A question about polar speeds

[PCP]

Look at this Polar speed : How can the boat sail faster upwind with 8K of apparent wind than with 8K of true wind? with 8k of true wind, against the wind, the speed of the boat is making wind, so with 8K of true wind the sais are having about 11K of apparent wind. Probably 5 to 6K of true wind corresponds upwind to 8 k of apparent wind.





So, how it is possible for the boat to make upwind faster speeds with 8k of apparent wind than with 8K of true wind if to a of 8K of apparent wind corresponds only 5 or 6K of true wind? The boat goes faster with less wind on the sails?

[blt2ski]

Probably the same way the last multi's in the AC were able to do 20+ knot speeds in 8 knots of wind. They are able to manufacture there own wind upwind or equal! so they are able to go faster than the wind. Same as Hydroptere is able to do 50+ in 30-40 knots of wind.

Then again, maybe I am not seeing what you are!?!?!?!?

Marty

[PCP]

Marty, If they can make their own wind (and the boat form the polar can in a certain measure) than for instance 8K of true wind will correspond upwind to 11 or 12 apparent wind. The boat is making wind.

So the boat should be faster with 8K true wind than with 8k apparent wind because to those 8k true will correspond 11 or 12 apparent and to the 8 apparent would only correspond 5 or 6k true wind.

With weak winds this boat can also go faster than the wind.

That is what you are saying regarding modern racing trimarans, in a much bigger scale and not only on weak winds.

When I saw this polar I thought that someone had made a mistake about the winds and where it is written true wind should be apparent wind and vice verse, but after that I had saw other polar the same way:

SaareYachts.com - Saare 38 - Specifications

So I guess I am missing something here. It would be possible that anybody here has a clue?

Regards

Paulo

[ebs001]

A boat cannot sail faster in 8K apparent than in 8k true assuming its going forward. V(A)=V(Boat)+V(T)

[miketucker]

Hi PCP,

I think I understand the source of your confusion. I assume what you're doing is looking at the TRUE wind side of the plot, following the 8kt wind curve to, say, 45 degrees, and seeing that the boat will go 6.5 knots. Then you look at the APPARENT wind side of the plot, follow the 8kt wind curve to the same 45 degrees, and notice that the boat speed is 7.5 knots. So it seems that, as you said, the boat is going faster in 8kts apparent wind than 8kts true wind. That seems counter intuitive, for the reasons you have explained.

The problem with this comparison is that the angles on the TRUE wind side of the plot are angles relative to the TRUE wind, while the angles on the APPARENT wind side of the plot are angles relative to the APPARENT wind. Thus a course of 45 degrees to the TRUE wind is NOT the same as a course of 45 degrees to the APPARENT wind. You're comparing apples to oranges.

The two sides of the plot are showing the same information, just in different frames of reference. There is no conflict (assuming the plot is accurate of course).

Does that answer your question, or have I misinterpreted the issue you're having?

Mike

[blt2ski]

paulo,

I would say mike has the answer. 45True is probably around 30-35 apparent plus/minus

marty.

[PCP]

Originally Posted by miketucker

Hi PCP,

I think I understand the source of your confusion. I assume what you're doing is looking at the TRUE wind side of the plot, following the 8kt wind curve to, say, 45 degrees, and seeing that the boat will go 6.5 knots. Then you look at the APPARENT wind side of the plot, follow the 8kt wind curve to the same 45 degrees, and notice that the boat speed is 7.5 knots. So it seems that, as you said, the boat is going faster in 8kts apparent wind than 8kts true wind. That seems counter intuitive, for the reasons you have explained.

The problem with this comparison is that the angles on the TRUE wind side of the plot are angles relative to the TRUE wind, while the angles on the APPARENT wind side of the plot are angles relative to the APPARENT wind. Thus a course of 45 degrees to the TRUE wind is NOT the same as a course of 45 degrees to the APPARENT wind. You're comparing apples to oranges.

The two sides of the plot are showing the same information, just in different frames of reference. There is no conflict (assuming the plot is accurate of course).

Does that answer your question, or have I misinterpreted the issue you're having?

Mike

Thanks Mike. I guess you are right. To a 45º angle of true wind will correspond probably something like a 35º of apparent wind. As you say two completely different reference systems.

I was just thinking about wind speed, apparent and true, forgetting that the angle is also different

After having seen two Polars with bigger speeds on the apparent wind I knew it was my fault and that I was missing something but I could not figure what. Must be getting old

Thanks again,

Regards

Paulo