freediver wrote on May 11
th, 2022 at 7:33pm:
Some good explanations and diagrams of how modern sailboats can outrun a balloon.
https://sailing-blog.nauticed.org/americas-cup-apparent-wind/
Basically, if the boat is moving at an angle to the wind, when the vector component of your speed in the downwind direction matches the wind speed, you do not cease to get a propulsive force from the wind.
Assuming you are at 45 degrees to the wind direction, and moving with the wind (keeping pace with the balloon in the wind direction) the relative wind speed sideways across the boat is the same as when the boat is stationary. The relative wind speed parallel to the boat direction is also equal in magnitude, but has shifted from a tailwind to a headwind. The headwind produces drag, which a modern boat sees little of, and the sail uses the sideways wind component to generate a forward thrust. That thrust only has to be enough to overcome the drag for the boat to accelerate so that it's downwind speed becomes greater than that of the wind.
The boat moves in the direction the boat is pointing, because a good keel makes it move through the water like a strain stuck on a rail. A tight sail still presents a wall to the wind that appears to be stationary relative to the water, if you cannot see it's movement in the direction the boat is moving, and is thus still able to generate thrust from it.
With sailing any course heading to true wind direction greater than 90 deg (perpendicular to wind) is considered downwind.
When travelling directly downwind ie when course heading is parallel to wind direction we call that running square.
A sail is like an aeroplane wing it needs airflow over both sides to produce lift. When running square you don't have airflow over both sides of sail so sail or wing has stalled. When you come up into the wind from dead square say about 45 deg you get airflow over both sides of the sail so it works as intended like a wing.
A keel stops the boat sliding sideways (leeway) that's another topic.
When sailing downwind at 135 deg from true wind then the apparent wind will move towards the bow when the boat is moving at a good speed. The faster you go the further forward the apparent wind will move so in effect the fastest boats always sail with apparent wind coming from a forward direction which means technically they're always sailing upwind.
If you are sailing to windward (upwind) ie 45 degrees from true wind doing 20 knots in 20 knots of wind then you will have around 40 knots of apparent wind coming over the bow.
The first boats to exploit apparent wind sailing were the 18ft skiffs on Sydney harbour over 40 years ago. Take note of how they let mainsail out so it's perpendicular to boat centreline while hoisting spinnaker (running square) then they point boat up around 45 deg get on trapeze the boat takes off then sails are trimmed closer to centreline as apparent wind increases just like if boat was pointing much higher into wind.
With the 18 video you will notice sails flogging and going inside out because they're overpowered while going upwind.
The advantage of using a wing is they don't go inside out stuffing up lift-drag coefficient so more efficient. Hydrofoils reduce skin friction drag from water to near nothing along with wave impact drag. These Catamarans are the current state of the art.
Video has graphics showing wind direction with white streaks along with windspeed and boatspeed. Take note of how they never run square.
It's basic trig to work out even though they travel much further distance tacking downwind compared to running square they're going a lot faster so they get there quicker.
Older keel boats are limited by hull speed which is 1.34 x square root of waterline length in feet so around 6.7 knots for a 25ft boat so people in those boats always sail shorter distance for better VMG ( velocity made good).
The origins of apparent wind sailing was developed on Sydney harbour and with sail gp video the skippers of the Japanese and USA boats are Aussies.
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