J/30 Class Association Forums J/30 Forum Racing Tools and Techniques VMG -- Problems and reflections

So OK ... decided we are going to get more techie and naturally wanted to play with VMG under spin to a downwind mark.

Two Garmin 440s didn't display VMG data, just a empty data box.
While VMG is set up for the numbers display as an option it is not covered in the garmin manuals we have.

We did however find a convoluted list of instructions
Link to Garmin VMG instruction

Which we will attempt to try this weekend.

We are trying to resolve a question on dead down spin vs broad reach. in rough water we only do broad to beam reach, but we've had some flat, light wind days lately and trying to zero in on a repeatable program and angle.

Vic,

That would be VMG to waypoint on the Garmin. VMG is usually calulated to wind and really needs a calibrated wind instrument.

During a race however, VMG to way point (rather than to wind) is really what matters. Important piece of data to have during a race.

Use your iPhone or go out and buy a cheap, reconditioned iPad. You can download apps that will give you that info cheaper than buying a new chart plotter. An iPad can slip easily into a gallon-sized Baggie/Zip Lock container to keep it dry. iNavX and Navionics both make cool apps for the iPad. You can touch and drag waypoints.

Bob Rude
#352
"Mmmmm"

Help. Explain the circumstance of the photo banner I see often when I come to this website. It shows a tacktick displaying a VMG 8.8 with a boat speed of 6.
How does that happen?

Easy - that was me returning from Cedar Point YC after the 2009 NAs. That is what tidal currents will do for you on Long Island Sound. The speed displayed is the measured speed by the paddle wheel through the water. The speed over ground was much greater due to the ebb tide currents pushing water to the east. Thus the GPS speed over ground increased the speed in the VMG measurement. Notice that the display shows VMGWP - meaning the TackTick was calculating VMG to waypoint using GPS data.

It's not uncommon to get a 2-3 knot tidal current. We planned the departure time to maximize the current push, rather than fight it.

Which APPS do you recommend?

So OK ... continuing report on VMG to waypoint via Garmin 440s

today initial test of following Garmin's secret method of getting VMG to waypoint on 440. It is not in the manual. we input the above referenced procedure and with a couple of coin flips we actually got VMG to work, but after about 15 minutes it would crash and the screen Wpt VMG would return to _._ kt display. requiring us to reset the waypoint.

However ... as with any new application it is not just the instrumentation, but what we were trying to interpret that is in a learning cycle.

so bear with us on this quick report


1. the course Photo ... start at tower leading into Gordon Pass [GP Safe] ... then tight haul on port about 3 miles then tack to point ~ dead upwind of GP to start a spin run back. this is sort of standard race sequence. The photos are not taken at exact points of the test.

[our objective was to see if running hotter by 10 degree hits would change the VMG in a beneficial way. ]

2. magenta line is course to turn
3. Black line is track/heading marked in miles for the std tack
4. Red line is return to GP under spin.


top display picture is the VMG on the cross leg ... it started around 2.3 kts and dropped to ~ -.2 or so when we launched spin. normally this would have been a set windward mark, but we used the lowering of the VMG to simulate the arrival at the mark.

wind was high enough to keep us over 5.5 with gusts taking us to 7.

bottom display picture is running back to GP pretty much on course.

a 20 degree heat up did give us +1 kt but no real increase in
VMG.

Rough lessons learned.
1. Running hot in 10+ kts doesn't seem to help, which is reflects J30 SOP
2. the speed readings on GPS Auto are too fast to easily follow ... will take averaging time to 5 seconds, wind was out of the south with 2-3 waves and some boat motion.
3. matching the speed and VMG is nice to have a good idea of the right course to steer
4. Garmin 440 either lacks decent VMG to waypoint power ... or we are going to have to find more secrets to its operation.

Vic,

You may want to look at the J/30 polars and make some tables to paste by the gps that give optimal course to steer for wind angle and speed.

I've found that unless wind is about 14 or above, you don't do as well headed near dead down wind and are better sailing angles, as confirmed by the polars. The polar charts are available at this link.

note to bill re photo of trip cedar point. it was the trip west to cedar point ... setting sun nails it.