Speed Through GPS Versus Old Fashioned Paddle Log

[rwidman]

Originally Posted by Dockhead

................ 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. ..........

OK, but if you are sailing from point "A" to point "B", speed over ground is most important. Your speed over ground, not speed over water is what tells you how soon you should get to your destination.

If you made a modification to your boat and want to see if it makes the boat faster, speed over water would be the important number.

[gbgreen59]

Originally Posted by Dockhead

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.

As long as there have been carrier tracking loops in GPS (which is always), velocity has been derived from Doppler. Every GPS receiver from inception has had a carrier tracking loop. Besides that, the code loop cannot track without aiding of Doppler measurements from the carrier loop. This has also been true from inception. If a receiver manufacturer chose to derive velocity from range measurements, it is because velocity was not important to them (they bypassed the calculation using Doppler). We could go back and ferret out which receivers chose not to use Doppler to derive velocity. I would be interested to know. But velocity derived from Doppler is not new or a new technology.

Using position or range measurements to derive velocity is very much inaccurate relative to velocity derived from Doppler measurements.

[Dockhead]

Originally Posted by gbgreen59

As long as there have been carrier tracking loops in GPS (which is always), velocity has been derived from Doppler. Every GPS receiver from inception has had a carrier tracking loop. Besides that, the code loop cannot track without aiding of Doppler measurements from the carrier loop. This has also been true from inception. If a receiver manufacturer chose to derive velocity from range measurements, it is because velocity was not important to them (they bypassed the calculation using Doppler). We could go back and ferret out which receivers chose not to use Doppler to derive velocity. I would be interested to know. But velocity derived from Doppler is not new or a new technology.

Using position or range measurements to derive velocity is very much inaccurate relative to velocity derived from Doppler measurements.

I'll defer to your superior knowledge and say -- if you say so. On this subject, I know only what I've read, and what I have observed as GPS receivers have evolved.

In any case, the main point is the same -- current GPS receivers give tremendously high accuracy speed measurement.

[Dockhead]

Originally Posted by rwidman

OK, but if you are sailing from point "A" to point "B", speed over ground is most important. Your speed over ground, not speed over water is what tells you how soon you should get to your destination.
.

Not necessarily!

In reasonably still waters, and for motorboats, what you say is true. But:

If you are sailing from point "A" to point "B" in tidal waters over a period of time which includes a change of tide, you will be doing water-referenced, not ground-referenced navigation -- navigating based on course and speed through the water using a calculated CTS. Your COG and SOG during such a journey -- say an English Channel crossing -- will vary wildly and look even crazy at times, and you will be unable to navigate without STW and heading. COG and SOG will be completely irrelevant over the whole journey. Your chart plotter will not do this navigation for you (although I have recently started using a really cool program called Neptune Plus which does do it).

And in general, in strongly tidal waters, you need to understand your speed through the water. In still waters it's less important.

The other case is sailing -- you can't understand the effectiveness of your sail trim without knowing your STW. SOG doesn't help you with this if there's any current running, even a small one, since even 10ths of knots are sometimes relevant to sail trim.

I don't know how fast your boat is, but a fast motorboat feels currents proportionately less than a slow blowboat. You will be less and less interested in STW if you're motorboating, and especially if you are in a fast motorboat, and/or the currents are weak.

[transmitterdan]

Dockhead,

It depends on how you have made your voyage plan. If you estimate set and drift and plot a course then that can be easily transferred to a planned route over ground. If you find that you can't hold the route because the current isn't what you thought then you have to readjust the plan. But the paddlewheel won't help much with that because it can't tell you what is going to happen. For that you need to predict the current speed and direction.

It may be better to rethink our strategy when we have more accurate instruments instead of trying to use old navigation methods. It is possible to plan and execute a voyage using a chart plotter with course and speed over ground even when fast currents are present. You just have to do it differently than before. The old way is not always better.

[Dockhead]

Originally Posted by transmitterdan

Dockhead,

It depends on how you have made your voyage plan. If you estimate set and drift and plot a course then that can be easily transferred to a planned route over ground. If you find that you can't hold the route because the current isn't what you thought then you have to readjust the plan. But the paddlewheel won't help much with that because it can't tell you what is going to happen. For that you need to predict the current speed and direction.

It may be better to rethink our strategy when we have more accurate instruments instead of trying to use old navigation methods. It is possible to plan and execute a voyage using a chart plotter with course and speed over ground even when fast currents are present. You just have to do it differently than before. The old way is not always better.

That works only if the current is constant. In that case, the easiest way to do it is just set your pilot to track mode and let your electronics do the rest.

But when the current is changing, ground-referenced navigation techniques will not get you across efficiently, and in some theoretical cases you don't even arrive, unless you use water-referenced methods and calculate CTS. We had a big discussion about this a couple of years ago.

[Ericson38]

OK, in another case, motoring along at night in the Chesapeake with no wind in unfamiliar water, SOG looks good, then round a point, heading up a river to a marina a few miles away.

Rounding the point to head up the river, I note that SOG has dropped a full knot. Don't have STW reading, it is fouled, but wish I did. RPM gage didn't change, still 3000 rpm.

If I had STW, and it had also dropped, would it indicate I have possibly picked up a crab pot, as I have less ability to see them, although I am outside their area on the chart plotter. Hope the transmission isn't slipping, and go down below and look and feel for heat, etc. Pretty sure the RPM is accurate and hasn't changed.

But if the STW was working, and after rounding the point was the same as before rounding, I would conclude we are now bucking an outflowing current, hence the drop in GPS SOG. Good to know the boat's performance is the same.

This condition can be predicted with the current tables, sometimes, depending on where you are. It is a good cross check of events. But the magnitude of the prediction at your location is not going to be very precise, and with these crab pots all over the place....

[Robin3]

Originally Posted by Dockhead

That works only if the current is constant. In that case, the easiest way to do it is just set your pilot to track mode and let your electronics do the rest.

But when the current is changing, ground-referenced navigation techniques will not get you across efficiently, and in some theoretical cases you don't even arrive, unless you use water-referenced methods and calculate CTS. We had a big discussion about this a couple of years ago.

And another is under way here!

I find when talking to our new boaty friends now that we live/cruise in Florida that most of them do not understand the concept especially of 'variable' currents and see no reason for STW paddlewheels. I had to firmly insist that I wanted STW transducer input, to the electronics 'expert' who fitted our new network as he saw no point for it . There is no easy access data here for current predictions in the ICW or offshore either although I think NOAA have looked at providing it somehow . I would like the equivalent of a Euro style tidal atlas or the equivalent of the Neptune program you have which I had too back then from which I could input anticipated boatspeed ( STW ) and then discover if there was an optimum time to depart for my intended destination, other than simply to get there say before dark or before the pubs shut! In the ICW it seems you just go whenever and hope the current flow 'withs' cancel out the 'againsts' OR You have a motor boat with big hp available and forget to look at the rate the fuel guage is dropping

[Robin3]

Originally Posted by Ericson38

OK, in another case, motoring along at night in the Chesapeake with no wind in unfamiliar water, SOG looks good, then round a point, heading up a river to a marina a few miles away.

Rounding the point to head up the river, I note that SOG has dropped a full knot. Don't have STW reading, it is fouled, but wish I did. RPM gage didn't change, still 3000 rpm.

If I had STW, and it had also dropped, would it indicate I have possibly picked up a crab pot, as I have less ability to see them, although I am outside their area on the chart plotter. Hope the transmission isn't slipping, and go down below and look and feel for heat, etc. Pretty sure the RPM is accurate and hasn't changed.

But if the STW was working, and after rounding the point was the same as before rounding, I would conclude we are now bucking an outflowing current, hence the drop in GPS SOG. Good to know the boat's performance is the same.

This condition can be predicted with the current tables, sometimes, depending on where you are. It is a good cross check of events. But the magnitude of the prediction at your location is not going to be very precise, and with these crab pots all over the place....

AHA, somebody does understand!!

[valhalla360]

Originally Posted by Ericson38

OK, in another case, motoring along at night in the Chesapeake with no wind in unfamiliar water, SOG looks good, then round a point, heading up a river to a marina a few miles away.

Rounding the point to head up the river, I note that SOG has dropped a full knot. Don't have STW reading, it is fouled, but wish I did. RPM gage didn't change, still 3000 rpm.

If I had STW, and it had also dropped, would it indicate I have possibly picked up a crab pot, as I have less ability to see them, although I am outside their area on the chart plotter. Hope the transmission isn't slipping, and go down below and look and feel for heat, etc. Pretty sure the RPM is accurate and hasn't changed.

But if the STW was working, and after rounding the point was the same as before rounding, I would conclude we are now bucking an outflowing current, hence the drop in GPS SOG. Good to know the boat's performance is the same.

This condition can be predicted with the current tables, sometimes, depending on where you are. It is a good cross check of events. But the magnitude of the prediction at your location is not going to be very precise, and with these crab pots all over the place....

Did the speed drop suddenly or gradually (I'm usually watching things closely when I come to changing conditions such as rounding a point or entering a channel)?
- If it's sudden, you need to check what happened (crab pot, rpm drop, something else)
- If it gradually drops as you come around the corner, it is most likely a current to turned into.

If you are unsure, do a quick u-turn. If your speed is now the former speed plus 1kt, it's a current if it stays steady, you know your STW is down. Remember, we are talking about cruising, you aren't going to lose the race if you take 30seconds to spin the boat around.

It's not bad have to have speed thru the water and it does tell you some information but if you are alert, it doesn't tell you much more than you can figure out if you pay attention.

[Dockhead]

Originally Posted by valhalla360

Did the speed drop suddenly or gradually (I'm usually watching things closely when I come to changing conditions such as rounding a point or entering a channel)?
- If it's sudden, you need to check what happened (crab pot, rpm drop, something else)
- If it gradually drops as you come around the corner, it is most likely a current to turned into.

If you are unsure, do a quick u-turn. If your speed is now the former speed plus 1kt, it's a current if it stays steady, you know your STW is down. Remember, we are talking about cruising, you aren't going to lose the race if you take 30seconds to spin the boat around.

It's not bad have to have speed thru the water and it does tell you some information but if you are alert, it doesn't tell you much more than you can figure out if you pay attention.

I've used that u-turn technique myself! Very handy if you lack STW data, and IF you're not under sail and IF there's little or no wind.

If you're under sail, it doesn't work, because you will be on a different point of sail after your u-turn. Likewise, if there's wind -- your speed will change with different heading in relation to the wind, and you won't be able to tell how much of the resulting change of speed is due to that, and how much is due to the current.

[valhalla360]

Originally Posted by Dockhead

I've used that u-turn technique myself! Very handy if you lack STW data, and IF you're not under sail and IF there's little or no wind.

If you're under sail, it doesn't work, because you will be on a different point of sail after your u-turn. Likewise, if there's wind -- your speed will change with different heading in relation to the wind, and you won't be able to tell how much of the resulting change of speed is due to that, and how much is due to the current.

True but I usually cheat. I look over the stern to see if anything is hanging off the outboard or rudders since I can see them.

Of course if you round a point while sailing, the speed difference could easily be due to the new point of sail also.

[rwidman]

Originally Posted by Dockhead

Not necessarily!

In reasonably still waters, and for motorboats, what you say is true. But:

If you are sailing from point "A" to point "B" in tidal waters over a period of time which includes a change of tide, you will be doing water-referenced, not ground-referenced navigation -- navigating based on course and speed through the water using a calculated CTS. Your COG and SOG during such a journey -- say an English Channel crossing -- will vary wildly and look even crazy at times, and you will be unable to navigate without STW and heading. COG and SOG will be completely irrelevant over the whole journey. Your chart plotter will not do this navigation for you (although I have recently started using a really cool program called Neptune Plus which does do it).

And in general, in strongly tidal waters, you need to understand your speed through the water. In still waters it's less important.

The other case is sailing -- you can't understand the effectiveness of your sail trim without knowing your STW. SOG doesn't help you with this if there's any current running, even a small one, since even 10ths of knots are sometimes relevant to sail trim.

I don't know how fast your boat is, but a fast motorboat feels currents proportionately less than a slow blowboat. You will be less and less interested in STW if you're motorboating, and especially if you are in a fast motorboat, and/or the currents are weak.

A lot of typing but it doesn't change what I posted.

[Dockhead]

Originally Posted by rwidman

A lot of typing but it doesn't change what I posted.

Of course it does. The shortest path from A to B over moving water is a straight line -- through water. Not over ground. That is an objective fact.

Here is the shortest path across the English Channel on a average day:



You are holding a constant heading and sailing a straight line through the water, which is the fastest way. But over ground your path can be very crooked.

The reason for all of this is that your boat goes through water. It does not have legs or wheels which reach the bottom.

And in a nutshell that is why STW is important.

[rwidman]

Originally Posted by Dockhead

Of course it does. The shortest path from A to B over moving water is a straight line -- through water. Not over ground. That is an objective fact.

Here is the shortest path across the English Channel on a average day:

Attachment 99050

You are holding a constant heading and sailing a straight line through the water, which is the fastest way. But over ground your path can be very crooked.

The reason for all of this is that your boat goes through water. It does not have legs or wheels which reach the bottom.

And in a nutshell that is why STW is important.

Again, if you are sailing from point "A" to point "B", speed over ground is most important. Your speed over ground, not speed over water is what tells you how soon you should get to your destination.

Sure you are travelling through the water but there is ground under it and the speed of the water (current) doesn't change the actual distance (over ground) between point "A" and point "B".