is not really very useful, for all the reasons others have explained very well, at least on a sailboat. For sailing, you are primarily interested in STW. You sail in water, not on ground. If you have only weak currents where you sail, you can use COG as a very crude proxy for STW -- as many people do. But many of us sail with strong currents (I've seen up to 12 knots of current in the English Channel -- Alderny Race at springs ) -- and in such places, COG is completely useless for sailing. I wish it were not so, but it is . . .


Edit: Oops -- I see Dave has already addressed this with far more technical depth. Note to self -- read the rest of the thread before spouting off

[Sailor Doug]

English GPS must work different, that's ok if you don't worry speed latency on a light air sunny day, opps you don't get many of those.


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[Ericson38]

Originally Posted by goboatingnow

Yes but in most places the tidal stream is not much and anyway your goal is an ep , which the gps gives you.

All I'm saying is that there is actually little use for a log on a modern day boat, perhaps justified as a backup, but since it's now electronic too, I don't think it's much of a backup. Occasionally as a STW speedo it's useful for sail trim, that's about it.


It's worth remembering that mariners actually always wanted distance over the ground. It's just till recently they had no practical way of determining it. It's the same way we call true wind , even though in reality true wind is ground wind.


Dave


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Set and Drift are important things to know when passing through narrow inlets from offshore. They can be estimated from the Tide and Current Tables for the selected reference and subordinate stations, for trip planning, and mid range Garmins like the 741 will display tide level and set/drift, based on solar and lunar effects, but the data is located in a way more aligned for commercial ship traffic (in the Chesapeake).

Air pressure effects and storm induced tides can swamp solar/lunar effects. With a GPS receiver, a magnetic compass (with deviation), and STW sensor, set and drift can be measured directly and locally, with much better here and now awareness than using the posted tables.

B&G Hydra for example (there are others) will display a running set/drift for the body of water you are cruising in, based on STW, GPS, COG (from GPS), and boats heading. Using this, you can verify directly whether your trip plan for going up a river from the ocean was realistic or not, based on predicted versus measured.

A good general non receiver specific discussion on GPS Doppler derivations from carrier phase processing is here-http://www.gpsworld.com/wp-content/uploads/2012/09/gpsworld_Innovation_0511.pdf

[Dockhead]

Originally Posted by 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.

That paper was published in 2004. In those days, consumer GPS receivers were not capable of independent speed measurement using analysis of the Doppler shift of the satellite carriers. They used a technique called "track pointing" -- they took a series of position fixes and calculated time versus distance.

Nowadays, the accuracy of GPS receiver speed measurement is better than 0.1 knot, maybe much better.

Don't confuse this with Doppler speed logs, which measure the speed of the water flow using the Doppler principle, thus giving you very accurate STW data. Different technology. I wish they made affordable Doppler speed logs for small boats.


Many people finding it hard to believe the accuracy of GPS speed readings have not used the new type which has this Doppler technology. They only started to be widely sold maybe four or five years ago.

[transmitterdan]

^+1

Ericson 38 is exactly right. I would just add for clarity that with GPS speed and position is not necessary to do all those set and drift calculations to know your exact position and VMG during the actual passage.

[Dockhead]

Originally Posted by Sailor Doug

English GPS must work different, that's ok if you don't worry speed latency on a light air sunny day, opps you don't get many of those.

Speed latency issues occur with older GPS receivers using "track pointing" techniques of speed measurement. Newer receivers update speed at 10hz and respond faster than regular logs -- seemingly instantaneously. They do not require a series of positions to calculate speed.