Speed Through GPS Versus Old Fashioned Paddle Log

[OceanSeaSpray]

Originally Posted by Dockhead

Dave, you have a deep understanding of all of the issues, and I respect that. You have a totally ground-referenced view of all of these problems which does work mathematically, and I respect that, too. We know all of this from our earlier discussions on the lee bow effect.
However, I must disagree with a couple of your points here.
First of all, CTS navigation is not a “party trick”. It is applicable to a wide range of real life situations, and profoundly useful in many of them. The usefulness of CTS navigation does not depend on high precision of the data inputs, either. You would see it instantly and instinctively if you could see the world in water-referenced terms (where that is appropriate), as many of the rest of us do. When you “get” the water-referenced frame of reference, then you realize that even a very imperfect CTS will usually be better than intentionally going off in the wrong direction by trying to crab along the rhumb line.
....

Thank you for taking the time... to thoroughly respond to what was essentially a huge confused blend of loose conceptually correct elements and invalid allegations aimed at trying to justify the preconceived statement that "STW doesn't matter". Speed through the water is key in tracking whether things are going according to plan, or whether the plan needs to be recalculated and there is no other way of obtaining this information than measuring it directly.

The suggestions that one should follow "agreed ground courses and not deviate" combined with statements of "one-hour CTS" in fact just equate to bucking the current unnecessarily for no other reason than lack of simple planning. Back to square one again.
The statement that the ground track cannot be evaluated in advance and will take the boat into dangers is just plain nonsense. An elementary projection of the anticipated speed and current contribution hour-by-hour (if relevant) shows immediately where the vessel will be taken.

Some have picked on the "symmetrical tides" of the English channel as a "particular case". It is not. What such a symmetry does, in reality and on the water, is that in the right circumstances (i.e. open waters) quicker calculations can be made by only considering the period of time exceeding a multiple of 12 hours on the basis that in any 12-hour period currents will more or less cancel out. Crossings seldom take exactly 12 or 24 hours and the drift to account for is the one that occurs in the "excess" after the last 12-hour period of course.
If the symmetry doesn't exist, then the benefit from the simplification is lost.

The concept is important not only in tidal areas, but equally for larger scale passage planning with ocean currents offshore. Some years ago I had left Mangareva for the Marquesas, and then the Marquesas for Hawaii, and each time this involved sailing across the strong equatorial currents (and the counter-current). A little planning not only means a shorter passage, but also better wind angles. It can eliminate having to sail hard on the wind, or reduce/eliminate the need for tacking.
Offshore, I always have both the routing chart and the Pilot Chart out on the table. I record a position once a day and compare observed currents with averages from the Pilot Chart each time. I also keep an eye on sea temperatures. There is a wealth of information available for the one who cares about understanding and observing.

Provided the distance is long enough, even fast motor vessels suffer from not integrating currents. High speed commonly translates into high costs and it is very easy to be forced to travel extra distance for not having accounted for the current correctly.

It truly is essential knowledge and there is no such thing as heading out and watching the little display and everything will be ok, no need to learn anything else because we know where we are and it is all there in a jumble of 3-letter acronyms. It isn't.
Forums quickly end up cluttered with a jumble of opinions and contradictions, great to raise awareness maybe, but not so much for actually studying/understanding something... There are plenty of navigation manuals and courses for that however. Some concepts that may seem unintuitive at first quickly become obvious after spending some quiet time studying them.

[OceanSeaSpray]

Originally Posted by El Pinguino

'Bucking the tide'? No wind has been mentioned so what difference will it make what direction you are pointing... you aren't bucking anything. If there is a wind.. lets say from the west... you are going to have a lovely flat water ride while steering 000*. Unfortunately when the tidal stream comes away from the east you are going to be a long way down wind from your destination with an interesting wind over tide situation, esp in this 5 knot stream of yours.....

This has been interesting exercise in theory... in the real world that most of us inhabit not of much practical use. In 99% of the world tidal information such as is available in the English Channel just does not exist.
It has limited application elsewhere, SF Bay it seems, and also when crossing the Rio Pongo 20 miles up stream where you know that being swept down river is not an issue as when you near the northern shore the eddies below Bing Bong will lift you back up to the village.

This is not the Rio Pongo but somewhere else in the 'world of limited info'...

My friend, the problem here is that you don't appear to even have the faintest clue about how little you actually know and understand. It may be a harsh thing to say, but that's what it is. "World of limited info" you said, indeed.
Beyond this, there is no reason why you can't improve on that. It is not always intuitive for everybody at first and that's fine.

As soon as you are sailing against the tide, some or all of it, so it doesn't take you where it is going, you are bucking the tide. What the wind is doing is completely irrelevant. If you are sailing at 6 knots in the 304 true, you are making 3.35 knots North and 5 knots into the West. Work it out for yourself. As in my basic example there is 5 knots of East-going tide, you are bucking all of that while slowly gaining north.

How do you think I came up with the course angles? You can make a simple little diagram with the directions and speeds to work them out. Any decent navigation course will show you how and it isn't hard.

When it comes to applicability, I have just touched on this subject. It ranges from crossing a bay to crossing an ocean. As soon as the water is in motion.

[Stu Jackson]

Thank you, thank you, thank you for a well written piece of good information.

Sometimes it's like rolling a rock uphill, but we're here to share, maybe not always can convince...

Originally Posted by OceanSeaSpray

Thank you for taking the time... to thoroughly respond to what was essentially a huge confused blend of loose conceptually correct elements and invalid allegations aimed at trying to justify the preconceived statement that "STW doesn't matter". Speed through the water is key in tracking whether things are going according to plan, or whether the plan needs to be recalculated and there is no other way of obtaining this information than measuring it directly.......................................... ..................................................
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It truly is essential knowledge and there is no such thing as heading out and watching the little display and everything will be ok, no need to learn anything else because we know where we are and it is all there in a jumble of 3-letter acronyms. It isn't.
Forums quickly end up cluttered with a jumble of opinions and contradictions, great to raise awareness maybe, but not so much for actually studying/understanding something... There are plenty of navigation manuals and courses for that however. Some concepts that may seem unintuitive at first quickly become obvious after spending some quiet time studying them.

[Robin3]

Originally Posted by transmitterdan

These days a boat can have 5-6 GPS receivers without trying hard. It's hard not to have that many.

Indeed and we have 2 on the Garmin network ( one for the AIS, one the plotter) another on the laptop with openCPN, another on the smartphone with a mapping app and a Lat/long display app and yet another now since I just acquired a new notebook with inbuilt GPS. I think that adds up to 5 and add yet another if my wife takes her laptop with OpenCPN and GPS along too to make 6. I even have a spare Bu353 puck GPS in case...

None of the above tell us the distance we have sailed through the water like the paddlewheel log does. COULD I do without it? , yes for sure, just like I could do without all the GPS receivers and the chart plotter/radar display. do I want to -Heck no! When Homeland Insecurity decide to turn off GPS Signals because... aliens arrive by UFO, AL QUAEDA or ISIS have acquired missiles with nukes or even... well just 'because' I can however still get out of dodge if needs be.

Am I joking - yes. But I have a paddlewheel log it feeds my network and/or reads as a standalone, it didn't cost as much as the other stuff, I'm very used to having it and I'm not ready to ditch it yet.

It amazes and also worries me that so many cruisers nowadays navigate solely by GPS track and fail to understand any other method. I guess this is the marine equivalent of dumping the multiplication tables and totally relying on calculators instead of brain cells. Progress I think it is called?

[valhalla360]

Originally Posted by Robin3

.

It amazes and also worries me that so many cruisers nowadays navigate solely by GPS track and fail to understand any other method. I guess this is the marine equivalent of dumping the multiplication tables and totally relying on calculators instead of brain cells. Progress I think it is called?

It is also amazing how many fail to understand that GPS skips a lot of the interim steps which is basically what STW is. By itself STW tells you nothing.

I can deduce with reasonable accuracy everything STW can provide. And I do, by watching wakes around daymark poles and comparing GPS speed and heading to throttle setting, boat wake and compass heading.

GPS could fail but your paddle wheel is more prone to fail.

Then again in most situations coastal situations, I can get by without any speed data by using simple line of sight navigation. I don't like to do that but I can.

[Dockhead]

Originally Posted by goboatingnow

Never said that CTS was a party trick, I said that using symmetrical reversing tides is a party trick

I say again Dockhead, multi hour CTS is complex tides , is nonsense , you cannot easily determine ground track and you cannot hence decide on safety of the route. Single or 2 hours is acceptable , especially where tides dont significantly change as you will in fact proceed along the ground track.

Why in the world would you bother with a one or two hour CTS where "tides don't significantly change"? If you have a working GPS on board, the only point to doing this is to impress a yachtmaster examiner. CTS nav is not useful UNLESS your CTS path departs significantly from the rhumb line.

Let's put it another way: The constant heading path to your waypoint is the ONE TRUE PATH to get there. The rhumbline can be a useful way to get there -- much easier to use with modern electronics -- IF it does not depart too far from the constant heading path. But once the two diverge, you are flogging yourself and your crew to stay on the rhumbline, which as you perfectly well know is the longer way to get there.

Originally Posted by goboatingnow

You can of course do a chart work up.

But of course. Which you do exactly to the extent you need to for safety.

Originally Posted by goboatingnow

no argument, once you do not stray too far from the course line

See above. If you're not straying far from the rhumb line, then there's no point to CTS nav.

Originally Posted by goboatingnow

Yes Yes, lets not do the suck eggs issues here. The fact remains that in a complex non reversing , angular tides, the ground path cannot be easily computed using a single CTS and hence ITS NOT A SAFE WAY TO NAVIGATE

This is clearly the voice of theory without practical experience with this particular problem. When you are doing water-referenced navigation, you translate to ground coordinates exactly to the extent you need to, for safety. A passage of many hours which needs CTS navigation is in 90% of cases in open water -- because currents don't run perpendicular to coasts. You have said several times that the heading of a long CTS passage is counterintuitive -- I say -- to whom? Certainly not if you do this regularly. You know instinctively which way you will be going by looking at the tide charts. If there are hazards anywhere in the general area you will be passing through, then you do a chart workup. If you are passing near hazards, you check your position frequently against the chart of on your plotter.

If you have to get through a really hazard-littered area, then you do exactly what you would do with a rhumbline passage -- you work it up in detail, and set waypoints at places you need to pass to safely clear hazards. If that means that you deviate from the perfect constant heading path, it will still be better than going down the rhumbline. It's somewhat more work than doing a rhumbline passage -- so what? It's better and shorter and worth it, when the CTS path deviates significantly from the rhumbline path. And we're talking about doing it by hand -- with a computer program, absolutely none of this applies -- at any given moment in your passage you can have a fresh updated predicted ground path in a second.

You make a bugbear out of this -- it's really not a problem in practice at all. Especially in this day and age where we have chart plotters which show us effortlessly where we are and where we're going. If this were a YM exam problem and we had to work it up on paper with no electronics -- yes, it would be a fair amount of work in a hazard-littered sea area to keep your position updated frequently enough to be sure you were not getting into trouble. That does not mean it's not safe, or that you wouldn't do it, but it would be a certain amount of work. But with a chart plotter, this is just irrelevant. You have what amounts to a new "fix" (we don't do actual "fixes" anymore, except to keep our skills up) every few seconds, all plotted for you instantly. It is extremely obvious if you get into a hazardous area, and when there are a lot of hazards, then you you're already out of the realm of navigation anyway, and into pilotage.

Originally Posted by goboatingnow

Indeed, but multi hour complex CTS has you sailing off initially at sometimes bizarre headings

So what? And bizarre to whom, by the way? It's not bizarre when you think in water-based terms. The rhumbline is what is actually bizarre -- it's a zig-zag through the water -- why would you sail that way if you have the brains to work up a CTS?

Originally Posted by goboatingnow

NO not correct, you merely uses an estimated speed ( which is actually ground distance by time ) to estimate the number of tidal hours you will pass through to therefor extract the tidal information. This isn't actually STW its actually a form of SOG. ( actually distance over the ground) remember all of this stuff predates GPS.

Sorry, this is simply wrong, and it's an error I am really surprised you, of all people, could make. One leg of the vector triangle is miles run through the water, so it's STW which is the operand. You would use ground position only if you are translating back and forth to ground coordinates -- not the way it's one. It's possible, of course, but ridiculously awkward, and so not the way it's taught or practiced.

Originally Posted by goboatingnow

I see you are suggesting a navigator does not fix his position for the duration of a multi hour CTS !!!!!!!!!!!!!!

Dave. Please. Be serious for a moment. We were talking about how to check if you are off your CTS passage plan. I said you never use a position fix for this. I never said -- and you can't seriously believe that I would say -- that you never check your position. On the contrary, I specifically said you must keep an accurate watch on your plotter (which is the way we do it in practice in this electronic age, even though I am one of the few remaining dinosaurs who insist on having paper).

You suggested using position fixes to check your deviation from plan. That's not the way it's done. That would require translation of everything back to ground coordinates, then back from that to water. The way it's done is to check miles run through the water against the STW assumed in your plan. I did not invent this technique.

Originally Posted by goboatingnow

The reality Dockhead is that multi hour complex CTS ( i.e. a combination of tides resulting in a combined CTS vector) is a complete nonsense because you cannot determine in advantage the ground track and hence evaluate the dangers

Very bad navigation to only determine haxxards as you come among them. ( or use a chart plotter exclusively )

The fact Dockhead ( and try this yourself ) if you work up a course to steer over many hours , firstly you cannot determine your ground track , hence you cannot determine in advance what hazards are important. You could for example stray in shallows, sand banks.

This is not the way it works in practice. First of all, in 90% of real life applications of CTS nav, you are in open water. If there are hazards somewhere in your possible ground path, you study it more closely, and work up a predicted ground path to whatever level of detail is needed. If necessary, you set an intermediate waypoint to keep you clear of hazardous areas. This is elementary navigation and not actually all that much different from the way you do a rhumbline passage -- it's just that a rhumbline -- since it is rigidly related to ground -- is easier to use for this purpose.

Originally Posted by goboatingnow

Its no use them dumping the whole CTS calculation, AT this stage you are in effect in the wrong place to begun with. You may find that you now have to set an uncomfortable direction or even have to stem a tide, simply to remove yourself from the hazard

Have you ever heard of this happening to anyone in practice? I have never heard of such a case. This is pure theoretical paranoia. It would take exceptionally poor passage planning to end up in such a situation. Once you are accustomed to CTS nav, you instinctively visualize your ground track, and the possible ranges of your ground track, and you work it up in whatever detail you need to stay out of such places. If necessary, you set intermediate waypoints.

Originally Posted by goboatingnow

1-2 hour CTS calculations are useful, I never argued CTS as a whole isn't useful

For what? You don't need CTS nav if your CTS path is close to the rhumbline. Why bother?

Originally Posted by goboatingnow

The first priority of navigation is safety. Multi hour CTS on complex tides can have you sailing over a ground track you cant easily determine. Thats NOT good navigation

The second is efficiency of progress.

One does not have to crap along the rhumb, you can calculate simple 1-2 hour CTS as you go along, on such single triangular vectors, you will therefore remain in the course line , i.e. the designated ground track.

I really don't understand what you're saying here -- surely you are not saying that a series of 1 or 2 hour CTS legs along the rhumb line is equivalent to a real CTS passage? The closer to the rhumb line you stay, the closer the results are, to just staying on the rhumbline -- this is elementary. Why would you even bother to even do it in one or two hours bits? I'm sure you understand this, so I guess I must be missing your point here.

In fact any passage along a rhumbline is a series of little CTS legs -- the size determined by how actively you or the pilot is steering to stay on the rhumbline.

Originally Posted by goboatingnow

I personally run 1-2 CTS as I progress , thats effectively the same as what you do ( because all CTS's become out of date fairly rapidly in a small boat)

I don't understand what you're doing -- you run a CTS based on your destination? Or a based on a waypoint along the rhumbline?

If the former, then what's the difference to what I do? You will go just as far out from the rhumbline as I do. Do you mean you are working up an estimated position one or two hours out every time? I would also do that, if it would be needed because of possible hazards.

If the latter, then why would you bother? You are getting little to no benefit from doing this, compared to just sailing down the rhumbline. It is certainly NOT "effectively the same" as what I do, if this is what you are doing. On the contrary, it is "effectively the same" as setting your pilot on track mode and forgetting about it.

Originally Posted by goboatingnow

Great more computers to rely on, personally I dont want to be left on an unknown ground track by a computed CTS, and then find I have to rebuild everything by hand

What are you saying here? That you would never use a plotter or a GPS because they might fail and force you to do a three-point fix and plot it on a chart? Please don't be silly.

Originally Posted by goboatingnow

Dockhead

This is the single point Im making about multi hour CTS . You cannot easily determine you ground track nor can you determine where you should be at any given point in your journey.

Hence (a) you may stray into an areas with hazards that you have not considered

Not indeed if you have even the most basic nav skills. Drawing a straight line on a chart is not indeed the only way to check for hazards. I've described the techniques above.

Originally Posted by goboatingnow

(b) you cannot use positions fixes to determine your progress as a function of your CTS plan. SO you cannot easily evaluate whether your CTS is correct

False. You can easily evaluate your progress by tracking miles run through the water against plan. To check whether other factors (wind-induced currents, deviations from predicted leeway, etc.) are putting you off plan, you simply run the whole thing again from time to time based on your current position. If you're running well, you don't need to do that very often. If it's tough --- for example, you're hard on the wind, so not holding a very steady heading -- you run it every hour. It's all in a day's work -- no big deal. A small price indeed to pay for sailing straight or nearly straight to your destination, instead of zig-zagging along the rhumb line.


Thanks for the stimulating debate.

[Ericson38]

Originally Posted by valhalla360

It is also amazing how many fail to understand that GPS skips a lot of the interim steps which is basically what STW is. By itself STW tells you nothing.

I can deduce with reasonable accuracy everything STW can provide. And I do, by watching wakes around daymark poles and comparing GPS speed and heading to throttle setting, boat wake and compass heading.

GPS could fail but your paddle wheel is more prone to fail.

Then again in most situations coastal situations, I can get by without any speed data by using simple line of sight navigation. I don't like to do that but I can.

"GPS skips a lot of the interim steps which is basically what STW is."

GPS is like doing celestial navigation to 8 bodies with very precise ephemeris and ship's clock, and an optical grade telescope, all at 10 times a second.

I used to have a Standard Horzion knotmeter that was self contained between the paddle wheel, the harness, and the instrument head. No external power required. Just a sensitive dc rectification meter movement taking in ac pulses from the paddle wheel. That and a MAG compass and you could maintain a running DR fix.

"By itself STW tells you nothing."

"I can deduce with reasonable accuracy everything STW can provide."

These two statements are at odds.

" And I do, by watching wakes around daymark poles and comparing GPS speed and heading to throttle setting, boat wake and compass heading."

You could also just glance at the STW for all the above, but what happened to the midnight scenario we were talking about ? You aren't seeing a lot of the features you listed, unless you have night vision goggles. I wish the price would come down on those, like they have on GPS equipment over the last 15 years.

"GPS could fail but your paddle wheel is more prone to fail. "

Considering availability and reliability together, I think the paddlewheel and direct reading knotmeter is more robust, and it doesn't need external power to boot.

[valhalla360]

Originally Posted by Ericson38

"GPS skips a lot of the interim steps which is basically what STW is."

GPS is like doing celestial navigation to 8 bodies with very precise ephemeris and ship's clock, and an optical grade telescope, all at 10 times a second.

I used to have a Standard Horzion knotmeter that was self contained between the paddle wheel, the harness, and the instrument head. No external power required. Just a sensitive dc rectification meter movement taking in ac pulses from the paddle wheel. That and a MAG compass and you could maintain a running DR fix.

I'm not clear what your point is. With half a dozen GPS units on board, the odds of losing power to all of them is small enough that I'm not worried about it.

"By itself STW tells you nothing."

"I can deduce with reasonable accuracy everything STW can provide."

These two statements are at odds.

What is at odds about it? STW by itself tells you nothing. It is when you combine it with other data and look at the results, you have something useful. I can deduce that well enough without a paddlewheel since it's just a rough estimate when you do use the paddlewheel.

" And I do, by watching wakes around daymark poles and comparing GPS speed and heading to throttle setting, boat wake and compass heading."

You could also just glance at the STW for all the above, but what happened to the midnight scenario we were talking about ? You aren't seeing a lot of the features you listed, unless you have night vision goggles. I wish the price would come down on those, like they have on GPS equipment over the last 15 years.

No night vision but I would be pulling the spot light out in such a scenario and have all hands on deck to help with spotting. More likely I would avoid the scenario described since regardless of the instrumentation available, a slip of the attention for just a couple seconds can run you aground.

The midnight scenario in a channel only 2' wider than the boat (that's your 20' channel) with a 4kt cross current and sharks leaping on deck trying to bite us? Yes, if we keep adding complications, we can come up with a strange situation where it might be of marginal use but as I mentioned, I wouldn't put the boat in danger just to prove the instruments allow it.

"GPS could fail but your paddle wheel is more prone to fail. "

Considering availability and reliability together, I think the paddlewheel and direct reading knotmeter is more robust, and it doesn't need external power to boot.

The GPS failure comment was in relation to the implication that a paddle wheel provides backup. Reality is it's drastically more reliable than navigation techniques of 20-30yrs ago.

It's not a bad thing to have and it does provide another data point but when you compare cost to benefit, it really isn't worth it unless you are racing and need to wring that last 0.1kt out of the boat.

[Ericson38]

Originally Posted by valhalla360

The GPS failure comment was in relation to the implication that a paddle wheel provides backup. Reality is it's drastically more reliable than navigation techniques of 20-30yrs ago.

It's not a bad thing to have and it does provide another data point but when you compare cost to benefit, it really isn't worth it unless you are racing and need to wring that last 0.1kt out of the boat.

The after dark scenario back up Jackson Creek is common, coming back from dinner in Urbana or Reedville for instance. Dredged channel is not wide. Not like our 'what do we care about draft or pots' west coast sailing out of San Pedro. The spread between the SOG and STW is insightful, and obtained instantly. I like your comment on comparing HDG and COG, detecting the crosswise current.

[Sailor Doug]

GPS's fail a lot more frequently than paddle wheels. Haul outs can be hard on paddle wheels, I have got in the habit of pulling mine before haul out. Totally relying on multiple gps's is not having total backup. I believe most air craft use non gps navigation for a fail safe.
I can remember running dr's before electronic navigation. We use to write down coarse and speed every 10 minuets. I enter my routs into two gps units and check them on to the other and then check distanced travelled to knot log . Have arrived at a unknown shore and not know quite where on it you are is unpleasant. Belts and braces, log calibration vs fractal distance.



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[valhalla360]

Originally Posted by Ericson38

The after dark scenario back up Jackson Creek is common, coming back from dinner in Urbana or Reedville for instance. Dredged channel is not wide. Not like our 'what do we care about draft or pots' west coast sailing out of San Pedro. The spread between the SOG and STW is insightful, and obtained instantly. I like your comment on comparing HDG and COG, detecting the crosswise current.

As stated, if I'm not familiar, I'm not running a 20' channel in a 14' beam boat in the dark with strong currents even with the STW unless I have no option.

If I'm familiar, I may think otherwise. Of course with 3' to spare, and the paddlewheel only a point source of data no where close to the width of the hull, it will be of very little use. The paddlewheel may be in deep water while the hull is aground or vice versa so it's of limited information.

If for some reason, I can't see anything on the water and my spot light is not working and I feel the need to enter a narrow unfamiliar channel with flying shaks attacking regardless, I'm probably going to be watching my GPS track closely to make sure it stays centered in the channel but mostly just go very slow and try to feel if she goes aground (and have a crewman with a bat repelling the flying sharks).

There is still nothing about your scenario that suggests any significant advantage to STW.

[Ericson38]

Originally Posted by valhalla360

As stated, if I'm not familiar, I'm not running a 20' channel in a 14' beam boat in the dark with strong currents even with the STW unless I have no option.

If I'm familiar, I may think otherwise. Of course with 3' to spare, and the paddlewheel only a point source of data no where close to the width of the hull, it will be of very little use. The paddlewheel may be in deep water while the hull is aground or vice versa so it's of limited information.

If for some reason, I can't see anything on the water and my spot light is not working and I feel the need to enter a narrow unfamiliar channel with flying shaks attacking regardless, I'm probably going to be watching my GPS track closely to make sure it stays centered in the channel but mostly just go very slow and try to feel if she goes aground (and have a crewman with a bat repelling the flying sharks).

There is still nothing about your scenario that suggests any significant advantage to STW.

Not a STW advantage over GPS SOG, a tactical advantage of having both. But to each his own, as they freely decide.

[valhalla360]

Originally Posted by Sailor Doug

GPS's fail a lot more frequently than paddle wheels. Haul outs can be hard on paddle wheels, I have got in the habit of pulling mine before haul out. Totally relying on multiple gps's is not having total backup. I believe most air craft use non gps navigation for a fail safe.
I can remember running dr's before electronic navigation. We use to write down coarse and speed every 10 minuets. I enter my routs into two gps units and check them on to the other and then check distanced travelled to knot log . Have arrived at a unknown shore and not know quite where on it you are is unpleasant. Belts and braces, log calibration vs fractal distance.

Define failure? If you are talking about the entire system needs to be replaced...maybe, maybe not. I've never seen a breakdown. If you include the paddle wheel becoming jambed or just becoming unreliable due to bottom growth, I'll put my bet on GPS while keeping an eye on surrounding conditions.

Aircraft having airspeed information is a different issue. If a plane doesn't maintain sufficent airspeed, it will fall out of the air. If a small plane is doing 100mph SOG and the pilot doesn't realize the storm they are flying thru is providing a 50mph tailwind, bad things happen. Generally, it's not a big problem is STW drops to low on a cruising boat.

[valhalla360]

Originally Posted by Ericson38

Not a STW advantage over GPS SOG, a tactical advantage of having both. But to each his own, as they freely decide.

And the tactical advantage is?

In principal, more information is good but the incremental advantage is negligible in your scenario and potentially distracting.

In your scenario, I really don't care much about speed (STW or SOG). I care about position and heading. GPS gives me that directly without going thru mental gyrations to compare compass heading, STW and estimated current. In your scenario where you are blind and purely on instruments (poor idea in the first place), this is one time where the joke about driving like it's a video game is probably your best bet beyond listening and feeling how the boat is reacting so you can stop when you hit bottom because with 3' to spare, you probably will hit bottom a couple times if the channel is of any length.

[Sailor Doug]

"You don't know what you don't know" aircraft have multiple navigation, air speed is another issue.
With the price I paid for my boat the knot log was very little more. Keeping a paddle wheel clean is quick and simple. I don't understand why you would not want one. Redundancy is great.


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