Thinking too much about keel forces
- Thread starter Daveinet
- Start date
Lift for a symmetrical hydrofoil is forward towards the angle of attack and dependent on speed thru the WATER and independent of the wind direction. Angle of attack is determined by hull shape and the rudder, any keel force against the wind is drag. The angle of attack for the hull is combine with the keel angle of attack to give forward motion to the boat by the sail.
The only thing I didn't like about that drawing is the vectors imply pulling, as opposed to pushing, which is what is really happening.
I think the vector diagram is the correct way to depict the relevant forces. The sum of those vectors is a vector with magnitude and direction, in the case of a sailboat under sail, the direction is forward. There's no implication of pushing or pulling (that's covered in the earlier fluid-dynamics posts).
Well technically the vectors don't imply that at all. They only show direction and strength.
There really is no pushing or pulling. Both of these would imply something to push or pull against (like solid ground).
Think about this way. Does an airplane wing 'push' or 'pull' a plane up?? Do you think of it pushing its way up?? No. Lift works against the pressure diff of the surrounding media alone.
EDIT - What KingJim said.
Exactly. No differentiation in physics between a push or a pull. It's just the same vector with either a 180 degree difference or positive/negative sign swap in magnitude.