| I am not speaking from a theoretical background. It is something I have experienced.

Experience is a hard thing. In that it is known, but what is known from it can be less obvious than we might think.

| A rigid wing has rotational inertia and an air flow lift moment as well as a bearing support. It is not possible to get them aligned longitudinally. That means when a boat rolls the inertia will delay its alignment under the lift moment. The delay means there is drive.

I am gonna take a stab at this. I think I might have this right, but I get things wrong often, so here goes.

First lets start with terms.
Lift will be from the angle of attack (aoa) of the wing to the relative airstream.

I think there is 2 rotational moments for the wing.
The rotational moment due to lift, commonly called Cm for aero heads. Also, the rotational moment due to movement of the mast. As the mast swings the wing perpendicular to the wing cord, if the center of mass of the wing (and Center of mass is kinda being abused because it includes a bearing friction factor too, but, if there are no objections, lets just call it the center of mass).

So, given the mast moves the wing some amount, I think we can mostly ignore the component of the movement that is in line with the wing cord. If that's true, then we only care about the part perpendicular to the cord of the wing.

This gives us 3 cases. The perpendicular thrust is ahead of the center of mass, behind the center of mass, and on the center of mass.

I think it is obvious the aerodynamic center is always aft of the mast, or the wing would not feather.

So, the aerodynamic center being aft, any mast thrust causes the wing to increase its angle of attack in the direction of the thrust, which creates lift in that direction.

So, perhaps it is obvious that wing masts do the wing mast dance in the wind.

Now, lets go back to mass. If the center of mass of the wing is forward of the mast (rotational center) a wing naturally immediately makes lift opposite to the thrust vector.

This should have a dampening effect.

Thus, I suggest, we can do math and figgure how much the center of mass of a wing needs to be ahead of a given mast (center of rotation) to dampen the out of plane thrust if a boat at anchor.

We should be able to plot the expected mast oscillations in different weathers and waves, and those movements map to an aerodynamic center force for a given wing and should outline what kind of center of mass for said wing needs to be to dampen these oscillations.

|That drive adds to the roll. As the roll reaches its peak, the wing is rotating into alignment and the boat gets to a point where it begins to right. Now the wing overshoots the other way and accelerates the restoring force but that keeps driving the roll in the opposite direction and so it goes on. In a given wind for a particular boat the process will pump itself into violent oscillation.

This, if this lead filled head remembers right, is a classic description of a system without dynamic dampening. And you are right, it is a BFD!

But positive dynamic dampening is, imho, just a mater of the design. I mean, at least some rigid wings have weathered storms, so the imminent harmonic destruction of every wing mast ever is a little exaggeration, right?

Now I probably overlooked something important, and obvious. I am kinda distracted trying to figure how to get bell cranks to work while spinning like records.

| The cat Ozone with its two wing masts found the only way to keep it from sailing on the mooring was to lock the masts at opposing angles.
| That is just masts. In fact the owner cut 3m off the top of the masts to stop the boat dragging its mooring.

What I find odd is that people seemed surprised that wing masts make lift when naked. It is a wing, right? And it does crap like this if you don't pay attention. https://m.youtube.com/watch?v=j-zczJXSxnw

Tom Speers consulted on a land sail speedster thats rigid wing fluttered to destruction too!

Now, afaik, we do build bridges like that any more. And Tom makes sure rigid wings are not built that way any more.

| There is no doubt a small rigid wing with a tail plane could have insufficient mass and rotational inertia to create the harmonic condition but that size wing is unlikely to have much influence in moving the boat in light air..

To be clear, this dynamic instability can effect small things too.

| The force on a sail is a function of wind velocity squared. A moored boat could reasonably expect winds to reach 60kts. On the other hand there is some hope of the boat moving efficiently in 5kts of wind.
| Most racing boats set sail area to begin reducing around 12kts. Cruisers maybe 20 to 25kts. So a wing of fixed area that can produce
|useful force in 5kts could easily experience 150 times that force on a mooring or at sea on long distance cruising.

Yep. Assuming it is not dynamically dampened by design.

Thank you Rick!