The physics of turning in a boat

Question

I am writing a small simulation of a boat (a sailboat under power rather than sail). It has a rudder and a keel and I have most of the physics working for the thrust and drag. I have modelled the drag across the boat to be higher than along the boat to reduce the sideways slip.

However it doesn't behave quite right. Am I right in saying that the keel will not only provide resistance but also contribute to taking the boat through the turn by channelling water?

The final model doesn't have to been scientifically accurate it just needs to "feel" like a boat.

Any pointers would be greatly appreciated.

Best regards and thanks in advance.

Dave

Answer 1

OK, realised I'm missing the hydrodynamic "lift" generated by the keel. Seems like it acts like a sail underwater and generates a force vector proportional to the flow of water over it and the angle of that flow. The force vector is in the opposite direction to the movement of water across the keel and toward the direction the boat is pointing. In theory this should reduce the sideways slip and also pull the boat through the turn.

Would appreciate if anyone out there who knows about this stuff if they could add an answer that makes some sense of my basic understanding. I am trying to distill this down to a formula that is related to the direction the boat is pointing and the direction and speed of travel of the boat.

Answer 2

When a displacement hull heels over one way (ie. to leeward), it causes the boat to turn in the opposite direction (ie. to windward); I've been told to do with the contour of the wetted area. I don't think the keel generates lift because the angle of attack through the water barely changes.

So when the boat turns, it heels outwards under centrifugal force (the centripetal force acts below center of mass, so there is a moment that tips the boat outward), and that changes the shape of the hull in the water in such a way that it acts to turn the boat slightly faster.

Another factor if the boat is under power is that the saildrive is usually directly in front of the rudder, which allows some thrust vectoring when the wash hits the rudder (ie. when the engine is going ahead, after a ½ second lag or so).