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Old 04-10-2012, 05:57 PM   #21
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Hello Uwe, With respect, allow me to suggest a clarification... The VMG that you are referring to on your GPS is not actually an acceptable measure of velocity. You cannot, for example, see that your VMG is 10 knots, and calculate that since you have 5 nautical miles to go (in a straight line without accounting for tacking distances), that therefore you will arrive in 2 hours.

One problem with this is that it doesn't account for the actual tacking distance you will travel (or your actual boat speed during the length of the tack). Did you know that if you are on the correct tack going upwind with a constant speed, that the VMG you are referring to will decrease all by itself the longer you stay on the tack? It will go all the way down to zero, and even into negative numbers. Not a very good measure of velocity.

There is a detailed explanation of how this happens with Velocity Made Good (VMG), if a geometric proof is helpful, in this article from GAM on Yachting magazine (863 k PDF). Or, you can just try it yourself sometime and see it happen on your GPS. Tack to a waypoint upwind, and watch the VMG decrease even though your speed and heading remain constant, the longer you stay on a tack.

That is why it is better to see a display with your optimal tacks, the exact distance on each tack, and the Tacking Time to Destination (TTD).

You are right that The Sailing GPS does not account for GRIB data or water currents, although the same SailTimer calculations are in the full-featured chartplotter software from NavSim (for Windows) and MacENC (Apple) mentioned on page 1, and these do include many other parameters such as GRIB, water currents and AIS.
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Old 04-10-2012, 07:07 PM   #22
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SV Adventure--you are surely in love with the technology--that is certain

Can you please, as I asked you before, explain your relationship to the manufacturer? Also, in what sailing venues have you personally used the technology? Do you sail? I almost get the impression that you do not sail or at least don't cruise based on your responses.

Thanks.

VMG

I think of the times when wave conditions require us to change tack to non-optimal so we'll have a decently comfortable ride or not be soaking wet Also, the funny sails when the wind and waves are just right to generate the perfect sailing at hull speed one one tack and...70% of hull speed on the other tack

Knowing one's boat and that boat's responses to particular conditions makes a huge difference to knowing when one is going to arrive--and where!

Time is most definitely on the side of the cruiser. The cruising sailor is in luck, though, often our passages are long enough that we can adjust and trim to arrive to a port or anchorage during daylight hours vs in the night. Luckily, our "window" of daylight hours is pretty big. And, luckily, we can often choose to simply heave to and await the daylight hours, or the right tide to clear the bar on a harbor entrance, or what ever the challenge may be.

Winning a race, no we are not. Living a lifestyle at sea, yes.
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Old 04-10-2012, 07:32 PM   #23
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Default Other competing technologies

There are other competing technologies out there (for the wind indicator). The compass, gps, and wind instrument in the Airmar weather station, for example, can provide NMEA 0810 or 2000 data to your computer. I've been intrigued about the Airmar because the wind instrument can't be killed by a passing Osprey who decides to sit on it (as our last wind instrument was!). Having said that, we had the older version of this Airmar (ours a PB100) on the boat but we've never used it as our (new in 2009) wiring inside the mast is bad so we've not "fixed" it yet by running a new power line to it. That's on the 2012 list of things to do.
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Old 04-10-2012, 08:46 PM   #24
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Hi Aquaria, you r opinion regarding a combination of a windvane and a gps is opposite to mine. I think its about time and an excellent developement. Add some paper charts and you,ve got it covered.
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Old 04-11-2012, 04:32 PM   #25
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... s/v adventure,
I hope you find it okay that I put my answers right into your detailed reaction


With respect, allow me to suggest a clarification... The VMG that you are referring to on your GPS is not actually an acceptable measure of velocity. You cannot, for example, see that your VMG is 10 knots, and calculate that since you have 5 nautical miles to go (in a straight line without accounting for tacking distances), that therefore you will arrive in 2 hours.

you are right - absolute numbers do not really help and I do not get to know the ETA. But when tacking I can compare the VMG-data of the starboard- and port-tack and then I love to do the trial and error thing, which is acceptable to me: sailing on one tack for some time and when the VMG deteriorates under a certain number (which has to do with experience), I change to the other bow and check the new data and when they meet my expectations, I continue on this tack. SO, it's just another philosophy of tacking: I'm doing my legs to windward like I learned it before the time of electronic navigation devices, and then (GPS is a fine thing to have) I love to look at the GPS to see, if the VMG is better than before... and so on!

One problem with this is that it doesn't account for the actual tacking distance (...which I personally don't really need to know, tacking can be awful: double distance, triple time and four times as much hastle) you will travel (or your actual boat speed during the length of the tack (... I know my speed very accurately - either as speed through the water given out by the traditional speedometer or by GPS which shows the exact speed over ground!)). Did you know that if you are on the correct tack going upwind with a constant speed, that the VMG you are referring to will decrease all by itself the longer you stay on the tack? Yes. It will go all the way down to zero, and even into negative numbers.Right, and here has do be made a decision way before it reaches the Number Zero, as I explained above. Not a very good measure of velocity but a pretty good measure deriving out of experience over the miles you drive your boat to windward.

There is a detailed explanation of how this happens with Velocity Made Good (VMG), if a geometric proof is helpful, in this article from GAM on Yachting magazine (863 k PDF). Or, you can just try it yourself sometime and see it happen on your GPS. Tack to a waypoint upwind, and watch the VMG decrease even though your speed and heading remain constant, the longer you stay on a tack. ... yes and in this very situation I love to decide when to tack or not and (referring to Fig 2 on page 29 I know out of own experience and with a brief look into the chart that I should continue on the track, even though the VMG will deteriorate still below 1,5kn/2,6 hours, because after another mile the vmg will be at its optimum!!! For this I don't need a display and it's playing with the elements and leaving decisions to myself and not to the onboard electronic devices.
That is why it is better to see a display with your optimal tacks, the exact distance on each tack, and the Tacking Time to Destination (TTD). If the display leads to the situation that the helmsman never learns to decide himself when there is the best time to tack, it is not so good and has nothing to do with learning how to sail.

Maybe it might be a thrilling toy to test the own abilities and experience to drive your boat to windward, comparing the GPSs optimal tacks with our own decision.

But in normal sailing life these data would drive me nuts. It's okay to have navigational raw data to work with, but I love to work with them and do the decision making and the navigation.


You are right that The Sailing GPS does not account for GRIB data or water currents, although the same SailTimer calculations are in the full-featured chartplotter software from NavSim (for Windows) and MacENC (Apple) mentioned on page 1, and these do include many other parameters such as GRIB, water currents and AIS.

... so, interesting idea to combine wind data with GPS, but not really necessary...

Uwe
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Old 04-11-2012, 05:48 PM   #26
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Well at least we are in agreement on the technical points, Aquaria. :-) And I agree that instruments are no replacement for good judgment and seamanship. The instrument readouts allow us to make good judgments.

Aside from all of the details though, for me one important thing is knowing when I am going to get there. Crew and family members may ask, too. It is difficult to give an accurate answer to that question when tacking upwind, no matter how much experience one has. You said you don't want to think about the hassle, which is fine for some people. But many people don't realize that ETA will not give an accurate answer, even though their GPS knows exactly where they are on the face of the earth. The Sailing GPS is an easy way to see what the correct Tacking Time to Destination (TTD) will be.

Different people have different preferences for how they like to sail, of course. (And however you approach it, it is good to know how to get the correct information if you do have to make a decision.) Thanks for sharing your approach -- it was interesting reading.
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