Speed Through GPS Versus Old Fashioned Paddle Log

[Ziggy]

Originally Posted by Dockhead

[...] my best friend and business partner here is an honors graduate of Eton and Cambridge -- so the best education you can have hereabouts -- and he drives me up the wall saying things like "with my wife and I" and using apostrophes to make plurals . . . yes, I am not too shy to correct him [...]

It seems to me the British upper classes make it a point of pride to speak as they please and not give a damn about what anyone else thinks is "proper" grammar or pronunciation. And they seem to take special pleasure out of butchering the pronunciation of French words and place names.

[El Pinguino]

Originally Posted by Dockhead

....My perception is that Yanks tend to speak about the sea like land people even when they're seasoned seamen; the English tend to speak about the sea like seamen -- even when they are land people.....

Such as saying things like 'full left rudder' and stuff?

[goboatingnow]

Originally Posted by chala

I know for a fact that that if I drive up or down a hill with my car the car GPS speed will be inaccurate. Also when the boat is ashore, due to the relative accuracy of the GPS, the COG will be everywhere and if Tracks is on, the track distance after 24 hours recording can be as high than 10 nautical miles with the boat not moving at all.

different issue , mostly due to the information that is fed out a NMEA port as opposed to what the GPS is calculating

[goboatingnow]

Originally Posted by Dockhead

Your issue with different AWA on different tacks has nothing to do with Ground Wind or True Wind. That's one of two things: (a) rigging not tuned well; and/or (b) wind instrument not calibrated.

Best way to calibrate the wind instrument is to go out in a dead calm and motor at top speed. That will give you 000 AWA; zero out the instrument. Then check your calibration by motoring in a circle when there's some wind -- True Wind should hold steady in all directions.

True Wind is what you sail in -- it's the relationship between wind and the surface of the water. It's what you need if you want to know what the wind will be like when you tack, head off, etc.

Ground Wind is the relationship between the land and the wind. You don't sail in it so you don't want to confuse it with True Wind. It's useful for knowing what will happen to the wind when the tide changes. It's "true wind" as experienced by landlubbers and is equal to True Wind when you're at anchor or tied up.

I would argue differently, Mariners use the term True Wind, because historically they had no way to measure the actual Wind. Meterologists will say that True wind is the wind their instruments measure

At sea , True Wind as mariners call it, is not " what you sail in" , at all times from when the boat is stopped to moving , you sail in the " apparent wind".

Again when you tack you do not experience True Wind.

in reality , most sailors in fact use True Wind measurements as a comparison to forecast winds. in practice currents have little real impact on true wind calculations, often the measurements inaccuracies are greater then any current.


Lest simply say that True Wind is a misnomer , but we live with it

[goboatingnow]

Originally Posted by Dockhead

I was taught it -- in the U.S. even ("it's not taught at all"). And all of the experienced Channel sailors I know use it, and use it instinctively, and use it all the time -- they will use it with the "eyeball" method for a passage which is hard to calculate. Since even a rough approximation of the correct CTS is better than an intentionally wrong course. Of course the Channel is a pretty rigorous laboratory for learning the method, since you really just can't navigate without it -- but the Channel is not a "trick case", or even a unique one.

This is simply not true, that "no prudent mariners would use it outside of a few trick cases." It is a basic technique wherever you have moving water on any scale.

Dockhead, please point to me a US reference manual that details multi hour CTS in a non trivial situation

Answer these issues

(a) Who in reality sails an unknown ground track

(b) in most cases , you dont have accurate position dependant hourly tidal data , multi hour CTS cannot be commuted with any precision at all, and what actually happens is a kind of hourly " eyeball" CTS ( which is fine)


in a complex non symmetrical non reversing multi hour tide, none uses a single vector computation to sail off into the sunset. in reality its just not safe

[Dockhead]

Originally Posted by goboatingnow

I would argue differently, Mariners use the term True Wind, because historically they had no way to measure the actual Wind. Meterologists will say that True wind is the wind their instruments measure

At sea , True Wind as mariners call it, is not " what you sail in" , at all times from when the boat is stopped to moving , you sail in the " apparent wind".

Again when you tack you do not experience True Wind.

in reality , most sailors in fact use True Wind measurements as a comparison to forecast winds. in practice currents have little real impact on true wind calculations, often the measurements inaccuracies are greater then any current.


Lest simply say that True Wind is a misnomer , but we live with it

Well, I think that the distinction between Apparent, True, and Ground is very useful:



But your point about measurement inaccuracies is well taken. In many cases, STW measurement is too far off to give a reliable True Wind calculation. You're right about that.

But still -- you do sail in True Wind -- that's the sheer between air and water which is our source of power. Apparent Wind is only what you sail in on this point of sail -- because your speed vector is added. You need True Wind to know how you can tack, what DDW is, etc., etc, etc. -- and only True Wind. Ground Wind is completely irrelevant to actual sailing on this tide, and you will get in trouble if you use Ground Wind to decide what course you can make a safe number of degrees from DDW, for example -- the answer may be very different than if you correctly use water-referenced True Wind. Ground Wind is useful to compare to forecasts and to know what will happen when the tide turns, but not for figuring out changes to points of sail.

The difference between Ground Wind and True Wind is relevant mostly to people with strong currents. As I said, if like a lot of sailors you never experience currents of more than a knot -- then you won't care about the distinction. Especially since the sloppiness of True Wind calculations based on speed readings from dirty paddlewheel logs may make them completely incoherent.

As to the terminology: You're right, of course -- there was no such thing as water-referenced True Wind until we had electronics capable of calculating it. It is a term invented by the instrument makers. But I think they have done us a big service -- it's much more precise, and the precision is highly meaningful, if we are dealing with at least moderately strong currents.

"True", as someone above said, just means corrected from some kind of deviation or error -- it's a fairly generic term, and it does not cause me any problem that people on land use the term "true wind" differently from what our instruments say. For someone on land, naturally true wind is ground-referenced -- that's the only reference they have. For us, I submit, it's much more natural to reference "true wind" to the water, since that is what we sail in. And therefore, I personally am really glad that the terminology is now like this and that our instruments work that way. I think it's valuable progress that we now have True, Apparent, and Ground Wind -- which as you said our ancestors did not. Progress which was made possible only with modern electronics.

Different people will have different taste about this, but I think you're swimming against the tide (so to speak! pace Shakespeare) if you try to stick to the old ground-referenced True Wind, at sea. You won't even be able to get your instruments to display it (unless you have Furuno, which does have that option, unique among the instrument makers AFAIK).

[MarkSF]

Originally Posted by Dockhead

Well, I think that the distinction between Apparent, True, and Ground is very useful:

Attachment 99360


Different people will have different taste about this, but I think you're swimming against the tide (so to speak! pace Shakespeare) if you try to stick to the old ground-referenced True Wind, at sea. You won't even be able to get your instruments to display it (unless you have Furuno, which does have that option, unique among the instrument makers AFAIK).

The manual for my Simrad IS20 wind instrument is a little (OK, very) vague on the subject, but it has a true wind mode, and it can be told to use either the GPS course over ground, or the paddle speed and heading sensor, to correct for true wind (all via NMEA 2000 / Simnet)

I have thought about this until my head span, but I think if you use GPS course, you are getting true wind relative to ground. If I use paddle speed and heading, I am getting true wind relative to the water.

This is somewhat academic as I never use the true wind function, and I don't have a heading sensor - so I am stuck with true wind relative to ground, anyway.

The nice thing about NMEA 2000 networking is that everything on the network is available to every instrument. So the IS20 does true wind as long as the plotter is booted up.

[goboatingnow]

[QUOTE=Dockhead;1783011]Well, I think that the distinction between Apparent, True, and Ground is very useful:

Attachment 99360

But your point about measurement inaccuracies is well taken. In many cases, STW measurement is too far off to give a reliable True Wind calculation. You're right about that.

But still -- you do sail in True Wind -- that's the sheer between air and water which is our source of power. Apparent Wind is only what you sail in on this point of sail -- because your speed vector is added. You need True Wind to know how you can tack, what DDW is, etc., etc, etc. -- and only True Wind. Ground Wind is completely irrelevant to actual sailing on this tide, and you will get in trouble if you use Ground Wind to decide what course you can make a safe number of degrees from DDW, for example -- the answer may be very different than if you correctly use water-referenced True Wind. Ground Wind is useful to compare to forecasts and to know what will happen when the tide turns, but not for figuring out changes to points of sail.

Dockhead, the only wind you sail in is the one blowing over the boat. Thats what your instruments read at all points of sail and at all speeds. That is traditionally called the apparent wind

True wind as its meant by mariners, is a mathematical calculation , its a derived number obtained by vector subtracting the effect of boat sped through the water.


It has been referred to as True wind, simply because for generations, SOG was impossible to calculate at sea. SO what was possible , become the accepted

The difference between Ground Wind and True Wind is relevant mostly to people with strong currents. As I said, if like a lot of sailors you never experience currents of more than a knot -- then you won't care about the distinction. Especially since the sloppiness of True Wind calculations based on speed readings from dirty paddlewheel logs may make them completely incoherent.

Typical tidal drifts, outside of certain estuaries and bores etc , run to 1-4 knots, when the vector is added unless directly into the or away from the boat, the use of inaccurate STW and sometime poor AWS, combined with heel , effects make the computation of True wind somewhat ridiculous Whether one uses ground wind or wind related to the water, makes little difference with such accuracy.

As to the terminology: You're right, of course -- there was no such thing as water-referenced True Wind until we had electronics capable of calculating it. It is a term invented by the instrument makers. But I think they have done us a big service -- it's much more precise, and the precision is highly meaningful, if we are dealing with at least moderately strong currents.

see accuracy above

"True", as someone above said, just means corrected from some kind of deviation or error -- it's a fairly generic term, and it does not cause me any problem that people on land use the term "true wind" differently from what our instruments say. For someone on land, naturally true wind is ground-referenced -- that's the only reference they have. For us, I submit, it's much more natural to reference "true wind" to the water, since that is what we sail in. And therefore, I personally am really glad that the terminology is now like this and that our instruments work that way. I think it's valuable progress that we now have True, Apparent, and Ground Wind -- which as you said our ancestors did not. Progress which was made possible only with modern electronics.

"True" is used in different sectors to mean different things. in meteorology , True wind is ground wind

Different people will have different taste about this, but I think you're swimming against the tide (so to speak! pace Shakespeare) if you try to stick to the old ground-referenced True Wind, at sea. You won't even be able to get your instruments to display it (unless you have Furuno, which does have that option, unique among the instrument makers AFAIK).

at sea I use all the variables my instruments can compute, what i dont persist in doing is making claims that my accuracy is better then my precision , which you continue to confuse over. if you use SOG or STW it really matters not a lot , only when you have a complete instrument system that can accuracy deliver precision ( the two together) is it worth making subtle differences as to what use you can make of each.

[Robin3]

On my last boat in the UK I had a 'spare' ST50 multimeter instrument which I mounted in the wheel pedestal consol display, connected on the Seatalk Bus and had it set most of the time to read 'Beaufort' wind strength as all the other data it could also show if asked was shown on other and 'main' intruments in the consol mounted over the mainhatch It was quite educational to see the Beaufort strength referenced I think to he 'true' wind calculated from paddlewheel log STW, compass heading and masthead apparent windspeed


More importsant to me however than true versus ground versus apparent was the separate closehauled meter which showed a magnified wind direction from say zero to +/-60 degs off either dead upwind or dead downwind. however I did also use the 'multi meter at the wheel set to read apparent wind angle Port or starboard and displayed digitally. my then boat had an optimum upwind of 7kts STW at 28 degs to the apparent wind although at a pinch ( pun intended) she woul manage 22 degs but speed dropped to 6kts. The VMG displayed in both cases was the same, but at 7kts/28 degs she was not slowed by a ripple or two larger than the normal on that heading. pinching up to 22 degs was useful on occasions to make it round a race mark or to nudge an opponent foolishly attempting an overtake 'up' and maybe to make them tack away.


Now who will be first to ask how we could say tack though 56 degs ( 2x28) let alone 44(2x22)? because the difference between apparent and true is not fully understood?

[goboatingnow]

Originally Posted by Robin3

On my last boat in the UK I had a 'spare' ST50 multimeter instrument which I mounted in the wheel pedestal consol display, connected on the Seatalk Bus and had it set most of the time to read 'Beaufort' wind strength as all the other data it could also show if asked was shown on other and 'main' intruments in the consol mounted over the mainhatch It was quite educational to see the Beaufort strength referenced I think to he 'true' wind calculated from paddlewheel log STW, compass heading and masthead apparent windspeed


More importsant to me however than true versus ground versus apparent was the separate closehauled meter which showed a magnified wind direction from say zero to +/-60 degs off either dead upwind or dead downwind. however I did also use the 'multi meter at the wheel set to read apparent wind angle Port or starboard and displayed digitally. my then boat had an optimum upwind of 7kts STW at 28 degs to the apparent wind although at a pinch ( pun intended) she woul manage 22 degs but speed dropped to 6kts. The VMG displayed in both cases was the same, but at 7kts/28 degs she was not slowed by a ripple or two larger than the normal on that heading. pinching up to 22 degs was useful on occasions to make it round a race mark or to nudge an opponent foolishly attempting an overtake 'up' and maybe to make them tack away.


Now who will be first to ask how we could say tack though 56 degs ( 2x28) let alone 44(2x22)? because the difference between apparent and true is not fully understood?

I see the " lee bow " effect myth about to hit us :LOL:bang head:
ps big thread on this last year

[Robin3]

Originally Posted by goboatingnow

I see the " lee bow " effect myth about to hit us :LOL:bang head:
ps big thread on this last year

OMG now that does produce serious brain ache.


IIRC our actual tack angle was around 75-80 degs. Not many got inside us back then except the real race machines and these days I plan cruise destinations to give a us free wind. Is that English sailing phrase universal I wonder?

[StuM]

Originally Posted by El Pinguino

Such as saying things like 'full left rudder' and stuff?

Don't you mean "left hand down a bit"?

Navy Lark Special | Left Hand Down a Bit, with Leslie Phillips: Navy Lark Special | Radio Times

[StuM]

Originally Posted by Dockhead

Different people will have different taste about this, but I think you're swimming against the tide (so to speak! pace Shakespeare) if you try to stick to the old ground-referenced True Wind, at sea. You won't even be able to get your instruments to display it (unless you have Furuno, which does have that option, unique among the instrument makers AFAIK).

So do the GRIB files we use for passage planning show predicted "ground" or "water" referenced winds?

[Seaworthy Lass]

Originally Posted by StuM

So do the GRIB files we use for passage planning show predicted "ground" or "water" referenced winds?

As far as I'm aware, all weather forecasts give ground wind.

[Robin3]

Originally Posted by StuM

Don't you mean "left hand down a bit"?

Navy Lark Special | Left Hand Down a Bit, with Leslie Phillips: Navy Lark Special | Radio Times

left hand 'dine' a bit from the officer, aye aye 'darn' a bit it is zur. from the crewman on the wheel.