Instruments set to True or Apparent wind

[dom]

First point to remember is that apparent wind is what the windex measures and therefore the closest thing to pure data. Even then it's not that pure as the airflow at the top of the mast is somewhat distorted by the sails below. It is also affected by pitch, yaw and roll, which can only be properly filtered out if the equipment includes a decent 9-axis gyro. Deducing the TWS will in turn require a well calibrated log (across the full speed spectrum, or one can think about groundwind by using a GPS input).

After that it probably depends on context and crew. Polars are generally based on true wind with wind angle on the left hand side of a table and wind speed at the top, but more detailed polars consist of columns for every TWS, including STW, TWA, AWA, AWS, Vmg, Heel, S/M, etc. The later is really too complicated unless pre-loaded into complex electronics.

Upwind most folk will set APs to AWA, in which case it's nice to see AWA to judge how well the AP is doing. Downhill in a blow it is usually best to select TWA to prevent the boat bearing off dangerously into puffs as the AWA comes forward. Then it's nice to see both AWA and TWA.

At night one needs both or a means of illuminating telltales that doesn't screw night vision. And as johnalison says some people hate looking up at the windex, whereas others find it easy.

I guess for me there's no right or wrong answer: it depends on context, it's not rocket science, but thinking it properly through will ensure that everyone's system provides what they want, when they want it.

 

[capnsensible]

First point to remember is that apparent wind is what the windex measures and therefore the closest thing to pure data. Even then it's not that pure as the airflow at the top of the mast is somewhat distorted by the sails below. It is also affected by pitch, yaw and roll, which can only be properly filtered out if the equipment includes a decent 9-axis gyro. Deducing the TWS will in turn require a well calibrated log (across the full speed spectrum, or one can think about groundwind by using a GPS input).

After that it probably depends on context and crew. Polars are generally based on true wind with wind angle on the left hand side of a table and wind speed at the top, but more detailed polars consist of columns for every TWS, including STW, TWA, AWA, AWS, Vmg, Heel, S/M, etc. The later is really too complicated unless pre-loaded into complex electronics.

Upwind most folk will set APs to AWA, in which case it's nice to see AWA to judge how well the AP is doing. Downhill in a blow it is usually best to select TWA to prevent the boat bearing off dangerously into puffs as the AWA comes forward. Then it's nice to see both AWA and TWA.

At night one needs both or a means of illuminating telltales that doesn't screw night vision. And as johnalison says some people hate looking up at the windex, whereas others find it easy.

I guess for me there's no right or wrong answer: it depends on context, it's not rocket science, but thinking it properly through will ensure that everyone's system provides what they want, when they want it.

Indeed it's very subjective. I find that most people learning find apparent wind easier to comprehend.

I, like others, nip the instrument onto true if we have the wind well aft or motoring.

But seeing as most yacht paddle wheel logs are mostly rubbish, it often doesn't help much. And yes I do pull it to clean it nearly every week and show people how to try calibrating. But generally, the results are rubbish!

 

[bbg]

True, always....

Apparent windspeed is almost irrelevant to any decisions about which sail to hoist, whether to reef, or if you can lay that headland. (If indeed you base these decisions on a readout of windspeed at all) True is the measure of how much power is available to your sails. If you used apparent you reach a different conclusion between turning head to wind to hoist the main, and actually sailing upwind.

Apparent wind direction is ascertained by looking at the windex. The only reason I can think of that I ever need to know the wind direction other than looking at the windex is to know if I am sailing the boat to its polars. And polars are always in true. So I know (for example) that in 10 knots of true wind I ought to be doing 6 knots on a TWA of 150 for the best VMG downwind. Apparent gets me nowhere with that.

Apparent is indicated by telltales, not needles and numbers.

+1

 

[John 32i]

RYA say 'apparent' wind if sailing upwind and 'truewind' if sailing downwind IIRC

 

[thinwater]

Apparent. If you speed is read with a paddel wheel, selecting true introduces error. And if your trig is any good at all, you should be able to convert approximately in your head.

 

[Luffe44]

The problem with true wind is that its not true. That is if the speed comes from the log. True is a calculated vector from apparent wind and measured speed. The logs I have seen only allows for a linear calibration of speed, meaning it gives the precise read at the calibrated speed only. When you add tidal and wind driven current, the "true" speed and direction gets un-true. If you can get the GPS speed into the calculation, true makes more sense. I still have not figured out if I can get this out of my package.

 

[flaming]

The problem with true wind is that its not true. That is if the speed comes from the log. True is a calculated vector from apparent wind and measured speed. The logs I have seen only allows for a linear calibration of speed, meaning it gives the precise read at the calibrated speed only. When you add tidal and wind driven current, the "true" speed and direction gets un-true. If you can get the GPS speed into the calculation, true makes more sense. I still have not figured out if I can get this out of my package.

Not this stuff again...

Wind calculated using GPS is known (in the instrument world at least) as ground wind. It is of very limited use to a cruising sailor. Mostly because the tide induced wind is frequently a large factor and the end result is absolutely nothing to do with the power or direction of wind that is actually available to move your boat.

Calibration isn't really a big issue to be honest, most logs these days are pretty good. Granted they aren't 100%, but if you sit at rest and then accelerate under engine to a range of speeds without any significant change in TWS, then it's basically good enough.

 

[dom]

Not this stuff again...

Steady, GWD, Coriolis Effect and Wind/Tide effects are treasured forum Whack-a-Mole games

 

[capnsensible]

I would love one of those pretty good logs!

 

[johnalison]

I would love one of those pretty good logs!

Mine is quite good but not especially pretty. It is pretty reliable and I reckon to be able to keep it within 5% or usually less! which is good enough to give meaningful True readings.

 

[RichardS]

I would love one of those pretty good logs!

Mine is guaranteed 100% accurate. But I'm in the Med and don't have a paddlewheel at all so the true wind calculation uses GPS.

Richard

 

[capnsensible]

It can be difficult sometimes to find a volunteer toothbrush to clean the paddle wheel....

I sail several different yachts quite a lot. None of the logs work particularly well.

 

[[70521]]

I don't have any posh instruments that calculate true wind etc so I work on apparent.

 

[savageseadog]

I'd like an instrument to show Best Wind when racing.

 

[KellysEye]

Since you are sailing in apparent wind it makes sense to use that.

 

[bbg]

Since you are sailing in apparent wind it makes sense to use that.

Maybe to you, but I can detect the apparent wind through telltales and Windex.

To me it makes more sense to use the instruments to provide you with information that is not immediately obvious: true.

I can't think of any boat that I have raced on that has had the instruments set to apparent. Or cruised on, for that matter.

 

[Yealm]

Mmmm

In defence of ground wind...

1) it tells you reliably if the (atmospheric) wind is changing over the course of the day - that's quite useful/interesting ? (can't do this with apparent or true)

2) allows you to compare wind on different days/outings - useful to decide choice of sails etc

3) means you never have to clean a paddlewheel

4) it's very accurate (true wind at best pretty approximate)

5) where/when I sail (SW UK) tides are usually <1 knot and ground wind 10-25knot - so tide wind fairly small effect (and I know pretty well what the tide is doing from the tidal arrows on the chart plotter)

Not this stuff again...

Wind calculated using GPS is known (in the instrument world at least) as ground wind. It is of very limited use to a cruising sailor. Mostly because the tide induced wind is frequently a large factor and the end result is absolutely nothing to do with the power or direction of wind that is actually available to move your boat.

Calibration isn't really a big issue to be honest, most logs these days are pretty good. Granted they aren't 100%, but if you sit at rest and then accelerate under engine to a range of speeds without any significant change in TWS, then it's basically good enough.

 

[Tomahawk]

Apparently true.

Sorry, .... couldn't resist it.

 

[dom]

Mmmm

In defence of ground wind...

1) it tells you reliably if the (atmospheric) wind is changing over the course of the day - that's quite useful/interesting ? (can't do this with apparent or true)

2) allows you to compare wind on different days/outings - useful to decide choice of sails etc

3) means you never have to clean a paddlewheel

4) it's very accurate (true wind at best pretty approximate)

5) where/when I sail (SW UK) tides are usually <1 knot and ground wind 10-25knot - so tide wind fairly small effect (and I know pretty well what the tide is doing from the tidal arrows on the chart plotter)

Fair points, but accuracy, only as good as windex itself, which still needs some sort of algorithm to damp out pitch, yaw, roll, etc. A 9-axis gyro helps here.

Come to to think of it the data from a handy buoy, or shore station wld prob be mostly fine for most people

 

[Luffe44]

#37: Me too &