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Old 29-05-2011, 04:04   #181
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Re: What Heading Am I Really On ?

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Originally Posted by Jim Cate View Post
Really? Seems to me that if you "motor ahead" the apparent wind moves forward, not aft.
Of couse you are right!
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Old 29-05-2011, 04:40   #182
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Re: What Heading Am I Really On ?

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Originally Posted by DotDun View Post


NO! The NavNet3D allows for calculating True Wind using either STW or SOG, but they still call it True Wind. There is no such thing as Ground Wind in the NavNet3D..

Really?

Wind
Apparent wind speed<FONT face=StoneSans><FONT size=1><FONT color=#221815><FONT face=StoneSans><FONT size=1><FONT color=#221815>
Apparent wind direction
True wind speed
True wind direction
Beaufort scale and cardinal point

<FONT size=1><FONT face=StoneSans><FONT color=#221815>Maximum wind sp
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Old 29-05-2011, 05:01   #183
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pirate Re: What Heading Am I Really On ?

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Originally Posted by Jim Cate View Post
What an interesting theoretical discussion, guys. Well done.

But, as far as I can see, in the real world, these nits have been over picked. In order for the "steer a constant course" method to work, one must have consistent wind speed and direction and to have tidal and current info that is completely correct as well.

These two conditions don't often occur in any part of the world that I have yet sailed. My experience does not include "The Channel", and I do understand that because of the strong tidal currents and heavy traffic it has been well studied and charted, but I don't believe that the tidal atlas considers the effects of wind driven currents, atmospheric conditions and all of the other minor things that add up to the actual flow on any given day/time. And in most other parts of the world, there is no tidal current data available at all.

And as to the wind (which greatly affects one's boatspeed and perhaps course), it seems to vary a lot when I'm sailing. One can use various means to predict what it will do over the course of a voyage, but IME none of these are accurate enough to make the "constant heading" method work over a whole voyage, or I suspect, even a channel crossing.

So, while the theoretical advantage of that method of navigation may be real, in practice frequent corrections must be made to reflect actual conditions, and these in turn make rhumb line sailing look pretty practical.

On ocean passages I reckon one is doing well just to avoid areas of known adverse currents, and even this is more difficult than it appears. Those nice broad arrows shown on the pilot charts are fictitious -- in actuality, the currents are made up of lots of eddies and swirls that are only averaged out on the charts. The advent of GPS has for the first time made observation of these small scale phenomena possible. We have experienced large variations in speed and set of currents in mid ocean, events that would not have been measurable by traditional navigational methods, and which would destroy the usefulness of the constant heading course.

At any rate, what Ann and I have done for our passages is to lay out a course that avoids (on paper) the worst adverse currents, avoids seamounts when possible, avoids reefs and such impediments by reasonable margins and eventually arrives at our destination. Then we set out in the best weather conditions we can find and then sail in whatever conditions Neptune throws at us, attempting to stay near enough to our proposed track to avoid the hard bits. It isn't as elegant as the preplanned constant heading, but it has gotten us where we wanted to go in reasonable time frames for a long time now, and I suspect that most oceanic voyagers do something similar.

Cheers,
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Old 29-05-2011, 05:03   #184
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Re: What Heading Am I Really On ?

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Originally Posted by hellosailor View Post
Dockhead-
"lee-bowing" is a racing maneuvre/strategy, it has been misused and misappropriated by a few folks as a navigational concept. EIther they're misunderstanding the earlier use of the word (which goes back decades in racing) or they've borrowed it badly. See:
32nd America's Cup Official Website
That is an entirely different tactic from what we were talking about. I have been studying it, and have come to the conclusion that there are three entirely different phenomena referred to by the same phrase:

1. The racing maneuver mentioned above, which has nothing to do with the other two meanings -- REAL.

2. The idea that if you cross a body of water with cross currents heading up wind, that if you plan your tacks to keep the current on your lee bow, you will have more apparent wind (more water-referenced true wind, too), which can be a lift if you are in need of more wind -- REAL.

3. The idea that you get some kind of magical lift from sailing a shorter distance or sailing closer to the ground wind -- MYTH.

That's my conclusion so far, although I am still studying it.
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Old 29-05-2011, 09:24   #185
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Re: What Heading Am I Really On ?

lee-bowing is not a "racing maneuver", its that the effect is small and only useful over a small course with a consistent tide.

Lee bowing merely means that the tide push the boat up towards an upwind mark. This has the effetc of bring the apparent wind aft ( slightly) which frees the boat and allows it to point slightly higher towards the upwind mark. Hence there is a "real" lift , ie anything that allows you to sail closer to the mark is a lift.

Its not a navigational concept only in so much that it tends to apply over short distances with consistents effects and a desire to reach a upwind mark, but then thats racing isnt it, For cruisers we start the iron topsail.!!

Dave
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