Usefulness of True Wind Data for Sailing Performance

[Dockhead]

In another thread, we were discussing the nature and usefulness of True Wind data, with one of our esteemed OpenCPN developers, Rhythm Doctor. He requested to move this part of the discussion out of the OpenCPN forum:

Originally Posted by RhythmDoctor

If you’re fine with it, that’s great. For this thread, I just would prefer to focus on “How to do it,” and let the discussion of whether it’s a good thing to do happen in a different thread.

FWIW, I am aware of the extreme instability of apparent wind readings when heading DDW. But I’m unconvinced that using calculated true wind provides anything more than false confidence. I see it as GIGO. But I’ll continue to read the arguments as time allows.

RhythmDoctor, like many of us, has problems with his paddlewheel log, and is trying to get OpenCPN to accept SOG as a substitute for STW. We were discussing the pros and cons of doing that, inasmuch as you won't get actual True Wind with SOG -- something close to it when there's no current, but something quite far from True Wind when there is current.

And so the question arose -- do we really need True Wind anyway?

I suggested that there are a number of situations where True Wind is really important. Racers will have a much longer list (and it would be interesting to hear about it), but for me, at least, True Wind is of supreme importance when running DDW or nearly so, and also for tactics for tacking.

RhythmDoctor asked whether our True Wind is not so unreliable that we are just getting false confidence from it.

Given the unreliability of paddlewheel logs, probably there is something to that, but on my boat, at least, the pilot works very well indeed in wind mode and using True Wind going DDW, steering a lot better than I could do by hand. I've done thousands of miles wing on wing like this.

It would be interesting to hear others' views on this, and useful for all of us and not just for RhythmDoctor.

[Paul Elliott]

I agree with the GIGO principle when it comes to True Wind calculations. On our boats, the fundamental inputs are Apparent Wind (speed and angle): AWS and AWA. When running deep downwind, especially on a boat that can sail fast, the AW is low and easily overwhelmed by the motion at the top of the mast (most of us have seen the windex spinning in 360-degree circles during light air). With this wildly fluctuating AW input, it really doesn't matter (in theory) how you calculate True Wind.

Yes, to do a proper water-referenced True Wind you should use Speed Through Water (STW) and Heading in your AW to TW calculations. You should also try to compensate for leeway, which is often calculated based on boatspeed, but this assumes a properly-trimmed boat.

If you are not under the influence of significant currents, then GPS-derived COG and SOG are fairly adequate substitutes for STW and HDG. These are also used to calculate ground-referenced TWS and TWD.

Dockhead, I suspect that any improvement you see in downwind autopilot function when using TWD is based on filtering, and not the conversion of AW to TW itself. I've written some programs that do a reasonably good job of compensating for masthead motion by using vector math on the combined AWA and AWS, but this requires fairly slow filtering. The high-end racers use inertial sensors (pitch, roll, yaw, heave, etc.) to compensate for the boat motion factors when using AW/TW as an autopilot input.

[estarzinger]

it depends 100% on the particular users gear and knowledge/interest inn performance.

we had high end B&G systems, and knew our polars, and I calibrated every 6 months or so, and kept the paddle wheel clean (less frequent work in cold water btw). With that, true wind, in lightish winds, provided information which allowed us to accurately (or rather to be precise, much more accurately than we could without true wind) sail our polars downwind - a small but noticeable incremental gain.

In stronger winds, we switched from polar sailing to double jib DDW sailing where true was less important.

but I would think that (first) situation represents a very small fraction of the cruising population sailing time, and for pretty much the rest true wind is not very useful.

Beth liked to have true because she was a data person, and could then log/track the actual wind for use in her various analysis and articles.

[Dockhead]

Originally Posted by estarzinger

it depends 100% on the particular users gear and knowledge/interest inn performance.

we had high end B&G systems, and knew our polars, and I calibrated every 6 months or so, and kept the paddle wheel clean (less frequent work in cold water btw). With that, true wind, in lightish winds, provided information which allowed us to accurately (or rather to be precise, much more accurately than we could without true wind) sail our polars downwind - a small but noticeable incremental gain.

In stronger winds, we switched from polar sailing to double jib DDW sailing where true was less important.

but I would think that (first) situation represents a very small fraction of the cruising population sailing time, and for pretty much the rest true wind is not very useful.

Beth liked to have true because she was a data person, and could then log/track the actual wind for use in her various analysis and articles.

Did you ever use your pilot in wind following mode?

I find this extremely useful in two situations -- hard on the wind, and DDW or close to it. I sailed thousands of miles wing on wing (no spinnaker! ) like this.

Autopilots use True Wind for sailing downwind in wind following mode.

[Dockhead]

Originally Posted by Paul Elliott

. . .

Dockhead, I suspect that any improvement you see in downwind autopilot function when using TWD is based on filtering, and not the conversion of AW to TW itself. I've written some programs that do a reasonably good job of compensating for masthead motion by using vector math on the combined AWA and AWS, but this requires fairly slow filtering. The high-end racers use inertial sensors (pitch, roll, yaw, heave, etc.) to compensate for the boat motion factors when using AW/TW as an autopilot input.

It could be. But I think there is also a very sharp wedge between True and Apparent when you are DDW or close to it. I mean that a tiny change of True Wind angle will produce a large different in Apparent Wind angle. This makes it very difficult to steer to Apparent Wind. One moment you are 10 or 15 degrees off DDW and with the tiniest deviation of course suddenly you are by the lee. If you look at True Wind, the change in wind angle corresponds better to changes in course. I suppose damping has something to do with this. And you're right of course that masthead motion really screws everything up in light wind, and the True Wind calculation damps that out usefully. But I can adjust the damping for Apparent Wind also in my B&G system, and my ultrasonic wind instrument doesn't have any inertia effect problems related to masthead motion. YMMV.

As far a GIGO goes -- obviously, this is a fact, but the degree of degradation of the output data is a non-obvious function of the degree of degradation of the input data. In other words, how bad does the STW data need to be, to make True Wind useless? I don't know. I know that True Wind was still very useful with my old instruments, including an old Airmar paddlewheel and Raymarine mechanical anenometer, especially with a reasonably clean paddlewheel. But the last five years I have been doing both STW and wind with ultrasonic transducers (Airmar CS4500 and Maretron WSO100) providing much higher quality data, so in my particular case True Wind is quite good, which in my particular case, is quite useful in a number of different situations. YMMV.

[Paul Elliott]

Originally Posted by Dockhead

One moment you are 10 or 15 degrees off DDW and with the tiniest deviation of course suddenly you are by the lee. If you look at True Wind, the change in wind angle corresponds better to changes in course.

Yes, but the TW numbers are derived from AW. If you are by the lee on AW, but not on TW, then there is some heavy filtering going on since no possible instantaneous calculation, even using STW, will switch wind from port to starboard.

I believe my old B&G autopilot, even when in Wind mode, uses the compass heading to correct for fast boat heading changes. It also uses STW and of course AW inputs. I don't know if the AW>TW conversion is a factor at all. In theory it shouldn't really matter since the autopilot has all the raw data.

If your point is that the STW input is important to AP performance, I can easily believe that. Compared to STW, the GPS SOG is usually quite filtered and this would lead to poorer dynamic performance.

[Paul Elliott]

Originally Posted by Paul Elliott

If your point is that the STW input is important to AP performance, I can easily believe that. Compared to STW, the GPS SOG is usually quite filtered and this would lead to poorer dynamic performance.

And I should add that STW is also used by the AP to scale the rudder gain factor. So even ignoring any possible AW -> TW conversion, STW is important, and probably superior to SOG.

For my sailing, The main reason I even look at TW is for the TWD calculations, which I use in my weather routing. TW and STW are also used to see how well I am approaching my polars (such as they are). But the boat and sails see AW, so that's what I trim with. Actually, I trim by looking at the sail shape, since wind at the masthead may not be the same as wind closer to the water, but AWA and AWS are a useful guide.

[RhythmDoctor]

Sorry to come in late, I'm just getting caught up with this thread.

First, as someone who sailed in a river with currents of 3-4 knots, I know the effect currents can have on tacking decisions (though that boat had no wind transducer). Currently I sail in the Chesapeake, with very weak currents and fast fouling of paddlewheels. Hence my desire to use SOG instead of paddlewheel. I do tacking judgments "by eye" with local knowledge of currents (and OpenCPN current estimates). I'm not a racer, so if I misjudge, I just do another tack. I know that can be very different from ocean sailors on a cutter rig with all sails up, where an extra tack can be a PITA.

Originally Posted by Paul Elliott

Yes, but the TW numbers are derived from AW. If you are by the lee on AW, but not on TW, then there is some heavy filtering going on since no possible instantaneous calculation, even using STW, will switch wind from port to starboard...

What he said! This is exactly what I meant by "false confidence" and GIGO. True wind is a calculated value, and is only as good as the apparent wind that it's calculated from. If you add in a relatively steady STW vector while sailing DDW, your little direction needle will bounce back and forth less severely (since it's closer to 180°), but it will still bounce back and forth.

There have been several good points made here, and I have a lot to learn on this topic, so those points are duly noted. The possibility of using HDG sentences off the autpilot could help to reduce some of the bouncing of TWA, especially if a swell off your quarter is causing some of the bouncing. Of course, if that's what's happening, you need to look up from your console and rig a preventer (or, in my case, either bear off to avoid DDW or drop the main and pole out the genoa for DDW sailing). I wonder if integrated wind/autopilot systems actually correct for HDG fluctuations when autopilot is set in wind mode.

Also, as for dynamic gain adjustments based on boat speed, I think my old Raymarine ST4000+ autopilot uses SOG if STW is not available. I'll have to check. I've had ST4000+ on two different boats, and I hooked both of them up to take GPS input through the NMEA input terminal. (Actually, I did something fancier, and hooked a Bluetooth receiver to the NMEA input, and a Bluetooth transmitter to my GPS puck.)

I have zero experience with a paddlewheel, so take this with a grain of salt: I wonder whether a paddlewheel helps stabilize TWA in lumpy conditions. It seems like, whether close-hauled into chop or being overtaken by a swell, there might be large deviations in the speed of surface water past the boat that are not reflective of the boat's actuall progress through the water. Couldn't this increase the fluctuations in calculated TWA?

[44'cruisingcat]

Originally Posted by Dockhead

Did you ever use your pilot in wind following mode?

I find this extremely useful in two situations -- hard on the wind, and DDW or close to it. I sailed thousands of miles wing on wing (no spinnaker! ) like this.

Autopilots use True Wind for sailing downwind in wind following mode.

For wing on wing, TWA works great. It prevents major course changes due to shifts in AWA when surfing, which would result in the mainsail being backwinded frequently.

However, with the kite up, I switch to AWA. Not so worried about the kite going by the lee from time to time, but certainly want to avoid big shifts in AWA and the resulting big increases in AWS.

[Dockhead]

Originally Posted by Paul Elliott

Yes, but the TW numbers are derived from AW. If you are by the lee on AW, but not on TW, then there is some heavy filtering going on since no possible instantaneous calculation, even using STW, will switch wind from port to starboard.. . .

You are absolutely right about being by the lee -- bad example and my bad.

But I think one of the main reasons for steering by True Wind when DDW or close to it is the fact that you (or your pilot) will not hold a steady course at all if you steer by Apparent. Small variations in wind speed or boat speed can make big changes in AWA when you're near DDW. That's not a damping issue, although damping issues may add to the problem.

This effect, as a matter of geometry, is strongest when just slightly off DDW, which is where I like to be when sailing wing on wing without the pole, with the main just 5 degrees or so by the lee.

We do sail in Apparent Wind and Apparent Wind only -- that is true. But Apparent Wind can become extremely flukey near DDW, and impossible to steer to. That's where True can be worth its weight in gold.

[Dockhead]

Originally Posted by 44'cruisingcat

For wing on wing, TWA works great. It prevents major course changes due to shifts in AWA when surfing, which would result in the mainsail being backwinded frequently.

However, with the kite up, I switch to AWA. Not so worried about the kite going by the lee from time to time, but certainly want to avoid big shifts in AWA and the resulting big increases in AWS.

That makes sense.

[RhythmDoctor]

Originally Posted by 44'cruisingcat

For wing on wing, TWA works great. It prevents major course changes due to shifts in AWA when surfing, which would result in the mainsail being backwinded frequently...

But isn't this just just appearances? Since TWA is derived from AWA, it's not really preventing anything. I think it's just making your display bounce around less violently. The wind is doing what it's doing regardless of what the instrument needle does. TWA just makes it look less chaotic - which I realize can be helpful if you're hand steering based on the motion of that needle.

Some have mentioned that the autopilot automatically switches to TWA in downwind headings. However, I suspect it's impossible to know what input data the AP is actually using in its control algorithms. It's possible that the autopilot is merely displaying TWA for the soothing reasons that you guys are describing. This could be false confidence, unless you're looking up and also relying on your eyes and ears to supplement your instruments. You should never rely on only one source of information.

EDIT: Let me add a few words to further explain why I call this "false confidence". I suppose it's soothing to display TWA because it's less chaotic than AWA. It's nice to see on a gauge that if your boat is bouncing around, the true wind is not actually shifting. However, if your boat is bouncing around due to swells and sea state, you may actually be at risk of a crash gybe, since your sails get pushed around by apparent wind changes resulting from your boat's movement. This is what I suggest that using TWA may be false confidence.

[Paul Elliott]

Originally Posted by Dockhead

But I think one of the main reasons for steering by True Wind when DDW or close to it is the fact that you (or your pilot) will not hold a steady course at all if you steer by Apparent. Small variations in wind speed or boat speed can make big changes in AWA when you're near DDW. That's not a damping issue, although damping issues may add to the problem.

I do understand your point. AFAIK my B&G "Network" series AP (autopilot) doesn't have a AW/TW option, it just steers to "Wind" or "Heading" (I haven't hooked up waypoint steering). The AP gets STW from the paddlewheel sensor, wind from the wind sensor, and heading from the electronic compass. I don't know if it tries to do the AW/TW conversion internally, but it does have all the info it would need.

But it seems to me that the AP really *ought* to steer to AW, since that's what your sails are trimmed for. Yes, major speed variations do require trim changes as well, and if you're racing hard you ease/trim on every puff or push. But in more casual sailing you try not to fuss with the sheets more than you have to and in either case keeping a constant AW angle seems like a good idea. After all, when hand-steering we head up in the puffs as the AW moves forward, and ease off in the lulls.

If we want to sail a straight line then we switch the AP to compass or waypoint mode.

Of course in practice not all APs are the same, and some do better than others in dynamic situations. Perhaps some APs prefer TW as an input?

[Paul L]

Originally Posted by Paul Elliott

I do understand your point. AFAIK my B&G "Network" series AP (autopilot) doesn't have a AW/TW option, it just steers to "Wind" or "Heading" (I haven't hooked up waypoint steering). The AP gets STW from the paddlewheel sensor, wind from the wind sensor, and heading from the electronic compass. I don't know if it tries to do the AW/TW conversion internally, but it does have all the info it would need.

But it seems to me that the AP really *ought* to steer to AW, since that's what your sails are trimmed for. Yes, major speed variations do require trim changes as well, and if you're racing hard you ease/trim on every puff or push. But in more casual sailing you try not to fuss with the sheets more than you have to and in either case keeping a constant AW angle seems like a good idea. After all, when hand-steering we head up in the puffs as the AW moves forward, and ease off in the lulls.

If we want to sail a straight line then we switch the AP to compass or waypoint mode.

Of course in practice not all APs are the same, and some do better than others in dynamic situations. Perhaps some APs prefer TW as an input?

If your on a boat that surfs having the AP steering to AW drives the boat in the wrong direction. We use AW when close hauled/close reaching. The rest of t h e time we use compass course on our AP.

[Paul Elliott]

Originally Posted by Paul L

If your on a boat that surfs having the AP steering to AW drives the boat in the wrong direction. We use AW when close hauled/close reaching. The rest of t h e time we use compass course on our AP.

I'm pondering this... My boat "surfs", but it doesn't start planing and speed up the way a more modern racing boat does, so the AW doesn't move very far forward. Sailing deep downwind with the sym spinnaker I don't want the AW to move too far forward else we end up on our ear and possibly overcorrecting into a round-down and wipe out.

On a hotter boat, I can see that you would be happy to let the AW move forward, where your polars are faster. In this case an AP that steered to TW might be the better choice.

With the spinnaker up we only use the AP in gentle conditions -- we don't have any of the fancy inertial compensation stuff feeding the AP. In stronger winds we hand-steer with one eye on the wind dial and one eye on the binnacle compass -- and both eyes on the windex and the leech of the kite. That takes a lot of eyes! And the wind on the back of our neck, and the feel of the waves as the stern starts to rise, and the heel of the boat and the pressure on the helm and the sound of our wake. That's a lot of inputs and processing, even for a human.