Adding wind speed to a small Garmin

Jan11 · May 11, 2020

dlochner said:
To calculate true wind, the instrument needs speed through water (not GPS), apparent wind speed and angle, and boat direction.

Are you suggesting that two boats, one anchored in a 6 kt current right off shore and doing six kts through the water at some angle to the wind, and the other boat sitting on land will have different true winds?

Different truths sounds like politics which we aren't supposed to do here.

kappykaplan · May 11, 2020

DrJudyB said:
We install the transducer almost every time we head out, and remove it when we put the boat away. It stays perfectly clean.

Same here. Never dirty that way.

DrJudyB · May 11, 2020

Ground wind is referenced to a fixed point on earth. Ground wind has a direction referenced to North.
Apparent and true wind are referenced to the boat. TWA has an angle referenced to the bow of the boat. TWS is referenced to the water.

A better terminolgy might be “water true wind speed” and angle. vs “ground wind speed” and ground-wind direction. They are different frames of reference and are useful in different ways.

If you use the gps with SOG and COG to calculate true wind angle and speed, you will get different numbers for “true” wind speed and angle for starboard vs port tack whenever there is a cross current . Using the paddle wheel’s speed through the water gives consistent results from tack to tack.

Sailors often want to know how close to the wind and how fast through the water they can go. For that you need the true water” wind direction and speed. sail boat performance is evaluated with water-relative wind speed and direction.

Current: with both a paddle wheel and gps you can calculate or estimate current, by comparing the SOG/cOG to the knot meter speed. If you are traveling around islands, where current direction and speed vary greatly, that information can add or subtract miles from how far you can cruise.

A knot meter can tell you if you’re pinching too high or footing too low. GPS doesn’t tell you that if there is any current.

(On edit - knot meter CAN tell you if you’re..... not “can’t”)

Hunter216 · May 11, 2020

RussC said:
I'm not willing to spend a fortune but just looking at options

I don’t know if this would have value for you but there are inexpensive small handheld wind instruments.

I have carried one of these in my windsurfing bag for years. Part of windsurfing is picking the right size sail for the conditions, the little battery operated unit gives a reading to help me assess the situation at a site I am new to.

LloydB · May 12, 2020

OK this thread is just hit about a half dozen of my pet peeves and it woke me up too early this morning. First the original poster sometimes known as OP stated that he understood the difference between true wind an apparent wind in his initial post. TW and TW are both spelled the same way however true wind is a real thing that is also reported by the Weather Service and refers to the wind over the earth from a specific direction and force measured with wind instruments and has a specific definition in that the wind is actually measured. The other TW is simply an expression of somebody's idea of what the real wind or actual wind feels like or where the wind hits the boat and because it's spelled the same they have forgotten that the wind speed and direction needs to be measured relative to the earth in order to be defined as true wind. Apparent wind is a real thing so it can be measured with calibrated wind instruments or by you if you have experienced comparative wind speeds. If a Garmin brochure or any other chart plotter says it will display true wind it probably says upon further reading 'if wind instruments are connected to the GPS device' because only then can it calculate true wind speed and direction if the instruments have been set up properly. If you are on the pond in a boat that is moving with a GPS and see that the water is not moving then you can calculate the speed through the water using a GPS. The GPS is not that smart it needs to have a flow device to find out how fast the water is moving past it. Sorry about the rant and even posting this but I didn't want to wake me up tomorrow maybe I should say I hope it doesn't.

DrJudyB · May 12, 2020

meteorologist don’t even mention “true wind”except for the purpose of distinguishing magnetic north from true north. I’m not a meteologist, but I haven’t found any reference to “true wind” in any other context.

Meteorologist define and use “ surface wind” or “ground wind” and their frame of reference is the earth’s surface.

True wind, by definition, evolved historically as a mariners’ term, to help measure and analyze currents and boat performance. the frame of reference is the surface of tidal waters and currents. It is an essential metric for planing voyages, and for making the boat perform to a standard.

As far as I can tell, meteorologists use only one reference frame, I.e., the surface of the earth. They call it “surface wind,” and they typically measure at a distance of 10 meters above the surface of the ground (or the surface of the water). Meteologoists have no need for apparent wind, because their frame of reference is the surface of the earth at at a specific altitude above the surface.

DrJudyB · May 12, 2020

From the following website

Airmar CS4500 Part II (Reliable True Wind) - Panbo

In Part I of this series, Dan Corcoran, and many commenters, got into not just the value of accurate boat speed, but the true meaning of True Wind. Click on on Dan's illustration above and check out Part II...

www.panbo.com

Joe · May 12, 2020

dlochner said:
AWS and AWD are far more important for sailing than either TWS or TWD.

Respectively, I must disagree. True wind speed and direction are what nature is giving you and is not affected by a moving vessel's course or speed (through the water, over the water... irrelevant.) We use true wind information to navigate.
Apparent wind speed and direction is you affecting nature. Your boat's speed and direction changes the way you feel the true wind....that direction and speed you feel become altered.... that's why it's called APPARENT. Because AWD and AWS are the factors the boat senses when it's moving, we use apparent wind information to trim, or manage, our sails.

Both are important, both are used whether you have instruments or not. You can't leave the dock without knowing which way the wind is blowing. You can't plan a route, or get around a buoyed course without knowing the true wind direction. You make reefing decisions based on true wind speed, not how fast your boat is moving.
When we're optimizing performance we need to watch our apparent wind information to adjust the sails... we can do that by monitoring the sail's telltales, watching the boat's flag, feeling the hair on the back of our necks... that's apparent wind.

Detecting changes in true wind direction..... lifts and headers... can be done by paying attention to your boat compass when sailing close hauled. Averaging your course directions on port and starboard when sailing close hauled will give you true wind direction (assuming current is minimal). Tracking those headings will tell you about wind shift patterns. That's what racing compasses do, btw. The headings are put into memory, with each new one compared to the previous to detect patterns.

IMHO, knowing your true wind speed and direction is much MORE important because it affects the decisions you make. Those decisions will, in turn, affect the boat's apparent wind information... and it's constantly changing with each course change, or change in sail trim. You could say the apparent info is telling us what we are doing. But there is nothing we do that influences the true data... that's nature telling us what we can or can't do.
Oh well... just my thoughts.

LloydB · May 12, 2020

Thanks Joe well said

Dr Judy thanks for the diagrams and I think you're right about meteorologist not calling it true wind. I think they just call it wind then describe it well. If they happen to be an an airport meteorologist and tell you the wind at the surface they don't name it surface wind however the direction it is coming from is given relative to magnetic North rather than geographic North and speed in knots.

Joe · May 13, 2020

RussC said:
The way I see it (be it right or wrong ) : TWS and TWD is what you need to know in order to sail. AWS and AWD is an indication of how well you've managed the first two.

.02 and back to the sidelines
RussC

Well stated, makes too much sense. True wind... is always the starting point. You don't go down to the dock, looking at the weather and ask your self: " I wonder what the apparent wind speed and direction are today?" . (btw, I've got a headache reading Scott's redux thread... Not you scott... the topic I mean... sounds like a room full engineers, if you know what I mean.

)

Pizzazz · May 13, 2020

Answering the OP's question. You need to feed the Garmin NMEA 0183 MWD sentence. This can come either from the NASA Clipper wind instrument (<$200) or from another chartplotter, head, converter that takes your existing instrument proprietary analog output and converts it to NMEA 0183.

In my case, I have a Garmin DV44 at the helm and a Raymarine C80 under the dodger. The C80 outputs the wind sentence (and water speed) that I feed to the Garmin to show apparent, true and ground wind. As discussed above, the difference is simply apparent wind is what you measure, true wind is referenced to the water around you (using water speed from the paddle), ground wind is referenced to the surface of the earth (using the GPS information).

In practice, apparent wind direction is most stable and used by the autopilot after a good deal of averaging. Apparent wind speed is sometimes useful but typically, when sailing in swell, the mast moves with the waves and the indicated wind speed shows rapid swings. True wind is useful sometimes when steering downwind. True wind speed is often inaccurate because the paddle wheel has a high margin of error. Ground wind speed is the best indication of true wind speed because whatever it loses by not accounting for current it gains by the improved accuracy of using GPS speed. All wind measurements become better with increasing wind speed. At low wind speeds and with a swell, the combined errors from the paddle surfing down the waves, the mast swinging, etc. make wind steering by autopilot very difficult and often useless.

SV Pizzazz