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Old 07-10-2010, 16:46   #1
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True Wind Measurement

I sail a 2003 Hunter 326 sloop. It is equipped with a Raymarine ST60 Wind Instrument. One can select a True or Apparent wind display. I see no evidence that there is any "input" that would facilitate a True wind reading. The question is: How do these instruments provide True wind information? What input do they need to provide True wind information Thank you
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Old 07-10-2010, 17:19   #2
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I sail a 2003 Hunter 326 sloop. It is equipped with a Raymarine ST60 Wind Instrument. One can select a True or Apparent wind display. I see no evidence that there is any "input" that would facilitate a True wind reading. The question is: How do these instruments provide True wind information? What input do they need to provide True wind information Thank you
If the Knotmeter/Log and the Wind speed/Direction indicator are linked (and the knotmeter is relatively accurate) the True wind relative to the water will be given. Of course, unless the water is dead calm/still, that will not be True relative to the earth but, unless you're in a raging tidal stream, close enough for government work, eh?
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Old 07-10-2010, 17:19   #3
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Well the couple I've had that came with the boats had a little paddle wheel affair at the top of the mast... bit like a rev counter in a car... X x Y = Z.... or.. A + B = C... or do I have to divide something somewhere.... ???
Sheesh... I hate Math...

OR................

Did I miss something in the post...

DUHHHH.... Gorrit... you weren't saying SPEED....
Damn glasses.... gonna have to stop reading as I sip the wine...lol
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Old 07-10-2010, 17:28   #4
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if the Knotmeter/Log and the Wind speed/Direction indicator are linked (and the knotmeter is relatively accurate) the True wind relative to the water will be given. Of course, unless the water is dead calm/still|
True wind is relative to speed through the water ground wind is relative to the earth

Dave
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Old 07-10-2010, 17:42   #5
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True Wind Measurement

Indeed if you can somehow input boat speed and apparent wind direction into an intelligent instrument the true wind (magnitude and direction) can be calculated. Knotmeters are problem prone and the value of True wind relative to water can be debated. I do not see any provision on the ST-60 to provide boat speed input (Konometer or GPS). The instrument however, is equipped with True wind mode. I just do not understand how they intended to do it for the ST60. The manual is not helpful or not explicit enough to clarify this mode of operation. Cheers
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Old 07-10-2010, 17:55   #6
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True wind is relative to speed through the water ground wind is relative to the earth

Dave
David-- I love ya like a brother but you've got your reference frames goofed up.

The knot meter reports only speed through the water to the Wind Indicator. E.g., if you're making 6 knots through the water heading east (090) under the Golden Gate Bridge, but the ebb is running at 7.5 knots (270), you are making 1.5 knots west (270) relative to the earth. If the apparent wind (aboard the boat) is reported as 15 knots from the east at 0 degress relative, the "True Wind" (relative to the boat) will be reported as 9 knots (apparent less boat speed through the water) which will be true relative to the body of water, but the True Wind relative to the earth will be 16.5 knots, the apparent true wind (at 270) relative to the water plus true speed of the water (at 270) traveling away from the apparent true wind. No?
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Old 07-10-2010, 18:00   #7
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Boat speed would come in over the Seatalk connector from a speed instrument. You'd just plug it in and, presto. I'm surprised you don't get an error message like 'NO DATA' when your select TRUE ... but that's Raymarine for ya...
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Old 07-10-2010, 18:07   #8
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It's probably my typing. True wind is relative to the water motion ie is the wind you feel stopes in the water ( but moving relative to ground). Ground wind is earth referenced

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Old 07-10-2010, 18:12   #9
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I think you can bypass electronic calculations and simply note the compass reading that is midway between your port and starboard tacks. This, of course, assumes that there is not a significant current factor and that your vessel performs with bilateral symmetry.
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Old 07-10-2010, 18:15   #10
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True wind is relative to speed through the water ground wind is relative to the earth
In the context of boating, and especially in the context of boat instruments, this is correct. The marine industry is not engineering oriented like say the aircraft industry, where the various speeds are strictly defined.

Computing True Wind Relative to the Ground would require an input from the GPS. Haven't seen it, and it would be even more useless to a sailor than True Wind numbers, which are only occasionally interesting.
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Old 07-10-2010, 18:41   #11
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I need true wind reading so I know when to panic when surfing downwind
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Old 07-10-2010, 18:47   #12
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I need true wind reading so I know when to panic when surfing downwind
Yeah, good to think about how the apparent weather is going to change when we slam to a stop into the back of a wave But we can do that simple addition in our head, SPEED+APPARENT, without loosening our death grip on the helm.
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Old 07-10-2010, 18:53   #13
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I realize now that when people state "true wind reading" on this post they are not specifying speed or direction. My post above was only to determine direction. If you're "surfing downwind", as Richadhula, it is a simple practice to add your boat speed to the apparent wind speed to have a satisfactory estimate of true wind speed.
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Old 07-10-2010, 23:26   #14
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True and Apparent are correctly used when describing wind speed. Apparent Speed vs. True speed.

The angle is always referenced as magnetic.

True Wind Speed TWS could be described as 16kts @ 330 magnetic. Apparent Wind Speed may be 18.5 kts @ 350 magnetic.

To get true wind direction you have to adjust for magnetic variation and compass deviation. It is important to understand if your weather report is reporting wind as "true wind direction" - it should. It is the only way for a wide area forecast to be accurate over a range of magnetic deviation areas.

So we care about true wind angle when passage planning. We must know how to convert that to expected magnetic wind angle at our location. Magnetic variation can be +-14 degrees. Once you convert to magnetic angle and apply your compass variation you will know from which angle to expect the wind in relation to your course.

Let's say the wind is forecast from 360 and your rhumb line is 330. You are pretty sure you can sail 30 degrees to the apparent wind. However the report is true and you are in a 14 degree east variation. The magnetic wind direction is 345 and you probably can't sail 15 off the apparent so your magnetic heading ends up as 315 and you need to plan for the eventual tacks.

True speed and apparent speed are important to know if you are sailing to the polars. i.e. am I trimmed efficiently.
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Old 08-10-2010, 02:13   #15
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The question is: How do these instruments provide True wind information? What input do they need to provide True wind information Thank you
You need:
Boatspeed data - so it knows how fast you are moving through the water.
Compass data - so it knows which direction you are traveling through that water.

This is enough for it to calculate wind information relative to the moving water - It's not strictly "True" wind as it is relative to the moving body of water that the boat is sitting on, but it is useful information none the less.

To get "proper" true wind information, the unit will also need position data so that it knows how you are moving over the ground.

Some systems allow you to define whether the "True" is related to the water or the ground. Some system automatically default to ground if they have position data. I don't know what the ST60 does, but probably the latter

The ST60 will get this data from whatever it is connected to via the SeaTalk bus.
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