have two speed logs - what if I remove one ?

[Daydream believer]

Pretty silly actually as GPS data is more useful. e.g. Holding station in a strong current should show apparent and actual as being the same. However, my Raymarine plotter only uses the log. Imagine moving at 6 knots into a 6 knot current. If wind is 5 knots from ahead GPS would show 5kts apparent but log data makes it 11kts as the log has 6kts..

Yes it is silly because if the wind is dead ahead you would not be able to sail dead into the current. The wind indicator would show 11 knots if set to aparent wind because the boat in your ficticious example is, as you say 5 plus6 = 11.
.
Somebody may apply physics & disprove that, because the boat has not moved forward- I could not argue with that only bow to those with greater knowledge
.
But that is not log speed it is windspeed. Log speed through the water is still 6Kts. You just flick the switch on the readout & it will tell you real wind speed.5 kts
But in your example my GPS shows how far the craft has moved ie zero. it does not show wind speed. That is a separate calculation

 

[LittleSister]

When I was doing a sailing coarse many years ago out of Maldon in Essex the instructor towed a log 50 meters behind the boat. He said what a useless piece of equipment it was with any weed in the water and that the results were never correct.
That stuck with me and with the advent of satellite navigation logs seems pointless. Its a bit like having a cranking handle on a new car.

I towed a trailing log for years, and really liked it. I can't recall it ever getting clogged with weed, and if it ever did it was so rare as to be negligible (unlike the through-hull ones I've had since!). This was a dry cell powered electronic log, not ye olde mechanical type, which probably has a bigger, more weed attractive, rotor, and the line was certainly much shorter than 50m (perhaps 6m?).

There was something satisfying about having it spinning away behind me. Deploying and hauling it in were generally a pleasing little seamanlike ritual rather than a chore. My only concern with my trailing log, and it was an ongoing one, was that I'd forget to haul it in when reversing and the cable would get chopped up by the prop. In fact I forgot, or didm't have time, to get it in on numerous occasions, yet that never happenned.

I believe that a trailing log is more accurate, or at least certainly much more consistent, than a through-hull one. I liked having a trailing log so much that I bought a second-hand Stowe one at a boat jumble years ago, even though I had by that time a boat with a through-hull one. I still have it, and have occasionally used it to calibrate/check my through-hull log, or when that was playing up.

The chief disadvantage with a trailing log, I found, was that people would often call from other boats to warn me that I had a line trailing in the water. (I suspect that these days many of those doing so would not understand me calling back 'Trailing Log!')

 

[Dockhead]

I
I find it annoying if the log stops working due to fouling. I lose true wind as that's used to compare movement and calculate from apparent.

Pretty silly actually as GPS data is more useful. e.g. Holding station in a strong current should show apparent and actual as being the same. However, my Raymarine plotter only uses the log. Imagine moving at 6 knots into a 6 knot current. If wind is 5 knots from ahead GPS would show 5kts apparent but log data makes it 11kts as the log has 6kts.

I don't know if Raymarine changed this on more modern kit. I never understood their logic as the plotter has a built-in GPS and therefore assuming you'd only have a paddlewheel log made no sense.

It makes sense because true wind (the way sailors use the word) is referenced to water, not to ground. Referenced to ground it's called "ground wind".

The reason why the difference is important is because our boats have no connection with ground. They sail in water. So all sailing is done in the sheer between water and air, and true wind (water referenced), both speed and direction, is exactly the expression of that.

That's the logic. And that's why top racing teams spend millions trying to get better STW data (it's hard to measure accurately), rather than just using the GPS (which is trivially easy). Even ships need STW -- they use expensive doppler speed logs to get it. They don't sail, but you can't calculate a tidal vector without STW, so they go to a lot of trouble and expense measuring it.

As to your example, I think you've confused some of the terminology. If you are moving at 6 knots STW due East against a 6 knot current running due West (or in 10 knots or 0 knows current, the current doesn't matter), and you measure 5 knots from directly ahead, apparent, then the true wind is 1 knot and from behind you, from the West.

And that's what you want to know. Because if want to turn around to head due West, you know you will measure 7 knots of wind apparent, and the same 1 knots of true.

If you switch your instruments to use SOG and to display Ground Wind (my B&G instruments can), you don't get that useful information. If the current is 6 knots from the East, and you measure 5 knots of apparent wind from dead ahead, your instruments will tell you that you have 5 knots of wind from the East, ground wind. That tells you nothing about what the wind will be like when you turn, unless you do an additional operation to add the tide vector, but you can't know the tidal vector if you don't have STW. Do you see now? There is no way to predict what the apparent wind will be when you turn, if you don't have STW.

And it's even worse than that -- because it's not just speed, but direction, of the true wind, which is determined by STW. So ground wind direction will not correspond to true wind direction if there is any current.

You sail in the sheer between water and air. STW measures that sheer. It's fundamental and essential data, and SOG is no substitute.

 

[shanemax]

I think possibly you could do with a shore based nav course.

All my courses have been at sea for 7 days including night passages and including navigation with paper charts To get three bearings on three different points on land with a hand bearing compass when there is any sort on sea running is not like in the classroom I understand what they teach in a classroom is interesting but very rarely practical.

 

[Refueler]

I
I find it annoying if the log stops working due to fouling. I lose true wind as that's used to compare movement and calculate from apparent.

Pretty silly actually as GPS data is more useful. e.g. Holding station in a strong current should show apparent and actual as being the same. However, my Raymarine plotter only uses the log. Imagine moving at 6 knots into a 6 knot current. If wind is 5 knots from ahead GPS would show 5kts apparent but log data makes it 11kts as the log has 6kts.

I don't know if Raymarine changed this on more modern kit. I never understood their logic as the plotter has a built-in GPS and therefore assuming you'd only have a paddlewheel log made no sense.

I suggest that the argument of STW vs SOG be put aside as it will lead to a discussion that has strong support on both sides ...

Plus there is a lot of BS applied to STW and its use ...

 

[Refueler]

I think possibly you could do with a shore based nav course.

mmmmm think that STW is being confused in use earlier !!

 

[Refueler]

Even ships need STW -- they use expensive doppler speed logs to get it.

You are putting two items together in sheer error.

Doppler logs had been worked on to produce as near geographical actual distance sailed instead of the values given by other forms of water based log. I didn't spend 17yrs of my professional Shipboard navigation career to read that !!

As to : "but you can't calculate a tidal vector without STW, so they go to a lot of trouble and expense measuring it." the far more prevalent Pitot Tube based speed Log gives STW directly ... unlike Doppler which uses water boundary layers to work ..

 

[Dockhead]

You are putting two items together in sheer error.

Doppler logs had been worked on to produce as near geographical actual distance sailed instead of the values given by other forms of water based log. I didn't spend 17yrs of my professional Shipboard navigation career to read that !!

As to : "but you can't calculate a tidal vector without STW, so they go to a lot of trouble and expense measuring it." the far more prevalent Pitot Tube based speed Log gives STW directly ... unlike Doppler which uses water boundary layers to work ..

That doesn't contradict what I said.

No matter how you measure it, you need STW even on ships. And even after GPS gives you a much more accurate way to measure "geographical distance sailed" (what, since 30 years already?). The point is that SOG is not a substitute for STW, contrary to what some people on here think.

 

[Refueler]

All sorts of silly ideas coming up in mind ... what if I put plug and socket in each line .. then I can swap logs if one goes down .... put switch in for same ...

Until I finally sort out which cable etc - its all thoughts.

One item and this was posted on another thread :

The wind display (before the above work on wiring) when set to TRUE - displayed the same speed as STW from the speed log ... set to APP and the speed window on the wind meter showed zero. The direction showed direction most time OK ... and never changed APP or TRUE setting - which confirms speed was not being rec'd from MHU.
The wind part is a factor to be sorted later .... first is to sort out this twin speed log matter and remove what is regarded as unnecessary complication in the wiring setup.

 

[Mistroma]

Yes it is silly because if the wind is dead ahead you would not be able to sail dead into the current. The wind indicator would show 11 knots if set to aparent wind because the boat in your ficticious example is, as you say 5 plus6 = 11.
.
Somebody may apply physics & disprove that, because the boat has not moved forward- I could not argue with that only bow to those with greater knowledge
.
But that is not log speed it is windspeed. Log speed through the water is still 6Kts. You just flick the switch on the readout & it will tell you real wind speed.5 kts
But in your example my GPS shows how far the craft has moved ie zero. it does not show wind speed. That is a separate calculation

Yes it is silly because if the wind is dead ahead you would not be able to sail dead into the current. The wind indicator would show 11 knots if set to aparent wind because the boat in your ficticious example is, as you say 5 plus6 = 11.
.
Somebody may apply physics & disprove that, because the boat has not moved forward- I could not argue with that only bow to those with greater knowledge
.
But that is not log speed it is windspeed. Log speed through the water is still 6Kts. You just flick the switch on the readout & it will tell you real wind speed.5 kts
But in your example my GPS shows how far the craft has moved ie zero. it does not show wind speed. That is a separate calculation

Obviously didn't explain correctly. I was trying to point out that Raymarine didn't have a fallback position for times the log gets clogged. I chose to an example that would obviously be motoring to exaggerate the error. I was trying to point out that I thought Raymarine should have the option to fallback on GPS when log fails as it would often give a reasonable figure.

 

[Refueler]

That doesn't contradict what I said.

No matter how you measure it, you need STW even on ships. And even after GPS gives you a much more accurate way to measure "geographical distance sailed" (what, since 30 years already?). The point is that SOG is not a substitute for STW, contrary to what some people on here think.

It does actually contradict you - because you mistakenly state Doppler is for STW ... its not. Its an attempt to get the most accurate ... I use the word Geographical to stay away from the word Ground ... geographical speed ... what it produced actually in deep water was a figure that wavered between STW and SOG in effect. As you entered shallower water and the Doppler could use sea bottom instead of water layers - the speed was more SOG ...

Don't know what ships you sailed on ... but in all the 17yrs I sailed as Nav Officer in all Oceans / Seas ... I only ever calculated tidal vector once or twice ... and I cannot recall any others doing it either !

 

[Refueler]

Obviously didn't explain correctly. I was trying to point out that Raymarine didn't have a fallback position for times the log gets clogged. I chose to an example that would obviously be motoring to exaggerate the error. I was trying to point out that I thought Raymarine should have the option to fallback on GPS when log fails as it would often give a reasonable figure.

Wouldn't it depend on the NMEA sentence ?? Have to have a look - but is STW sentence diferent to SOG (GPS) speed sentence ??

 

[RunAgroundHard]

Wouldn't it depend on the NMEA sentence ?? Have to have a look - but is STW sentence diferent to SOG (GPS) speed sentence ??

Yes STW is different to SOG and transmitted separately via NMEA becuase the values are used differently in MFDs. Discussed in this article.

Raymarine FAQs

 

[shanemax]

Speed over the ground tells me if I am going to make it before the tide turns or before its dark and speed over the ground allows me to come up with a pretty good ETA. Speed through the water tells me nothing. I could be traveling at 6 knots STW with a 5 knot tide against me .

 

[Refueler]

Yes STW is different to SOG and transmitted separately via NMEA becuase the values are used differently in MFDs. Discussed in this article.

Raymarine FAQs

Thought so - I rarely look at the sentences as when I connect NMEA - the items connected accept what they look for .. no need for my clumsy intervention.

 

[Chiara’s slave]

All my courses have been at sea for 7 days including night passages and including navigation with paper charts To get three bearings on three different points on land with a hand bearing compass when there is any sort on sea running is not like in the classroom I understand what they teach in a classroom is interesting but very rarely practical.

You might learn what STW and SOG are, and the use of both. STW is vital for sailors particularly. Like Refueler says, if you can just dial up 15 knots and someone else pays for the fuel, not such an issue.

 

[RunAgroundHard]

... Speed through the water tells me nothing. ...

It can be used, as per the link in post 33, which allows MFDs to calculate tacking, gybing angles to a waypoint, or just show the laylines. Without an MFD it is essential to show the impact on sail adjustments based on boat speed being free from any latency or smoothing that happens with SOG.

 

[Chiara’s slave]

Speed over the ground tells me if I am going to make it before the tide turns or before its dark and speed over the ground allows me to come up with a pretty good ETA. Speed through the water tells me nothing. I could be traveling at 6 knots STW with a 5 knot tide against me .

There’s a lesson for you then. The difference between STW and SOG is how you tell you’ve got a 5 knot tide, and where it’s taking you.

 

[rogerthebodger]

Sa Refuler sails mainly in the Baltic Sea which has little tided like the Med STW would have little real use except maybe on his river

I can recall some time ago there was a twin speed log setup posted in PBO using a tilt switch to change the speed log that feed the indicator depending on which tack the boat was on

 

[RunAgroundHard]

The RYA initiative Digital First (DF) is about sailors using the digital data that the majority of us have these, days; because it is fairly ubiquitous. Recognised Training Centres, practical courses, now have to incorporate DF into the course. One issue that is recognised is that data sources and how they are used is not always clear. In particular, what does it mean to the data being shown if certain transducers are wrong. There is work going on to ensure that there is an understanding. However, for most instructors or skippers, leisure or commercial, RTFM is usually a good start.