Speed Over Ground Data Collection

[Paul Elliott]

Originally Posted by goboatingnow

I know stw has historically been used to trim against , because that was all that was available. But in reality we trim to apparent wind and that wind is in effect SOG referenced. In your example with the Gulf Stream , the boats are most definitely not trimmed the same even though STW is the same

Why do we persist with stw then ?

Dave

SOG is an easy number to get, and is very useful when you're trying to get somewhere, but STW is the best parameter to use when evaluating sailing performance. This is because of the current. Our boats sail through the water, and they see the wind relative to the water.

There is confusion as to the proper reference to use for TWS (True Wind Speed) wind measurements. Weather reports and forecasts use ground-referenced wind. Your boat sees water-referenced wind. Both of these can be called TWS, but they can be quite different numbers. In order to eliminate the confusion there have been proposals for calling these something other than TWS and TWA in one case or the other, but no consensus has been reached.

Boat performance measurements (polars) are based on water-referenced TWS and TWA. This removes the difficult to measure current effects from the equation. Otherwise, your performance data would be different when sailing up-current or down-current.

There is no confusion about AWS and AWA, these are referenced to the boat. When talking about boat performance, TWS and TWA are properly referenced to the water.

[goboatingnow]

Originally Posted by caradow

STW is the bottom line!!!
I do not understand why you trim to AW......not near as accurate.
AW is not an important part of trimming other than the obvious.
To get the most out of any boat you must always be cognizant of STW!!
Then after that is maximized I would turn to SOG in my strategy planning.

You are merely restating the received wisdom. Stw was used because until GPS that's all we had.

Dave

[caradow]

Dave,
Doesn't sound like you come from a racing background.
Either that or we are talking about two different things.

[goboatingnow]

Originally Posted by Paul Elliott

SOG is an easy number to get, and is very useful when you're trying to get somewhere, but STW is the best parameter to use when evaluating sailing performance. This is because of the current. Our boats sail through the water, and they see the wind relative to the water.

There is confusion as to the proper reference to use for TWS (True Wind Speed) wind measurements. Weather reports and forecasts use ground-referenced wind. Your boat sees water-referenced wind. Both of these can be called TWS, but they can be quite different numbers. In order to eliminate the confusion there have been proposals for calling these something other than TWS and TWA in one case or the other, but no consensus has been reached.

Boat performance measurements (polars) are based on water-referenced TWS and TWA. This removes the difficult to measure current effects from the equation. Otherwise, your performance data would be different when sailing up-current or down-current.

There is no confusion about AWS and AWA, these are referenced to the boat. When talking about boat performance, TWS and TWA are properly referenced to the water.

Boats sail to the apparent wind. There is no other actual wind. A boat does not experience wind referenced to water. A boat stopped in the water , that is moving due to a current is experiencing apparent wind.

Historically , unlike your comment , instantaneous SOG was virtually impossible to measure and was typically post processed from distance run over time , often 24 hours. STW can be easily measured by simple devices in real , or near real time and therefor in my opinion this is why it was used to trim against, or there wasn't anything else.

Polars are a different issue , what I was merely commenting on is that I see no issue in trimming to SOG , or using increases or decreases in SOG to evaluate trim. I appreciate the ' current' issues but I don't beleive its a factor

STW evolved in instrumentation because it was all we had at the time.

The issue over nomenclature is entirely different , in my opinion TWS is ground wind , but the term has been used by boat instrument companies to mean wind speed referenced to water , this is again because that's all they had.

Dave

[goboatingnow]

Originally Posted by caradow

Dave,
Doesn't sound like you come from a racing background.
Either that or we are talking about two different things.

No I've done a fair bit of racing, but at heart I'm a bluewater cruising man. I'm well aware of the debate etc. and I'm well aware of the maths and the terminology.

What I was saying is that I see no reason why one cannot trim to SOG.

Ultimately if you think about it , the wind is not referenced to the water , when I say a 50 kn wind from the SE , I am inherently meaning ground wind ( to use the corrupted marine terminology )

When I refer to the boats progress, I am referring to SOG

But if I want to extract maximum speed at a given moment , in effect I am comparing Apparent wind with SOG. Ie I have 25kn " over the deck " and I am progressing at say 5kn SOG. , if I improve my trimming and my speed increases , then I'm sailing better. I fully except that equally I could use STW. But fundamentally I am comparing apparent wind , which is in effect ground referenced with boat speed which is water referenced and that's apples and oranges.

That's the debate !


Dave

[roetter]

There still seems to be some confusion for some what the wind terms mean.

Ground Wind - wind speed and angle over the ground relative to the ground (the ground stands still, the water does not)
True Wind - wind speed and angle over the water relative to the boat. This is the wind the boat would experience standing still in the water, which might be moving at considerable current speed.
Apparent Wind - wind speed and angle relative to the moving boat.

The boat instruments can only measure apparent wind speed and angle. All other value are computed using simple vector algebra from the apparent wind, the heading and STW, plus the COG and SOG.

However:
- The wind instrument probably over or under reads the wind speed
- The wind instrument is probably not properly aligned with the center line of the boat. Just 3 degrees off to one side is significant when pointing. This makes for 6 degrees difference reading at the same real AWA.
- The angle transducer is probably not linear.
- The water speed transducer is not properly calibrated and not linear. I have seen them over reading by as much as 1 knot in the 7 knot range. On my mono it always reads higher on the port tack as it is slightly off center on the hull.

Sailing the trade wind routes, you almost always have 1+ knots of current going with you. That means that SOG is very bad in comparing numbers. With just 1 knot of current two identical boats on a beam reach going (approximate) opposite direction and the same STW will show a 2 knots difference in SOG. Going 6 knots STW, one boat will show SOG 4 knots while the other shows 6 knots. Also, if they are sailing the same apparent wind angle, they are not going directly in opposite direction, or if going in opposite direction they are not sailing the same AWA.

Using spot readings is useless. Besides all the other problems with obtaining performance numbers discussed in this thread, You would need long time measurements and then build statical averages. In short, any data collected without properly calibrated instruments and without long term data collection (at least one hour of data for each sail point) without statistical analysis is useless.

Crossing the Pacific on a cat 3 years ago, I used a board computer and averaged the boat speed over 30-45 minutes at a time to detect the performance changes when changing the AWA the autopilot was steering by. It is amazing how much difference just 5-10 degrees in AWA can make when sailing on a broad reach. Coming up 10 degrees could increase the average boat speed by about 1 knot, while only making 2 degrees difference in COG. The boat speeding up brought the AWA so much forward that the autopilot had to fall off again (measured in COG).

[Paul Elliott]

Originally Posted by goboatingnow

Boats sail to the apparent wind. There is no other actual wind. A boat does not experience wind referenced to water.

I knew that someone was going to say this.

Sure, apparent wind is what you actually measure on a boat, and yes, that's what the boat and sails are sailing in, but are you somehow claiming that AW exists without a reference? That reference is, or should be, the water.

A boat stopped in the water , that is moving due to a current is experiencing apparent wind.

It's also experiencing True Wind, and in this case they happen to be the same.

This isn't just a pointless philosophical debate over terminology. This isn't a case of "Well, they didn't have GPS back when the term was invented." It's for a very practical reason that True Wind should be water-referenced if you are describing sailing performance, such as Polars. If you use a ground-referenced TW then any current you sail in will completely screw up your measurements and analysis.

Ground-referenced wind is used in wind forecasts (GRIBS, etc), but you have to adjust those for predicted current to convert to water-referenced wind before you use polars for weather routing.

If you want to call water-referenced True Wind some other name, by all means do so, but please define your terms if you expect to have a useful discussion of boat performance.

[noelex 77]

On a practical note most cruising sailors would get far more accurate and meaningful displays if they switched their instruments to ground wind instead of "true" wind.

Unfortunately many instruments don't allow this. This is silly legacy left over from the days when reliable SOG data was not available. (To echo Dave's comments about nomenclature). It should be an easy software upgrade. When you speak to a rep from an instrument company tell them to unlock this option on their instruments.

We are constantly getting better and better SOG data with the availability of SD fixes, more satellites, better chips and 5hz GPS units. Most instrument makers still regard SOG data in the same way as they did when SA was in operation.

Accurate, consistent, reliable "true" wind in either water or ground referenced formats is a great bit of information to have, even on a cruising boat, but very few boats achieve this.

[belizesailor]

Originally Posted by noelex 77

On a practical note most cruising sailors would get far more accurate and meaningful displays if they switched their instruments to ground wind instead of "true" wind.

Unfortunately many instruments don't allow this. This is silly legacy left over from the days when reliable SOG data was not available. (To echo Dave's comments about nomenclature). It should be an easy software upgrade. When you speak to a rep from an instrument company tell them to unlock this option on their instruments.

We are constantly getting better and better SOG data with the availability of SD fixes, more satellites, better chips and 5hz GPS units. Most instrument makers still regard SOG data in the same way as they did when SA was in operation.

Accurate, consistent, reliable "true" wind in either water or ground referenced formats is a great bit of information to have, even on a cruising boat, but very few boats achieve this.

Definition of "ground wind"?

...never mind found it...learn something every day...

[goboatingnow]

.

Sure, apparent wind is what you actually measure on a boat, and yes, that's what the boat and sails are sailing in, but are you somehow claiming that AW exists without a reference? That reference is, or should be, the water.

It's also experiencing True Wind, and in this case they happen to be the same.

Firstly , only mariners claim teh term True Wind is water referenced. That is solely because for 100s of years that all that could be deduced. There was simply no way to determine real time ground speed.

Wind is NOT referenced to water, it cant be as teh medium is moving relative to the earth. Wind is referenced to the earth , the reason as sea we use water is thats all we every had

This isn't just a pointless philosophical debate over terminology. This isn't a case of "Well, they didn't have GPS back when the term was invented." It's for a very practical reason that True Wind should be water-referenced if you are describing sailing performance, such as Polars. If you use a ground-referenced TW then any current you sail in will completely screw up your measurements and analysis

.

Polars are a very particular case. They show performance against true wind. The wind is merely given in speed over distance as is the boat performance. You then must convert those polars into whatever reference system you wish, taking into account whatever refernece frames you want.

But I can quite easily trim to SOG from Apparent wind speeds. and I can state

at x m/s at an angle of 'theta' AW, the SOG is yy m/s,

Ground-referenced wind is used in wind forecasts (GRIBS, etc), but you have to adjust those for predicted current to convert to water-referenced wind before you use polars for weather routing.

Only boat instrument makers , hijacked the term True Wind and then frantically looked around and invented "ground wind". In all other uses TRUE WIND is ground referenced wind. Ground wind suddenly appeared due to the availability of real time SOG.

[goboatingnow]

Originally Posted by noelex 77

On a practical note most cruising sailors would get far more accurate and meaningful displays if they switched their instruments to ground wind instead of "true" wind.

Unfortunately many instruments don't allow this. This is silly legacy left over from the days when reliable SOG data was not available. (To echo Dave's comments about nomenclature). It should be an easy software upgrade. When you speak to a rep from an instrument company tell them to unlock this option on their instruments.

We are constantly getting better and better SOG data with the availability of SD fixes, more satellites, better chips and 5hz GPS units. Most instrument makers still regard SOG data in the same way as they did when SA was in operation.

Accurate, consistent, reliable "true" wind in either water or ground referenced formats is a great bit of information to have, even on a cruising boat, but very few boats achieve this.

correct , because for most sailors, what they want when they press 'True Wind' button is actually ground wind. Ie the forecast say 40kn, what actually out there where I am. Thats True , ie ground referenced Wind.

If you want to construct Polars, then you need a speed that is solely a function of boat performance, that is why you use STW, becuase you need a reference frame to allow you to transport the resulting graph to another boat in different circumstances. It is in essence a mathematical construction as in reality no such effect is occurring. ( you are always referenced to the planet, one way or another)

Its about the only use for water referenced speed.

Dave

[Paul Elliott]

Originally Posted by goboatingnow

.
Polars are a very particular case. They show performance against true wind. The wind is merely given in speed over distance as is the boat performance. You then must convert those polars into whatever reference system you wish, taking into account whatever refernece frames you want.

Polars show performance against water-referenced true wind.

But I can quite easily trim to SOG from Apparent wind speeds. and I can state at x m/s at an angle of 'theta' AW, the SOG is yy m/s,

Yes, you can trim for max SOG, and that's usually what you want. But if you are trimming for a target speed based on your polars, if you use SOG you *need* to know the current. If you don't care about polars, or recording your sailing performance data, then I guess it doesn't matter.

If you use STW and apparent wind (and leeway), you can calculate water-referenced TWS/TWA. This is what the polars use, and you don't need SOG or current data.

For those who wonder what all the fuss is about, remember that the OP was asking about sailing performance, and we started discussing polars.

[psneeld]

SOG is usually very accurate STW affected by set/drift/leeway. If you use SOG in perfectly calm water with no current and sort of dismiss leeway...then it should be OK...and probably better than STW as most STW instruments are not accurate...it you can get all of the STW instruments accurate...then STW would be the better parameter.

[FSMike]

I'm amazed we haven't had any ice boaters or desert sailors complaining about the lack of discussion of "speed over frozen water" or "speed over sand".
I'm sure that there is some wonderful info on this thread and I'm also sure I don't need it to go sailing.

[caradow]

Hey FSMike
also don't forget about keeping a sharp eye out for your TWA's down wind so you can slam those jibes in at the perfect time. Otherwise how can you truly enjoy the beauty of sailing without those little babies.