I tried to google it and it left me very confused.
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Think about a blade on a wind turbine. Because it is fixed on the hub and has a cool shape all it wants to do when the wind blows is move sideways.
Now imaging sticking one on top of a boat. It still wants to move sideways but without a hub it just goes downwind instead of upwind.
A solution is to stick another blade under the boat and because it is in “wet” wind which is a lot more dense it can be a lot smaller and all it wants to do is also move sideways too.
They work against each other in a simple upwind or downwind consideration but they work in concert for sideways movement because they are both headed in the same direction. As the speed increases the wet blade “lifts” more that the dry one so the boat heads upwind at an angle.
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