Calibration with AIS Buoy

[SailingHarmonie]

Originally Posted by Dockhead

Great idea about the sea anchor Thank you!

Yes, I realize there are many fewer racers here than on, for example, SA. But out of loyalty to CF, I wouldn't post anything on SA until I've exhausted resources here.

Every time I look at SA I see such nasty, evil, snarkiness I need to take a shower. It is a great example of the internet dragging the worst out of people. I assume there are other racing oriented forums out there, but if not, I am very glad that is a community I have no part of.

[wholybee]

I see the usefulness of the data, but how would you then incorporate it into the calibration of the instruments? If the instrument doesn't provide a way, then the data is useless.

I _had_ an airmar ultrasonic, and it was fabulous and very accurate, until it wasn't - twice. And I can't afford yet another $1000 on something that seems to fail every 1-2 years.

[Dockhead]

Originally Posted by wholybee

I see the usefulness of the data, but how would you then incorporate it into the calibration of the instruments? If the instrument doesn't provide a way, then the data is useless.

I _had_ an airmar ultrasonic, and it was fabulous and very accurate, until it wasn't - twice. And I can't afford yet another $1000 on something that seems to fail every 1-2 years.

Well, all instruments have a simple offset to calibrate the speed log data. With my instruments (first generation B&G Zeuses and old Tritons), if not using a sailing computer, you have to set the offset in every instrument separately -- a PITA.

But I have a sailing computer, an H5000, which allows multipoint calibration with different offsets at different speeds, plus a correction for heel. You connect the speed log directly to the H5000, then your network sees the H5000 as the source for STW.

This works also for an N2K transducer -- you just choose the H5000 rather than the transducer as your source.

Your Airmar must have been the old CS4500 -- I also had one of those. It was far from perfect, but still the best speed log I've had so far. It lasted me a good while -- I guess 5 or 6 years -- before it failed. When I replaced it, I had high hopes that newer models would be better, but have been disappointed so far.

[Off Trail]

And if you have a cannon, or catapult, you could shoot it out in front on a long line and retrieve it after you pass. ;-)

Heck if the line is long enough you could just drop it and then pull it back once you got a measurement.

[Dockhead]

Originally Posted by Off Trail

And if you have a cannon, or catapult, you could shoot it out in front on a long line and retrieve it after you pass. ;-)

Heck if the line is long enough you could just drop it and then pull it back once you got a measurement.

No need for any line.


It broadcasts its position, remember? Just steer back to it and haul it on board with a boat hook.

[Off Trail]

Originally Posted by Dockhead

It broadcasts its position, remember? Just steer back to it and haul it on board with a boat hook.

I was thinking of something to solve your goal of offshore current measurement underway. OK maybe a bit tongue in cheek, but in theory....

[TeddyDiver]

Originally Posted by Off Trail

I was thinking of something to solve your goal of offshore current measurement underway. OK maybe a bit tongue in cheek, but in theory....

That's easy to see after you have calibrated before leaving..

[Dockhead]

Originally Posted by Off Trail

I was thinking of something to solve your goal of offshore current measurement underway. OK maybe a bit tongue in cheek, but in theory....

I don't get it.

You would want to do this when there is as little current as possible, offshore where there are less swirls of current, on as calm a day as possible.

You would want to log your data when you're close to the buoy. So you would toss it out, then turn around and run past it, repeat in the opposite direction, and maybe from 90 degrees as well.

Calculate set and drift from the buoy's COG and SOG, apply to your own COG and SOG, and compare to STW reading from the instrument you are calculating.

That would be the way to do it.

You won't be able to get any readings while heeled because that implies wind which will induce drift in the buoy and invalidate the data. You'll have to use standard coefficients for that, and then see whether your true wind data looks ok when you're heeled. Tweak the coefficients as necessary.

You won't get it perfect but perhaps -- provided you have a transducer which has high repeatability, sub tenth of a knot (accuracy to that degree is not essential, just repeatability, so your calibration is effective), you might get close enough to have usable true wind.

With usable true wind, you have:

1. Valid laylines

and

2. Valid target speeds in reference to your polars.

I realize most cruisers couldn't care less about this, but if you really care about sailing, even if you're not racing, you might. I do.


As a bonus:


3. Your pilot will work reliably downwind in wind mode.

[Off Trail]

I was thinking about live operation, not just calibration.
Imagine (and I would never advocate this) that you had a box of dozens of small disposable gps bouys with radio transmitters.

Underway on passage, drop one. Collect its cog/sog. You now have a direct measurement of current. Your cog/sog vector minus the current's gives you the elusive stw vector, does it not? Repeat whenever you think the current might have changed.

If you could both 1) get a good measurement and 2) find a way to retrieve the device, then wouldn't this do what you are looking for?

Just an idle thought, sorry if it confused your thread.


Dan

[Dockhead]

Originally Posted by Off Trail

I was thinking about live operation, not just calibration.
Imagine (and I would never advocate this) that you had a box of dozens of small disposable gps bouys with radio transmitters.

Underway on passage, drop one. Collect its cog/sog. You now have a direct measurement of current. Your cog/sog vector minus the current's gives you the elusive stw vector, does it not? Repeat whenever you think the current might have changed.

If you could both 1) get a good measurement and 2) find a way to retrieve the device, then wouldn't this do what you are looking for?

Just an idle thought, sorry if it confused your thread.


Dan

The whole thread is mostly idle thoughts, so welcome!


Yes, you could do it that way, but why would you, if you managed to get your speed log calibrated?


The current changes constantly, so you really want to measure STW directly as much as you can.

[Bill_Giles]

Having sailed for 50 years I’ve never felt the need for this. Perhaps if I were a dedicated racer I might.

[Dockhead]

Originally Posted by Bill_Giles

Having sailed for 50 years I’ve never felt the need for this. Perhaps if I were a dedicated racer I might.

Sure, to each his own.

I sailed for many years with nothing but a compass and a windex. It can be done!

Accurate apparent wind from an instrument is extremely valuable and quite realistic to obtain. All you need a decent instrument, mounted high enough to get out of the upwash from the sails

Valid true wind is really hard because of the inherent difficulty in measuring speed through water. It's awfully useful to have, but surely not worth the trouble for most people!


It's worth mentioning that valid STW data is useful not just for calculating true wind, but also to know your performance! Are your sails trimmed right? You can't know exactly without this.

[Bill_Giles]

I race a small boat as well as cruise in a 36 footer. Racing with no instruments and only a windex does sharpen the senses. Even the mast head indicator does not get much use as my neck is not as flexible as it was. But my ears remain attuned to the wind and I don’t tend to cover them up.

[Dockhead]

Originally Posted by Bill_Giles

I race a small boat as well as cruise in a 36 footer. Racing with no instruments and only a windex does sharpen the senses. Even the mast head indicator does not get much use as my neck is not as flexible as it was. But my ears remain attuned to the wind and I don’t tend to cover them up.

I sometimes think that sailing with no instruments at all (I wouldn't want to be without a windex, however) is better than sailing with instruments which are uncalibrated.

[TeddyDiver]

Originally Posted by Dockhead

I sometimes think that sailing with no instruments at all (I wouldn't want to be without a windex, however) is better than sailing with instruments which are uncalibrated.

Can agree with that.. especially with novice sailors. They tend to stare the instruments all the time not checking sails at all. Sometimes Iv'e covered the panel to force them go "analog"