How to calculate best VMG from polars?

[danstanford]

Jackdaw sent me the polars for my boat and we are trying to use the chart as we learn to race faster, but I need to understand how to calculate our best upwind and downwind VMG values from this chart. I watched a video on youtube talking about this topic and they showed the fastest points of sail by drawing vectors on the polar chart but didn't explain how they arrived at them.
Can anyone help?
We are in the whitesail class so they don't have us going downwind much but both intrigue me especially since I recently had a big downwind run and the best VMG I could get on the chartplotter was dead downwind which was unexpected.

Dan

 

[LeslieTroyer]

can you import the polars into your chart plotter?? I've read that I can do this on my e7D Raymarine - but have never taken the time to do so....
Les

 

[jviss]

Off the top of my head, not having thought of this before, I'd guess you would make a plot of velocity times the cosine of the angle from the head, for upwind, and the stern, for downwind, in the simplified case that the mark is directly upwind or downwind. For a mark not aligned with the true wind it's more complicated.

 

[Davidasailor26]

For a mark not aligned with the true wind it's more complicated

Not really. Until you get to the lay line your VMG relative to the wind is what matters. Although it seems like sailing toward the mark might be making more progress, you're going to sail the same overall distance as someone who sails on the other tack until they get to the layline.

I've seen some polars that say what the optimal angles are. Without that precalculation I'd guess you'd need to do the math like jviss says.

 

[jssailem]

Dan, while studying Polars, I came across this site that may be addressing the issue of identifying best VMG from a polar diagram of your boat.
https://76trombones.wordpress.com/2009/10/17/polar-diagrams-vmg/

 

[Davidasailor26]

Dan, while studying Polars, I came across this site that may be addressing the issue of identifying best VMG from a polar diagram of your boat.
https://76trombones.wordpress.com/2009/10/17/polar-diagrams-vmg/

"To find the maximum VMG, you just pick the point furthest forward on the polar." I think I knew that trick but forgot it somewhere along the way.

 

[Jackdaw]

Or you could just ask....... ;^)

raw numbers.

Twind 4 6 8 10 12 14 16 20 25 30
Up.Vs 2.73 3.99 4.92 5.52 5.72 5.85 5.95 6.11 6.22 6.28
Up.Bt 45.0 44.0 42.5 41.9 40.7 40.0 39.7 39.9 40.5 41.8
Up.Vmg 1.93 2.87 3.63 4.11 4.34 4.48 4.57 4.69 4.73 4.68

Dn.Vs 2.65 3.89 4.82 5.16 5.90 6.43 6.87 7.67 8.90 10.84
Dn.Bt 138 140 144 157 161 165 168 169 165 161
Dn.Vmg 1.99 3.00 3.92 4.76 5.57 6.23 6.73 7.54 8.61 10.2

For any Twind speed
Up.Vs boatspeed
Up.Bt Twind angle
Up.VMg VMG
same down.

The numbers assume:
1 lightship displacement + crew
2 clean bottom
3 folding prop
4 good sails
5 good trim
6 good driving

anything less and then you have to start to fudge. And it means spin downwind.

The BEST way for a learning crew to use polars is to focus on: target:

BOATSPEED upwind
TWIND ANGLE down.

Than means that if you know true wind is say 10 knots, your target is 5.52 knots. If you are slower you are pinching (or 1-6). If you are faster you are footing, no two ways about it!

Downwind sail to the true wind angle. Easy.

 

[danstanford]

Dan, while studying Polars, I came across this site that may be addressing the issue of identifying best VMG from a polar diagram of your boat.
https://76trombones.wordpress.com/2009/10/17/polar-diagrams-vmg/

JSSAILEM, this is perfect and kind of what I interpreted by watching the video I saw from the angles they chose, but they did not tell me how they picked the angle like this article.
Thanks!

 

[danstanford]

Jackdaw, that is a lot to digest but I am working on it. I do understand your point about using boatspeed for the upwind target but please help me with the downwind target? Ultimately we have decent factory sails but are pretty heavy (because we cruise her also) and we do not have a folding prop so I will use the wind angle to help get us in the right neighborhood -Light winds 44-45 and heavy winds 39-41.
Can you clarify the sail to the true wind angle statement? Are you saying that we pick the angle from the chart for our windspeed and focus on staying on it? Are you using true wind angle in this case because the apparents are so hard to keep track of when going downwind?
Thanks for your help! I made a couple of bad calls last night and we finished last again but had we been near the line at the start we would have been mid-pack.
Dan

 

[danstanford]

So instead of working I have ben wrestling with these new ideas and I have a couple of questions I hope are not too tiresome:
-Given that we are not matching the assumptions in the polars (weight, folding prop), should we be applying some factor to the speed targets upwind? By Jackdaw's example, if we are chasing a speed we would be footing to achieve it and getting less than the optimum VMG. Wouldn't we? Hopefully this doesn't sound like one of those ridiculous examples we are all used to seeing on the net these days, but if I had to move down to a beam reach to achieve a speed my VMG would be zero. It seems to me that if this whole question is valid then I should go out and do some test sails and edit my polars to reflect real speed targets my boat is capable of....

-I have always worked off of apparent wind, and am now wondering about how you go about working off true wind? Do you set your wind instrument to true and work from there or do you observe it before race time against the compass and sail courses relative to what you got then adjust for shifts?

I am enjoying the thinking a whole lot more than working, so please take my questions as sincere. I will not take offense to any suggestions that I am an idiot either. I am new to racing in the skipper role having sailed for years in a fun cruising way and my prior racing was as a grinder and trimmer on bigger boats. Recent evidence shows I have tons of work to do on the tactical side, and I am doing that thinking and research also.
Dan

 

[jssailem]

Dan
Thinking, theory and research are all wonderful, but they are no replacement for getting out on the water and learning what your boat is capable of giving you. As I understand, you have been given polars for your boat. Have you taken your boat out on to the water and tested it against the polars?
My thinking has been that the data is all informative and usable, but the boat, the crew and the ever changing conditions play a critical role for the race skipper. High levels of competition have a legion of helpers, coaches, trainers... all requiring big boat dollars to support. I am guessing you are not in this class. Working to get the most of your Beneteau 323 in club competition.

Take the boat out on the water and pick two markers that are on an upwind track. Time your runs, speed, etc. and sail from marker 1 to 2. Then go back and repeat. Sail one at 40degress off true. Sail a second at 45 degrees, then a third at 50 degrees.
Try to make you tacks all the same. I am almost certain when you have made this run 4-5 times you will find your speed has increased... if nothing else your tacks will be more smooth and carry more speed through the evolution.

When you have some personal data developed, go back and do it again only this time change the sail trim. More outhaul or less. Forward - mid - or all the way back blocks for your jib/genoa. Weather helm or none.

Too few sailors really get to know their boats and what they have to give, or the waters they sail and what they do to the boats on them. The sailors that do these things either in heavy practice sessions or from years of repeating weekends are the racers who finish in front of the pack.