A few terms to explain me please

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cmedsailor

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Hello,
Could you please help me a little bit clear in my mind the following terms?
Heading: Is this the true course as shown on the map? So, depending where you sail there must be a constant deviation from compass reading (+ or – some degrees)?
COG: The course, the movement of the boat, in relation to the ground? Any deviation from heading could be due to currents etc?
Waypoint Bearing: The true (per map) angle between the place where the boat is and the waypoint? Or is it the course you need to sail to get to the waypoint (or probably I am saying the same thing)?
VMG Wpt ????
 
Heading is the compass direction that the boat is pointing. Ideally this is the same as the course but tide, current and leeway can affect this.

COG or Ground Track is the actual path of the boat over the sea bed.

Waypoint bearing is the bearing to a particular waypoint. To get to that waypoint you would need to set a course to that bearing which may require a different heading to accommodate tide, wind and current effects.

I suggest that you get hold of a copy of the RYA course notes for Day Skipper where all these terms and others are explained.
Good luck
Morgan
 
VMG= velocity made good. Speed in a direct line to the target/waypoint. If you travel in a straight line to your target, your COG=VMG.

If you need to zigzag/tack towards your goal, your VMG will be a lot lower. Why is this important: if you are tacking against the wind, pointing higher towards your waypoint means shorter distance, but slower speed. Sometimes by dropping a few degrees, you increase the distance by 10%, but your speed goes up by 20%. So when tacking always go for the optimum VMG.
 
Heading, COG and Waypoint bearing - what nobody has mentioned is how you have your GPS set up. You can set them so that info is shown as True (i.e. in accordance with the chart) or Compass (taking into account the variation). You need to know the set up.

Heading is where you are pointing, COG is where you actually go because of tides, leeway etc, Waypoint bearing is angle to next waypoint and VMG is rate of progress towards next waypoint.
 
[ QUOTE ]
VMG= velocity made good. Speed in a direct line to the target/waypoint. If you travel in a straight line to your target, your COG=VMG.



[/ QUOTE ]

To avoid confusion, SOG = VMG (SOG = Speed over ground)

COG = CMG (CMG=Course Made Good??? Cant recall that one, but it should exist if it doesnt)

I use the mnemonic CADET to convert from Compass to True, but cant remember what the letters stand for /forums/images/graemlins/smile.gif

Compass Add Deviation East True fits?

Cheers

Richard
 
[ QUOTE ]
I use the mnemonic CADET to convert from Compass to True, but cant remember what the letters stand for

[/ QUOTE ]

<span style="color:red"> C</span>ompas - <span style="color:red"> Ad</span>d <span style="color:red"> E</span>ast - <span style="color:red">T</span>rue

Works for me!
 
[ QUOTE ]
To avoid confusion, SOG = VMG (SOG = Speed over ground)

[/ QUOTE ]

I know you were correcting a post in which someone said (incorrectly) COG = VMG, but your correction - SOG = VMG - only applies if COG = Bearing to Waypoint - i.e. if you are moving directly towards the waypoint.

In any other case SOG > VMG.
 
VMG can cause confusion because of its different uses. Basically it means velocity made good in a specified direction. Long before GPS & Decca in yachts it was used for VMG to windward, calculated by those who could afford B&G stuff from wind transducer & log. This function is still present in Wind readouts as "VMG".

Alternatively, VMG can be used to mean Velocity Made Good to Waypoint, and I have this function on my Raymarine repeater, and occasionally use it. It describes the vector from the boat's speed and direction which is directly to the waypoint.

Thus, when using "VMG" one has to specify which VMG one is referring to.
 
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