Todd,
That makes sense.
As Rick says, the force due to lift is a lot higher than the force
due to drag, and it's highest when the boat is creating loads of
apparent wind. So the non-lift position, when the boat is at rest
during a shunt, would be the lowest force.
If you can put the wing in neutral by over-sheeting, which is what
I think you're saying, then the rig isn't nearly as worrisome as I
feared. That will allow the boat to stop before flipping the wing,
and once stopped, the flip should generally be benign.
Thanks for the explanation.
---
One comparable kite maneuver might be like with a Revolution,
which flies just about equally well in forward as reverse. There's
a short period of increased downwind drag during the change in
direction, but it's generally just a blip, and certainly less than
the lift generated when the kite is flying.
Another comparable maneuver would be the turn. As you said, the
kite is generating loads of lift when flying through the water at
just the right angle, then we put it closer to the edge where it
loses power, bring it straight up to near-neutral, turn while the
forces are low, and then take off again, with lift increasing
radically with the buildup of apparent wind.
I think the biggest difference between our viewpoints is my level
of caution.
Part of this stems from using kites on land and on frozen lakes.
I've been overpowered on these surfaces, to the point where even
neutral is bouncing me into the air, or dragging me across the
sand/ice towards fixed obstacles at high speeds, and that's really
uncool. I've never gotten hurt, but there have been close calls.
It takes a serious amount of skill to get an overpowered kite to
soften your landing when overpowered to the point where the jump
wasn't fully intentional. And if the wind picks up to that point,
wrestling the kite to the land, single-handed, and stowing it
without getting dragged about or losing the kite, can be a bit of a
crap shoot.
Once I had a wind shift so powerful, that even when I got the kite
down to the ground at the edge of the window, I needed two friends
to help: one to help hold me down, and the other to get to the kite
and wrap it up into a ball until we could find a sheltered area to
fold it properly. And that's after they had to run a quarter mile
to catch up with me. Without the buggy -- I was just on foot. I
went from having a great time, sliding about the sand, and taking
long three-foot-high leaps, to "holy crap!" in the span of a minute,
with sustained winds over twice what was predicted.
Maybe I shouldn't have been out in big winds as many times as I
was. Or should have started with a smaller kite. But when whole
new level of wind comes out of nowhere and stays, things that were
safe ten minutes ago can get pretty ugly.
As long as you're not trying to stay off a lee shore, kite surfing
presents fewer opportunities for crashing into hard objects. Jumps
that would have been scary or dangerous on land can be loads of fun
if there's water under you.
---
Boat-wise, I'd liken it to flying a hull on Hobie or Elementarry,
compared to flying a hull on a 30 or 40 foot catamaran.
On the Hobie, a capsize might mean having a bit of a rough landing
and getting wet, but then getting back to sailing in a few minutes,
to capsizing a 30 or 40 footer, where capsizing means you're in a
bad way. On the big boat, a capsize means a longer fall, probably
turning turtle and staying that way until righted by a powerboat or
crane, and damage to all sorts of electronics and cruising
equipment.
Basically, what's great fun on the Hobie, and basically safe, is a
different story on the cruiser.
Thus, my time with the kites, and with a catamaran I can't right
without help, has conditioned me to be cautious. Perhaps over
cautious. I'll admit it, and certainly wouldn't imply that anyone
else should be quite so safety-conscious.
---
You and Rick are actually convincing me that the cambered wing
shift isn't such a bad thing as long as you have the right amount of
sail area up.
My personal challenge, then, is in reefing (because the wing that
works in five knots won't work as well in fifty), and finding a
fail-safe neutral.
I'm still trying to work those out in my head.
I can see having a soft cambered bidirectional wing, where ribs
are hoisted up and down, supporting the fabric in between. If the
leeches can be tied into a rigid boom, the sail should be quite
controllable.
The fail-safe neutral is harder for me. I'm a fan of the
simplicity of a fixed mast with a junk/wing sail that will
weathervane in neutral on its own without help, and which can be
dropped quickly and completely when needed. But it won't likely be
as efficient as the bidirectional wing, nor will it shunt nearly as
quickly.
I understand that my focus on safety goes overboard in most
people's minds. The average multihull with a stayed rig stands
little chance of going to true neutral without input at any point of
sail, so what I'm talking about is abnormal. Ultra-fast and
ultra-safe rarely coincide in the same boat and/or rig.
- Mike
tsstproa wrote:
Mike real quick I think you asked me how Do I compare the solid bidirectional wing to a four Line Kite. I should have said semi rigid inflatable four line power kite. Pure parafoils probably don't maintain the rigidity when fully dump of their power through the eye of the wind.
I gave the answer of precision of stable daft and camber from wing being rigid.
But I didn't not explain the really simple fact that its exactly like a kite going from one edeg of the wind window to the other . Us Kiters make most of our turns at the edge of the wind windward far out infront pinch out as far as the kite will go. Kitesurf kite especially cause were over powered on land need that depowering more so. The solid rigid bidirectional wing does the same thing but way more precise due to its ridigity its small circumference rotating on its axis vs swinging through a huge arch on lines from one edge of the window to the other to change direction. I think we discussed this a couple of years ago. The effect is the same thats why I relate it to four line semi rigid inflatable power kite. The force on the wing never builds up in the center of the wind window directly down wind if you don't want it to. Over sheeting depowers the wing through the eye of the wind window just like on a four line power surfkites k but the wing is way faster and way more precis.
Todd
Sorry for the double talk.
--- In harryproa@yahoogroups.com.au, Mike Crawford <mcrawf@...> wrote:
>
>
> I's possible that I'm missing something.
>
> Could you show me how to stall a square/elliptical rig, and leave it
> stalled for a few minutes, even if the wind direction changes or the
> boat drifts? That would do a lot to help me see the safety of the rig,
> and should be easily demonstrated on a model if it's possible.
>
> I've flow four-line kites since the 1990's, so I'm familiar with how
> they power and depower. My favorite thing is to use the kites on a flat
> beach during low tide, flying along on a three-wheeled kite buggy.
> Great fun. That said, I've never been able to just leave my kite alone
> while I get up to pee.
>
> There's also the worry of the momentary shock loading of the system
> during a shunt as the wing turns from one side to another. Your
> "Planning Proa squared" appears to demonstrate this issue, with the
> windward hull popping off the water momentarily during many of the
> shunts. Get hit with a gust at the same time, and the boat is gone.
>
> Which is not to say it's a bad rig. Your tests have clearly shown how
> fast it can be. I just personally have an issue putting this on a boat
> with a stove, fuel, water, head, and passengers.
>
> ---
>
> This is actually one big reason that I'm planning on a proa in the
> first place.
>
> If I dump my current catamaran in water less than 30' deep, it will
> likely cost me between two and six thousand dollars to replace the
> equipment that could get damaged. I love having a boat with an SA/D
> over 50, but it does worry me when pushing the boat. That kind of cost
> figure for a capsize tends to lead to white knuckles when flying a
> hull. Especially if I'm at an angle where dumping the sheets will
> actually increase the heeling moment, leading to a knockdown or pitchpole.
>
> So it's my dream to get a boat of similar speed, but with greater
> righting moment and longer waterline, and a more powerful rig that I can
> always just depower by letting go. Up to this point, you wouldn't think
> that fast and safe could go together with the same boat. But a WTW
> proa, or catprao, can be an impressive design.
>
> - Mike
>
>