Speed Through GPS Versus Old Fashioned Paddle Log

[rwidman]

Originally Posted by Don C L

This thread should be published and made into an online navigation course.

Or added to the joke thread.

[Dockhead]

Originally Posted by noelex 77

The only thing your sails respond to is apparent wind. This is the simplest display shown by all wind instruments and subject to the least error.

As a second display if the instruments offer the option (as yours do) we can choose ground wind or true wind. To use your own (I hope hypothetical example) of 1020 knot ground wind and 1000 knot current, apparent wind and true wind would give no indication of the extreme conditions. Only ground wind would do this.

I think I would want to know about 1000 knots of wind.

However, the big advantage of ground wind is that is always accurate, always calibrated. There is no fouling or miscalibration of the log to give deceptive readings.

If you have a racing boat it is worth spending the time to ensure the log reading is spot on: identical on different tacks, calibrated regularly and regularly cleaned, but few cruising boats go to this much trouble. GPS units have improved leaps and bounds in responsiveness and accuracy over the last decade but many instruments only offer wind calculated from the frequently inaccurate log.

Ground wind is the best option if you want to answer questions like: Has the wind become weaker, stronger or changed direction? Does the wind match the forecast? Or what will be the wind strength and direction when I anchor or dock? True wind will not answer any of these questions correctly.

Information about both true wind and ground wind have their usefulness, but with the typical instruments fitted to cruising boats the option of accurate ground wind beats true wind that is vulnerable to bits of weed, or some hull fouling. At least companies like Raymarine could offer us the option.

Well, I guess I can't really argue with any of this, which is all well-reasoned.

Concerning this:

"Ground wind is the best option if you want to answer questions like: Has the wind become weaker, stronger or changed direction? Does the wind match the forecast? Or what will be the wind strength and direction when I anchor or dock? True wind will not answer any of these questions correctly."

All correct, except the first! " Has the wind become weaker, stronger or changed direction?" Here you really care about the True Wind -- because if the tide has changed the wind, you need to know about that. But temper this with your correct observation that the calculation of True Wind will be much less accurate than calculation of Ground Wind.


I don't know about other cruisers, but I care an awful lot about what my boat is doing in relation to the water. To this end, I spent a lot of money on an ultrasonic log, which unfortunately has not entirely lived up to expectations. If a better one comes out, no doubt I will spend money again. I would love it if they could give us direction of travel through water, as discussed above. I don't think you need to be a racer to care about such things; at least, those of us who sail in tidal waters.

[chala]

Originally Posted by Dockhead

Except that if you calculate True Wind with SOG instead of STW, it's not True Wind -- it's Ground Wind.

Absolutely not.

True. Frequently used to differentiate between a value that as been corrected for known errors and one that as not.

Apparent Wind that as been corrected by STW is True Wind.
Apparent Wind that as been corrected by SOG is True Wind.
Even Apparent Wind that as been corrected by many known “errors” cannot be called Ground Wind but should be called True Ground Wind.
It is interesting to note that long time professional mariners have never heard the term Ground Wind and I even doubt that True Ground Wind can equal (met) wind. A correct Deviation would be required which is hard to produce at sea.

[Seaworthy Lass]

Originally Posted by chala

Absolutely not.
True. Frequently used to differentiate between a value that as been corrected for known errors and one that as not.

Apparent Wind that as been corrected by STW is True Wind.

Correct.

Originally Posted by chala

Apparent Wind that as been corrected by SOG is True Wind.

Sorry, I think this is incorrect.

Are you saying true wind has two different values depending on which speed you use to determine it?

As I understand it, this is the relationship using vector addition:

AW = TW + STW = GW + SOG
(Note: STW is the vector sum of the paddlewheel log reading if it is accurate, plus leeway)

[Dockhead]

Originally Posted by chala

Absolutely not.

True. Frequently used to differentiate between a value that as been corrected for known errors and one that as not.

Apparent Wind that as been corrected by STW is True Wind.
Apparent Wind that as been corrected by SOG is True Wind.
Even Apparent Wind that as been corrected by many known “errors” cannot be called Ground Wind but should be called True Ground Wind.
It is interesting to note that long time professional mariners have never heard the term Ground Wind and I even doubt that True Ground Wind can equal (met) wind. A correct Deviation would be required which is hard to produce at sea.

Deviation? I think you are confusing this with true and magnetic compass bearings -- not the first time that this has entered this discussion.

I would suggest that you read this:

Of Instruments and Real Angles | Sailing World


And have a look at this diagram:



You might also refer to the B&G Glossary of Sailing Terms:

"Ground Wind
Ground wind is the speed and direction of the wind referenced to land.

Ground Wind is often used by sailors in a strong tidal current when trying to anticipate what the wind will be like when the tide changes.

Weather forecasts provide Ground Wind. If sailing in tidal areas the navigator should be aware of the additional "Tide Wind" effect and compensate for it in their decision making. For example, if the forecast is for 18 knots of wind, 2 knots of tide could be the difference between 16 and 20 knots of True Wind."

Glossary

[chala]

Originally Posted by noelex 77

In my view most cruising sailors would be better if they switched to ground wind.

Originally Posted by Dockhead

But I don't think it's a very good idea to use Ground Wind for sailing. It's not what you sail in. It will lead to big mistakes in areas with strong tidal streams. You can be 5 knots off, in a place where the tide regularly reaches 5 knots, like in the Channel, and the direction may be significantly different, too.

Yes Wind Direction can be way off, mostly if the boat is made of steel.
In the pic everything looks fine. Wind Direction not referenced to boat can be an annoyance.

[chala]

Originally Posted by Ericson38

GPS receiver is directly calculating your position in ECEF coordinates, the same frame as the satellite positions and orbits. Then the receiver converts the ECEF X,Y,X into your local reference frame, say LLA. So the calculated speed is in 3-D. Going up a hill or down, your 3-D speed accuracy is not effected. Going up a 45 deg incline, your speed on a horizontal plane is reduced by square root of 2. ( or .707).

I must have a different GPS, if I was to follow the GPS speed indication the difference in speed will be enough to get me a speeding ticket.

Originally Posted by Ericson38

Depending on where your boat is now, there can be a lot of wander in the static position calculated, from multipath induced by nearby structures (masts and corner reflectors are good sources of this). Over 24 hours, this could end up as quite an accumulated trip log distance, but all the moving was within a 10-20 foot radius of your boat.

Yes and this will also happen underway, which make trip log distance not that accurate.

[chala]

Originally Posted by valhalla360

If you are regularly getting hits 10 miles from your true location.

Originally Posted by chala

the track distance after 24 hours recording can be as high than 10 nautical miles with the boat not moving at all.

Track distance, not location.

[Wotname]

OK, with regard to all the various winds referenced above, how do you reference the the various wind speeds that occur between boom height and mast head height.

Note, I am presuming the wind direction does not change significantly between thes points.

Do you use an average speed where the wind has most influence on the sail, or just use the wind speed transducer's position and ignore the other speeds or something else?

Maybe it's WMH, AWB, AWA or should that be MHAW, BAW, AAW and then work out TWMH, TWB, TWA or of course it could be MHTW, BTM, ATW.

[valhalla360]

Originally Posted by chala

Track distance, not location.

Then what's your point?

Who cares if it moves 100miles over the course of a day if it's never more than a few feet from it's true location at any point in time.

Heck, that might actually be accurate, If the boat is moving back and forth in a slip, 2-3ft every 10 seconds, run the math.

[Dockhead]

Originally Posted by Wotname

OK, with regard to all the various winds referenced above, how do you reference the the various wind speeds that occur between boom height and mast head height.

Note, I am presuming the wind direction does not change significantly between thes points.

Do you use an average speed where the wind has most influence on the sail, or just use the wind speed transducer's position and ignore the other speeds or something else?

Maybe it's WMH, AWB, AWA or should that be MHAW, BAW, AAW and then work out TWMH, TWB, TWA or of course it could be MHTW, BTM, ATW.

Now you've opened a can of worms!

Actually, they say it DOES change direction as you go up, which is supposed to be the reason why you need twist in the sail.

I do not understand it at all, and would be grateful if someone who does, would explain whether this phenomenon exists or doesn't.

[El Pinguino]

Originally Posted by Dockhead

.....Ground Wind is often used by sailors in a strong tidal current when trying to anticipate what the wind will be like when the tide changes.
......

This debate seems to have centered mainly around sailing in the English Channel. Therefore I would suggest that the term 'tidal stream' should be used... not 'tidal current'

....for the avoidance of confusion etc.

[El Pinguino]

Originally Posted by Wotname

OK, with regard to all the various winds referenced above, how do you reference the the various wind speeds that occur between boom height and mast head height.

Note, I am presuming the wind direction does not change significantly between thes points.

Do you use an average speed where the wind has most influence on the sail, or just use the wind speed transducer's position and ignore the other speeds or something else?

Maybe it's WMH, AWB, AWA or should that be MHAW, BAW, AAW and then work out TWMH, TWB, TWA or of course it could be MHTW, BTM, ATW.

The world's met departments standardise wind measurements at 10 metres above ground. I'm sure someone here can come up with a simple system for wind speed adjustment using instrument height/angle of heel as arguments.

[chala]

Originally Posted by Seaworthy Lass

Are you saying true wind has two different values depending on which speed you use to determine it?

I suppose the quest for true wind started with somebody throwing a log overboard. Since then many other devices as been used to find true wind as pointed out by the OP.

Originally Posted by Jonathancpwalsh

Can anyone tell me why so many boats still use the "old fashioned" impeller log to record their speed? GPS obviously gives much more accurate speed over the ground which is, ultimately all you really want, isn't it? Or am I missing something? I'm not a racer - just cruising! Would appreciate your thoughts!

Nothing wrong about using a device that is more accurate if manufacturers provide it.
Yes true wind can have different values depending on the accuracy of the device that someone uses.

[Wotname]

Originally Posted by Dockhead

Now you've opened a can of worms!

Actually, they say it DOES change direction as you go up, which is supposed to be the reason why you need twist in the sail.

I do not understand it at all, and would be grateful if someone who does, would explain whether this phenomenon exists or doesn't.

Worm pie I wonder what dish SWL can make with a COW.

I used to know why the wind direction changed with increasing alitude and in what direction (its hemisphere dependent) but right now I have forgotten. Google will know, it's standard stuff any proper pilot will know ( or did know )
But does it change significantly over a range of say 15 metres very close to sea level - I dunno, clearly the strength varies and that is easy to observe and measure, almost always ( or perhaps always) increasing strength with increasin height.

Anyone else know for sure about the wind significantly changing direction between boom and mast head?