Polar diagrams

[Twister_Ken]

The windspeeds shown on polar diagrams (like the attached) are true, not apparent, but are the degree radials shown from the true wind or the apparent?
Logic says apparent, but you never know.

 

[Evadne]

Plotting true windspeed against apparent wind direction would be more difficult than plotting true vs true and apparent vs apparent, I'd have thought.

 

[snooks]

True wind speed and true wind angle

 

[Twister_Ken]

So the boat in question is able to sail at 30º to the true wind?

 

[lenseman]

. . . Logic says apparent, but you never know.

There is an interesting article about this here:

http://www.secondsailing.org/wiki/index.php/Sailing_in_SL_-_Introduction+

The section on 'Helms' Role' true v apparent.

 

[snowleopard]

I suspect they are all made up anyway. Ever tried to gather the real-life data to plot a polar? I spent 2 hours trying to do it, in flat water, and gave up. Variations in wind speed and direction make it near impossible to get consistent data.

 

[snowleopard]

So the boat in question is able to sail at 30º to the true wind?

I suspect the angle is heading, not course sailed. At 30° the leeway would be massive.

 

[snooks]

So the boat in question is able to sail at 30º to the true wind?

Well the designer's computer can predict that boat can sail at 30º to the true wind. With imaginary sails and sea state

 

[dt4134]

So the boat in question is able to sail at 30º to the true wind?

Yes, but not efficiently according to the polar diagram. i.e. you can pinch up that high.

In fact with 3m/s true wind (approx 6Kts) her best close hauled course looks like about 43 degrees TWA.

 

[Deleted member 36384]

There is an interesting article about this here:

http://www.secondsailing.org/wiki/index.php/Sailing_in_SL_-_Introduction+

The section on 'Helms' Role' true v apparent.

I wracked my brains trying to understand RL and SL with regards to VMG and the Polar Diagram. I couldn't understand what the text on the instrument was and assumed they must be racing with real time recording, so I had to check out the yacht club - who are these guys; Nantucket Yacht Club.

Then it dawned this is Second Life: RL is Real Life, SL Second Life. Virtual sailing, whatever next?

 

[Twister_Ken]

Yes, but not efficiently according to the polar diagram. i.e. you can pinch up that high.

In fact with 3m/s true wind (approx 6Kts) her best close hauled course looks like about 43 degrees TWA.

Yes but at higher windspeeds just below 30 looks best. My boat's polar BTW and I've maintained 25 to apparent but she's slowing down when that high. That's why my suspicion is that radials are apparent.

 

[dt4134]

Yes but at higher windspeeds just below 30 looks best. My boat's polar BTW and I've maintained 25 to apparent but she's slowing down when that high. That's why my suspicion is that radials are apparent.

I noticed that too. Looks like roughly 33 degrees to me in 20 Knots of wind. I've raced on a 370 and she could certainly give a good account of herself and I'd expect the 340 to be pretty good too, but I'd not have guessed at a figure that high. I'd have though maybe 36-37 degrees.

I race on a boat that can tack through 65 degrees TWA but she is a bit extreme and has an IRC handicap to match whereas I see the Arcona as a good modern cruiser racer without being extreme.

Every polar diagram I've seen uses TWA & TWS so I'd be very surprised if this one is any different. I usually like looking at the actual numbers themselves so I know what speed we should be getting close-hauled for a given wind speed. Sailing the numbers and all that. Do you have any tabulated data to go with the diagram? That should be more accurate than trying to read the numbers off a graph.

 

[dt4134]

I couldn't find anything for the Arcona 340, but here's the 430.

http://www.blur.se/polar/arcona430_vpp.pdf

She gets the best VMG at 37 degrees TWA in 20 knots TWS. Presumably they improved the hull form a little for the 340 which was a later boat but I'd still be surprised if it is much better than 37 degrees.

If it is keep stumm and go racing.

 

[Woodlouse]

I suspect they are all made up anyway. Ever tried to gather the real-life data to plot a polar? I spent 2 hours trying to do it, in flat water, and gave up. Variations in wind speed and direction make it near impossible to get consistent data.

Generally polar diagrams are created using a Velocity Prediction Program, which uses a formula that takes into account all the factors of hull shape, weight, rig, sail area etc. This then works out the theoretical speed the boat can achieve at various wind angles.

The problem with trying to create one with real life data is you are not strictly measuring what the boat can do, but rather the crews ability, and so the diagram is flawed from the very out set.

Generally speaking VPP diagrams are pretty accurate and any boat in good condition and sailed well should be able to attain 100% of it's theoretical potential. Top end racing boats, such as TP52's and the like can sometimes achieve up to 105% of their theoretical speed, which is what makes a boat successful.