True Wind Discussion Redux

[jon hansen]

scott, one of my very favorite drivers was dennis connor. find some old video or clip from his cup days, the guy moves the wheel so little yet keeps the boat in the groove or perfornance zone like few others.
on the ships the old story was "the laziest wheelsman is the best wheelsman" , he kept the ship going straight with only the slightest wheel commands.
you overdrive a ship you can get it snaking through the water, oh they can flex big time.
same with sailboats, i'll bet almost all of you oversteer big time. ( my standard bet is a nickel. )

if you find the sweet spot on the wheel for any given course, hold the wheel there with only subtle variations.
again, watch dennis he's a true master. i teach what dennis does.
i'm always instructing crew as i was instructed.
and for those that say 'if you ain't racing man it don't matter' Pushaw

 

[jon hansen]

"you steer with the sails, you trim with the wheel"

 

[jon hansen]

but gentlemen, don't get me wrong. just messing around with boats is all great. those videos of terry cox sneaking around up in the great NW are spectacular. i get it. i love that too

thank you terry for all those great videos. i love watching those videos

 

[dlochner]

So now I am going full circle and back to relying on an accurate paddlewheel.

This is an issue with all instruments, how accurate are the instruments? Every instrument has error, the more inputs the algorithm has the more error it has. The error for all instruments, wind speed, water speed, compass heading, GPS COG and SOG all have error and the errors are cumulative. And simply because of the delay in processing the information, the news is yesterday's news and it is not all that accurate.

There is an illusion of accuracy because the instruments can give readings to the 1/100 of whatever, so we think we have accurate readings, but we don't have any where near that precision. Instrument data is simply a quantified and easily recorded display of what we should know using our 5 senses.

 

[Will Gilmore]

Pushaw

Unfortunately, I am the antithesis of the lazy helmsman. My whole life, the Old Man has tried to hammer into my head, "Don't over steer!" I'm sure we've put in nearly twice the miles at sea when ever I stand watch than the same trip without me. I know not to over-steer, I've always known not to do that, but, at least with a wheel, I suck at managing to do what I strive to do.

-Will (Dragonfly)

 

[Will Gilmore]

adding a heading sensor to the network, the current set and drift is accessible. But I'm not sure how the chartplotter knows set and drift (it is displayed when the heading sensor is active).

Adding a heading sensor shows the computer what the difference is between the direction the boat is pointing and the direction it is traveling. GPS will tell it what way it is traveling. Now it just needs to break out the forward direction component of the direction travelled and the remainder is drift and current set. If your computer is able to break out drift from current, I couldn't say how it does that. Perhaps it estimates drift based on wind speed and tack, the remainder is current set. Or, it uses published current data and extrapolates drift from that.

-Will (Dragonfly)

 

[Davidasailor26]

From what I’ve seen on my chartplotter it’s using the comparison between heading and COG, and STW vs SOG to determine current set and direction. You can’t really determine current by heading and course alone. If you are going directly into or with the current, for example, your heading will match your COG, but there is still current to account for.

I don’t think the plotter does any math to account for leeway. Maybe it assumes some, but as far as I can tell it just does the math as if the boat’s movement through the water we’re based strictly on its heading, not sliding sideways through the water. Leeway is not constant and not really measurable (unless you know the current, or have some kind of sideways facing paddlewheel), so there’s going to be some imprecision introduced there.

 

[Captain Larry-DH]

All of the good racers I know only care about apparent wind.

I sailed with Cam Lewis for a day of instruction, aboard another trimaran, and all he cares about is true wind. I even tried to debate him because I think only about apparent wind. He wasn’t having it. (If you don’t know who he is use Google.). I remember only one thing from that day as a take-away, and it was how to make the boat go in light air.

 

[Will Gilmore]

Current induced wind is called "apparent wind" in the Artemis video.

-Will (Dragonfly)

 

[Davidasailor26]

Current induced wind is called "apparent wind" in the Artemis video.

-Will (Dragonfly)

Not exactly. “That movement generates apparent wind”. The current moves the boat, and the resulting movement of the air relative to the boat is the apparent wind. To extend their bike riding analogy a bit - if you put your bike on a hill and start to roll down, you feel apparent wind from the air flowing past you, not from the air relative to the ground.

Also, now I’m having flashbacks of the original thread about the Artemis video, so thanks for that.

 

[Davidasailor26]

By the way, here’s an article that gives some insight on how chartplotters calculate current - Pro navigator's tips on how to use the ocean's currents.

In particular, see the quote from Navico -

On B&G Instruments tide is calculated from the difference between boat speed and SOG/heading and COG, whereas in Deckman the tide is calculated by comparing dead reckoning (course and boat speed) and the GPS fix position. In this case, it is really important to have calibrated boat speed, heading and leeway (if enabled). In Deckman the output can be adjusted to average over a specified number of minutes depending on the user’s preference.

Leeway can be taken into account in current calculation but is not necessarily measured on a simple instrument or software package. However, when more boat speed devices come onto the market that can measure leeway accurately, there will be improvement in tide and drift calculations.

So calculating current always requires input for the boat’s heading and speed through the water, and compares that with either instantaneous SOG and COG or change in position over time. Since there’s no way to calculate current without STW, you need an accurate paddle wheel for that.

 

[Scott T-Bird]

By the way, here’s an article that gives some insight on how chartplotters calculate current - Pro navigator's tips on how to use the ocean's currents.
So calculating current always requires input for the boat’s heading and speed through the water, and compares that with either instantaneous SOG and COG or change in position over time. Since there’s no way to calculate current without STW, you need an accurate paddle wheel for that.

That pretty much sums up my suspicion! Any way you want to arrive at the answer, TWS & TWD depends on that little paddlewheel or some other devise that measures boat speed accurately. I like that taffrail log that @Will Gilmore posted! I would like to have one of those! In the meantime, I just might have to pull the ducer out after every sail and see just how much of a nuisance that is.

 

[shemandr]

... Or, it uses published current data and extrapolates drift from that.-Will (Dragonfly)

Notoriously inaccurate. The best way I know to estimate current is to sail by a lobster pod, crap trap, conch trap and observe with your eyeballs how fast and what direction the current is moving.
In another vane, I do not understand the obsession with true wind. 'splain to me Paco. I understand it's favored by real racers but I don't get it. To be it's like navigation using True North. You're supposed to do it but it's so much easier to use Magnetic North. No conversion. Why think of something that is at best a construct.
By the way, I think the wind you experience just drifting in the current, is known as "Current wind" and is incorporated in the term "Apparent wind."

 

[DrJudyB]

That pretty much sums up my suspicion! Any way you want to arrive at the answer, TWS & TWD depends on that little paddlewheel or some other devise that measures boat speed accurately. I like that taffrail log that @Will Gilmore posted! I would like to have one of those! In the meantime, I just might have to pull the ducer out after every sail and see just how much of a nuisance that is.

I think you will find it very interesting to watch your speed through the water when going upwind closehauled (or on a very deep run downwind). It's a Goldilocks thing: Sometimes upwind you're pinching, and sometimes you're footing, and sometimes you're doing it just right.

When we put the paddle in or out at the dock, about a cup of water comes in. We use a synthetic chamois cloth to wipe it up. Be sure to have the alignment tab facing forward on the transducer before you pull the plug.

 

[Will Gilmore]

I like that taffrail log that @Will Gilmore posted! I would like to have one of those!

They are notoriously susceptible to shark attack.

-Will (Dragonfly)

 

[markwbird]

That pretty much sums up my suspicion! Any way you want to arrive at the answer, TWS & TWD depends on that little paddlewheel or some other devise that measures boat speed accurately. I like that taffrail log that @Will Gilmore posted! I would like to have one of those! In the meantime, I just might have to pull the ducer out after every sail and see just how much of a nuisance that is.

No. You need SOG to calculate TWS. That is all. If you want to know the speed of the current then you need the paddle wheel so that you can compare speed through the water to speed over ground. These are two completely separate things. Also, that video is misleading. You may be able to turn the boat around and go upriver for awhile at 30 mph but will come to a stop without some energy source moving it. You don't get something for nothing.

 

[Davidasailor26]

You need SOG to calculate TWS.

You need SOG to calculate speed of the wind over ground, which most people call TWS, but the original Panbo article doesn’t. To calculate speed of the wind relative to the water, which is what racers care about, you need either speed of the boat through the water, or both SOG and current.

Also, that video is misleading. You may be able to turn the boat around and go upriver for awhile at 30 mph but will come to a stop without some energy source moving it. You don't get something for nothing.

We’ve already been through 301 posts on this. The water moving relative to the air is the energy source. See Wow...

 

[Will Gilmore]

We’ve already been through 301 posts on this. The water moving relative to the air is the energy source. See Wow.

Yes. I resisted the idea at first, but came around when I realized the system did have a continuous source of energy, just as no sailing could happen in a ten knot current with a ten knot wind going the same direction. That is a system with no energy relative to the boat. Ten knots true wind but zero knots relative wind or apparent wind.

Amazing what a good thought experiment can reveal.

However, the video is misleading, but only in that it shows a sailboat sailing head on into the wind.

-Will (Dragonfly)

 

[markwbird]

You need SOG to calculate speed of the wind over ground, which most people call TWS, but the original Panbo article doesn’t. To calculate speed of the wind relative to the water, which is what racers care about, you need either speed of the boat through the water, or both SOG and current.


We’ve already been through 301 posts on this. The water moving relative to the air is the energy source. See Wow...

No. Speed over ground is what racers care about. I’m not sure how this is even a discussion.

 

[Davidasailor26]

No. Speed over ground is what racers care about. I’m not sure how this is even a discussion.

I respectfully disagree. From the standpoint of measuring the boat’s performance the speed through the water, and using the speed of the wind relative to the water, are what matter. This is what your polars measure.

You don’t have to take my word for it. Here’s a write up from an instrument manufacturer on the differences and some reasons why measuring water-relative wind speed is preferred:True wind - Ockam Sailing Instruments. The links on that page also go to some good discussions of the relationship between current and wind. Ockam says they default to using water relative wind speed. Navico (B&G, Simrad, Lowrance) does the same as well, allowing the user to switch to ground relative wind, but burying that way down in the settings, not making it the default.