Calculating True Wind Direction

[flaming]

This is true of true as well. most instument make no alowance for leway.

It's true that it's true of true. (however just re-read my original post and realised that I menat tide, not wind, but it's still true)

However, from a cruising point of view I would argue that the most important use of TWS is judging windspeed when running downwind, which can be deceptive. There would of course be little or no leeway in this instance.

 

[noelex]

I can't think of a single possible scenario where the difference between true and ground wind would influence where you dropped the hook. This would impy significant tidal flow, which I'm sure would be far higher up my list of things to consider than the <10 degree difference between the two, don't you?
.

I agree that for the great majority of cases either true wind or ground wind is perfectly adequate (providing they are accurate). To decide which is best we need to consider those occasions where one would be significantly superior.

You could not "think of a single possible scenario where the difference between true and ground wind would influence where you dropped the hook"
I guess that’ means I have got to come up with such conditions

As an example an area I have cruised a lot. The outer reef of Australia’s great barrier reef. The anchorages are marginal at best, many only offering protection from a narrow range of wind directions. Wind strength is also critical above 15K you don’t want to be there. The tidal streams in the channels can be fierce.. Selecting an anchorage in these conditions the ground wind speed and direction (and how its been changing over the day) are important in staying safe. Current is important, but is much less in the anchorages and is predictable.

This is obviously an extreme example, but there are many anchorages where a small change in wind direction or strength can impact how good your sleep will be.

I still feel however that the overwhelming difference between true wind and ground wind is that I can get accurate ground wind because my my GPS doesn’t alter its calibration with a bit of weed on the hull or paddlewheel. The log does.
The difference between true wind and ground wind is largely academic and of no consequence unless there is a significant tidal stream, but the difference in accuracy of the two alternatives, on most boats is significant and this difference is present with and without current.

 

[Keen_Ed]

The other thing is that calculating true wind is actually a significantly non-trivial task, and there's a lot more than just taking the apparent wind and boat speed vectors into account. For example:

a) as has been mentioned, there's leeway, which varies with TWA, seastate, TWS etc etc...
b) upwash at the masthead, which (AIUI), varies with different TWAs/TWS
c) movement of the masthead as the boat moves
d) heeling
e) windsheer - which can vary significantly

etc etc...

Again AIUI, at the Volvo race/Americas cup level, one of the major tasks for the navigator is to keep ontop of the calibration of the instruments. Without that level of attention (i.e., someone doing it all/nearly all the time), it might be better to stick to apparent wind numbers, experience, and a healthy dose of caution.

 

[noelex]

The other thing is that calculating true wind is actually a significantly non-trivial task, and there's a lot more than just taking the apparent wind and boat speed vectors into account. For example:

a) as has been mentioned, there's leeway, which varies with TWA, seastate, TWS etc etc...
b) upwash at the masthead, which (AIUI), varies with different TWAs/TWS
c) movement of the masthead as the boat moves
d) heeling
e) windsheer - which can vary significantly

etc etc...

Again AIUI, at the Volvo race/Americas cup level, one of the major tasks for the navigator is to keep ontop of the calibration of the instruments. Without that level of attention (i.e., someone doing it all/nearly all the time), it might be better to stick to apparent wind numbers, experience, and a healthy dose of caution.

b, c, d and e distort apparent wind numbers as well.
I am going back to a wet finger

 

[trapezeartist]

Have a play and report back.

I will indeed, though we're probably lifting out at the end of the week so it may be a little while before I can.

 

[trapezeartist]

Reporting back as promised.

We went outsailing today, so once everything settled down I started to play aaround with the chart plotter. I couldn't find the wind vector anywhere! I gave up in the end.

I did find Ground, True and Apparent wind on the data bar though. I was able to display ground wind without losing anything else or covering up any more of the chart. It displayed direction in degrees true and speed in knots, so ideal for the log and pretty good for deciding which tack a run will be on. Unfortunately, I became gradually less and less convinced that the speed figure was true as it just didn't seem to correlate to what we were experiencing. Ho hum. At least the direction seemed good and that was what I was really after in the first place.