Velocity Made Good

[trapezeartist]

My post on how to display true wind direction drifted off into an interesting discussion of true wind and ground wind. Now what about VMG? I think Flaming may be able to offer some wise words here.

Last time out a few weeks ago, I started playing with the VMG reading on my wind instrument. We were sailing close-hauled in wind varying between 10 and 13 knots apparent. There was a tide of about 3 knots running with us. With the course adjusted to get maximum boat speed we were doing about 5.5 knots through the water, and the apparent was about 40 to 45º off the bow. When I turned over to VMG it was only showing about 1.8 knots.

I then started experimenting to find the highest VMG and ended up sailing 10º higher. The speed through the water dropped to 4.5 knots but VMG increased to 2.7 - 2.8 knots. At the same time heel was significantly reduced.

As the tide was a significant figure in comparison with boat speed and VMG yet is not taken into account in the electronics' calculations, I'm not sure how meaningful these figures are. Do the numbers make sense? Have I just discovered how to sail my boat better?

I suppose doing a couple of races against the usual opposition would be a good test of whether we can improve significantly. However now being out of the water at the moment in order to paint myself, the yacht cradle, the ground and perhaps even the boat, I wasn't able to do the race this morning.

 

[Doug_Stormforce]

vmg

VMG is simply a calculation of a vector. It is the speed you are doing in relation to your destination or next waypoint. If you were heading directly towards your mark it would be the same as SOG

As it is reactive (ie based on what you have just done and what has just happened to you) its not the best instrument to trim or steer by, however its a useful tool for the tactician to monitor.

 

[noelex]

Glad you enjoyed the previous post. It did get a bit OT which was probably my fault.
VMG do you mean VMG to the waypoint or VMG upwind?
If the latter 1.8K is very low, if you had 5.5K STW @ 40 to 45 apparent and 3K of favorable tide your upwind VMG should be much better. Something is wrong with these numbers.
When you pinch the VMG will initially rise, so it is not a good instrument for short term helm trim.

 

[snowleopard]

There are two different types of VMG as noelex says. There is VMG to a waypoint which is calculated entirely by the GPS. That takes tides into account and gives you a true speed over ground. The VMG given by a wind instrument defining your angle to the wind and log giving you speed through the water gives you an indication of how efficiently you are sailing upwind but is not as good an indication of your speed towards your destination. As I can't carry a wind instrument I'm not up to speed with the latest software but I presume there is provision for combining wind direction with GPS track that would give the most accurate reading of windward performance.

 

[Angele]

From the context of the previous thread, I am assuming the OP is talking about VMG to windward. If so (i.e. you are reading it off your ST60 wind display) then the tide is not taken into account in that calculation. It is simply that part of your velocity vector through the water that it directly into the wind, as opposed to that part which is at 90 degrees to it.

Clearly, this is different to VMG to a waypoit which, using GPS location, will include a tidal adjustment.

But, yes, you probably have found a way of sailing your boat to windward more quickly. As you have found, there is an optimal point as you luff up when, although speed falls, your VMG increases to its maximum. If you turn further into the wind then both boat speed and VMG fall away.

 

[sarabande]

you need to consider VMC


And, Angele, it's definitely not the case that VMG is at the optimal point you describe. Quite definitely.

 

[flaming]

As others have said VMG off a wind instrument is VMG to the wind, without taking into account tide.

This is the instrument to use when trying to improve the way that your boat sails upwind, balancing speed and pointing and seeing what effect small changes in trim make to your progress.
However, a couple of words of caution. This number does not take leeway into account. As you pinch closer to the wind you'll make more leeway, but your VMG may look better than it actually is. How important this is depends on your boat.
Also, if you're using a stand alone wind instrument you often cannot see the windspeed at the same time. A windspeed change will have an effect on VMG, so it's important to check that you're comparing apples to apples. The ideal is to have VMG, speed and windspeed displayed at once, but this is not always possible on some setups.

A tactic often recommended on these forums is to use the GPS VMG to get VMG to windward by putting in a waypoint a long way (100 miles+) directly upwind and using the VMG to waypoint function. This is fraught with errors, as a small windshift that would often go unnoticed by cruising sailors would have a much bigger effect on VMG than changing your trim.

 

[snowleopard]

The way to get an exact VMG upwind taking tide into consideration is to put a waypoint into the GPS a long way off directly to windward. The problem with this is that you have to manually move the waypoint when the wind shifts.

 

[bedouin]

If you do the figures it seems that you changed from sailing about 70 degrees to the true wind to about 50-55 degrees. So congratulations - you have certainly discovered how to sail your boat better!

Tide does not come into it - but leeway does and a VMG derived purely from Wind and Log cannot include that, which probably rather flatters the tighter angle

 

[davidej]

If the tide is running parallel with your course to the waypoint (either with or against you) then maximising VMG thru the water will maximise VMG over ground.Not so if the tide is not exactly parallel.

 

[johnalison]

Trapezeartist is correct about the use of traditional VMG when sailing close-hauled. I have used it from time to time with friends and by myself and in spite of the fact that the figure can't be entirely accurate the results can be surprising. In still water we found that the best VMG was very hard to achieve as it needed the helmsman to sail a very accurate course much closer to the wind than seems natural. It certainly improved our helming but required more concentration than desirable when cruising. I can also bring up VMG to waypoint but have found little use for it.

 

[trapezeartist]

Thank you , guys. Yes I am talking about VMG to windward as given by my ST60 wind instrument. On the occasion mentioned the tide was running more or less parallel with out course so I was viewing the VMG reading as an aid to helming.

Noelex, I thought the numbers seemed low too. For simple maths, isn't sine 45º = 0.707? Which would mean that a speed of 5 knots could be expected to deliver a VMG of somewhere about 3 knots. Am I getting that roughly right?

This also ties in with Bedouin's suggestion that we moved from 70º off true wind to 50-55º. The same wind instrument was showing about 35º off apparent and I think from memory 45º off true (but that's when the tide starts to interfere with things).

Flaming, you're right the VMG readout occupies the space normally occupied by wind speed, so you can't have both at the same time. On the day in question the wind was fluctuating about 3 knots which didn't seem to be enough to affect boat speed.

So it seems the numbers are questionable, but the general concept of steering to optimise VMG (on the instrument) is probably good.

As an aside, I have had a few issues with log (speed) calibration this year. Apart from looking at the speed to cheer me up (or depress me), I occasionally use it when motoring flat out to just the state of fouling. I was a bit alarmed when I suddenly seemed to have lost over a knot from my normal flat out speed. Subsequently I realised that the log calibration must have gone awry. After checking that the impeller was clean, I have made several attempts to calibrate the log against GPS (in non-tidal water) but I've not really been satisfied with the result so far.

 

[Twister_Ken]

I then started experimenting to find the highest VMG and ended up sailing 10º higher. The speed through the water dropped to 4.5 knots but VMG increased to 2.7 - 2.8 knots. At the same time heel was significantly reduced.

(

That bit worries me and suggests that you might have had the sails oversheeted when you were sailing broader, then as you luffed up you found the wind angle to which the sails were trimmed. If so, having the sails working well would certainly have upped your ability to work upwind and that would (I guess) have been reflected in a better VMG to windward.

 

[trapezeartist]

Well, they weren't oversheeted for maximum boat speed, but they were for maximum VMG.

 

[noelex]

Noelex, I thought the numbers seemed low too. For simple maths, isn't sine 45º = 0.707? Which would mean that a speed of 5 knots could be expected to deliver a VMG of somewhere about 3 knots. Am I getting that roughly right?
.

No you need true wind to calculate VMG.
If your apparent wind was 11.5K @ 42.5 degrees (the mean of the figures you quoted) my maths says the true wind was 8.3K @ 69 degrees

VMG is given by the formula
VMG = VCosA

Therefore your VMG with the above figures is 2.0K