I want to try VMG to the TW by using the 2000 mile waypoint. Would substantial error be introduced if I use the GPS course reading when passing through head to the wind? Do I need a fluxgate compass linked to the GPS or independent compass to get good results ?
GPS to find wind direction
- Thread starter Quoddy
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GPS course when you're turning through the wind? You head the boat up into the wind, the sails luff, you look at your compass. Then input that bearing into the GPS. If you have time to head into the wind for a longer time, sure, use the GPS. The simple point is to get as close to TW when you're heading directly into it as you can. Since the waypoint is 2000 miles away, it ain't an exact science either, because the local winds WILL change direction, right? Maybe not substantially, or maybe major, but just remember what it is you're trying to do.
I don't know how much lag there is on the GPS course, but I suspect that you would want to hold your upwind course for 10-20 seconds at least to let the gps course settle out.
Sounds good
It sounds like that could be quite useful in finding the TW at anytime, without passing through the eye and without instruments that have that computing capability.
It sounds like that could be quite useful in finding the TW at anytime, without passing through the eye and without instruments that have that computing capability.
I am not sure what the purpose of this exercise would be. The true wind direction will change withing 100 feet of your current location let alone any distance away. VMG will be determine as the boat moves toward a waypoint over time and not for any one given moment in time. Chuck
Re: Sounds good
More than you care to know can be found here...
http://www.foundationwebsite.org/Miscellany51.pdf
More than you care to know can be found here...
http://www.foundationwebsite.org/Miscellany51.pdf
What are you trying to determin?
VMG toward a point 2000 miles directly upwind from the boat will tell you what? Trying to calculate the best wind angle to beat at? While that is an interesting thought experiment I can't think of a thing I can do with that info. The fact is you typically don't want to sail to a point directly up wind but to one that is off the wind to one side or the other. So VMG to the point is what you would want in most situations. There will be one tack that is "better" VMG wise. The may be that that is also your best angle but probably not. You can pick the better tack, come off close hauled beat a little and make better VMG. Or stick with the worse tack, stick to the best wind angle VGM you determined and suffer beating all day.
Perhaps I'm missing something.
VMG toward a point 2000 miles directly upwind from the boat will tell you what? Trying to calculate the best wind angle to beat at? While that is an interesting thought experiment I can't think of a thing I can do with that info. The fact is you typically don't want to sail to a point directly up wind but to one that is off the wind to one side or the other. So VMG to the point is what you would want in most situations. There will be one tack that is "better" VMG wise. The may be that that is also your best angle but probably not. You can pick the better tack, come off close hauled beat a little and make better VMG. Or stick with the worse tack, stick to the best wind angle VGM you determined and suffer beating all day.
Perhaps I'm missing something.
Maybe wrong title for posted question
My original answer assumed (doh!) that the OP was asking about using a 2000 mile ahead waypoint to determine best VMG to the TW. And not to the next waypoint.
That "way out waypoint" has been mentioned a bit recently on this board to use for determining best upwind VMG.
Of oucrse, it assumes (drats, there it is again!) that the wind stays constant.
It's just a tool, not necessarily one to quibble with...
My original answer assumed (doh!) that the OP was asking about using a 2000 mile ahead waypoint to determine best VMG to the TW. And not to the next waypoint.
That "way out waypoint" has been mentioned a bit recently on this board to use for determining best upwind VMG.
Of oucrse, it assumes (drats, there it is again!) that the wind stays constant.
It's just a tool, not necessarily one to quibble with...
TW
I want to maximize speed to a mark. If I use VMG to that mark, the closer I get the lower the VMG, as I come along side it for the final tack it drops to zero.
These are two places which describe how and why VMG to TW can be a better method than straight VMG.The results can be similar to using polars.
http://www.wingsailor.com/html/vmg___gps.html
http://knol.google.com/k/john-navas/why-vmg-matters/qefv1b2dsnit/2
It seems that keeping track of the TW, to know if its oscillating or shifting and if you are near the median is key to making this work. How to do that was the question.
I want to maximize speed to a mark. If I use VMG to that mark, the closer I get the lower the VMG, as I come along side it for the final tack it drops to zero.
These are two places which describe how and why VMG to TW can be a better method than straight VMG.The results can be similar to using polars.
http://www.wingsailor.com/html/vmg___gps.html
http://knol.google.com/k/john-navas/why-vmg-matters/qefv1b2dsnit/2
It seems that keeping track of the TW, to know if its oscillating or shifting and if you are near the median is key to making this work. How to do that was the question.
We have a Ockam 4 display system with about 48 display chips and DO NOT use VMG or about 44 of the other chips 
True wind direction made easy
True wind direction can be easily determined by averaging your closehauled headings on starboard and port tacks. That how's the TackTick tactical compass does it. All you need to do is jot down the compass headings on both tacks, add them together, then divide by two. example.... starboard 270 + port 360 = 630 / 2 = 315 true wind...... there I just saved you 1200 dollars.
VMG/GPS info is way more useful with a thorough understanding of your boat's tacking angles. You also need to be able to read wind shifts. For that reason you need to keep track of your headings as you sail towards the mark to determine if there is a wind shift pattern you can utilize to your advantage. The vmg to mark feature of a gps can help that, but you can't luff the boat up everytime you want to know what true wind is.
Another thing you need to be able to estimate is the lay line. If you know true wind, your tacking angles and the bearing to the mark you can figure it out. If you wait for vmg to tell you you've sailed passed the lay line by getting smaller... it's too late, you've lost.
True wind direction can be easily determined by averaging your closehauled headings on starboard and port tacks. That how's the TackTick tactical compass does it. All you need to do is jot down the compass headings on both tacks, add them together, then divide by two. example.... starboard 270 + port 360 = 630 / 2 = 315 true wind...... there I just saved you 1200 dollars.
VMG/GPS info is way more useful with a thorough understanding of your boat's tacking angles. You also need to be able to read wind shifts. For that reason you need to keep track of your headings as you sail towards the mark to determine if there is a wind shift pattern you can utilize to your advantage. The vmg to mark feature of a gps can help that, but you can't luff the boat up everytime you want to know what true wind is.
Another thing you need to be able to estimate is the lay line. If you know true wind, your tacking angles and the bearing to the mark you can figure it out. If you wait for vmg to tell you you've sailed passed the lay line by getting smaller... it's too late, you've lost.
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Quoddy,
Simple answer is: you need BOTH. Just like the age old argument of "I don't need no stinkin' paddelwheel knot meter in MY boat! I have a GPS!", it is helpful to know BOTH the VMG to the next mark AND the VMG upwind.
Here on San Francisco Bay the TW is relatively steady, but relative is a, well, relative term! It backs coming westbound around Point Blount, and veers going westbound near the South Tower of the GGB. You know, a little local knowledge.
The steadily decreasing VMG to the next mark is simple: when it gets to zero, TACK, assuming you can do 90 degrees. On older GPSs without the VMG displayed, they still did the math but thery called it time to go (TTG). when it went ballistic and then to infinity (and beyond) you needed to tack.
Unless you know where your wind shifts are coming from before they do, you can usually only input one at a time, so oscillating wind directions that vary much during a day aren't going to let you make those changes in your GPS on the fly. You could always pre-input them, and then label them so you can always go to a waypoint named: "Wind 2000 miles away, from 240 Mag", etc.
Get both and see what I mean.
GPS VMG Wind
Thanks a lot. I’ve got lots of ideas to work with. I keep finding more as I reread.
Thanks a lot. I’ve got lots of ideas to work with. I keep finding more as I reread.
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