Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 07-01-2011, 08:57   #1
Registered User

Join Date: Jan 2009
Location: Home port is San Diego, California but currently cruising the East Coast of the USA.
Boat: Contest 48
Posts: 638
Images: 1
Velocity Made Good Calculator

As we're planning a significant amount of down wind sailing soon (Mexico to the South Pacific) and I'm hoping someone can share with me a simple formula or calculator to determine Velocity Made Good. Obviously, sailing deep down wind is slower than a broad reach but one covers more distance sailing off the rhumb line.

My new Garmin chartplotter has a Wind Velocity Made Good field but there's no explanation for it. When I'm sailing straight to a waypoint, it shows a negative number, however, when I'm tacking or jibing to a destination, it shows a positive number.

Perhaps some will say that after sailing for 45 years, I should know the answer to this question and I won't argue. Regardless, I'd appreciate any feedback.

Fair winds and calm seas.
__________________

__________________
nhschneider is offline   Reply With Quote
Old 07-01-2011, 09:09   #2
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,150
VMG is the straight line speed towards your waypoint or destination.

VMG can be calculated using a plotting sheet or using trig. Some GPS units and electronic charts have this feature.

The only time it should be negative is if you are getting further away from your destination as measured by a straight line between you and your waypoint or destination. Your VMG will always be less than or equal to your SOG.
__________________

__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 07-01-2011, 14:05   #3
Senior Cruiser

Cruisers Forum Supporter

Join Date: Mar 2007
Location: Santa Cruz
Boat: Boatless Again
Posts: 4,324
VMG=speed x COSINE(course-bearing to mark)
__________________
donradcliffe is offline   Reply With Quote
Old 07-01-2011, 14:32   #4
Registered User

Join Date: Jul 2010
Location: Seattle, Wa
Boat: Spencer 31
Posts: 62
Believe on the Garmin unit, it is showing you your VMG upwind... so positive for heading up and negative for downwind. If you are trying to make the best time downwind, you would want to sail with the largest negative number.
__________________
jaredko is offline   Reply With Quote
Old 07-01-2011, 15:32   #5
Registered User

Join Date: Jun 2008
Posts: 64
VMG to upwind = speed * cos( true wind angle )

I assume that you Garmin is hooked up to the wind instruments, in which case what jaerdko said. Positive VMG is upwind, negative VMG is downwind.

But some chartplotters display VMG as toward the waypoint, not the wind. So double check what you have displayed.
__________________
zydecotoad is offline   Reply With Quote
Old 07-01-2011, 15:47   #6
Senior Cruiser

Cruisers Forum Supporter

Join Date: Dec 2006
Location: Seattle
Boat: Cal 40
Posts: 2,401
Images: 7
A lot of GPSs give VMG to the next waypoint. I've read that some will give VMG for upwind, which obviously requires a wind direction and I would assume a wind speed input so it can calculate true wind direction (also requires boat speed.) I don't have this function on my GPS, but I would assume that somewhere in the menu is a choice between waypoint and wind VMG.

Plotting sheet solution that David pointed out is simply find the straight line distance from where you started to where you are on the chart and divide by the time it took. Plotting sheets will have speed, time, distance nomogram so you don't even have to do the division.

John
__________________

__________________
cal40john is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Celestial 229 vs 'Calculator Method' ? sneuman Navigation 43 08-11-2012 14:07
Amp Hour Load Calculator - Did I Miss Anything? GeoPowers Electrical: Batteries, Generators & Solar 29 13-01-2010 07:41
Electrical Calculator GordMay Electrical: Batteries, Generators & Solar 0 17-04-2007 03:02
Can anyone suggest a good european made bluewater cruiser (27-32) uki Monohull Sailboats 27 03-11-2005 07:39
Wire size calculator Alan Wheeler Electrical: Batteries, Generators & Solar 0 21-09-2004 14:42



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 07:26.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.