Paddle wheel speed sensor, who needs ‘em!

[Dockhead]

Originally Posted by hjohnson

Well, except that the forces on the boat (and on the water) aren't related to ground wind, they're related to wind in relation to the water.

The example I always use is First Narrows in Vancouver, BC. During a decent Ebb tide, you can easily have 3 knots+ of current under the Lions Gate. At the same time, if you have a 12 knot westerly, what are the forces acting on your boat? It's much more like being in 15 knots, which makes for a washing machine experience in the shallows there. Conversely, if you have a 12 knot easterly, while on a 3 knot ebb, it's a pleasant sleigh-ride.

But in effect, the speed of the wind w.r.t. the shore is irrelevant. The only thing that matters is the speed of the wind w.r.t. the water.

Indeed! We sail in the sheer between air and water; the ground is irrelevant. In still water, SOG may approximately work, but with a current running all calculations of True Wind angle and wind velocity go to the dogs. There is no data more important for sailing than STW, but unfortunately it is the very devil to measure it accurately.


I spent some years with an Airmar CS4500 ultrasonic transducer which worked reasonably well, but failed after 7 or 8 years. I now have the UST850 which is the same technology (apparently) but in native N2K. It seems to be worse than the old CS4500. I've just bought the elusive DX900+ which is electromagnetic rather than ultrasonic, and which can directly measure leeway. I'll post here after I've had a chance to use it.


True wind is really really hard to calculate accurately. I've been chasing it for years. This year I will bring a B&G H5000 Hercules computer and new wind sensor to the fight. I'm thinking about doing an RORC ocean racing series in 2023, so really hoping to get this nailed down this year.

Originally Posted by PineyWoodsPete

I've been a sailor for over 60 years, and never heard "True wind" speed and vector being defined as RELATIVE to water. "True wind" - sailing or otherwise can only be defined as relative to the immovable earth SOG/COG. Period.

No, sorry, for sailors there are three winds -- Apparent Wind, True Wind, and Ground Wind. True Wind is relative to the water and Ground Wind is relative to the earth. The confusion comes about because meteorologists on land refer to True Wind as ground referenced. It's not for us because ground-referenced true wind is not directly useful -- we sail in the sheer between wind and water, so wind relative to the water is what is important to us.


There's a good treatment of the subject on the Raymarine site:


"What Raymarine refers to as True Wind (TWA, TWS) is always wind-over-water, not wind-over-ground (more on this below.) We calculate this from AWA and AWS, plus speed-through-water (STW, from a paddle-wheel or equivalent.) It's not possible to calculate proper True Wind from GPS speed (SOG.)"


https://raymarine.custhelp.com/app/a...calculate-them


And from Ockham Instruments, makers of fine racing instrumentation:


http://www.ockam.com/docs/WhyPaddles.pdf


This matters because even a small amount of current can cause a dramatic difference between Ground Wind and True Wind, especially the wind angle. Ground Wind is not really useful for sailing, as explained in the Raymarine resource linked above.

[PineyWoodsPete]

Originally Posted by noelex 77

“True wind” is calculated relative to the water.

You are not alone in misunderstanding what the “true wind” display is showing. “True wind” is a rather misleading term. It came about as before GPS there was no possibly of calculating the actual actual true wind on a boat.

“True wind” should have more accurately have been called “water wind”, but we are stuck with terminology. True wind does not eliminate movement of the water. Even if the instruments are 100% accurate it will not display the wind that would be experienced at anchor or the wind that is forecast, or measured by a station on the ground.

That's exactly what most of us (I think!) would call AWS - affected by both sea and ship's motion vectors, and to some extent height of sensor above water - eg going from 10m to 20m mast height, there's about a 15% increase in velocity.

So how do you define AWS as different from your definition of TWS?

[noelex 77]

Originally Posted by wingssail

We use the knotmeter (paddle wheel) and we remove it whenever the boat will be stationary for more than a couple of days. When out, we clean it with a soft brush if needed. Frequently we check the hull in the vicinity of the paddle wheel to remove weed which can affect the knotmeter.

Excellent. You are far more diligent than most cruising sailors. For an accurate true wind display the STW also needs to be accurate. Ideally the reading needs to calibrated for different angles of heel (Airmar have just released a consumer rated paddlewheel where this is possible) as well boat speed and angle of attack at the keel. Even if you can dial out all these factors a small amount of weed or debris on a passage will destroy the calibration.

Unfortunately, even when sailors know their paddle is fouled or otherwise inaccurate many continue to leave their wind instruments displaying true wind rather than switching to ground wind.

Ground wind uses SOG data and therefore requires no calibration, or adjustment.

Originally Posted by wingssail

While sailing in areas of no known tide or current we check the Boat Speed compared to the SOG and adjust the calibration.

We certainly do the same. Unfortunately, when sailing areas of high current, such as the west coast of Scotland where we are at present, it is very difficult to find stretches of water where you can be sure the current is zero. Of course these waters are the very areas where accurate calibration of the paddle wheel is most critical.

In short, I maintain that for most cruising sailors switching to ground wind will result in more accurate and therefore useful information, especially when combined with the information from apparent wind. It is certainly worth a trial.

Note: Switching to ground wind does not mean doing without STW information. STW and current information can still be displayed together with ground wind.

[Dockhead]

Originally Posted by PineyWoodsPete

That's exactly what most of us (I think!) would call AWS - affected by both sea and ship's motion vectors, and to some extent height of sensor above water - eg going from 10m to 20m mast height, there's about a 15% increase in velocity.

So how do you define AWS as different from your definition of TWS?

Apparent wind (both speed and angle) is what is directly measured.


True Wind is corrected for Speed Through Water and is normally expressed as angle to the bow


Ground Wind is corrected for SOG and SOG and is normally expressed as a compass direction


There's a good explanation on the Raymarine site linked above.

[Dockhead]

Originally Posted by noelex 77

. . . In short, I maintain that for most cruising sailors switching to ground wind will result in more accurate and therefore useful information, especially when combined with the information from apparent wind. It is certainly worth a trial.. .

The problem with that is that Ground Wind when expressed as angle to the bow is nonsense, even with very small currents, and with big currents fuggedaboutit. Tacks will show wildly different angles even in slight currents. You can't derive any useful sailing information from this.


Unfortunately True Wind coming from an uncalibrated and/or inaccurate paddlewheel sensor is also nonsense.


It's a conundrum I've been struggling with for years.

[flightlead404]

Originally Posted by noelex 77

True, but ground wind angle and ground wind speed can be calculated and this is arguably more use to cruising sailors, ...
Ground wind is best thought of as “the real true wind” as it is the wind that would be experienced when completely stationary.

I'm not understanding your differentiation between True and Ground Wind.

Coming from an aviation background what I think of as True Wind, ie the speed and compass direction of the wind, matches what you seem to describe as "Ground Wind" - a term I've never encountered before.

True Wind speed and direction and relative to the compass and the surface of the earth. Please explain the difference, thanks.

EDIT: Nvm I see you did that. My "true wind" is your "ground wind" which is what I use (when my @#$@#$^ paddle wheel is working lol)

[noelex 77]

Originally Posted by Dockhead

It's not possible to calculate proper True Wind from GPS speed (SOG.)"[/I]

Agreed. GPS speed will calculate ground wind.

Originally Posted by Dockhead

There's a good treatment of the subject on the Raymarine site:

https://raymarine.custhelp.com/app/a...calculate-them

Raymarine instruments will not calculate or display ground wind (at least last time I checked).

This is a serious oversight in my view. Even if you prefer true wind over ground wind if the paddlewheel fails users are left with apparent wind as the only option.

Rivals such as B&G incorporate this option as a simple software setting. I may be cynical, but but I suspect the quoted article has been written to appease customers complaining about this oversight.

Originally Posted by Dockhead

This matters because even a small amount of current can cause a dramatic difference between Ground Wind and True Wind, especially the wind angle.

Of course this also means that a small error in in the STW reading, even in the absence of any current will cause a dramatic error in the true wind especially the wind angle. I wonder how many crusing sailors have an acurate enough STW calibration to avoid misleading numbers?

[PineyWoodsPete]

Originally Posted by noelex 77

“True wind” is calculated relative to the water.

You are not alone in misunderstanding what the “true wind” display is showing. “True wind” is a rather misleading term. It came about as before GPS there was no possibly of calculating the actual actual true wind on a boat.

“True wind” should have more accurately have been called “water wind”, but we are stuck with terminology. True wind does not eliminate movement of the water. Even if the instruments are 100% accurate it will not display the wind that would be experienced at anchor or the wind that is forecast, or measured by a station on the ground.

That's exactly what most of us (I think!) would call AWS - affected by both sea and ship's motion vectors, and to some extent height of sensor above water - eg going from 10m to 20m mast height, there's about a 15% increase in velocity.

So how do you define AWS as different from your definition of TWS?

[noelex 77]

Originally Posted by flightlead404

Coming from an aviation background what I think of as True Wind, ie the speed and compass direction of the wind, matches what you seem to describe as "Ground Wind" - a term I've never encountered before.

Yes, it is confusing. It is not my terminology, but it is what we are stuck with in the sailing world.

What anyone with an aviation or meteorological background would call true wind is called ground wind in sailing terminology.

Here is a brief summary:

Apparent wind: the wind felt on the face when sailing. This wind results from the combination of the motion of the boat through the water the movement of water itself and the real wind (or wind on the surface of the earth).

True wind: This wind is a result of the movement of water itself and the real wind (or wind on the surface of the earth).

Ground wind: This is just the real wind (or wind on the surface of the earth).

Thus when a meteorologist measures the wind strength as 10 knots for example, a sailor with perfectly calibrated instruments would concur that the ground wind was indeed 10 knots but the same perfectly calibrated instruments would report a different strength of both true and apparent wind (unless the boat and current speeds were zero).

[wingssail]

Originally Posted by Dockhead

...True wind is really really hard to calculate accurately. I've been chasing it for years. This year I will bring a B&G H5000 Hercules computer and new wind sensor to the fight. I'm thinking about doing an RORC ocean racing series in 2023, so really hoping to get this nailed down this year...

It is so hard to caclulate that even top boats accept that True Wind Speed readings are generally more useful to track changes in velocity, not absolute velocities.

I'm interested to know which displays you will be using when you install a H5000 computer, will you also add the performance processor, and what wind sensor?

I've noticed that the Type213 wind sensor seems to be either in short supply or not available from B&G any more, yet all the high end yachts use that sensor, and even if they are available, the vertical spar, which is the choice of all GP yachts, is absurdly expensive.

Some replacement pieces, such as wind sensors, are apparently available from A&T instruments.

Being on the cheapo side, when recently upgrading my Hercules system, I made my own Type 213 vertical spar from used parts . If you have a bracket and correct cable connection then a simple pipe bender will result in the correct orientation.

Originally Posted by Dockhead

Apparent wind (both speed and angle) is what is directly measured.


True Wind is corrected for Speed Through Water and is normally expressed as angle to the bow...

\
I'm not sure about "normally".

In all of my experience any high end instrument system, if equipped with a compass can and usually does express "True Wind" in reference to the magnetic direction.

TWA (True Wind relative to the angle to the bow) is always going to be the same regardless of windshifts as the helmsman or automatic steering turns the rudder to maintain the correct wind angle, so TWA is useless to track shifting winds. TWD (relative to the magnetic north, is much more useful and in my experience, that is how it is normally expressed.

[Dockhead]

Originally Posted by noelex 77

. . . Of course this also means that a small error in in the STW reading, even in the absence of any current will cause a dramatic error in the true wind especially the wind angle. I wonder how many crusing sailors have an acurate enough STW calibration to avoid misleading numbers?

Indeed. That is the whole rub. Why I have been friends with my windex all these years despite having dumped tens of thousands into electronics . . .

[goboatingnow]

Originally Posted by PineyWoodsPete

I've been a sailor for over 60 years, and never heard "True wind" speed and vector being defined as RELATIVE to water. "True wind" - sailing or otherwise can only be defined as relative to the immovable earth SOG/COG. Period.

Glad your not instructing on navigation then

[goboatingnow]

True wind as regards sailing is a wind expressed as a angle from the bow , that would be experienced in the vessel was stopped in the water ( but not necessarily stopped regards the ground ). It’s a sailing parameter

Ground wind , or True wind as regards meteorology, is a wind that is blowing over the earth referenced to magnetic north. This is a weather forecast / weather reporting parameters

A true wind meter on a boat is not there for. weather forecasting , it’s a sailing instrument.

For sailing you use STW and TWA from the bow

For weather you use SOG and a north referenced angle.

[Dockhead]

Originally Posted by goboatingnow

True wind as regards sailing is a wind expressed as a angle from the bow , that would be experienced in the vessel was stopped in the water ( but not necessarily stopped regards the ground ). It’s a sailing parameter

Ground wind , or True wind as regards meteorology, is a wind that is blowing over the earth referenced to magnetic north. This is a weather forecast / weather reporting parameters

A true wind meter on a boat is not there for. weather forecasting , it’s a sailing instrument.

For sailing you use STW and TWA from the bow

For weather you use SOG and a north referenced angle.

There's no inherent reason why you couldn't express True Wind as a compass direction (then it would be TWD) or Ground Wind as an angle to the bow, just that these expressions would not be very useful.


That's why the convention is to express True Wind as TWA and Ground Wind as a compass direction. As you say -- Ground Wind is for weather -- you need to know its compass direction; True Wind is for sailing -- you need to know angle to the bow.


And as Noelex and others said -- if you don't have usable STW data, as many of us don't, you can fiddle your network settings to use SOG instead of STW and get Ground Wind Angle on your instruments as a poor substitute for TWA. Which might be better than nothing at all, slightly, in very still water.

[noelex 77]

I find ground wind very useful, particularly when combined with apparent wind. Ground wind keeps track of the environment (what is the wind strength is the wind increasing, decreasing or changing direction).

Apparent wind, both strength and direction is used to trim the sails.

The combination works well.

Give it a try.