Association Forum - Measuring real and apparent wind

Assn Members Area · Join

Home.

Association Forum

Ships Store

Cruising

Racing

Technical Tips

Photo Gallery - New

Boat Search - Updated

Manuals & Brochures

Swap Meet

Links

Officer Flag Line & Staff

Bylaws & Constitution

Members Area

Join the Association

Notice:
The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.

Association Forum

Username:	Password:
Save Password
Forgot your Password?

All Forums

Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

Measuring real and apparent wind

New Topic

Topic Locked

Printer Friendly

Author

Topic

Page: of 2

Voyager
Master Marine Consultant

Members Avatar

USA
5389 Posts

Response Posted - 09/19/2009 : 07:23:19

Sorry, I equate VMG to the speed the boat and I are moving over ground.

Velocity Made Good is, IMHO, the real speed you are making on your heading.

Now, I realize there is leeway that affects the speed you are making toward any particular object (x cosine of the angle between your intended versus actual course).

If the GPS heading and my compass don't agree within about five degrees, then I compensate by steering up until I am on my intended course.

If I am also compensating for current as well as leeway, then I may have to tack from time to time.

When I use the goto function on the GPS, it will tell me whether I'm going the right way and my ETA to the waypoint. While I can pilot without a GPS, why should I?

Edited by - on

Dave Bristle
Master Marine Consultant

Members Avatar

Djibouti
10005 Posts

Response Posted - 09/19/2009 : 08:44:16

My understanding of the definition of VMG (sailing to windward) is the speed at which you are closing on an objective directly to windward or, with a GPS, a specific waypoint. "Directly to windward" is, for our boats, generally not less than about 45 degrees above your course on a beat, even though it appears you are sailing much higher than that (due to apparent wind).

VMG is an excellent tool for determining what compromise between speed (steering a little lower) and course (steering a little higher) will get you to an upwind objective most quickly. A GPS will show you both speed over ground (SOG) and, given a waypoint, VMG--the thing you want to maximize. But VMG is equal to SOG only if your objective is directly on your present course--or for a racer, if you're on the layline.

Off the wind, VMG can tell you whether it's better to run directly at your objective or to sail a little higher and jibe back, for more boat-speed.

Edited by - Dave Bristle on 09/19/2009 08:51:33

Stu Jackson C34
Admiral

Members Avatar

844 Posts

Response Posted - 09/19/2009 : 23:31:12

VMG has nothing to do with the direction or point of sail. It is simply the trigonometric calculation of speed toward a given point when other than aiming directly at said point.

Edited by - on

Steve Milby
Past Commodore

Members Avatar

USA
5904 Posts

Response Posted - 09/20/2009 : 06:18:52

Edited by - on

Dave Bristle
Master Marine Consultant

Members Avatar

Djibouti
10005 Posts

Response Posted - 09/20/2009 : 09:09:55

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stu Jackson C34
 VMG has nothing to do with the direction or point of sail. It is simply the trigonometric calculation of speed toward a given point when other than aiming directly at said point.
<hr height="1" noshade id="quote"></blockquote id="quote">I think that's what I said--although that definition is more specific to the use of a GPS so that you can specify the point. The more general definition I've read (probably from pre-GPS days) is the speed toward the direction of the true wind. So on a windward leg of a race, you make the mark in the shortest time if you maximize VMG.
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Isn't that the purpose of polar plots, i.e., to plot the different VMGs at different points of sail?<hr height="1" noshade id="quote"></blockquote id="quote">I don't think so--a polar shows SOG at various point of sail. If it were VMG, it would go to zero at 90 degrees--the plots I've seen don't look like that--they build steadily from a beat to a broad reach, and then decline toward a run.

Note here the VMG table next to the polar chart... The optimum VMG downwind angles are represented by the little boats on the chart. I'll guess optimum upwind VMG is at the beginning of each curve.

Edited by - Dave Bristle on 09/20/2009 10:10:23

Stu Jackson C34
Admiral

Members Avatar

844 Posts

Response Posted - 09/20/2009 : 11:49:14

Dave and Steve, you are confusing the quantity of the VMG with the definition of VMG itself.

Garmin defines it as "The rate of closure on a destination based upon your current speed and course..."

If you are heading directly at any waypoint, the SOG and VMG will be equal. If you are moving away from a waypoint VMG will be zero. Anything in between, the VMG will be less than SOG. VMG will reduce as you get close to but then start to pass any waypoint when you become dead abeam of said waypoint.

You can most likely Google the trig calc for VMG, it's out there.

OK, here's the first Google result from wikipedia: http://en.wikipedia.org/wiki/Velocity_made_good

Your discussions revolved around the use of VMG, and how much VMG was on different points of sail, not the meaning of VMG itself.

The more general definition I've read (probably from pre-GPS days) is the speed toward the direction of the true wind. 

That's simply not true. The definition of VMG does NOT change just because someone invented GPS! Using the wind as a waypoint is what I mentioned in an early post on this topic: find true wind and make the waypoint 1,000 miles to that direction. That determines best VMG to the given wind (assuming wind direction doesn't change), not to a local waypoint, like a buoy or racing mark. There are two VMGs to work with - wind and local waypoint (or race mark).

So on a windward leg of a race, you make the mark in the shortest time if you maximize VMG.

Be very careful here, because you haven't defined VMG to WHAT? There are TWO important VMGs to consider. You can maximize VMG to a waypoint, but you also have to factor in VMG to the wind. At the start of a race, going upwind, they "should" be the same if the RC has set the line perfectly perpendicular to the wind and the first mark is dead upwind. In the real world that simply doesn't happen, which is why there is a "favored" tack to the first mark, defined not by the VMG to the first waypoint (mark) BECAUSE the mark doesn't MOVE, does it, but by the wind direction VMG to work "uphill" to the mark based on that 1,000 mile away "wind" waypoint.

POLAR PLOTS are completely different than VMG and quite often confused. The polar "predicts" what the boat's designer says the boatspeed will do on different points of sail. Most boats don't sail at all directly into the wind, right?, and go fastest on a beam reach. Most all of us know that "cracking off" from a close haul to a close reach will increase boat speed.

It is the VMG calculation that will determine how much extra speed over a longer distance will improve the slower speed over a shorter distance. That's all, and that's how polars work WITH VMG.

And while this "racing" stuff / explanation might make some yawn, it works in the real world for cruisers, too, by choosing to use the 1,000 miles away wind idea (which took me 15 years to learn - even Erick Hiscock wasn't too clear on the concept!) AND the local waypoint concepts and PUT THEM TOGETHER to work anywhere, not just upwind (although the 1,000 mile wind is only applicable upwind). The VMG link to wikipedia also helps explain some issues.

Edited by - Stu Jackson C34 on 09/20/2009 12:18:51

Dave Bristle
Master Marine Consultant

Members Avatar

Djibouti
10005 Posts

Response Posted - 09/20/2009 : 13:21:37

We're not arguing. Of course Garmin says it's too a waypoint--that's the only way a GPS can figure it. The "1000-mile waypoint" simulates "to the wind" because the angle to that point changes only infinitesimally on each tack. I've used it, although at something like 100 miles. (The shorter the tacks, the fewer miles you need for the simulation.)

BTW, I think the VMG on my GPS goes negative when I'm sailing away from the point--only to zero when it's dead abeam.

Edited by - on

bigelowp
Master Marine Consultant

Members Avatar

USA
1786 Posts

Response Posted - 09/22/2009 : 05:26:14

It looks like the answer is "all of the above"! SOme trandom thoughts:

I use/look at the hand-held AND the Raytheon Wind Indicator AND the local "flags" AND listen to the weather radio AND . . . Local knowledge is always the best. Common sense is second best. Instruments just help.

One additional resource is a website caled "sSailflow" I frequently look at "sailflow.com" which offers the best regional wind information. It is looking backward, but after using it for a year or so you see the typical types of wind conditions caused when the wind is coming from different directions, at different power levels. It is amazing how predictably different the real wind was vs. the NOAA forecaster wind.

ALL of the above makes for the knowledge base that enables safe passage in different wind conditions (and/or better race results). No one item is the all telling, user friendly "silver bullet".

Finally, in some areas, especially narrow coastal areas, the overall "traffic" from other boats can have more impact on sailability than the wind -- cutting through lots of wakes when their is little wind, or wakes action from channels that create waves moving in a different direction than the natural flow from wind and seas can make any instrument mute.

Edited by - on

Dave5041
Former Mainsheet Editor

Members Avatar

USA
3758 Posts

Response Posted - 09/22/2009 : 08:01:31

There are no opinions on what VMG is, only a definition. Opinions can can vary on how you use it. VMG entails a destination point; it can be a waypoint, port, rock, or anything else. Sailing in a circle around your destination will result in a VMG of 0, toward it is always positive, away is always negative. The definition is absolute and independent, only the resulting number is changed by what your boat is doing.

Edited by - on

Dave Bristle
Master Marine Consultant

Members Avatar

Djibouti
10005 Posts

Response Posted - 09/22/2009 : 09:38:02

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave5041
 There are no opinions on what VMG is, only a definition...<hr height="1" noshade id="quote"></blockquote id="quote">...or several definitions:
<blockquote id="quote">quote:<hr height="1" noshade id="quote">VMG may also refer to the upwind vector of boat speed (this is often the VMG expressed on sailing instruments).<hr height="1" noshade id="quote"></blockquote id="quote">
...similar to definitions from several sources I've seen over the years. This one is Wikipedia, so who knows... The purpose of the "1000-mile" waypoint is to simulate the measurement of upwind VMG so the optimum VMG remains a constant target, rather than varying widely on each tack. This tells the sailor where the "sweet spot" is for any upwind work, and also helps determine the angle of the boat's layline in the particular wind conditions (although currents mess that up). The GPS VMG to a nearby point will vary along each tack as your angle to the point varies, making the "sweet spot" harder to recognize.

Which definition and technique you choose is up to you and your understanding of how to use them.

Edited by - Dave Bristle on 09/22/2009 09:39:45

Stu Jackson C34
Admiral

Members Avatar

844 Posts

Response Posted - 09/22/2009 : 12:53:58

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave5041
 There are no opinions on what VMG is, only a definition. Opinions can can vary on how you use it. ... The definition is absolute and independent, only the resulting number is changed by what your boat is doing.
<hr height="1" noshade id="quote"></blockquote id="quote">

Thanks, Dave, just what I was trying to say but you said it much better.

Opinions on the use of the VMG function vary widely, but the definition doesn't, at least mathematically, because it was challenged as being "from wikipedia" in Mr. B's last post.

Dave B, I'm not arguing anything BUT the definition. I may disagree about how you explain how to use it, and won't go into that here, but was simply trying to teach folks what it is and how there are two very different VMGs used for sailing.

Motorboats, or when we're motoring, could care less about the "1,000 Miles or More" (Alison Krauss album, too!

) windward VMG.

Edited by - on

Page: of 2

Topic

New Topic

Topic Locked

Printer Friendly

Jump To:

Association Forum

Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.