Planing upwind

Friends,

I'm brand new to this forum and this is my first posting.

I'm hoping that somebody can explain to me the physics that allow a boat like BMW Oracle or Alinghi to sail upwind at double the true wind speed for a prolonged period of time. I've sort of been curious about this phenomenon for some time but my interest was sparked again this week when BMW Oracle released a photo of the vessel sailing upwind at 20 kts. in about 10 kts. of true wind.

I understand how planing monohulls like dinghies and sportboats can exceed the wind speed for a periods of time when broad reaching. But I've never understood the science of how it happens upwind.

I imagine a bell curve (or polar) comes into play. In other words, an upwind multihull like BMW Oracle cannot immediately exceed the wind speed upon first trimming its sails. And at the other end of the bell curve must be another limit, i.e., a boat cannot go, say, six times faster than the true wind.

Thanks for any help you can provide.

 

Are you sure it was sailing upwind? I wouldn't bet the money on it. Plus all speed and wind speed numbers are pure speculation - unless you followed them in Alinghi's chase/spy boat.

Anyway, the apparent wind builds and the boat sails faster and faster until they outrun the sails or their course is way off and vmg suffers.

 

It was the writeup from BMW/Oracle that said the boat was going upwind. If you go to the syndicate's Web site, I think you can see the photos.

Meanwhile, I'm curious. What leads you to doubt that it's possible to exceed the true wind speed when going upwind?

 

I don't doubt it for a second that they go multiples of wind speed in any direction. With that kind of money spent, they better.

 

As to the physics..remember that the sail is not pushing the boat...it's a big wing generating lift and pulling the boat along, much like a plane.