| The problem with a rigid wing is a combination of its inertia and having a very narrow angle where there is no drive.

| A soft sail can be easily feathered by releasing the sheet. It will flog in high wind but will not offer drive; just drag. A rigid wing offers drive as soon as it moves a fraction off alignment.

I am not sure how a rigid wing moves off alignment in wind, as it is the wind that creates the alignment. Perhaps bearing drag?

But lets just assume its reasonable and possible.

Is not a rigid wings drive proportional to its distance off alignment. While a small drive might be noted, that same drive returns the wing to neutral. Should we not assume the wing will follow the wind closely?
And this small drive must be compared to the random and more severe drive of the comparable bare pole. Or, i guess, the flogging soft sail?

| As soon as it moves, the platform rolls due to the drive and that will pump ever increasing oscillations until something breaks or the platform capsizes.

This cannot be known to be true. Dynamic dampening is well understood in fluid dynamics. The idea that your outcome is the only possible seems wrong.

I am much more experienced with wing tail planiforms, and they are designed for dynamic dampening. This is well understood and nearly trivial to accomplish.

This phenomenon you mention actually happens in poorly designed flying aircraft. Also it is exaggerated with pilot input known as pilot induced oscillation. This kills pilots.

But it is relatively easy to design a wing tail combination that has very aggressive dampening. I imagine given the interesting roll problem boats have with rigid wings, the typical techniques might require tweaking.

| A soft sail flogging violently is bliss compared with a rigid wing rolling the whole boat violently.

And a reasonable, properly designed, rigid wing has less drag/thrust/drive than the pole left behind by other sails.

Rigid wings have had very little development, and there have been several successful ones, so the idea that this event should be assumed as the one true outcome seems unsupported. But I know nothing.

| And the flogging sail can be lowered in a matter of seconds or loaded to stop it flogging.

Leaving behind the driving pole. Which, if I am not wrong, have been known to rock boats violently.

Also, I assume one could demount the rigid wing and drop it in the water if one had need.

| As far as I know there is only one water based platform with a rigid. wing that has bettered the speed of kites, which are not rigid.

There are reasons to pursue rigid wings beyond speed.

The ability of a builder who just built a foam and plastic boat to build an inexpensive, relatively impressive performing sail using the same techniques.

A lower maintenance sail.

An easier to repair/rebuild sail.

A simpler to control sail. One tail control only!

The ability to have a drive system that maintains angle of attack to the incoming wing automatically.

Of course, as mentioned, the bearing is a concern. Single points of fail are bad.

hello luc

> Aircraft dynamic stability (wing + tail + aircraft cg position) is very different than a free rotating wing on a pole (no tail, gravity play no role in wing rotation unless the boat is rocking).

Aircraft have a number of design solutions for controlling pitch, there are tails with shared distribution of load, there are tails that oppose lift popular for STOL, and there are all flying tails which provide +/- . They all depend upon the size or area associated with the couple or distance from Cl CG.

The one complication is that the shorter the couple, the more the tail is influenced bu the flukey turbulent downwash of the mainplane. From memory (always dangerous) something like 3.5 chord lengths distant presents a good numerical case. (I can check if needs be)

> Best case scenario is a symmetrical wing rotating on a pole at the aerodynamic center of the foil so there is no varying pitching moment with wind speed (CM=0). But also the CG of the wing need to be at the center of the pole otherwise when the wave rock the boat, it would create a moment that will rotate the wing and create lift, so a driving force and more boat rocking moment.

Probably the best case, except that depending on the section it is usual that the moment alters by angle of attack (AoA). I dont think this is the moment our learned friend was referring to though, I think he was talking about fore and aft at the masthead. Its a plausible concern.

> But still, nothing to do when the bearing jam! Something will go wrong!

In defence of that position, there have been hard wing sailing autonomous trimaran drones at sea for weeks at a time covering many thousands of miles in the most treacherous seas the Bearing Sea can offer. without a pilot. a crew, and with no ability to reef. To my knowledge non of these boats have been lost.

I think its wise to be very cautious over the bearing issue, but it seems technology and maintenance can get on top of the situation. We've come a long way from the Walker Wingsail 1982.

> PS They use crane to remove rigid wing from a boat ..

We used one to get my sticks in and out too ...