True wind on a handheld

kloudie1 · Feb 7, 2012

The vector math is not really hard but not trivial either..Since the wind is variable, every data point would have to have a TW calculation.. time consuming.. It takes a while and you'd be much better off having the real time instrument read-out ..

EDIT : To do it "manually" you could set up a spread sheet that could handle the math and simply take all readings and enter them and let the spread sheet generate the polar.

Don S/V ILLusion · Feb 7, 2012

As a practical matter, true wind in the cockpit (presumably where your handheld instrument would be) can be quite different than direction (and speed) higher up where the sails are located.

Jackdaw · Feb 7, 2012

As others point out this would be tricky to do by hand. The really issue would not be wind speed, but ANGLE.

Its too bad that Hunter did not generate a set of Polars for the 260. With Glen Henderson there, I figured he'd throw ALL their hulls thru the VPP.

If you every want to get REALLY fancy, the iRegatta ipad app with generate polars by collecting data in real time.... Sadly that requires NMEA data available wirelessly..... ;^\

RichH · Feb 7, 2012

If you have a mast head 'wind speed/dir' instrument and that is integrated to a GPS .... such calcs. are automatic .... just push the TRUE button on the wind machine readout instrument.

Ditto if the wind machine is integrated to a GPS and computer navigation system and the 'track' data will automatically record all the TRUE/MAG, windspeed, etc. data. Such 'tracks' usually record all the NMEA083 sentences, just learn how to read them if not converted to output instrument data and make your corrections manually.

OR

You can go to the pulpit and read your hand-held, and simultaneously read the magnetic compass as the boat s-l-o-w-l-y goes through the eye of the wind on a tack, momentarily going into irons for a few seconds so that the wind will 'attach' equally to both sides of the sails while taking the average of several magnetic compass and hand held readings, etc.

Polar plots are usually always based on windspeed and true wind dir. *at the mast head* ... so you might need to estimate some correction when you plot data taken near the surface of the water.

Bill Roosa · Feb 7, 2012

I don't use polars much as there are none for my boat, however how would you use them if they were for true wind speed and angle? You never set the sails by true values. With that said, I thought all polars were referenced to apparent wind angle and speed. My reasoning is they could never quote a value of true wind speed that also incorporated the speed of the boat so they can't quote the true values.
Clearly I'm missing something.

RichH · Feb 7, 2012

There is a relative better way to do this.

Go to http://www.phrfne.org/page/567, and find your PHRF number.
Then lookup similar PHRF ratings for boats similar to yours in underwater shape, keel configuration, sail area, etc. and search for polar plots for these boats.

Take the polar plot from one or two of these similar boats, ..... and assuming perfect boat handling, sail trim and shaping, and helmsmanship .... do several comparison trials to make corrections of actual differences to your boat -- when sailing on magnetic courses in relation to the true wind referenced on the polar plots. Will be 'close enough' if you are 'club racing' or just want to have a 'target' for 'performance cruising', etc.

Jackdaw · Feb 7, 2012

Bill Roosa said:
I don't use polars much as there are none for my boat, however how would you use them if they were for true wind speed and angle? You never set the sails by true values. With that said, I thought all polars were referenced to apparent wind angle and speed. My reasoning is they could never quote a value of true wind speed that also incorporated the speed of the boat so they can't quote the true values.
Clearly I'm missing something.

Bill,

Polar are always done true wing speed and angle. That have to be, else the curve would wildly distort as the AWS dropped and the plot turned downwind. Remember true wind speed does not factor in the boat speed or angle. Thats apparent wind.

Looking at the polars gies you an idea of your best windward and downwind angles at any true wind speed. And you are right, crews don't sail to a true wind ange at any true wind speed, they sail to a optimal true BOAT SPEED for any true wind speed. If you look at any racing boat, they will have along with optimal angles target upwind and downwind boat speeds for a list of true winds. Set to this upwind, and you are optimal. Slower, you are sailing too high, faster you are footing off but sailing too far.

IN the chart below you can see the 'Up.Vs'. This is the optimal boat speed for max VMG on our First 260, based on true wind speed. Monitor true, drive to that, you'r golden. As long as the polars are right! ;^)

Bill Roosa · Feb 7, 2012

OK
thanks Jackdaw

Gudenuph · Feb 7, 2012

Re True Wind on a handheld

Quoddy said:
If I make my own polars or target speeds (none published for H260) the reference point is true wind. Any suggestions on easily determining true wind from a hand held wind meter while underway?
I suppose to make this work, we basically need a wind instrument linked to GPS on our Raymarine multifunction display for TW on an instantaneous basis.

IMHO, the published polars and target speeds have to assume zero current and at best, estimate the sea state. Current and fetch affect sea state significantly! So, unless you are interested in the intermediate detail, why not just trim for maximum boat speed and steer for the best Velocity Made Good on the GPS to automatically factor in all of the conditions pertaining at the time? Don't forget the other tack when you cross the rhumb line , and of course it helps to be on the favored side of the future wind shifts and in the best current.
That's the way I see it, Geohan

Jackdaw · Feb 7, 2012

Gudenuph said:
why not just trim for maximum boat speed and steer for the best Velocity Made Good on the GPS to automatically factor in all of the conditions pertaining at the time?Geohan

But thats the point of the polars, max boat speed is only sometimes equal to VMG.

And BTW, the VMG feature on almost all GPS devices is bad because it calculates VMG towards a POINT (the waypoint) and not a DIRECTION. That means that the reported VMG speed will constantly change and get worse as you approach the laylines, even as everything else stays the same.

RichH · Feb 7, 2012

Jackdaw said:
But thats the point of the polars, max boat speed is only sometimes equal to VMG.

And BTW, the VMG feature on almost all GPS devices is bad because it calculates VMG towards a POINT (the waypoint) and not a DIRECTION. That means that the reported VMG speed will constantly change and get worse as you approach the laylines, even as everything else stays the same.

not if the POINT you are steering for is at at very long distance 'directly behind' the actual POINT your are steering for .... like at near infinity distance away.

Jackdaw · Feb 7, 2012

RichH said:
not if the POINT you are steering for is at at very long distance 'directly behind' the actual POINT your are steering for .... like at near infinity distance away.

You're right Rich, that's the 'trick' to fool it into a more sailboat-like VMG! ;^)

But that can be a PITA to set up on the water in the heat of the battle. When I ran Product Planning at Magellan I lobbied hard for a sailing version of our marine devices. No dice.

Joe · Feb 7, 2012

How to determine true wind direction......... sail close hauled on port and starboard tack, note each course on the boat compass, not the handheld..... then average the two readings. If you know true wind direction, you can put the boat at 45 degree wind angle and do a simple vector calculation (hah, hah) or... heave to for a moment.

Keep track of each heading on each tack... to determine wind shifts.

That's the essence of the tactical electronic compasses, such as the tacktick products.

For instance, if you sail close hauled on starboard with your heading at 315, then tack over to port and sail close hauled at 45, then you would have a true wind direcection of 360 or 0 deg. If you note a +5 degree change on starboard then you know the true wind has shifted also....in this case a lift....

Compare the boat's heading to the true wind direction to determine true wind angle when incorporating your polar data as exampled in Jackdaw's piece on VMG strategy.

Sailing off the wind is similar but you find the optimal wind angle and seek the target speed..... comparing that to the polar figure. Then, when you jibe over, you seek the same speed number.... and note the wind angle... same as going up wind, changes will alert you to wind shifts.

Obviously, it's important to log the data for ongoing analysis to aid your decision making.

Bill Roosa · Feb 7, 2012

note the GPS SOG, true course over ground and apparent wind speed and direction
SOG and true course are the boat vector (speed and direction)(makes an arrow) Choose a length to represent 1 knot, 1"/knot, 1cm/knot, 3.1417 quatlues/knot it does not really matter except it all has to fit on the graph paper.
Apparent wind speed and apparent wind direction make the apparent wind vector
Place the apparent wind and boat vectors head to head (so the arrow points touch)
Draw a new vector from the tail of the apparent wind vector to the boat vector tail and that is the true wind vector. The length is the true wind speed and the angle is the true wind direction
Note that the wind arrows point "as the wind flies" and not "into the wind"
Polar plotting paper makes this a snap

Gudenuph · Feb 7, 2012

Re True Wind Determination

Quoddy said:
If I make my own polars or target speeds (none published for H260) the reference point is true wind. Any suggestions on easily determining true wind from a hand held wind meter while underway?
I suppose to make this work, we basically need a wind instrument linked to GPS on our Raymarine multifunction display for TW on an instantaneous basis.

Perhaps the simplest (easiest?) way would be to come head to wind or dead down and then subtracting or adding, respectively, the boat speed from/to the Apparent Wind. That is if you can stand a bit of a loss and the having to ease and re-trim the jib to avoid back winding it. Going to windward, just work it into your weave through the wave crests and troughs. In light air just guess at it. LOL Anyway, first place is not cost effective!
Geohan

Stu Jackson · Feb 7, 2012

Jackdaw said:
You're right Rich, that's the 'trick' to fool it into a more sailboat-like VMG! ;^)

But that can be a PITA to set up on the water in the heat of the battle. When I ran Product Planning at Magellan I lobbied hard for a sailing version of our marine devices. No dice.

Actually, it's quite simple, just set up two waypoints: one to the infinite distance on the true wind angle and the other to your waypoint for the next upwind mark.

Gary_H · Feb 8, 2012

HMMM... seems like all this math and cyphering is gonna take the fun out of sailing.