VMG Wind
 

1) Both my Raymarine ST60 Wind Machine and E-80 Chartplotter have the
ability to display a value "VMG wind". What is this ? Why is it
negative sometimes ?

2) What does the TACK button do on the Wind Machine. Does it simply
make my heading display on the wind-machine ?

VMG Wind
 

"b393capt" wrote in
ups.com:

1) Both my Raymarine ST60 Wind Machine and E-80 Chartplotter have the
ability to display a value "VMG wind". What is this ? Why is it
negative sometimes ?

"Velocity Made Good (VMG): The speed of a yacht relative to the waypoint
it wants to reach, or toward or away from the wind."

Your VMG is negative when your tacking isn't working and your boat is
actually losing the battle to the waypoint, going AWAY from it, instead
of towards it. The waypoint keeps getting further away, an unobtainable
goal.

Just watch Lionheart at the start of a CORA race in light wind when the
tide is going the wrong way....(c; Negative VMG (the starting line)
causes a lot of cursing in the cockpit...hee hee. Hell, I had to tack
The Beast to get forward motion with the 50hp diesel, one day! Even with
our handicap, the race committee insists we at LEAST make it over the
starting line! We'll make it up later during that inevitable gale when
Lionheart is haulin' ass and those light boat smartasses are trying to
keep it from broaching when someone in a jacket stands up in the
cockpit...


2) What does the TACK button do on the Wind Machine. Does it simply
make my heading display on the wind-machine ?



http://www.raymarine.com/raymarine/S.../Instruments/6
0wind_prompt.pdf

Look on page 2. If you press TACK, the Wind instrument gives you the
correct tack heading for best VMG when you come around (smartly, of
course). It's other function is to hold it for 3 seconds to reset
Maximum Wind, the instrument constantly stores for comparisons.

Of course, with the captain screaming at the crew who dropped the genoa
sheet that's now flogging around in the block on its dead end knot.
Pressing the TACK button is a moot point....until they recover it so you
have steerage, again.....(c;

Sorta makes you hope your name is posted for the midwatch when things are
"less hectic" and His Majesty is snoring away in the aft cabin....(c;

--
There's amazing intelligence in the Universe.
You can tell because none of them ever called Earth.

VMG Wind
 

b393capt wrote:
1) Both my Raymarine ST60 Wind Machine and E-80 Chartplotter have the
ability to display a value "VMG wind". What is this ? Why is it
negative sometimes ?

2) What does the TACK button do on the Wind Machine. Does it simply
make my heading display on the wind-machine ?


It's the speed at which you are moving into (positive value) or from
(negative value) the direction of the wind. It is calculated as
cos(heading-wind)*speed.

So if you are sailing due East with a Northerly wind, your VMG is 0.
When you sail due North, it will be identical to your speed.

It is used when you want to sail to a target (almost) directly upwind.
You should then sail at maximum VMG. In other words it tells you whether
you will get there faster when sail at a different angle to the wind.

The Tack button will tell you what your course will be when you tack and
sail at the same wind angle.

Example: you sail close hauled on port tack, and there's a cliff you
need to clear when you tack. You wait with tacking until the bearing to
the cliff is "past" the tack heading the system predicts you will be
able to attain.

--
Kees

VMG Wind
 

Kees Verruijt wrote:
The Tack button will tell you what your course will be when you tack and
sail at the same wind angle.

Example: you sail close hauled on port tack, and there's a cliff you
need to clear when you tack. You wait with tacking until the bearing to
the cliff is "past" the tack heading the system predicts you will be
able to attain.

Is this a basic calculation (e.g. it expects the boat to have
symetrical performance on each tack), or does it do something fancy
such as take into account the direction & speed of water current, the
average wind direction for x minutes, etc ?

Dan

VMG Wind
 

And most important: the fact that you can go closer to the wind at higher
windspeed ....

Bjarke

"b393capt" wrote in message
oups.com...
Kees Verruijt wrote:
The Tack button will tell you what your course will be when you tack and
sail at the same wind angle.

Example: you sail close hauled on port tack, and there's a cliff you
need to clear when you tack. You wait with tacking until the bearing to
the cliff is "past" the tack heading the system predicts you will be
able to attain.

Is this a basic calculation (e.g. it expects the boat to have
symetrical performance on each tack), or does it do something fancy
such as take into account the direction & speed of water current, the
average wind direction for x minutes, etc ?

Dan

VMG Wind
 

b393capt wrote:
Kees Verruijt wrote:
The Tack button will tell you what your course will be when you tack and
sail at the same wind angle.

Example: you sail close hauled on port tack, and there's a cliff you
need to clear when you tack. You wait with tacking until the bearing to
the cliff is "past" the tack heading the system predicts you will be
able to attain.

Is this a basic calculation (e.g. it expects the boat to have
symetrical performance on each tack), or does it do something fancy
such as take into account the direction & speed of water current, the
average wind direction for x minutes, etc ?


AFAIK it's quite basic. Instantaneous calculation based on boat speed
(from log, not GPS), course from heading sensor and current wind angle.

--
Kees

VMG Wind
 

Actually, you can use the negative VMG to advantage when tacking downwind.
Maximise the absolute value.

"Larry" wrote in message
...
"b393capt" wrote in
ups.com:

1) Both my Raymarine ST60 Wind Machine and E-80 Chartplotter have the
ability to display a value "VMG wind". What is this ? Why is it
negative sometimes ?

"Velocity Made Good (VMG): The speed of a yacht relative to the waypoint
it wants to reach, or toward or away from the wind."

Your VMG is negative when your tacking isn't working and your boat is
actually losing the battle to the waypoint, going AWAY from it, instead
of towards it. The waypoint keeps getting further away, an unobtainable
goal.

Just watch Lionheart at the start of a CORA race in light wind when the
tide is going the wrong way....(c; Negative VMG (the starting line)
causes a lot of cursing in the cockpit...hee hee. Hell, I had to tack
The Beast to get forward motion with the 50hp diesel, one day! Even with
our handicap, the race committee insists we at LEAST make it over the
starting line! We'll make it up later during that inevitable gale when
Lionheart is haulin' ass and those light boat smartasses are trying to
keep it from broaching when someone in a jacket stands up in the
cockpit...


2) What does the TACK button do on the Wind Machine. Does it simply
make my heading display on the wind-machine ?



http://www.raymarine.com/raymarine/S.../Instruments/6
0wind_prompt.pdf

Look on page 2. If you press TACK, the Wind instrument gives you the
correct tack heading for best VMG when you come around (smartly, of
course). It's other function is to hold it for 3 seconds to reset
Maximum Wind, the instrument constantly stores for comparisons.

Of course, with the captain screaming at the crew who dropped the genoa
sheet that's now flogging around in the block on its dead end knot.
Pressing the TACK button is a moot point....until they recover it so you
have steerage, again.....(c;

Sorta makes you hope your name is posted for the midwatch when things are
"less hectic" and His Majesty is snoring away in the aft cabin....(c;

--
There's amazing intelligence in the Universe.
You can tell because none of them ever called Earth.