#### Blue Crab

##### benthivore

twice the distance from the middle to one end .

*That's*my kinda math.

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter matthewwhill
- Start date

twice the distance from the middle to one end .

VMG's with decimal points are not as good.

Normally “hauling ass” means faster than the other boatTried 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!”

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

0.7 times the target boatspeed is a decent guesstimate

Don't forget to factor in tidal current, unless you are sailing on a lake.

So I'd go with 'less than 12 Parsec'

Happy now?

VMG's with decimal points are not as good.

@matthewwhill, how is 'decent VMG directly upwind' applicable to your situation?What is a decent VMG directly upwind?

This thread is great if you want to know how to interpret polars and how polars are developed, but I'm thinking that's not what you are after.

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?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

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.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?

Doesn't that just mean the highest boat speed is at 109? Best VMG downwind varies on wind speed.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?

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.

Good VMG is anything better than your competitor's!

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…

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

If nothing else. This is one of the more entertaining threads going right now.

Crumbs!

your question is kinda vague... sailing version of how high is up.Thanks. But that is the kind of vague answer I was talking about. Care to express it as a number or percentage of the hull speed as calculated from the waterline length (and how powered up the boat is)?

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

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.

Last edited:

If you are going hull speed to windward, your boat goes upwind like a witch.