Theory of Sailing

[Christian Williams]

Put a sailboat dead in the water. Trim the sails.

It sails into the wind.

Now on the same boat, instead of the sail, raise a piece of plywood and trim it.

Will it still sail into the wind?

(Never mind the keel; my pram daggerboard is already flat plywood, no attached flow)

 

[footrope]

Well, don't be too hard on yourself if you don't "get" lift. It is not obvious to me why the curved surface accelerates the flow.

Those America's Cup multi-hulls really had a great time with wings for main sails though.

 

[Teranodon]

Put a sailboat dead in the water. Trim the sails.

It sails into the wind.

Now on the same boat, instead of the sail, raise a piece of plywood and trim it.

Will it still sail into the wind?

(Never mind the keel; my pram daggerboard is already flat plywood, no attached flow)

I think that a sailboat with a flat plywood sail will go to windward, no problem. But there has to be some lateral resistance in the water: a keel or its equivalent.

The big square-rigged ships of old had no keel, just an elegantly-shaped hull that must have provided a certain amount of lift.

Another guess: a sailboard with nothing sticking down into the water will just slide sideways. Am I wrong?

The simplest model of a lifting sail is: the air "piles up" on the windward side, thus increasing the pressure on the sail. The leeward side side is "shielded from" the wind. There is less air there, period. Hence low pressure. Hence a pressure gradient across the sail, hence lift.

The next simplest model of a lifting sail is: the sail deflects flowing air because it is tilted with respect to the wind direction. By Newton's Third Law, the sail has to be pushed the other way. Hence lift. I really hate this model.

 

[bgary]

I really hate this model.

It has some merit, though.

Think of the simplest form of rudder. It generates (effectively) no lift, it works by deflecting water. Push the tiller, the rudder presents an angled obstacle to the flow of water, that deflection results in a force which moves the back of the boat. It could be argued that the whole purpose of the rudder is that deflection... and/or the redirected "thrust" that comes from the deflection. Providing an elegant NACA-section shape to the rudder enhances flow and reduces drag, but doesn't change the fact that it's the deflection and/or redirected thrust that does the work of turning the boat.

Kinda like Loren's example of holding a hand out the window of the car. Not an airfoil shape, but clearly holding the hand at an angle to the airflow deflects the air, which results in a net force.

Or something.

An exercise for the physicists.... take the same principle and apply it to a headsail. Air comes in at an entry-angle (call it 30 degrees) and gets deflected, coming off the back of the sail at an angle of ~0 degrees relative to the axis of the boat. Could that simple deflection be what matters? And can the resultingly redirected airflow, aft along the boat's axis, be considered "thrust", pushing the boat forward, with the keel taking care of the sidewards force...?

Bruce (somewhere in the dark recesses of my brain lurks the memories of how to draw a force-vector diagram...)

 

[Mark F]

Something I have heard while racing is a few degrees of weather helm and resulting rudder deflected to the direction of the boat was advantageous to pointing because the rudder creates a small amount of lift to windward.

 

[Kenneth K]

Anyone who has sailed a light tiller boat probably knows that on a windless day, you can propel the boat solely by "sculling" the rudder repeatedly from port to stbd. Enough headway can be gained to maneuver the boat into a slip.....

One would think the forces from repeatedly wagging the tiller would be side-to-side forces which would (along with any lesser fore-aft forces) cancel each other out. But, all the forces are apparently seeking the net of their equal-and-opposite reaction and the boat is propelled in its direction of least resistance--forward.

 

[gadangit]

A great book by Brian Hancock called Maximum Sail Power devotes 20 pages of text and diagrams to describe what is going on. Not mentioned in any posts above is the Kutta Condition which doesn't get a lot of press.
Chris