<<The best wing profile from a performance perspective is
cambered as Todd has. To get benefit from this it needs to flip
through the wind per your observation>>
I definitely get why the camber is effective at generating lift,
why the wing can be a lot smaller than the equivalent sail, why the
apparent wind will be an even bigger benefit/effect, and why all
this can combine to make the momentary broadside position during
shunts a minor issue or non-issue. Comparing a perfectly-cambered
rigid wing to any normal sail is to compare apples to oranges.
For a coastal speed demon, as Todd defines it, this is a very
simple and efficient rig for a certain window of wind speeds. And
man, can it shunt quickly...
However, I'm going for a rig that will let me enjoy four knots of
wind, work at sixty knots (whether through reefing or shaping), and
then be able to ride out a hurricane as good as or better than a
bare tapered cylinder.
Even in our home harbor, there have been times we've gotten
nor'easters that were much more powerful than predicted, and I've
watched our moored boats ride out thirty to fifty-knot winds for two
days. I definitely want my next boat to be able to do the same
thing without my having to worry about it.
This is where a cambered bidirectional wing appears to come up
short. I can't yet envision a wing that works in four knots being
safe over forty knots, or being able to hang out in true neutral for
long periods of time, whether at sea or on the hook.
However, that doesn't mean it can't be done. I'd *love* to be
able to shunt as quickly as Todd's rig can shunt while still being
safe throughout a huge wind range. It's possible that I just need
to learn more.
<<The only way I see to make a rigid wing work reliably is
with a tail plane or flap. >>
Thus far, I agree. I don't see another way of allowing the wing
to tend itself, particularly in neutral, for any significant length
of time.
Two sets of questions:
1) Staying in Neutral. I understand how a flap could default to
neutral when the sheets are released, and stay that way for minutes
or even days (you'd surely want to lock in in neutral if
intentionally leaving it unattended for any length of time).
But what about a wing with a tail plane? Would the tail plane
be in conjunction with a symmetric foil with a single leading edge,
a wing with a single leading edge that has camber induced by a flap,
or a cambered bidirectional wing?
I'd love the auto-adjustment ability of a tail plane with a
bidirectional wing, but would definitely worry about the entire
system falling apart if the tail were to get damaged. It's probably
a very low probability of failure, but still the potential for a
single point of failure. Though I suppose you could make both the
tail and its control system really beefy. Would the tail have to be
switched from one side to another on each shunt?
2) Windage. Could any of the above wings end up providing the
same or less windage as the bare pole? I know that many foil shapes
will do this, but I don't know if that's true for a foil that's long
enough to be a sail.
<<If you are after a real easy to handle rig then maybe a
Flettner rotor would be of interest>>
Those rotors are awesome! I've only seen them on larger test
craft so far, not on smaller sailboats. Neat.
Alas, I may have to go with a more traditional rig on the next
boat. :-)
- Mike
Rick Willoughby wrote:
Mike
I believe all the issues you raise are valid.
The only way I see to make a rigid wing work reliably is with a tail plane or flap.
A tail plane can control the lift precisely and will automatically adjust to wind shifts or boat heading.
The best wing profile from a performance perspective is cambered as Todd has. To get benefit from this it needs to flip through the wind per your observation. However it is possible to use it in reverse in strong winds where you do not need the maximum lift it is capable of generating to avoid the flip if it is rigid.
A 20% thick solid section with a 10% camber will have a higher lift coefficient going to windward than drag coefficient when flat to the wind. So up to the point of being overpowered the force on the sail will be less during the shunt flip than when reaching.
Also for a large range of true wind an easily driven proa will gain apparent wind speed. So this increases the drive when moving compared with the overturning load during the sail flip.
A non-cambered wing can be fitted with a flap to achieve the same result as a cambered wing. It will not automatically adjust to wind shift but it can be depowered by releasing the sheet.
If you are after a real easy to handle rig then maybe a Flettner rotor would be of interest:These get very high lift coefficients compared with typical foil shape - Cl of 5 or 6. A shunt would simply involve changing the direction of rotation. The ratio of Cd when not rotating to best Cl when rotating is about 10 times. If it had a tail section that could be deployed in heavy wind the Cd could be reduced even more. They are easily depowered by reducing the speed of rotation.
Rick
On 06/06/2011, at 9:02 AM, Mike Crawford 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