Is it the apparent wind created by forward momentum? Is it possible to go downwind faster than the wind speed?
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I think most people are skipping this important point. Instead of thinking of wind in terms of absolute speed, think of it in terms of force applied.
Imagine an object without friction in outer space. If you apply force, it accelerates and then continues to move. It doesn’t slow down to a stop. Apply more force and it speeds up, even if that force is minimal.
A sailboat is similar. If you push it, it will continue to move. Now continuously apply more force from the wind. The sailboat accelerates. The wind is a force applied to the boat, not like a conveyer belt that drags it to a stop when the wind stops. If wind stops, the boat continues to stay in motion until friction and other opposing forces eventually cause it to slow.
There are two ways to understand this. The first is to think of it as a simple machine like a lever. Normally we use a lever to turn large, low force movements into small, high force movements. Here we are doing the opposite: turning a small, high force movement into a large, low force movement. The sail, however, resembles less a lever than an inclined plane, with fluid as one of the acting members.
The second way to understand it is through energy. A well-made boat takes relatively little effort to move forward. The wind, however, has practically as much energy as we are willing or able to capture, limited only by the size of the sail. I believe the equation is the same as that for a rocket: mass flow rate times velocity squared for the power, although even the best sail won’t be able to capture it all. The mass flow rate is given by density times area: in other words, the larger the sail, the more energy we can capture.
Yeah sail-CARS can also do this… Veritasium proved it on YouTube. Won a bet against a few actual physicists and whatnot
It works on land too. Using propellers that power a gear system, you can ride faster than the wind that pushes you. You use the wind more like a fuel than as a thing that directly pushes your vehicle.
Great replies and links! Makes sense now. I think the soap analogy and the idea that the boat will move even if the force of the wind were to stop really made it click. It’s funny because I am (what I would like to think) a pretty good sailor, so I use this principle regularly without fully understanding the physics of how it actually works.
Don’t think of sails at nets that catch the wind. Think of them as wings that ride the wind. They work in almost the exact same way that an airplanes wings work, except sideways instead of up and down.
I’m not sure I saw a good answer to this.
But first, most sailboats don’t travel faster than the wind, since the boat and keel have friction against the water.
However, with the right design, you can make a boat to hydroplane against the water while the sails act like wings, pulling the boat.
So, if you can get near frictionless and have a good wing, you can surf the boat.
the really fast ones use a hydrofoil type of hull
there are two main types of hulls
a) planing hull (think speed boat going fast it rides up out of the water and hydroplanes) the net result is a small amount of the hull is in the water thus it goes fast.
and b) a displacement hull – think aircraft carrier (and normal sailboats) these have to push their way through the water.
the third type is relatively new the hydrofoil hulls (discussed below) which i think is what you are asking about.
the problem for boat speed is the size of the hull in contact with water as you can tell a high speed boat gets up into planing speeds the surface area in contact with the water reduces so the boat goes faster.. imagine an air craft carrier going that fast
the hydro planing and hydrofoil boats take this to an extream
this site goes into more detai
[https://improvesailing.com/questions/what-are-the-fastest-types-of-sailboat-and-why](https://improvesailing.com/questions/what-are-the-fastest-types-of-sailboat-and-why)
here is a video about sail rocket a really fast boat.
[https://www.youtube.com/watch?v=K3m06731BQY](https://www.youtube.com/watch?v=K3m06731BQY)
one of the problems you have at this speed is cavitation… this is the white frothy air bubbles formed by a propeller when the propeller is turning really fast… in this case the foils are going fast enough to cause cavitation and that cavitation causes drag which slows you down.
Here is a simple toy analogy, which explains how it works:
[DOWNWIND VMG GREATER THAN WINDSPEED ANALOGY](https://www.youtube.com/watch?v=ZRnckbYeoRs)
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