Ultrasonic wind sensor [CV7]

[a64pilot]

Originally Posted by boat_alexandra

What about side slip through the water?

This seems to often be ignored, but I don't see how true wind (over water) can be calculated without it

Your a thinker.

Don’t know about boats, but in aircraft slideslip or drift is calculated by knowing airspeed, heading and ground track and ground speed.
If you ignore sideslip and are tracking to a fixed point, your course transcribes an arc and of course is not optimal, I would assume it’s the same for boats ?

[Capella]

These are great devices. I have the Maretron one, which does not have a compass. Once plugged in to an N2K network, the MFD or chartplotter works out true wind angle. But all I ever use is apparent wind, that nice display of your boat with an arrow pointing into the wind. Here in Wellington the wind blows hard and often, and the mechanical sensors take a beating.

Regards Michael

[GordMay]

Greetings and welcome aboard the CF, smiagol, & brodiej.

[Dockhead]

Originally Posted by lolly08

. . . The GPS can be used for Speed but the accuracy will be less.

Actually if you use the GPS for speed, it will be very accurate. Only, it won't be True Wind. It will be Ground Wind, which is an entirely different animal.

[Dockhead]

Originally Posted by a64pilot

Your a thinker.

Don’t know about boats, but in aircraft slideslip or drift is calculated by knowing airspeed, heading and ground track and ground speed.
If you ignore sideslip and are tracking to a fixed point, your course transcribes an arc and of course is not optimal, I would assume it’s the same for boats ?

I agree with you -- a very clever observation on Sean's part.

He's absolutely right. You're going to lose accuracy of True Wind calculations when you're hard on the wind and making leeway, and this will be a non-trivial error in many cases.

The new Airmar ultrasonic speed log does measure the DIRECTION of travel through the water, so will give data sufficient to correct this error. However, no currently produced marine electronics can use this data as far as I know.

[a64pilot]

All US built single rotor helicopters rotors turn counterclockwise. The tail rotor of course keeps the aircraft from spinning opposite to the main rotor.
In forward Flight the vertical fin usually is used to counter this spinning tendency.
For this reasons all US built single rotor helicopters drift or “translate” to the right and if you just fly directly toward your destination, your flight path will be an arc to the left.
If you have a fly to programmed and have a heading of say 180 degrees, you will “push the head” of the needle to maintain the course of 180, so you’ll end up flying say 175 or so to maintain a 180 course and fly a straight line.

You can do the same with a sailboat of course, if you find the heading required to maintain a course, then you will know how many degrees heading correction is required.
I’m sure a person who is good at math could calculate drift speed based on those two angles and Boat speed, but that is way beyond my capabilities, but I would assume an algorithm could be written so that you could enter the three variables and get speed of drift?

[Dockhead]

Originally Posted by a64pilot

All US built single rotor helicopters rotors turn counterclockwise. The tail rotor of course keeps the aircraft from spinning opposite to the main rotor.
In forward Flight the vertical fin usually is used to counter this spinning tendency.
For this reasons all US built single rotor helicopters drift or “translate” to the right and if you just fly directly toward your destination, your flight path will be an arc to the left.
If you have a fly to programmed and have a heading of say 180 degrees, you will “push the head” of the needle to maintain the course of 180, so you’ll end up flying say 175 or so to maintain a 180 course and fly a straight line.

You can do the same with a sailboat of course, if you find the heading required to maintain a course, then you will know how many degrees heading correction is required.
I’m sure a person who is good at math could calculate drift speed based on those two angles and Boat speed, but that is way beyond my capabilities, but I would assume an algorithm could be written so that you could enter the three variables and get speed of drift?

Heading vs COG will not indeed give you leeway, except in a total absence of current (which in my waters is, like, never), because COG is ground referenced.

This new Airmar transducer is the only way I know to calculate it, and degrees of leeway would be a hell of a useful thing to know -- I'd like to have it in big numbers on an instrument for sailing hard on the wind.

[ChrisseH]

As Jman said in post #10 you need info on boat speed to calculate TWA and TWS from AWA and AWS and info on heading for calculating TWD. Simple speedo, wind & compass instrument setups can perform this math. To get an approximate result this requires however that the AWA measurement is corrected for offset and that the boat speed measurement is calibrated. In order to get TWD referenced to North the heading measurement must be compensated for both magnetic deviation and declination. This in order to obtain a rough result...
If better accuracy is required the calibrated water speed and AWA measurements must be corrected for the effect of heeling. And, as concluded by several others, leeway needs somehow to be measured or estimated. Finally, depending on the wind sensor's location relative to the sails, the wind measurement might require correction for upwash. These latter corrections can normally not be done in simple instrument setups or basic plotters, but require some kind of black box, race computer or a PC with a suitable software.
Measuring leeway would obviously be nice, but is expensive and the measurement is not problem free. The measurement will be heeling dependent and in a sailboat where the speedo/leeway sensor typically should be installed well in front of the keel, you will measure not only leeway, but also transverse flow caused by steering, tacks etc. These disturbances can be filtered out, but will delay the leeway measurement.
A traditional way for estimating leeway is to use the old Pendrick formula: LWY = k*heel/STW^2 where k is the boat specific leeway coefficient having values between 9 and 13. The challenge here is to obtain k. The formula does neither work especially well at very low boat speeds.

[ronstory]

Yes, in the real world there is always that pesky sampling error whenever we try and use math to measure reality. The interesting thing about the boat moving in the water the longitudinal axis almost always points leeward of the wind unless you have big current going against the wind. And if that happens, prepare for bumpy ride, like the mouth of the Columbia River bar during ebb tide. Astoria is my home town too much experience at buoy 10, but I digress.

It would be handy to have a gauge for leeward slip to help see how the boat was trimmed, but after having really cool wind instruments like the CV7 or a season, I found I actually don't use them much. This surprised me since they are just a click away on the binnacle's MFD.

I typically just rely on the windex, telltails and feel of the helm to handle most considerations. Now when the winds are light and variable or heavy are typically the times it turn the instruments on.

Another data point, we were in the BVI at the end of April last year on a 50 foot aged sailing cat. The boat was at it's end of life with charter company and we had a young captain that loved to sail... and was happy there were other sailors on boat to share the load "for fun". When I asked about wind instrument on the helm, I was told they failed long ago... but the best wind instrument on the boat was a tattered pirate flag. After 6 days of sailing by skull&crossbones... a light flag and a windex is all i really use now.

My random observations, YMMV.

[ChrisseH]

Originally Posted by ronstory

Yes, in the real world there is always that pesky sampling error whenever we try and use math to measure reality. The interesting thing about the boat moving in the water the longitudinal axis almost always points leeward of the wind unless you have big current going against the wind. And if that happens, prepare for bumpy ride, like the mouth of the Columbia River bar during ebb tide. Astoria is my home town too much experience at buoy 10, but I digress.

It would be handy to have a gauge for leeward slip to help see how the boat was trimmed, but after having really cool wind instruments like the CV7 or a season, I found I actually don't use them much. This surprised me since they are just a click away on the binnacle's MFD.

I typically just rely on the windex, telltails and feel of the helm to handle most considerations. Now when the winds are light and variable or heavy are typically the times it turn the instruments on.

Another data point, we were in the BVI at the end of April last year on a 50 foot aged sailing cat. The boat was at it's end of life with charter company and we had a young captain that loved to sail... and was happy there were other sailors on boat to share the load "for fun". When I asked about wind instrument on the helm, I was told they failed long ago... but the best wind instrument on the boat was a tattered pirate flag. After 6 days of sailing by skull&crossbones... a light flag and a windex is all i really use now.

My random observations, YMMV.

Fully agree.

During day sailing and daytime cruising you only need a Windex, telltales and manual wind observations. For night sailing a wind sensor is not bad, and here you mainly need info on AWA. And sometimes you want wind info for your AP.

With the exception of dinghies and small multihulls, wind measurement is however essential in serious racing. Not so much for direct steering, but for detecting wind shifts, laylines, tactical decisions, double checking performance and to record the race for post analysis. And here it is common to have a tactician following the measurements so that the helmsman can concentrate on the steering.

Finally you have the boaters that are interested in meteorology and are closely following the weather. Long distance sailors looking for suitable weather windows could also be classified to this category. They study various sources for weather data and forecasts, but want also own measurements. I am mentioning this group because they might also appreciate trustworthy measurements.

[a64pilot]

Originally Posted by Dockhead

Heading vs COG will not indeed give you leeway, except in a total absence of current (which in my waters is, like, never), because COG is ground referenced.

This new Airmar transducer is the only way I know to calculate it, and degrees of leeway would be a hell of a useful thing to know -- I'd like to have it in big numbers on an instrument for sailing hard on the wind.

Heading and cross wind are the same. Drift can be calculated.
I can even show you how by using an E6B circular slide rule, but I’m not smart enough to know how to do it on paper, but I know it’s not a difficult problem for those smarter than me.
https://en.m.wikipedia.org/wiki/E6B
The back side of the E6B is used for wind calculations, wind and current ought to be the same?

[Dockhead]

Originally Posted by a64pilot

Heading and cross wind are the same. Drift can be calculated.
I can even show you how by using an E6B circular slide rule, but I’m not smart enough to know how to do it on paper, but I know it’s not a difficult problem for those smarter than me.
https://en.m.wikipedia.org/wiki/E6B
The back side of the E6B is used for wind calculations, wind and current ought to be the same?

I understand, but it's not the same - for flying you are calculating drift referenced to ground. True wind (sailor's true wind, anyway) is referenced to water. As is, of course, leeway.

Leeway is not the same as drift in an airplane.

A sailboat works in TWO fluids, which presents several problems which don't exist for aircraft, which work only in one.

[ronstory]

I pulled the manual and brochure for the DX900+. It looks to calculate the leeward angle directly, since it has a two axis speed sensor, one for longitudal speed and another for lateral.

Depth Frequency: 360kHz
Depth Range: 60m
Transverse Speed Range: ±6knots
Longitudinal Speed Range: ±60knots
Accuracy of Transverse and Longitudinal Speed:
±0.1 knots for speed under 10 knots
±1% for speed above 10 knots
Display Resolution: 0.01 knots
Operating Temperature Range: -15°C to 55°C (5°F to 131°F)

It also has a three-axis accelerometer and a three-axis rate gyroscope.

The brochure also claims it more accurate and faster than the calculated drift. Check out the graph on the right hand side.

Airmar

[a64pilot]

Originally Posted by Dockhead

A sailboat works in TWO fluids, which presents several problems which don't exist for aircraft, which work only in one.

I understand, but as both are accurately measured, it ought not be hard to calculate drift or whatever the proper word is.
Most sailboats have all the data available, speed through the water, speed over the ground, heading and track.
Assumption is it’s just not valuable to know drift?

[a64pilot]

See, I think I could calculate drift with an E6B, and once drift is known, then you either add or subtract that to “true wind” to get actual wind?
I know I’m butchering terms, sorry. For example apparent wind to me is relative wind