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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

VMG

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RoofRoof
Navigator

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USA
186 Posts

Initially Posted - 12/24/2003 : 10:02:07

I now have a new wind instrument linked via NMEA to my Garmin GPS/chartplotter 176C. Now, I need to know more about VMG.

Can anyone explain exactly what it is, and how it is used? Must I be heading to a specific point to use VMG?

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 12/24/2003 : 10:29:47

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 12/24/2003 : 12:40:57

"Must I be heading to a specific point to use VMG?"

Yes... simply put, it's "how well you're doing" at moving towards that point.
If you have no destination, you have no VMG and are simply sailing "by the wind"...
(salty talk there).

Thats part of the charm of sailing... figuring the fastest way to your destination which has to factor in speed at any point of sail, currents, leeway, yada. For a device with basically only 3 moving parts, a sailboat gets pretty complicated.

Edited by - on

Arlyn Stewart
Master Marine Consultant

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USA
2984 Posts

Response Posted - 12/24/2003 : 12:59:04

Yes, it comes into play relative to a specific waypoint. Think of it this way. If your heading directly at the waypoint, your boatspeed is the VMG or velocity made good toward the mark... but if unable to sail directly at, the numbers are crunched by the gps to produce this number (vmg) that is used to calculate among other things the elapsed time to arrival at the waypoint considering the lack of a straight course.

The wind instrument has traditionally been the tool of deciding favored tack after applying its observations to the chart. GPS makes that process a lot easier.

As far as the gps unit using any data from the wind, newer models might, I don't know, or they might simply be repeating it so that it will be available on another display. Auto pilots however, can use wind instrument data to steer a wind course rather than a compass or waypoint course.

I've sailed a good amount on a boat with a wind instrument and find them very nice. The substitute is a hand bearing compass and hand wind strength meter which work well but aren't quite as convenient.

Edited by - Arlyn Stewart on 12/26/2003 07:55:39

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 12/24/2003 : 15:03:24

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by RoofRoof
 I now have a new wind instrument linked via NMEA to my Garmin GPS/chartplotter 176C. Now, I need to know more about VMG.

Can anyone explain exactly what it is, and how it is used? Must I be heading to a specific point to use VMG?<hr height="1" noshade id="quote"></blockquote id="quote">

From a SailNet article titled, "[url="http://www.sailnet.com/collections/articles/index.cfm?articleid=leonar0021"]Basic Downwind Performance Part II[/url]"...

VMG

Performance sailors use the acronym VMG (velocity made good) to express boat speed made along the direction of the wind. The concept holds that for a specific sailing condition, there is a single combination of boat speed and true wind angle that equates to the optimum VMG. On any point of sail—upwind or down—the longer we can maintain the best VMG, the better our cumulative performance.

VMG for specific boats is best understood by way of a polar plot or polar diagram. The information that supports these diagrams is derived from velocity prediction programs (VPP), which take into account the boat's displacement, sail area, and righting moment. In the example above, a SailNet 30 will make six knots sailing at a true wind angle of 45 degrees. Downwind, it should move at 5.85 knots at an angle of 150 degrees, and so on. The actual VMG is listed at the left side of the plot. (VMG downwind is expressed as a negative quantity.) Using this polar diagram, the crew on board has a fixed idea of the speeds that they should attain for optimum performance on every point of sail.

Edited by - on

Derek Crawford
Master Marine Consultant

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USA
3324 Posts

Response Posted - 12/24/2003 : 20:01:23

Of course, you can always fall back to the KISS principle - if you are heading more than 45 degrees from the weather mark - then tack!
Derek

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osmepneo
Past Commodore

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USA
1420 Posts

Response Posted - 12/25/2003 : 07:56:23

If you ewven know that more or less than 45 degrees from the "point"

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Arlyn Stewart
Master Marine Consultant

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USA
2984 Posts

Response Posted - 12/25/2003 : 10:55:30

Well...actually, after sleeping on my comment yesterday... I have to retract

what I said about tacking at half the boat speed. It was flawed logic and in fact I should have grasped the flaw from an experience last summer.

My only redemption is that no one challenged me before my retract.

So, to restate, if sailing to a weather mark and able to sail at 45 degrees to the wind, the lay line is
NOT at a vmg of half the boat speed but rather at 0.

If considering the leeway of the c250 which very often in strong winds and seaway requires a much larger than 90 degree tack angle... the lay line would be reached at a negative VMG.

Edited by - Arlyn Stewart on 12/25/2003 17:51:15

Derek Crawford
Master Marine Consultant

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USA
3324 Posts

Response Posted - 12/25/2003 : 11:56:38

Don - the RC boat should have the compass bearing to the first mark as part of the course board. You calculate the 45 degrees from that.
Derek

Edited by - on

jwilliams
Captain

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USA
357 Posts

Response Posted - 12/26/2003 : 11:36:57

Hi all,

Thanks to my Buddy Paul (Old Neptune) I got a neat paperback manual for Christmas called Boat Navigation for the Rest of Us, byt Capn. Bill Brogdon. He defines VMG and explains a discrepancy between some manufacturers of nav devices:

Velocity Made Good
Velocity made good indicates the speed over ground in the direction of the destination waypoint. It is primarily useful to sailboat navigatos when beating to windward. A sailboat sailing as close to the wind as possible but unable to point toward the destination may be making 65 knots through the water, but only 4 kts toward the waypoint. Unfortunately, manufacturers have two definitions of VMG:

1. Speed along the line from the starting point to the destination waypoint. This is the more useful of the two VMG definitions.

2. Speed from the current positioin to the destination waypoint. A receiver using this definition shows VMG decreasing along a leg of the course, even thought speed and heading are constant. VMG decreases to zero as a sailboat approaches the lay line, preparing to tack for the next mark.

ps: this is an excellent companion book to the USCGA course I just took.

Jim Williams
Hey Jude C25fk 2958
SF Bay

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