| >How does twist help the need for reefing?
|
|If the top of the wing/sail is twisted to produce very little or no lift, it will add little or no heeling moment. The bottom will produce the lift for sailing. So this is similar to reefing. But with the added drag from the unused sail/wing area.
Sure, but, why add the complexity if twist, when one can just ease the angle of attack?

Because of wind gradients. Wind is always slower on the surface because of friction. Because of this, the angle of the apparent wind might be significantly different in the top compared to the bottom. So a sail always needs to have twist. In lighter winds,a rigid wing without twist might work. Then the top will work at a higher angle of attack than the bottom. Maybe the top will lift and the bottom will be at zero or negative angle of attack. The boat will move, but it would be better if the whole sail could provide lift.

In the case of strong wind sailing, if there is no twist and the top is the first part of the wing to produce lift, it will create a strong heeling moment, and the boat will be very sensitive angle of attack. The boat might flip as soon as there is a small gust. With twist/flex in a sail, the top will be flexible, and might spill this gust. The bottom of the sail doesn't have the same leverage, so the gust will not change the heeling moment as much as at the top.

| This is more or less what any sailor does when easing sheet tension. However, easing sheet tension generally adds camber though, which is the opposite to what one wants in a strong breeze.. On the foresail, there is the option to change the angle of the sheet in this situation, to make the bottom part tight and flat, and the top part twisted and spilling wind or even flogging. For the main, tightening the outhaul can flatten the bottom of the sail to get a similar effect.
Sure, but are we not talking about Robs wing sail? Which has no fore sail? This is a schooner or una rig, right? Does one not generally ease said sail by decreasing the angle of attack?

Most boats have a bermudan rig, so I used it as a reference, and I was saying that it has many ways to control the 3D-shape of the sails. All types of sails needs ways to control camber, angle of attack, and twist, to work on a sailing boat, I think. (Maybe not some downwind sails)

You are right that the harryproa sails with a wishbone boom doesn't change the twist with the sheet, like most other boats. The sheet only changes angle of attack of the boom, so it works like the traveler on a catamaran. Other ropes control twist and camber, but are not constantly controlled, because the flexible mast changes angle of attack/camber on the top of the sail somewhat automatically. I think you can compare it to a spring controlled flap on a wing. If the spring has the right rate and pre tension it can give the wing almost constant lift independent of speed. So if the mast flex rate and the tension of the twist control ropes fits the conditions and RM of the boat, it should be very forgiving in gusts. Because the heeling moment from the top of the sail should be constant, so the sailor only has to adjust for changes in lift of the bottom of the sail. And if he isn't flying the hull, there is likely enough margin that he doesn't have to adjust the sheet at when the gust hits.

| In any situation though, if the boat is steered in a direction which increases the angle of attack significantly, it will heel over anyway. A wing with a tail should solve that situation, since the tail works to keep the apparent wind at a constant angle of attack.
I don't think Rob's wing sail is expecting to have a tail. But yeah, a tail makes sense on a solid wing sail.

Rob has at least once indicated a desire to have a tail. I think multihulls are generally hard to trim because the sudden changes in apparent wind angle due to the fast acceleration and large speed range. The sailor really have to sheet a lot and develop a great understanding of how boat speed affect the apparent wind angle to avoid stalling the sail or heeling too much. So I think it makes a lot of sense with a tail which does a lot of the work automatically.

| Another problematic situation is accelerating to a speed where the lift is enough to heel the boat over. This is where the sailor has to change his input in some way, even with a wing with a tail. Perhaps by changing the angle of the tail, or by decreasing angle of attack in the top (twisting the flap and/or wing).

 

Should the lift not drop as you gain speed?

 

Lift is proportional to the square of the apparent wind speed, so higher speed has the potential to increase the forces significantly. But the apparent wind angle will decrease when the boat accelerates, so generally lift is reduced. However, when I'm sailing, I think often part of the sail doesn't have attached flow. It's easy to stall some part of the sail. When the boat accelerates, and the angle of attack decreases, flow might attach, and the sail suddenly develops a lot of lift in this now higher apparent wind. This leads to heeling and I have to release the sheet completely to prevent a capsize, or in a better case, ease the sheet a little, or steer a little, to find a good balance, and reach a great speed.

 

Of course one can change the angle of attack. Is that not easier than changing twist? And if so, I am back to why have twist control?

Depending on conditions, the wind gradient will be different. Depending on point of sail and speed, the resulting gradient in apparent wind angle will be different.

On Thu, Jun 21, 2018 at 12:56 PM, '.' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:

 

| >How does twist help the need for reefing?
|
|If the top of the wing/sail is twisted to produce very little or no lift, it will add little or no heeling moment. The bottom will produce the lift for sailing. So this is similar to reefing. But with the added drag from the unused sail/wing area.

Sure, but, why add the complexity if twist, when one can just ease the angle of attack?

| This is more or less what any sailor does when easing sheet tension. However, easing sheet tension generally adds camber though, which is the opposite to what one wants in a strong breeze. On the foresail, there is the option to change the angle of the sheet in this situation, to make the bottom part tight and flat, and the top part twisted and spilling wind or even flogging. For the main, tightening the outhaul can flatten the bottom of the sail to get a similar effect.

Sure, but are we not talking about Robs wing sail? Which has no fore sail? This is a schooner or una rig, right? Does one not generally ease said sail by decreasing the angle of attack?

| In any situation though, if the boat is steered in a direction which increases the angle of attack significantly, it will heel over anyway. A wing with a tail should solve that situation, since the tail works to keep the apparent wind at a constant angle of attack.

I don't think Rob's wing sail is expecting to have a tail. But yeah, a tail makes sense on a solid wing sail.

| Another problematic situation is accelerating to a speed where the lift is enough to heel the boat over. This is where the sailor has to change his input in some way, even with a wing with a tail. Perhaps by changing the angle of the tail, or by decreasing angle of attack in the top (twisting the flap and/or wing).

Should the lift not drop as you gain speed?
Of course one can change the angle of attack. Is that not easier than changing twist? And if so, I am back to why have twist control?

| I read somewhere that the AC50 cats could twist the top enough to produce negative lift, to actually get righting moment from the top of the wing.

The bell shaped wing guys are big on this. For flying machines, it allows the control of yaw while turning without a vertical tail (flying wings). Al Bowers calls it the bird wing solution, because that's how birds fly. No vertical tail.

For a boat though, I still do not understand why the added complexity is desirable. Yes, a small thrust at the top of the mast is super neat. But I am not sure how it works when I am expecting the mast to flex when gusting, and it stands rigid due to aerodynamic loading. That seems like it removes a safety feature.

Thanks for being patient with a sailing noob Björn