Hard to understand VMG and speed values

danstanford

danstanford

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Aug 2, 2010
502
J-Boat J/88 Cobourg
First let me say that I have been having some problems with my autopilot and seemingly sometimes with my flux compass, not sure how the two problems may be related but I am tightening wires and cleaning joints!
Now to the question. We were bringing up the back of the fleet in a race last night going dead down wind for just under 4 NM going east to west on the north shore of Lake Ontario. As I played around with VMG to waypoint (end of leg) I got VMG readings from my chartplotter that were higher than the speed on my knotmeter which I have always thought was a little optimistic. I understand that if I were going down current that this reading were possible but if there was any current we should have been going into it.
It struck me because going dead down wind wing on wing gave me the highest VMG and I have always believed a faster deep reach would give me the best VMG...now I should also admit that I was struggling to maintain 3 knots VMG (with a showing boat speed of 2.5) in just under 6 knots of breeze.
Any explanations you can think of?
Dan
 
M

markwbird

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Nov 26, 2012
1,653
Hunter 34 Berkeley
The only way the VMG would be faster than what the knot meter reads is if you are going with the current or there is something wrong with the equipment. It is not possible for VMG to be faster than the speed of the boat over the ground. It seems likely that your knot meter is reading slow. Maybe time to clean it..
 
B

Benny17441

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May 24, 2004
7,131
CC 30 South Florida
Ditto, something wrong with the equipment, my bet is also on the knot meter being the culprit.
 
danstanford

danstanford

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Aug 2, 2010
502
J-Boat J/88 Cobourg
Thanks for the answers folks, this is how I added it up too. Strangely, I would say the knot meter has always been optimistic so I will pull it out and have a look!
 
Davidasailor26

Davidasailor26

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May 17, 2004
5,069
Beneteau Oceanis 37 Havre de Grace
There is usually a way to calibrate knot meters, either electronically or with a calibration knob. To set it to the right value either match it against GPS speed or by seeing how far the knot meter says you went after traveling a known distance (preferably going out and back so effects of current are minimized).
 
uncledom

uncledom

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Jun 11, 2011
1,243
Hunter 41 Lewes
Perhaps your showing boat speed through the water andthe VMG is being calculated with GPS as speed over ground.
 
HMT2

HMT2

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Mar 20, 2014
899
Hunter 31 828 Shoreacres, TX
uncledom said:
Perhaps your showing boat speed through the water andthe VMG is being calculated with GPS as speed over ground.
Click to expand
That would be my guess too. GPS uses speed over ground in calculating VMG.
 
B

Benny17441

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May 24, 2004
7,131
CC 30 South Florida
uncledom said:
Perhaps your showing boat speed through the water andthe VMG is being calculated with GPS as speed over ground.
Click to expand
Yes down current but "danstanford" indicates if any he was likely pushing against current.
 
Jackdaw

Jackdaw

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Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
All VMG values should be speed over water, as should be their components. Never SOG.
 
HMT2

HMT2

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Mar 20, 2014
899
Hunter 31 828 Shoreacres, TX
Jackdaw said:
All VMG values should be speed over water, as should be their components. Never SOG.
Click to expand
So Jackdaw, other than the boat being on the water, thus SOW vs SOG is there any difference between SOW and SOG as calculated by the GPS? The point I was trying to make is that the speed through the water as indicated via the paddle wheel will not always match with the SOW. "Or SOG as my GPS reads" as calculated by GPS.
 
Jackdaw

Jackdaw

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Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
HMT2 said:
So Jackdaw, other than the boat being on the water, thus SOW vs SOG is there any difference between SOW and SOG as calculated by the GPS? The point I was trying to make is that the speed through the water as indicated via the paddle wheel will not always match with the SOW. "Or SOG as my GPS reads" as calculated by GPS.
Click to expand
Well first, from a sailing perspective, all performance calculations like VMG should only use over-water-based speeds, because things like the actual speed of the water will make the reading meaningless. And navigation techniques can factor them in later for actual navigation.

Now to your question, GPS cannot calculate SOW. The little box has no idea why you are going 5 knots, be it your sails or the current, or one of a million possible combinations of the two.

Now, in slack water, why are the two (SOW vs SOG) often different?

1) Calibration - most units have a user settable 'calibration factor (say 0.8 to 1.2) that they will multiply against the default turn rate to get actual speed close. If it's off you're off.

2) Dirty - If the wheel or aperture is dirty, it will not measure well.

3) Wear - If the wheelshaft it worn, it will not measure well.

4) Non-linearity - 1-3 all add up to this, making getting #1 right impossible

5) Wheel not on centerline - If the wheel is on the side of the hull, one tack will measure .2-.4 knots faster. This is because the wheel 'bites' better in more (deeper) water pressure. Race boats will always have the transducer on the CL, or even better have TWO transponders, each at the optimal heel angle on a tack.
 
uncledom

uncledom

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Jun 11, 2011
1,243
Hunter 41 Lewes
It's called SOG on every GPS instrument I have ever seen. It just means speed across the planet no matter what medium your using, water, land or air.
 
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