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 Post subject: Velocity Made Good (VMG)
PostPosted: Thu Mar 05, 2009 8:55 pm 
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Joined: Tue Sep 16, 2008 10:10 pm
Posts: 92
Location: Gilbert, AZ
I purchased a Garmin GPSmap 76CSx and would like to get some feedback on the topic of velocity made good. I choose the GPS because it's water proof (so to speak) and the VMG feature. I'm somewhat familar with VMG and how it can help you achieve the best angle to improve your speed/time to the waypoint. On my GPS screen, my display has waypoint pointer, ground speed and VMG speed displayed. I've been able to get the VMG speed to reach the ground speed, should I be able to exceed to ground speed?

I have a few questions, can VMG speed exceed ground speed, how do you have your GPS display setup when you view VMG and how do you adjust course heading to optimize your VMG speed?

I would really appreciate getting some feedback on the topic so I can learn how to use the VMG feature.


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 Post subject:
PostPosted: Fri Mar 06, 2009 1:21 am 
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Location: Terrigal NSW, Australia
Hi Bob,

I have also just acquired a Garmin GPSMap 76CSX. I am still figuring it out - its a great GPS, but the documentation is pretty basic. I think I have the VMG figured out.
Bob wrote:
I've been able to get the VMG speed to reach the ground speed, should I be able to exceed to ground speed?

- VMG cannot exceed ground speed, since for any given ground (or water) speed, the fastest you can approach your destination is to head straight for it, in which case, ground speed will equal VMG.
Bob wrote:
how do you have your GPS display setup when you view VMG

I have set up my trip computer page to display VMG - I believe you can do the same on the compass page.
Bob wrote:
how do you adjust course heading to optimize your VMG speed?

The whole point about VMG is that the maximum speed through the water is usually not the fastest way to reach your destination - there is usually a tradeoff. VMG is the speed at which you are actually approaching your destination. The AI usually reaches its maximum water speed on about a beam reach (around 90 degrees to the wind), but the shortest distance to your destination is more often than not at some angle other than this. If your aim is to reach your destination as quickly as possible, the trick is to adjust your helm (angle to the wind) until you achieve the highest possible VMG reading. For example, if you are sailing on a close reach and you start to turn away from the wind and the VMG reading drops, you should point higher into the wind, even though your boat speed may drop. If your VMG reading increases, you should gradually point higher until your VMG starts to drop. Whichever heading gives the highest VMG is the optimal heading for reaching your destination quickly.

To make things even more complex, you need to adjust the trim of your sail whenever you change your heading, so you should compare the VMG reading after you have trimmed the sail.

Personally, I'd rather take a bit longer and have fun with the boat moving faster 8)

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 Post subject:
PostPosted: Fri Mar 06, 2009 5:59 am 
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Location: Punta Gorda, FL
chrisj wrote:
Personally, I'd rather take a bit longer and have fun with the boat moving faster 8)


Hah...powerboaters need a destination. Sailors are already there. :D

For the most part, anyway. ;) I own several powerboats, including a jet ski. Jet skiiers are also already there. :D


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 Post subject:
PostPosted: Fri Mar 06, 2009 7:28 am 
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Location: South Florida
Bob,

Here are my typical "Map Page" and "Trip Computer" pages:
Image

Image

If, unlike Chris, you are heading to some destination, the VMG is a great tool. It can never be more than your "Speed" which is your current straight-ahead speed. When you are heading directly at your destination, the speed and VMG will be identical (+/- one or two tenths of a mph). More often, you will be forced, because of wind direction, to head off from the shortest line to your destination. The tack you choose will depend on which one gives you the best VMG--here is where VMG really shines. You stay on a tack until the VMG begins to drop off, then change to another tack which gives you a better VMG. It is a judgement call when to change your tack. Sometimes you must take a less desirable tack in order to get back on a good tack (i.e., one with your best VMG)

Your Bearing is always the direct line (direction) to your destination from your current position. Your Heading is the actual direction you are moving. These 2 items, Bearing and Heading, do not make a lot of difference as long as you have the best VMG possible. As you near your destination, you forget the GPS and simply make the necessary tacks to get you to shore.

I feel I need to explain the data on my Trip Computer page. It is from a recent 70-mile sea kayak trip.
http://www.hobiecat.com/community/viewtopic.php?p=65262#65262

The Average Speed of 4.1 mph is pretty good for a fully loaded (100-125 lbs of gear) kayak. I like to see the average speed because I can estimate how long it is going to take to reach my destination--the GPS ETA (estimated time of arrival) is pretty useless because it depends on your momentary speed, which can vary all over the place. I use routes frequently when paddling a sea kayak--then, I like to stay on course, the shortest distance to my next waypoint. I rarely use a GO TO when paddling. Again, I stay precisely on my route, so I like the OFF COURSE reading. You can also get a very good estimate of your course deviation (deviation from a route line) from the Map Page. Winds and tidal currents can push you all over the place when paddling or pedaling. Stay on course, use a route.

When sailing the AI, I do frequently use a GO TO, since it always tells you how far to your destinating and gives you a Bearing, Heading, and VMG. Staying precisely on a route is a near impossibility when sailing. Still, there is nothing wrong w/ having a route even when sailing. If, when becalmed, you may have to pedal. You will want to take the shortest, most direct route possible. In such a case, it is worth setting up a route "on-the-fly." That is, set a wpt at your current position, then go to the route page, start a "New Route". Your first point is your current position wpt, your next point is your destination. That is a simple route--a single leg, straight line. If pedaling you are going to precisely follow that line (all the while, praying for the wind to pick up.)

The Garmin 76 is a great GPS, and the more you use it, the more useful it gets. One bit of advice, especially if used in a salt water environment, keep it in a dry bag like an AquaPac. These devices are supposed to be waterproof; but, over 18-24 months, they will die if regularly exposed to salt water.

Keith

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"Any intelligent fool can make things bigger and more complex ... It takes a touch of genius and a lot of courage to move in the opposite direction." A. Einstein


Last edited by Chekika on Fri Mar 06, 2009 6:55 pm, edited 1 time in total.

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 Post subject:
PostPosted: Fri Mar 06, 2009 6:12 pm 
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Location: Northern VA
Chekika wrote:
The Garmin 76 is a great GPS, and the more you use it, the more useful it gets. One bit of advice, especially if used in a salt water environment, keep it in a dry bag like an AquaPac. These devices are supposed to be waterproof, but over time, 18-24 months they will die if regularly exposed to salt water.

Keith


Keith,

You're right about the Garmin 76 being a great GPS, and about the dry bag. I just had to get my 76Cx factory repaired (replaced with refurb) because the gasket on the battery compartment cover allowed salt water to seep in and corrode the battery contacts. Garmin was going to charge me about $125, but since the unit was less than 18 months old, I talked them down to $60. I haven't picked up a dry bag for it yet, but will do so the next time I'm into Boater's World or West Marine. Could probably find better pricing on-line, but want to test fit before buying. By the way, nice explanation of the features and use.

Happy Sailing,
- Jim L


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 Post subject:
PostPosted: Fri Mar 06, 2009 7:38 pm 
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Joined: Tue Sep 16, 2008 10:10 pm
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Location: Gilbert, AZ
Thank you all for your replies. I've been working with VMG the last few days and I’m starting to understand the concept. I’ll be heading back to Rocky Point in the Mexico Baja in a couple of weeks and will be able to use the unit allot while there. Now you’ve got me a little concerned about salt water damaging the unit. I purchased a Ram mount for it and had intended on mounting it on the rear point that I attached Reconlon’s splash shield attached to. What type of GPS mount are you using when the GPS in the Aqua Pac?


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 Post subject:
PostPosted: Fri Mar 06, 2009 8:42 pm 
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Location: Terrigal NSW, Australia
Bob L. wrote:
I purchased a Ram mount for it and had intended on mounting it on the rear point that I attached Reconlon’s splash shield attached to.


Ah, You've got the same logic as I do, Bob - once you've laid out $12 for the Ram ball, you've just got to get the $400 GPS to mount on it :)

The salt water corrosion issue is a bit of a concern. Jim, did the corrosion of the battery contacts happen gradually or was it too late to do anything by the time it was evident? Also, Jim & Keith, did you rinse your GPS's in fresh water after each use?

Clearly, you can't use a Ram mount if the unit is in an Aquapac, but I'm wondering if you could just pull a clear plastic bag over it while it was mounted. There would be little opportunity for water to get in, as it would never be immersed, or even subject to much splashing.

Comments???

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 Post subject:
PostPosted: Fri Mar 06, 2009 8:58 pm 
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Joined: Tue Sep 16, 2008 10:10 pm
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Location: Gilbert, AZ
Ram has an Aquabox with a soft touch clear screen, unfortunately it length is .025" to short for the 76Csx.

Anyone find a good solution to saltwater corrosion?


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 Post subject:
PostPosted: Fri Mar 06, 2009 9:57 pm 
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Location: Northern VA
Hi Chris,

Yes, I did rinse it off after sailing (most times, anyway). It built up gradually, but mostly in the last few months of the summer 2008 sailing. I noticed it a couple of times as I was changing the batteries, but didn't realize how serious it had gotten. I did clean them a couple times using a pencil eraser. It helped, but it just delayed the end result. In the end the contacts were just completely rusted. Unit would still work if connected to a USB power cable, but using batteries was a lost cause.

I also picked up the Garmin Marine mount for my 76. Hadn't mounted it yet, only because I hadn't figured out where I wanted it yet. When I'm using the unit, I would tie it off to the hooked bungee for the mirage drive pedals, and let it sit on the 8" hatch in the cockpit. It worked just fine there, to be honest. Good thing it floats! :D Obviously the mount isn't going to work when the GPS is in a drybag, so I guess it'll just end up floating in my lap for the time being. Oh well. It's worked well enough so far.

Not sure how well the idea of a plastic bag over the mounted unit would work. I think you would still get water up inside the bag, which even if it didn't knock the bag off, would at least make it difficult to read the GPS due to water drops on the inside of the bag. If you try that, let us know how it works out. That would restore the option of using the GPS on my mount.

Overall I think the 76 series is an awesome unit. I'm sure you'll be happy with it. I've owned 3 Magellan GPSs, and now have two Garmin GPS (76Cx & Colorado 400c). I prefer the Garmins, hands down. Only bought the 400c because the 76 was broken, and West Marine had a screaming Black Friday special this past Christmas. Too good to pass up. Wanted a second unit anyway for a 50 mile open water trip I'm hoping to make this summer. If schedule and conditions line up to allow me to make the trip, I'll be reporting on that later.

Happy Sailing,
- Jim L


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 Post subject:
PostPosted: Fri Mar 06, 2009 10:29 pm 
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Thanks for the info Jim. I think I'll try the plastic bag, possibly with an elastic band to close it off. I'll let you know how it works.

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 Post subject:
PostPosted: Sat Mar 07, 2009 3:43 am 
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Location: South Florida
Yes, I've tried the "rinse thoroughly with fresh water" routine. I think the situation is something like (1) over time, somehow water intrudes into the case--if you use no protection, it will show up on the LCD after 12-18 months unprotected use. (2) Once the water is inside, perhaps making contact through the external electrical connections, it begins to corrode the electronics—internal electronics (I'm not talking battery connections.) 18-24 months, the unit is dead. As with all corrosion, salt water will accelerate this process.

Here is a picture of a friend's homemade holder:
Image

He appears to be using AquaPac bag for protection. He does all his paddling in saltwater.

On both my AI and my sea kayak, I attach my Garmin 76 in its AquaPac bag by a suitable (2-3') cord. It cannot be lost overboard. At first I laid it on the AI center hatch cover, but it would not work correctly. Again, on the AI, I simply put it in one of the mesh pockets or just lay it along the side on the mesh pockets. I pick it up to read it. Sometimes, like maximizing the VMG, I hold it in my free hand.

Some people (WaterTribe) who do serious adventure kayaking like the Everglades Challenge which will start today, Mar 7, smear grease all over the openings of the Garmin 76 so it is "protected" but affords easy access to buttons and easy reading. I'm not sure that does much for the long term protection of the units. In any case, grease on my GPS is not my cup of tea.

Keith

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I sail: Biscayne Bay, Everglades to Cape Romano, Ft Desoto, Cedar Key

"Any intelligent fool can make things bigger and more complex ... It takes a touch of genius and a lot of courage to move in the opposite direction." A. Einstein


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 Post subject:
PostPosted: Sat Mar 07, 2009 8:29 am 
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"Only bought the 400c because the 76 was broken, and West Marine had a screaming Black Friday special ..." Jim_L


Jim -- Was just trying to decide between the 76cx and 400c. Leaning towards the 400c, except the comments on screen legibility in bright sun has me skittish. Any comments on which of the 2 you prefer?

Cheers, Joe


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 Post subject:
PostPosted: Sat Mar 07, 2009 1:40 pm 
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I wrote:
Clearly, you can't use a Ram mount if the unit is in an Aquapac


What was I thinking of? RAM has several universal mounts, including a side-clamping one, which should hold the Garmin in an Aquapac OK. Will investigate and report back.
Image

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 Post subject:
PostPosted: Sat Mar 07, 2009 5:58 pm 
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Hi Joe,

I've been using the 76Cx for two summers now, and have had no complaints about it (apart from the "not quite as waterproof as expected" thing). It has performed well, is quick to grab satellites every time, is simple to operate and easy to read. It floats, too. I purchased the US Inland Lakes and the City Navigator packages, as the built in maps are pretty basic. The only thing I found annoying was that when you load two map packages on a Garmin GPS, if one is "licensed" (City Navigator in this case), and the other one is not, then the licensed maps take precedence. This made it so that in at least one place that I sail, if I wanted to have maps of the backwater lagoons (contained in the Inland Lakes package), I lost the location of a bridge where a minor road (shown in City Navigator package) crosses that lagoon. One has the lagoon but not the road, while the other has the road but not the lagoon. Minor thing, I guess.

As for the 400c, I haven't had many opportunities to really use it yet. I do like the small size, easy to hold, and easy to operate with one hand. I think the wheel control seems cool, but the jury's still out on that. It's also supposed to be waterproof, and appears like it may be more so, as well as more rugged than the 76. The 400c, however, does not float. I also like the ability to create profiles of settings and features. Nice to not have to manually change units, display, and other settings when moving from the boat to the 4-wheeler. The D-ring attachment clip is a nice sturdy way to keep the unit from getting away. I hadn't heard the complaints about the screen legibility in sunlight, but I would have to agree. While the screen is slightly larger than the 76, I do find myself turning on the backlight in conditions I never had to with the 76. I do like the built in BlueChart maps. I've already verified that with the City Navigator package loaded, I was able to see both the backwater lagoon, and the road bridge.

I'll be taking the 400c with me on an AI sailing outing on the Chesapeake tomorrow. With 38 degree water temps, I'll be wearing gloves, so I don't know how much I'll be really using the GPS while underway. That is one thing, the bigger buttons of the 76 are easier to use with gloves.

I don't know if this helps any, but it's the best information I can offer at this point. I'll try to post a better comparison once I had more opportunity to use the 400c under real conditions.

Happy Sailing,
- Jim L


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 Post subject:
PostPosted: Sat Mar 07, 2009 6:57 pm 
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Jim_L wrote:
Hi Joe,

The only thing I found annoying was that when you load two map packages on a Garmin GPS, if one is "licensed" (City Navigator in this case), and the other one is not, then the licensed maps take precedence. This made it so that in at least one place that I sail, if I wanted to have maps of the backwater lagoons (contained in the Inland Lakes package), I lost the location of a bridge where a minor road (shown in City Navigator package) crosses that lagoon. One has the lagoon but not the road, while the other has the road but not the lagoon.
...............
I do like the built in BlueChart maps. I've already verified that with the City Navigator package loaded, I was able to see both the backwater lagoon, and the road bridge.


Jim, Isn't this more likely a property of the maps, rather than the GPS? I presume the GPS is only reading from one map at a time, so isn't it more likely that Bluecharts has the bridge, whereas Inland Lakes does not?

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