True Wind Discussion Redux

[Scott T-Bird]

I thought I'd start a new thread so that we don't take the wind speed topic (which is related to his Garmin instrument) too far off base. I've said that I use the GPS speed input for the true wind calculation. Thanks to @DrJudyB and the posted article (copied here) I have a better understanding why "boat movement thru water" or paddlewheel input is best. However, it seems to me that it is best simply because it best represents your targets with respect to the polars. Throw the polars out and what difference does it make? Of course that is heresy to racers. But the objective is to maximize VMG, isn't it? I understand that the way to maximize your VMG is to hit the targets relative to boat speed through water and true wind angle.

Airmar CS4500 Part II (Reliable True Wind) - Panbo

Believe me, I'm not trying to be contrary. My understanding is that the goal of VMG is to maximize your speed over ground to the mark, which is set on ground and doesn't move with the current (unless it comes loose). If your true wind input is GPS, then don't you know that current is going to offset the "true wind" heading (true wind as defined in the article, not "ground wind") and distort the boat speed so that it doesn't match SOG. You wouldn't look for the same targets on both tacks. Instead you will set your targets based on the best VMG. It sounds backwards, and I'm sure that it's just making it over-complicated. But the article led me down this path. The last paragraph (written mainly for power boats) talks about the value of "ground speed" if you have a good heading source and your chartplotter can input current set and drift.

So, my B&G chartplotter has that capability with the external GPS sensor B&G ZG100 (with heading sensor) all integrated on NMEA 2000 network. Wouldn't it be feasible for the chartplotter and displays to calculate and display "True Wind" and "Boat Speed" from the inputs that include Apparent wind from the wind sensor, GPS boat speed, and current set and drift from the chartplotter/heading sensor. In that sense, you would not be dependent on a paddlewheel input and you can have your True Wind heading and boat speed displayed to achieve the targets in the polars.

Just askin'

 

[markwbird]

They only way to get an accurate true wind speed is to use the velocity over ground when calibrating it. Period. Anything else is not the true wind speed and, therefore, wrong.

 

[Davidasailor26]

Wouldn't it be feasible for the chartplotter and displays to calculate and display "True Wind" and "Boat Speed" from the inputs that include Apparent wind from the wind sensor, GPS boat speed, and current set and drift from the chartplotter/heading sensor. In that sense, you would not be dependent on a paddlewheel input

The chart plotter can calculate “ground wind” from the apparent wind and GPS speed / COG. If the chart plotter were pre-programmed with current speed and direction it could use that to do the conversion from ground wind to true wind. The problem is that the only way to calculate current speed and direction on the fly is to compare heading with COG and speed through water with SOG.

I haven’t wrapped my head around your other questions or read the article yet. I’ll check back in when I do. One other nitpick though -

My understanding is that the goal of VMG is to maximize your speed over ground to the mark, which is set on ground and doesn't move with the current (unless it comes loose).

Strictly speaking, as long as you’re inside the lay lines, VMG is usually measured as the component of your speed directly into the wind. That will be different than speed toward the mark, unless you’re right in the middle of the lay lines. If you’re close to the starboard layline your GPS will report a higher VMG on starboard tack than port, but really the sailing VMG is the same.

 

[PaulK]

Could you heave to and get the wind speed with the boat stopped? Since you’re not racing it shouldn’t be a big deal.

 

[Scott T-Bird]

They only way to get an accurate true wind speed is to use the velocity over ground when calibrating it. Period. Anything else is not the true wind speed and, therefore, wrong.

Well, that argument has been made and rejected by sailors and I understand the reason why (not saying you're not a sailor! ). After reading the article that @DrJudyB provided, it's clear that the sailor definition of "True Wind" is in relation to the boat's movement through water and it is distinct from "Ground Wind", which is what you are defining as true wind. I accept the definitions as they are accepted by sailors, that there is a distinction between True Wind and Ground wind. Afterall, it's just semantics and I'll recognize the distinction.

 

[Scott T-Bird]

Could you heave to and get the wind speed with the boat stopped? Since you’re not racing it shouldn’t be a big deal.

No, because the wind is constantly changing in direction and speed.

 

[JohnShannon]

I think they use "true wind" which is perhaps poorly named as wind it seems it would be the wind measured by a drifting buoy. The issue is that you want to ignore the effects of current which can be significant. A 3 knot current is pretty important if you have a 6 knot hull speed. So if "true wind" is wind with respect to the water frame of reference I guess it works the best? If you are racing or really just trying to get anywhere the VMG to your waypoint or mark is I think the relevant number. So maybe you duck out of the current and have lower boat speed with respect to the water, but higher boatspeed with respect to your destination? I find it all very confusing

 

[Scott T-Bird]

The chart plotter can calculate “ground wind” from the apparent wind and GPS speed / COG. If the chart plotter were pre-programmed with current speed and direction it could use that to do the conversion from ground wind to true wind. The problem is that the only way to calculate current speed and direction on the fly is to compare heading with COG and speed through water with SOG.

I haven’t wrapped my head around your other questions or read the article yet. I’ll check back in when I do. One other nitpick though -

Strictly speaking, as long as you’re inside the lay lines, VMG is usually measured as the component of your speed directly into the wind. That will be different than speed toward the mark, unless you’re right in the middle of the lay lines. If you’re close to the starboard layline your GPS will report a higher VMG on starboard tack than port, but really the sailing VMG is the same.

Yes, the chartplotter already calculates ground wind easily based on GPS input. The trick is converting ground wind to true wind with either programmed or measured current set and drift. I know that the logical response is "just use the paddlewheel speed, stupid". The problem I have with that is even the article says that if the paddlewheel speed is off by just a little, the true wind calculation is useless.

The paddlewheel simply isn't reliable in my opinion. I'm not putting it in and pulling it out every time I sail. It's not happening - not with all the other chores that go with opening and closing a boat. And how often do you calibrate? Where can you calibrate? I've read the directions. It's rare to even find the conditions when you can calibrate accurately and who's going to spend the hours running a 1 or 2-mile course to make the calibration. Not going to find a location that doesn't have current and the repetition that is required to compensate for current is just too tedious. Again, it's not happening. Besides, it seems impossible to set the thru hull in a position so it is calibrated equally for both tacks. If I put the paddlewheel in just for 6 races per year, perhaps. But that's just silly.

The article did mention the CS4500 speed sensor, which seems like it should be much better technology. It seems that it isn't also a depth transducer, though. It would not be a good replacement for the DST800 that I have now. All considered, I think a paddlewheel is not reliable and the article says that true wind is not useful unless the speed through water input is accurate. Therein lies the crux of my argument that there must be a better way to compute true wind via the input of set and drift.

 

[Scott T-Bird]

I think they use "true wind" which is perhaps poorly named as wind it seems it would be the wind measured by a drifting buoy. The issue is that you want to ignore the effects of current which can be significant. A 3 knot current is pretty important if you have a 6 knot hull speed. So if "true wind" is wind with respect to the water frame of reference I guess it works the best? If you are racing or really just trying to get anywhere the VMG to your waypoint or mark is I think the relevant number. So maybe you duck out of the current and have lower boat speed with respect to the water, but higher boatspeed with respect to your destination? I find it all very confusing

The way I see it, the purpose for the True Wind conversion is mostly so that you can read and interpret the polars accurately to maximize VMG. There doesn't seem to be any other reason for knowing True Wind. If you just want to know wind speed and direction, ground wind speed should be fine, and you would use your tell tales for trimming. You know that set and drift are going to affect your angles and speed, and you can compensate intuitively without actually reading True Wind on a display.

 

[Will Gilmore]

There doesn't seem to be any other reason for knowing True Wind.

True winds is calculated during navigation. It is also what is relevant when judging your performance relative to past and future performance. If you want to know how your sailing is improving, it is meaningless to judge based on a GPS measurement. Current, windspeed and directions all vary from sail to sail. With True Wind, instead of Ground Wind, your comparing similar effects on your boat to get a better idea of your progress or growth or noting something wrong.

For navigation, unless you always sail line-of-sight in a bay or near shore, you want to know, in advance, what your expected True Wind will be. This is determined by the same vector addition shown in the article. It is an approximation, because the data you base it on is a forecasted wind and a seasonal average current, but that's what you use. You can adjust when underway, to get a more accurate reading. This gives you a better idea of course heading and expected time.

Could you heave to and get the wind speed with the boat stopped? Since you’re not racing it shouldn’t be a big deal.

Heaving to does not stop your boat. It only slows it relative to the wind and waves. Don't heave to near obstacles, go below, and expect to stay where you are.

The paddlewheel simply isn't reliable in my opinion.

There are more consistent instruments that are unaffected by tack:

A tafftrail log, for instance.
Another method is to time the passage of bubbles or a piece of t.p. as it floats from bow to stern. Use your LWL and a stop watch to get a pretty accurate STW. Make up a calculated chart of times. I used it on the OB pulling boats and my DR navigation was quite accurate, even in the Maine coastal fog.

-Will (Dragonfly)

 

[Stu Jackson]

The problem I have with that is even the article says that if the paddlewheel speed is off by just a little, the true wind calculation is useless.

Scott, that is not what he is saying. Like any calculation, GIGO. If any of the inputs is incorrect BECAUSE OF THE QUALITY OF THE MEASURING DEVICE, the answer will, of course, be incorrect. He does, after all, sell instrumention and the article is one that promotes the "higher" quality of his particular gear.
One should put side "cleaning knotmeters" and the like, and, rather, focus on the importance of the differences in the real definitions of the resulitng input and calculations to the two different resultants. There is a difference between ground and actual, and it's more than as you said "senamtics." (and spelling, too ).
Another way to say this is: "IF EVERY SINGLE ONE OF YOUR INPUTS (i.e., instruments) was PERFECT..."
Then let's have the discussion about ground and actual sailing in a current. That's all, pretty simple then.

 

[jon hansen]

i tried to understand his new to me made up terms. his changing definitions. i know allot about how the wind moves across the surface of the earth. i'm a student of such things. he is trying to sell a computer that has one needle to point you in a direction. BORING. what a waste. open your eyes, get your mind out of the cockpit and sail the damm boat.
i gave up on him as a sailor of any skill when he said something about the crew allowing him to sail to weather or not.
yeah, not much of a fan of robot sailing.

 

[jon hansen]

no computer can see upwind, read the signs, feel the helm, ... just saying

 

[jon hansen]

wana learn to read the wind, read Bethwaite

 

[DrJudyB]

Could you heave to and get the wind speed with the boat stopped? Since you’re not racing it shouldn’t be a big deal.

If you let the sails luff and let boat drift with the current and observed the wind, you’d have a good approximation of true wind speed. It would be pretty accurate in light winds, but in high wind, say over 5-10 kts, the boat would “sail” on her hull.

 

[jon hansen]

i do not accept his terms / definitions for the winds we sail in.

 

[BarryL]

Hey,

I think you are making way too big a deal from this. You understand the difference between True and Apparent wind. That is the main point. It doesn't really matter if you use GPS speed (SOG) or paddlewheel speed (STW). Assuming your paddlewheel isn't fouled, the calculated true wind will be pretty much the same for SOG or STW.

On my boat I use SOG for my wind instrument. The boat I usually race on uses STW for his wind calculations. It doesn't really make any difference.

HINT - All of the good racers I know only care about apparent wind. Yes, they want to know true wind as well but the sails are only trimmed for the apparent wind. We select the sail to use based on apparent wind. The helmsman steers to the wind he is seeing, and the helm and trimmers communicate based on what they FEEL, not by what the instruments show.

I understand polar tables and targets and I have generated them for my boat. During a race they are just about useless. We are always trying to get the boat moving as fast as possible. If we're sailing at 6.2 kts and the target is 6.5 it's not like we can say, "oh, we're down .3 kts, trim better, or hike harder, or adjust ...." We're always sailing as fast as possible.

My last point - To me, it seems like the best sailors spend way more time looking OUTSIDE the boat than INSIDE the boat (concentrating on the instruments). Those guys seem to recognize the shifts faster, get the boat positioned to take advantage of current, have the best mark approaches, etc. To have a real successful team you need both types. Personally I'm better with the electronics. I know where we're supposed to be going after a rounding, how long until we get to the mark or need to gybe, or where the predicted currents are, etc.The other guys are way better at feeling the wind, detecting puffs (and trimming and steering accordingly). That's why we're a good team.

Barry

 

[Davidasailor26]

i do not accept his terms / definitions for the winds we sail in.

Jon, How else do you propose that we measure our boat’s performance to see objectively how well we’re doing? I get what you’re saying about old salts being able to do it by feel, but that doesn’t seem very objective. If I’m sailing in a 2 knot current going 6 knots through the water is that good or bad? The best way I know of to objectively measure that is to know what the true wind speed (relative to the water) is.

 

[jon hansen]

why not read the speed of the true wind with your eyes!

 

[jon hansen]

thank you Barry!