What is a good windward VMG?

slug zitski

Super Anarchist
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Tried to ask this before but people don’t seem to like giving simple answers. I know its not a simple topic bit please humor me…..

How fast is fast upwind? Lets say in a racing boat like a j80 or take your pick. In decent 10-15 kt wind with reasonable 1 ft waves. You can even express it as a percentage of hull speed. What is a decent VMG directly upwind?

Just a ballpark estimate is fine. Some speed that would make you say “that boat goes to windward like a freight train!”
Normally “hauling ass” means faster than the other boat

numbers are bogus …almost impossible to calibrate instruments and make this calibration repeatable

once they are …perfect ! Then you face wind environmental conditions..5 degrees of wind sheer or a a few funny waves will turn your numbers into jibberish

nothing wrong with target boats speeds as a reference
 

HotSnail

Member
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Flânsum
The Olson 25 Table and Polar diagram both show much of the same data except the table misses the actual boat speed to windward. The numbers come from a VPP which is a physics model of the boat.


Here's a Farr 40 polar chart. It is in true wind direction. Look at red markings first (beating to windward). Start at blue curve with circle "1". This is the 5 knot WIND speed curve.

Follow the curve along until you see a little square marked with a "2". This is optimum VMG point in 5 knots of wind.

If you follow along the line toward the center to "3" you'll see the true wind angle of the boat is between 40 and 50 or about 45 degrees. Finally from square "2" follow the radial curves and you'll see the boat speed is ~5 knots. (not to be be confused with wind speed of 5 knots).

The VMG can be found by drawing a horizontal line to the vertical axis (in black). That shows VMG is about 3.5 knots in 5 knots of boat speed.

Now let's look at downwind speed in 10 knots. Start at green circle "1" where the 10 curve is circled. Follow the curve until you get to circled "3" square. That is downwind optimum VMG point. If you follow the green line outward you'll see angle is about 150 degrees. Follow the radial curves and you'll see boat speed is about 7+ knots. Finally black horizontal line is VMG downwind at about 6.2 knots.

View attachment 555476 .

Now below is the same data in a table. The table is much more readable and generally you look at the numbers not the pretty picture. Well at least NA's do.

The chart also gives predicted boat speed at various true wind angles. This is in case your course to the next mark is a reach, you want to know "in 8 knots of true wind speed and reaching at 55 degrees what is my target boat speed"? (7.44 knots)

Top row of chart is wind speed, left hand column is wind angle.

Upwind in 5 knots (column 1 in red)

VMG optimum Upwind boat speed = 4.95 knots
VMG optimum angle = 45.4 deg
VMG = 3.48 knots

Downwind in 10 knots (column 1 in green)

VMG optimum downwind boat speed = 7.47
VMG optimum angle = 152.3 deg
VMG downwind = 6.61

View attachment 555477
Thanks Zonker, that was exactly how I thought the run angle an VMG were determined. And now I still don't understand why they say the optimum run angle for the Ohlson is 109deg. in 8kts. of wind. Is it just an error in the polars?
o25polars-2-jpg.555451
 

Marty Gingras

Mid-range Anarchist
Thanks Zonker, that was exactly how I thought the run angle an VMG were determined. And now I still don't understand why they say the optimum run angle for the Ohlson is 109deg. in 8kts. of wind. Is it just an error in the polars?
o25polars-2-jpg.555451
The -y axis is a litte complicated in that it has tick marks for boat speed but labels for the apparent wind speed curves. Still works the same. No error.
 

danstanford

Anarchist
676
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Lake Ontario
Thanks Zonker, that was exactly how I thought the run angle an VMG were determined. And now I still don't understand why they say the optimum run angle for the Ohlson is 109deg. in 8kts. of wind. Is it just an error in the polars?
o25polars-2-jpg.555451
Doesn't that just mean the highest boat speed is at 109? Best VMG downwind varies on wind speed.
 

Zonker

Super Anarchist
9,754
5,723
Canada
No, 109 degrees apparent in 8 knots of wind. Find the 8 knot curve (circled with "1" beside it).

Follow curve until you find a boat. Read out the apparent wind angle on the perimeter -> 109 degrees.

I didn't mark it but if you look at the radial lines you'll see boat speed. So the boat is doing about 5 knots but because it's not pointing downwind too far, the VMG downwind is 3.911

I hate this chart with apparent wind angles. I really think in true wind angles when I read a VPP.

1669322074016.png
 
Well, if you're looking for a basic guide when you're sailing upwind, sans instruments except for boat speed and a compass, if you are sailing at about 45 true wind angle, which you can visualize by looking at the tiniest wavelets and realizing true wind is perpendicular, and where most boats seem to live naturally, if you pinch up 5 degrees and let the boat settle, and you lose less than 10% of your boatspeed, you have a faster vmg. Otherwise, it wasn't worth pinching. If you crack off 5 degrees and your boatspeed increases by more than 10%, cracking off was worth it. It's about 15% from 55 to 60, so diminishing returns. It's about 6% going from 40 to 35 true, but let's not get silly. This helps in chop, you dont need polars.
 

slug zitski

Super Anarchist
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worldwide
Well, if you're looking for a basic guide when you're sailing upwind, sans instruments except for boat speed and a compass, if you are sailing at about 45 true wind angle, which you can visualize by looking at the tiniest wavelets and realizing true wind is perpendicular, and where most boats seem to live naturally, if you pinch up 5 degrees and let the boat settle, and you lose less than 10% of your boatspeed, you have a faster vmg. Otherwise, it wasn't worth pinching. If you crack off 5 degrees and your boatspeed increases by more than 10%, cracking off was worth it. It's about 15% from 55 to 60, so diminishing returns. It's about 6% going from 40 to 35 true, but let's not get silly. This helps in chop, you dont need polars.
Polars and accurate instruments are handy downwind…..jibe angles , target speed

downwind the fleet spreads out , difficult to use your eyes to determine speed, spot bow up , bow down, boat lengths…
 

Not for nothing

Super Anarchist
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jupiter
This can't be answered, why?
Wind isn't constant speed or direction
Current favorable one tack vs the other
What's the fleet doing, follow them or not?
Projected wind shift at the windward mark
A J-22 race LIS, the fleet went left, Normal way to go, we weren't the fastest, we went right nearing the windward mark, we got a 20 degree lift and a favorable current, so the rest of fleet got the header into a bad current, we got a 10 boat lead and won the regatta
so get your head out of the boat and off the instruments
 

shaggybaxter

Super Anarchist
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2,551
Australia
I hate this chart with apparent wind angles. I really think in true wind angles when I read a VPP
Here ya go Zonker.


polars.png

The TWS is a bit hard to read.
Red - 35kn
Green - 30kn
Blue - 25kn
Dk Green - 20kn
Then 16,14,12..then I lose interest.
 
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