Velocity Made Good (VMG) on a tablet?

[Tessellate]

I use Navionics on Android devices as my main navigation. I love it and it has saved over $1000 on overpriced marine nav displays. But the one thing I'm lacking is a convenient way to calculate VMG (without having to do it constantly in my head).

Note there are two kinds of VMG people typically talk about - VMG to waypoint, and VMG to windward (wind angle). VMG to windward is simply wind angle + speed over water. (note: apparently there's some debate over acronyms: I don't care to discuss that, so will just state in words what I mean)

VMG to waypoint (GPS-based) seems more useful. We often sail in current, and GPS-based VMG would make it very clear which side of a channel has better current, which tack is favored, etc.

VMG to waypoint is simply angle to waypoint + SOG (gps speed). Navionics has that info, so it's surprising they haven't implemented VMG. Is there a different Android app I can do it with?

[conachair]

Originally Posted by Tessellate

Is there a different Android app I can do it with?

Opencpn does. Navigate to a waypoint you get XTE, bearing, VMG, range & time to go. This is from a PC but the android versian is the same -

[Tessellate]

Oh neat. I knew OpenCPN for PC could do that but didn't realize there's an OpenCPN for Android now, and it looks pretty decent (only found one assertion/exception error so far). I was able to set a waypoint and get VMG. Don't have charts loaded yet, but I can work on that later. Thanks!

[AedanC]

Note there are two OpenCpn releases for Android, one free and one paid. I’ve never used either so I can’t advise what to use but people in the OpenCpn forum will no doubt explain the differences.

[canyonbat]

Originally Posted by Tessellate

I use Navionics on Android devices as my main navigation. I love it and it has saved over $1000 on overpriced marine nav displays. But the one thing I'm lacking is a convenient way to calculate VMG (without having to do it constantly in my head).

Note there are two kinds of VMG people typically talk about - VMG to waypoint, and VMG to windward (wind angle). VMG to windward is simply wind angle + speed over water. (note: apparently there's some debate over acronyms: I don't care to discuss that, so will just state in words what I mean)

VMG to waypoint (GPS-based) seems more useful. We often sail in current, and GPS-based VMG would make it very clear which side of a channel has better current, which tack is favored, etc.

VMG to waypoint is simply angle to waypoint + SOG (gps speed). Navionics has that info, so it's surprising they haven't implemented VMG. Is there a different Android app I can do it with?

I don't get this speed plus angle idea in either example. VMG to either is going to be less than actual speed unless you are going directly to your waypoint or directly into the wind. Could you clarify?

[Tessellate]

Originally Posted by canyonbat

I don't get this speed plus angle idea in either example. VMG to either is going to be less than actual speed unless you are going directly to your waypoint or directly into the wind. Could you clarify?

Sorry, I didn't mean plus as in the mathematical operation of addition. Just meant it's the product of these two variables.

[conachair]

Originally Posted by AedanC

Note there are two OpenCpn releases for Android, one free and one paid. I’ve never used either so I can’t advise what to use but people in the OpenCpn forum will no doubt explain the differences.

This is the real one from the developers -
https://play.google.com/store/apps/d....opencpn&hl=en

The other one is old and isn't supported, might be of interest if you just want to have a play before paying a few peanuts but all the bells and whistles won't be there.

[canyonbat]

Originally Posted by Tessellate

Sorry, I didn't mean plus as in the mathematical operation of addition. Just meant it's the product of these two variables.

Thanks for the clarification. Now next question, given that you need apparent wind speed and angle plus SOG (I don't think you really need speed over water in either case) to calculate VMG, how is your tablet or PC getting the wind info?

[conachair]

Originally Posted by canyonbat

Thanks for the clarification. Now next question, given that you need apparent wind speed and angle plus SOG (I don't think you really need speed over water in either case) to calculate VMG, how is your tablet or PC getting the wind info?

Pretty sure Opencpn & probably most plotters/GPS just calculate VMG from SOG & the angle between COG & bearing to the waypoint.
VMG = Cosine(Angle between COG & bearing to waypoint) X SOG maybe?

Though would be more interesting if that is wrong

[barnakiel]

If you can't get what you want from ready made packages, simply build your own android vmg calculator after hours. MIT App Inventor will do. Very easy.

I simply dial the wpt into our Garmin 72 and that's that - Garmins display VMG as one of the screen box options.

Garmin 72 upwards of USD 50 at ebay. AA / 12V, data in and out and handheld/pocketable.



Cheers,
b.

[Tessellate]

Originally Posted by canyonbat

Thanks for the clarification. Now next question, given that you need apparent wind speed and angle plus SOG (I don't think you really need speed over water in either case) to calculate VMG, how is your tablet or PC getting the wind info?

See back to my original post where I mentioned VMG to windward vs VMG to waypoint. "VMG to waypoint is simply angle to waypoint + SOG (gps speed). Navionics has that info ..."

[canyonbat]

Ok, that works. I guess if you want VMG to wind you just set a waypoint upwind somewhere and not worry about minor wind shifts.

[Stu Jackson]

Originally Posted by canyonbat

Ok, that works. I guess if you want VMG to wind you just set a waypoint upwind somewhere and not worry about minor wind shifts.

Close, but...

The technique is NOT to get a waypoint to the wind, but rather a waypoint to a destination that is close to where the wind is coming from, and that destination (waypoint) for this purpose is like 200 or more miles away. It is ONLY a waypoint upwind, since the wind will always be shifting.

Once you set that, you are getting a VMG to that waypoint. Not the wind, although it may be close.

It's a subtle but important difference to know.

At first it will tell you the favored tack, but as the wind shifts, you MUST know that the VMG it is giving you is to a waypoint, NOT the wind.

Once the wind shifts, you can simply change the "far away" waypoint.

Hope that's clear. If not, ask away.

[rgleason]

See tactics_pi OpenCpn.
There are various alternatives for use of speedo or gps, water referenced or ground referenced.

There are formulas in Tactics and discussion about this in the tactics thread.

https://opencpn.org/wiki/dokuwiki/do...ics#actual_vmg

[FroggyKaf]

Hi,
In Boating from Navionics, you can show the time to go to the next way point in the upper left panel. This is not directly VMG but it looks to be calculated taking the boat speed, the COG and the BRG like VMG.