Apparent wind

[MAXDV]

Detect the true wind from the apparent wind without using the log but through the GPS
it's possible ?
Regards
Massimo

[AedanC]

Dashboard-Tactics can do that.

[Tupaia]

If there is any current then the results will be inaccurate. Think of it this way, going directly into the wind and it shows 15knots (app) your speed is 5 knots(SOG) so the true wind is 10 knots. It there is an adverse current of 2 knots (that you can't measure) you are actually making 7 knots through the water true wind will be 8 knots. Conversely you have a favourable current of 2 knots you are only making 2 knots through the water so true wind is 12 knots.

Now the actual current is at 37 degrees and the app wind is at 17 degrees ????

[rgleason]

Both Tactics and Dashboard_tactics can calculate and display aparent, true wind and current, given the correct data inputs. I believe there is an unknown which is the boats leeway/heel, which can be an instrument input or the plugins can use an input table to improve the calculation.

It is worthwhile reading the manuals for these two plugins to understand the pros and cons of which data inputs to use, but there are options.

[Quebramar]

Originally Posted by Tupaia

If there is any current then the results will be inaccurate. Think of it this way, going directly into the wind and it shows 15knots (app) your speed is 5 knots(SOG) so the true wind is 10 knots. It there is an adverse current of 2 knots (that you can't measure) you are actually making 7 knots through the water true wind will be 8 knots. Conversely you have a favourable current of 2 knots you are only making 2 knots through the water so true wind is 12 knots.

My experience goes in the same way. True wind is in my mind derived from SOG (ie GPS) not speed on water. Current will impact apparent wind hence the calculation to get true wind will incorporate this. Speed on moving water cannot provide a good basis for the calculation of TWS IMO.
I sometimes get out to sea for a couple of hours and am too lazy to put the loch in place, and I still ge true correct TWS, showing that GPS provides the data needed. That’s on Raymarine, YMMV

[goboatingnow]

Originally Posted by Tupaia

If there is any current then the results will be inaccurate. Think of it this way, going directly into the wind and it shows 15knots (app) your speed is 5 knots(SOG) so the true wind is 10 knots. It there is an adverse current of 2 knots (that you can't measure) you are actually making 7 knots through the water true wind will be 8 knots. Conversely you have a favourable current of 2 knots you are only making 2 knots through the water so true wind is 12 knots.

Now the actual current is at 37 degrees and the app wind is at 17 degrees ????

True wind doesn’t change because boats speed has changed. !!!! If there’s 10 knots of true wind it’s there irrespective of SOG STW or currents

[goboatingnow]

Originally Posted by Quebramar

My experience goes in the same way. True wind is in my mind derived from SOG (ie GPS) not speed on water. Current will impact apparent wind hence the calculation to get true wind will incorporate this. Speed on moving water cannot provide a good basis for the calculation of TWS IMO.
I sometimes get out to sea for a couple of hours and am too lazy to put the loch in place, and I still ge true correct TWS, showing that GPS provides the data needed. That’s on Raymarine, YMMV

STW was used to compute true wind because it was all we had before gps.

SOG is a far better means of converting apparent wind to true wind.

[Tupaia]

Originally Posted by goboatingnow

True wind doesn’t change because boats speed has changed. !!!! If there’s 10 knots of true wind it’s there irrespective of SOG STW or currents

Yes I agree, but if you are stopped on the ocean, no speed through the water, then you are most likely interested in the true wind relative to the boat. You could argue that this is also an apparent wind and I would agree.


For the true wind relative to the planet then SOG and COG need to be used.

[Rucksta]

Originally Posted by goboatingnow

STW was used to compute true wind because it was all we had before gps.

SOG is a far better means of converting apparent wind to true wind.

True wind based on boat speed provides performance /tactical information.
Can I lay that headland?
Is the opposite tack favoured?
What % of the polars is the boat achieving?

Boat speed gives symmetrical performance data (port/starboard) and lets you know quickly if you're back in the groove after a tack/gybe


SOG (Earth Wind) is useful for navigation and passage planning.
What is my transit time?
Is a route alteration indicated to take advantage of the weather predicted in the last GRIB download.

Both are useful although if I could only pick one it would be True Wind (water wind) based on boat speed.

Some instrument systems allow choice of the most appropriate data for the task in hand

[jordanbigel]

Raymarine published this article a few years ago to address this issue. I don't use Raymarine gear, but he seems to make some good points. Pretty sure all wind instrument manufacturers calculate TWS/TWD the same way, using boat speed through the water, not SOG.

Why is Ground Wind not very useful for sailing?
Put simply, your keel's in the water, not on the ground, so the wind angles you sail to are wind-over-water, not wind-over-ground. On a lake or at slack water the main difference will be your leeway, but in tidal waters or in offshore currents, ground wind can be very different from true wind, which is why our sailing-oriented wind instruments only show true wind (although on faster boats, apparent wind is probably more useful still of course.)


https://forum.raymarine.com/showthread.php?tid=987

[Hakan]

Also the inbound Dashboard calculates a true wind angle and speed providing its not received from any sensor and true heading (HDT), SOG and COG are present.
Also wind direction and speed are calculated with the same conditions.

Legend:
"Wind angle" is with respect to ships bow and what's used in many nav-instruments. (-180° : 0° : +180°)
"Wind direction" is with respect to north and not very useful for navigation/sailing. It's used for the wind history graph and the data instrument "wind direction" ( 0° - 360°)

[rgleason]

We had this exhaustive discussion years ago under the Tactics thread. Tactics represents all of these factors well and Thomas explains them very clearly, including the discussion about VMG (the sailors version and the powerboat version). The manual is available to all here , the new version missing a few links. There are so many opinions about this it is probably not fruitful to discuss it. Just understand it.

Petri has extended the manual for Dashboard_tactics and it is worth reviewing that as well https://opencpn-manuals.github.io/ma...0.1/index.html
His main manual under Read the Docs https://dashboard-tactics-pi.readthedocs.io/en/latest/

[goboatingnow]

Oh I agree it all comes down to what you regard as true wind. To me true wind = ground wind any wind generated cause the boat is moving with respect to the ground is apparent wind.

But I appreciate for VMG and tacking you need a sea water referenced true wind.

[seandepagnier]

True wind is the wind relative to the water. In any case, you need water speed, magnetic heading, leeway as well as apparent wind speed and direction to calculate it. The gps can then be used to calculate the current speed and direction from this.

The tactics plugin includes this calculation. true wind calculated from just wind and gps is only the true wind over ground not water and not as useful for analyzing sailing performance.

[rgleason]

Good explanations both of you. You just have to understand ground wind and water wind factors and use.