Speed Through GPS Versus Old Fashioned Paddle Log

[goboatingnow]

Originally Posted by rwidman

As simple as I can state it:

If point "A" and point "B" are seventy nautical miles apart and you start at point "A" and maintain a speed of seven knots over ground, it will take you ten hours to get to point "B".

indeed, in the absence of tides , and you point is

however if you have a tide at right angles etc then you must not only maintain a ground speed, as you say, you must also remain on the intended ground track , which will require a further increase in effective speed, since unfortunately your boat does not run on wheels

[goboatingnow]

Originally Posted by bigbrownpilot

I fly jets for a living. I have a digital readout of my INS/GPS ground speed. But I also have a big old indicated airspeed gauge and a true airspeed readout. All three are important and useful at different times. Just sayin'.


Sent from my iPhone using Cruisers Sailing Forum

airspeed is importent to a fixed wing airscraft for obvious reasons. instantaneous speed through the water is of little importance to a sailing boat , since nothing happens if it stops !!!

[valhalla360]

Originally Posted by Ericson38

Could also just put it neutral and after awhile measure the set and drift off the GPS SOG and COG and keep a mental note of what is going on (at that moment), but doing a 180 or what I just offered is not the way to get somewhere in a limited width channel in a strange area with degraded visibility to what is floating around.

The STW sensor is going to allow you to continually measure the difference between STW and SOG, as opposed to a one time measurement. Situational awareness is what this type of cruising is about, using diverse data.

Open ocean, the STW sensor has much less value, I would certainly agree. Same for charted depths in open ocean versus soundings.

99% of the time, you won't need to stop or do a U-turn. The issue is readily obvious by other means. This is cruising we are talking about so 30 seconds lost figuring out what's going on is no major problem.

It would have to be a pretty narrow channel before I couldn't do a u-turn and then the speed drop would likely be explained by the fact I intentionally slowed down due to restricted waters.

You just added the restricted visibility. While I'm sure we can come up with a scenario where it provides a nice benefit beyond racing, if you have to keep adding complications, it becomes an ever less likely scenario we trying to address.

[goboatingnow]

The STW sensor is going to allow you to continually measure the difference between STW and SOG, as opposed to a one time measurement. Situational awareness is what this type of cruising is about, using diverse data.

yes but the information derived from such comparison is not a whole lot of use. for example if you are maintaining a continuous plot , if will also be obvious, if you are using an autopilot to steer to a waypoint , current effects will also be obvious

simply seeing a difference between SOG and STW in itself could be made up of many things, sensor inaccuracy, wavemotion, current , leeway, etc etc

Yes you can loo at the two and make some assumptions about current, but you can also do that by several other ways too.

[goboatingnow]

put it simply this way folks


If you had to pick between loosing your GPS or loosing your paddlewheel , which would you pick.


I think that might end this debate

[valhalla360]

Originally Posted by Stu Jackson

For those of you who, misguidedly, believe the GPS and VMG approach, you might want to search out Dockhead's excellent thread about CTS with great details about crossing the English Channel. Anytime you sail through currents that are abeam, your GPS will seriously misguide you. Try it sometime. If you, and only YOU can the GPS can't, anticipate the current, then you'll sail a lot shorter distance. Please, think about it.

The GPS doesn't misguide you at all. The idea of picking a heading and letting the current push you to one side for a while before it reverses and pushes you the other way for a while, is easily understood with GPS output.

It is a special case but you just set your compass (traditional not GPS) to your desired heading. You then keep an eye on how your GPS calculated course deviates to make sure it does switch back to get you to your destination in the least time but not the least distance.

[goboatingnow]

Originally Posted by Stu Jackson View Post
For those of you who, misguidedly, believe the GPS and VMG approach, you might want to search out Dockhead's excellent thread about CTS with great details about crossing the English Channel. Anytime you sail through currents that are abeam, your GPS will seriously misguide you. Try it sometime. If you, and only YOU can the GPS can't, anticipate the current, then you'll sail a lot shorter distance. Please, think about it.

Again, your point is correct in theory, in practice , staying " reasonably " close to the course line, is a better navigation choice then the "theoretical " idea of setting course hours in advance based on assumptions and often inaccurate data into the future.

Forget crossing the English channel, why, because symmetrical reversing tides in effect cancel out errors, in real life most tidal journeys are not symmetrical nor as simple as a channel crossing

[valhalla360]

Originally Posted by Dockhead

What you say would be correct if the current were constant. In that case, you don't need to calculate a CTS. You can just put your pilot in track mode and relax. When COG matches bearing to waypoint, everything is fine. Your GPS can solve this problem.

The problem is when the current changes. The problem is dramatic when, like in the English Channel, it changes direction. But the problem is the same if even the speed changes -- crossing the Gulf Stream to the Bahamas from Florida is exactly the same problem. In that case, your GPS does not tell you a single useful thing.



Here's a great simplified illustration of the problem:

Let's say you're trying to get right across a wide river. The river is three miles wide. The current is slack for the first mile, runs at 5 knots for the second mile, and runs at 1 knots for the last mile. The river runs due South. You are trying to get from Point A to Point B, which is due East of you. How do you navigate it?

If you're in a fast powerboat, you don't -- you just pile on the coal, keep your COG at due East, and squirt across. No reason to bother to do anything else. RWidman knows that you have to keep your speed up, but at a certain multiple of the speed of the current, you can crab across that part without any great difficulty.

If you're in a slow sailboat on a calm day, so motoring at 5 knots, you know instinctively that you won't arrive at all if you just motor due East. Because once you get into the part of the river running at 5 knots, you can no longer go East. Note that as you motor through the first mile, your GPS will be telling you that everything is fine -- VMG to waypoint is 5 knots, COG is 90, SOG is 5 knots, who needs STW? Your GPS gives you zero useful information for this case.

So in order to get across at all, you have to steer upstream from the very beginning. But how far upstream to you have to steer? Ah, there's the rub. To know that, you have to run exactly the same calculation you need for the English Channel or the Gulf Stream to get a Course to Steer, which you then have to hold the whole way across. The inputs into this calculation are heading and speed through water -- COG and SOG are irrelevant; your GPS doesn't give you a single useful datum.

STW doesn't tell you how to steer across the river any better than the GPS heading.

This example is all about predicting the current and choosing the best route ahead of time. I can do that quite well with a GPS by comparing my compass heading to my GPS heading. If they are pretty close, I know I'm in the slow current. If they are off by 30degrees, I know I have a fair amount of current.

[goboatingnow]

Originally Posted by valhalla360

STW doesn't tell you how to steer across the river any better than the GPS heading.

This example is all about predicting the current and choosing the best route ahead of time. I can do that quite well with a GPS by comparing my compass heading to my GPS heading. If they are pretty close, I know I'm in the slow current. If they are off by 30degrees, I know I have a fair amount of current.

There are two different issues being mixed up here,

In itself STW conveys no information whatsoever about tidal set or drift , thats clear

Yes heading differences can be used to approximate the current one is subjecting yourself to at this moment in time.

CTS is a theoretical computation ( as its normally done in advance) attempting to select a heading that either cancels out the effect of the tide, or maximise the speed of advance. It has very significant consequences in real life if applied for more then an hour or two.

By having a GPS, hence SOG, COG and position, STW is much less useful and actually can be dispensed with in almost all situations other then sail trim and in fact in sail trim, once you are in slack or no current areas, its no better then SOG for that either

[Robin3]

Originally Posted by El Pinguino

Oh dear.... why do I bother...... I'm amazed I have survived as long as I have.... how does it go again..2 barks for port...one for starboard?

good job there is all that electronics on the bridge, 'cos the cute 'n cuddly OOW in that pic looks like he is fast asleep!

That's me done on this subject. I think next time we want to visit the Bahamas we will forget the traumas of navigation, join the crowd and jump on a cruise ship or let the plane take the strain... Now tell me again peeps what is the best anchor to use for a swivel and tilt rocker chair??

[Ericson38]

Originally Posted by valhalla360

STW doesn't tell you how to steer across the river any better than the GPS heading.

This example is all about predicting the current and choosing the best route ahead of time. I can do that quite well with a GPS by comparing my compass heading to my GPS heading. If they are pretty close, I know I'm in the slow current. If they are off by 30degrees, I know I have a fair amount of current.

You would of course limit your statement to cases where the current was crossing your course at close to right angles. Then you will get a difference as you note. I travelling along or against the current, not much difference in heading.

Do you cruise on the Chesapeake at night, by chance ? Some channels up rivers are marked with day markers and are 20 feet wide, to marinas that you have made a reservation to side tie at. There are S-turns and even 90 degree turns close to shore. The MLW depths are in the 6-8 foot range, and if you are outside the channel by a little, you can slow down. If the STW and GPS SOG both drop together in these tight maneuvers, you are possibly carving a little grove in the mud. I always look at the difference between these two sensors in such cases, along with the sounder.

Another poster just asked to pick between which is best, GPS speed or STW speed. That is not what the original OP asked. GPS is how I made my career since it went IOC in 1993 and FOC in 1996. Have been using it since 1994 in technical applications.

A very good general GPS measurements paper was published a while back, but is well written - http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf


A good treatment of GPS Kalman Filtering is in-

http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf

This is the second edition of a highly regarded textbook, I have the 1st edition hard copy dated 1996. Between pages 475 and 482 is a worked out example of a Kalman filter iteration for one satellite, with numbers assigned to values.

[valhalla360]

Originally Posted by Ericson38

You would of course limit your statement to cases where the current was crossing your course at close to right angles. Then you will get a difference as you note. I travelling along or against the current, not much difference in heading.

Do you cruise on the Chesapeake at night, by chance ? Some channels up rivers are marked with day markers and are 20 feet wide, to marinas that you have made a reservation to side tie at. There are S-turns and even 90 degree turns close to shore. The MLW depths are in the 6-8 foot range, and if you are outside the channel by a little, you can slow down. If the STW and GPS SOG both drop together in these tight maneuvers, you are possibly carving a little grove in the mud. I always look at the difference between these two sensors in such cases, along with the sounder.


.

If you are going with or against the current, your speed would go up or down accordingly. Again not hard to estimate if you are familiar with your boat and what speeds she runs at different throttle settings.

Yes, we've done plenty of narrow shallow channels including the cheasapeak. If I'm in a 20' wide channel, I'm slowing way down so I see the impacts of the current exagerated and watching those day markers to see how the current is reacting to them. It also helps to avoid plowing so deeply into the shallows that you can't get out.

At best you describe a very minor benefit for a rare situations. I usually don't enter narrow shallow unfamiliar channels in the dark and I often hear of cruisers doing the same by heaving to or anchoring out until morning light allows them to safely enter.

[Sailor Doug]

My Raymarine knot meter supplies the following in a large display
Speed through water.
Trip odometer through water
Odometer through water.
Water temperature.

If you don't find it use full pull the transducer or don't buy.
For me multiple sources of information improves navigation.


Sent from my iPad using Cruisers Sailing Forum

[transmitterdan]

I have done some calibration curves of my Raymarine paddle wheel. At 6.5 knots it is accurate +/-0.2 knots. At 3 knots it reads high by 1 knot. At 1 knot it reads zero. There is no calibration function to correct the nonlinearity.

[Ericson38]

Originally Posted by Ericson38

You would of course limit your observation to cases where the current was crossing your course at close to right angles. Then you will get a difference as you note, in COG versus Boat HDG. But, if travelling along or against the current, not much difference in the two angles and a significant difference between STW and SOG, if there is a current present as in your example. Glad you brought that up.

Do you cruise on the Chesapeake at night, by chance ? Some channels up rivers are marked with day markers and are 20 feet wide, to marinas that you have made a reservation to side tie at. There are S-turns and even 90 degree turns close to shore. The MLW depths are in the 6-8 foot range, and if you are outside the channel by a little, you can slow down. If the STW and GPS SOG both drop together in these tight maneuvers, you are possibly carving a little grove in the mud. I always look at the difference between these two sensors in such cases, along with the sounder.

Another poster just asked to pick between which is best, GPS speed or STW speed. That is not what the original OP asked. GPS is how I made my career since it went IOC in 1993 and FOC in 1996. Have been using it since 1994 in technical applications.

A very good general GPS measurements paper was published a while back, but is well written - http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf


A good treatment of GPS Kalman Filtering is in-

http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf


This is the second edition of a highly regarded textbook, I have the 1st edition hard copy dated 1996. Between pages 475 and 482 is a worked out example of a Kalman filter iteration for one satellite, with numbers assigned to values.

To expand on my last-You would of course limit your observation to cases where the current was crossing your course at close to right angles. Then you will get a difference as you note, in COG versus Boat HDG. But, if travelling along or against the current, not much difference in the two angles and a significant difference between STW and SOG, if there is a current present as in your example. Glad you brought that up.

I need to swing my compass, and this is much easier to do nowdays with GPS COG, than before.