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Old 12-08-2013, 22:54   #31
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Re: Speed Over Ground Data Collection

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Originally Posted by FSMike View Post
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.
No, you don't need it. I totally agree.

The thing is, the original poster was asking about sailing performance metrics. If he is going to get any useful data, he does need this information.
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Old 14-08-2013, 10:53   #32
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When sailing I always use a split screen display on my chart plotter. The data page is set up to show me, beside other data:

Apparent wind speed and angle
True wind speed and angle
Ground wind speed and direction

The need for all these has nothing to do with - they did not have GPS with SOG and COG in the o'old' days. True wind as a reference for boat performance is purely based on physics and nothing else.
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Old 14-08-2013, 18:08   #33
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Re: Speed Over Ground Data Collection

hi roetter
I am not sure I understand your conclusion/statement that
"True wind is based on physics and nothing else"
you certainly keep tract of a lot of data on your boat with your split screen. All which are related to physics.
maybe I do not get your point and I am misinterpreting your statement and for that I must apologize.
But is it not "physics what we use to make boats go faster??
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Old 15-08-2013, 20:17   #34
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Quote:
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hi roetter
I am not sure I understand your conclusion/statement that
"True wind is based on physics and nothing else"
you certainly keep tract of a lot of data on your boat with your split screen. All which are related to physics.
maybe I do not get your point and I am misinterpreting your statement and for that I must apologize.
But is it not "physics what we use to make boats go faster??
I wrote that true wind used as a REFERENCE for boat performance is based on physics because it is the true wind that provides the energy for the boat. All other wind values are just derived. That is why all boat polars are with respect to true wind speed and direction.

A really high performance multi hulls will go upwind to the apparent wind even though theynare going downwind (true wind). There are two videos on YouTube, one shown from a Lagoon 40 or similar the other one from the French trimaran Banquet Populaire. They are filming each other in the mid Atlantic. They are going the same direction. The Lagoon is going deep down wind, the Banque Popuaeire is on a close reach. On the lagoon the apparent wind is probably 8 knots, on Banquet Populaire it is more like 25 knots. BanqmPolpulaire is doing 30+ knots the Lagoon about 7 knots. This show clearly that you can not use apparent wind as a reference for boat performance.
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Old 16-08-2013, 01:53   #35
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Re: Speed Over Ground Data Collection

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Originally Posted by roetter View Post
This show clearly that you can not use apparent wind as a reference for boat performance.
If we know the apparent wind speed and direction and STW we can calculate true wind.
So for the purposes of data collection AWS, AWA and STW are all that sailors need to report. True windspeed data is not needed. Given the ease of calculating true wind its superfluous, unnecessary information.
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Old 16-08-2013, 10:43   #36
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Re: Speed Over Ground Data Collection

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If we know the apparent wind speed and direction and STW we can calculate true wind.
So for the purposes of data collection AWS, AWA and STW are all that sailors need to report. True windspeed data is not needed. Given the ease of calculating true wind its superfluous, unnecessary information.
You're right, but let me point out some of the difficulties in converting apparent wind to true:

To be more accurate we also need to know leeway before we can convert AWS/AWA/STW to true wind (water-referenced). We then need to know current to convert that to true wind (ground referenced).

The only wind we can directly measure from the boat is Apparent Wind. Leeway is not usually measured, unless you have a fancy multi-axis ultrasonic water-speed transducer (and I've never seen one of these myself). Leeway corrections can be made based on the observed angle of your wake, or calculations based on measured heel and known boat characteristics. Leeway can also be estimated from AWS/AWA/STW, and generic fudge-factor.

Many systems just figuratively throw up their hands ignore leeway, but that will result in inaccuracies that can be significant when close-hauled. For most of us that's good enough. If you are designing the next America's Cup boat you will want accurate data, but fortunately for most of us these numbers are merely a fun distraction.

The only reason it might matter here is if we are comparing the performance of different boats. The varieties of measurement methodology can result is significantly different numbers. Also fortunately for most here, these inaccuracies are most pronounced when sailing close-hauled, and that's a point of sail that most cruisers try to avoid on long passages.


So like many things, the closer you look the more complicated it becomes...
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Old 16-08-2013, 11:11   #37
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Re: Speed Over Ground Data Collection

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You're right, but let me point out some of the difficulties in converting apparent wind to true:

To be more accurate we also need to know leeway before we can convert AWS/AWA/STW to true wind (water-referenced). We then need to know current to convert that to true wind (ground referenced).

The only wind we can directly measure from the boat is Apparent Wind. Leeway is not usually measured, unless you have a fancy multi-axis ultrasonic water-speed transducer (and I've never seen one of these myself). Leeway corrections can be made based on the observed angle of your wake, or calculations based on measured heel and known boat characteristics. Leeway can also be estimated from AWS/AWA/STW, and generic fudge-factor.

Many systems just figuratively throw up their hands ignore leeway, but that will result in inaccuracies that can be significant when close-hauled. For most of us that's good enough. If you are designing the next America's Cup boat you will want accurate data, but fortunately for most of us these numbers are merely a fun distraction.

The only reason it might matter here is if we are comparing the performance of different boats. The varieties of measurement methodology can result is significantly different numbers. Also fortunately for most here, these inaccuracies are most pronounced when sailing close-hauled, and that's a point of sail that most cruisers try to avoid on long passages.


So like many things, the closer you look the more complicated it becomes...
Your right, but if the boat concerned quotes there true wind reading we won't be any better off (with 95% of systems) because leeway is not taken into account by their display, as you correctly point out.

My view is it is simple maths to calculate the true wind as shown on most CF members displays so there is little point requesting this extra information.

I am a big fan of ground wind as a superior display for a cruising boat, but it is irrelevant to a boats polar performance. True wind is the only important factor. There is nothing to gained calculating ground wind unless SOG is used instead of STW in which case the speed data is so wrong the data has little relevance anyway.
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Old 16-08-2013, 12:10   #38
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Re: Speed Over Ground Data Collection

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There is nothing to gained calculating ground wind unless SOG is used instead of STW in which case the speed data is so wrong the data has little relevance anyway.
Unless there's no current, in which case SOG is probably gives more accurate results than STW (since STW transducers are often inaccurate). SOG/COG also eliminate the leeway issue.

But where I sail, there's usually a significant current! Even a half-knot of current can truly screw up some of the SOG/COG-based calculations.

(I realize that we haven't heard from the OP in over a week, but that doesn't mean the rest of us can't have a fun discussion!)
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Old 16-08-2013, 12:36   #39
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Re: Speed Over Ground Data Collection

Collecting SOG is not the right data for the OPs purpose. STW is much more relevant for this purpose. I cannot see SOG working in this application.
If using STW there is no point calculating ground wind. It is irrelevant.

Its only when you get to high end instruments fitted to top racing boats that Ground wind (measured relative to the bow) uses the difference between COG and heading in its calculations of ground wind relative to the bow. (Although this may be used when displaying the compass bearing of the ground wind). There is good reason for this. Any discrepancy in the fluxgate compass heading would be reflected the ground wind angle relative to bow. Thus a very accurate heading sensor is needed. Top racing boats are not very interested in ground wind anyway.

In other words very few members will report ground wind relative to the bow with any allowance for leeway.

Ground wind is a great parameter for a cruising boat but it is of little use here.
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Old 25-08-2013, 12:58   #40
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Correct me if I am wrong, but I think leeway is not a factor. It just goes into the calculation of set and drift which is the difference between the heading and speed in the water vs SOG and COG.
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