Okay so this is obviously an America's Cup analysis now. We can play at being Ben Lexcen's. (Ben Lexcen was the designer of Australia two's winged keel back in1983) I think that if you can see air bubbles eg. in the wake, or waves formed from the objest when it is underway, then you must have turbulence or resistance ie energy lost. If it is smoothe and making no wake or frothy foam bubbles, then it must be pretty efficient. What I was finding on Sidecar, was that when the rudders were too small (0.8m underwater depth-draft) there was problems. I am talking about real visible stuff, not a number on a page. This can be where critics can question the Harryproa legitimately, because some problems seem to occur with steering as well as with
the drag. When people listen to the BD debacle it must be a put off. Rare Bird hardware failure is easier to explain as underestimated rudder loads. Doug
--- On Mon, 16/5/11, Micha Niskin <micha.niskin@gmail.com> wrote:
From: Micha Niskin <micha.niskin@gmail.com>
Subject: Re: [harryproa] Re: Ventilation
To: harryproa@yahoogroups.com.au
Date: Monday, 16 May, 2011, 5:40
This is how I understand the phenomenon. Please correct me if I got it wrong:
Atmospheric pressure is about 100kPa. This means that at the surface, the water pressure and atmosphere pressures are equal, at 100kPa. The static pressure (that is, the pressure exerted by still water at some point beneath the surface due to the weight of the column of water above it) is about 10kPa per meter. So at a depth of one meter the pressure is 100kPa + 10kPa = 110kPa (100kPa at the surface, plus the added 10kPa due to the weight of one meter of water column), and in general, at a depth of d meters, we have
static pressure = 100kPa + d * 10kPa/m
A foil, say a daggerboard, produces lift to counter the side force and prevents leeway by creating a low pressure region on the upwind side, and a high pressure region on the downwind side. For NACA sections the pressure is not constant along the chord, there is a peak pressure at some point between the leading and trailing edges. In Rick's case the peak negative pressure was 56kPa. That is to say, the pressure on the windward side is reduced by a maximum of 56kPa. This is the dynamic pressure.
The total pressure is the sum of the dynamic and static pressures.
Consider what happens to the water on the windward side of a surface-piercing daggerboard: at the surface, the minimum pressure is 100kPa - 56kPa = 44kPa (that's the static pressure at the surface minus the peak negative pressure produced by the foil). The atmospheric pressure is greater than the water pressure, so the air pushes down on the water surface, and the water surface must move downward until the pressure differential between the atmosphere and the water is zero. This will happen at a depth of 5.6m, because, at this depth we have:
total pressure = atmospheric pressure = 100kPa = static press. + dynamic press. = 100kPa + d * 10 kPa/m - 56kPa
therefore,
d = 56kPa / 10 kPa/m = 5.6m
Since the peak pressures occur close to the surface of the foil, fitting a fence would prevent the atmosphere from pushing down the span, because the air would need to push outward past the extent of the low pressure region to get around the fence. What about other ways? Could you decrease angle of attack at the surface? Maybe slots?
On Sun, May 15, 2011 at 12:17 PM, Mike Crawford <mcrawf@nuomo.com> wrote:
<<could I please get the layman's definition for what yer'all
are talkin' about please.>>
I shall attempt.
It's not about a problem, but instead, an issue regarding
optimization.
The surface-penetrating foils will suck down air at speed,
eliminating any lift the generate from their shape (in the way a
wing generates lift on its "upper" surface, or a well-tuned sail
generates lift on its leeward surface), and instead turning them
from foils to boards that just catch water (the way a spinnaker
catches air).
When compared to foils that extend from the bottom of a hull, the
surface penetrating foils will require a lot more surface area in
order to achieve effective steerage and leeway prevention. This
will result in extra drag, slowing the boat.
Rob's current two-foil setup is optimized for kickup without
damage, and hopefully, and easy reset to sailing after the kickup.
As a result, it needs larger foils than a boat with foils or foils
and a daggerboard that exit the hull bottom.
Rick's design is optimized for speed. Since the foils are all
under water, and can't suck in in any air at high speeds, or
interfere with various flows of water, they can generate lift as
well as just force water out of the way, and can therefore be
significantly smaller. Adding a knot or two to the top end. In a
three-hour race, that could result in an extra nautical mile (or
several, if you've got good wind and a lot of reaches).
Which is better? That all depends upon your criteria.
Since I don't race, I'll go for the kick-up without a second
thought. Not having to worry about my boat sinking, or becoming
inoperable, due to a sand bar, log, submerged container, or whale,
does a lot for me. We have a lot of shoals and lobster trap warps
in Maine, and there are times it's tough to avoid them, particularly
the trap warps -- boat traffic sometimes determines one's path.
The other benefit of the kick-up design would also like being able
to see the foils, raise them, clear them of seaweed, or work on
them, and so forth. On my current cat, I can easily kick up the
rudders to remove the weed, which slows me down like a drogue, but I
can't quite do the same thing with my daggerboard.
*However*, If I were to race, it would be tough to argue against
smaller, faster, non-ventilating foils. If the goal is shortest
elapsed time, this design wins. It is without a doubt more
optimized.
---
That said, it would definitely be interesting to see how large an
end plate is required to keep Rob's kick-up rudders from
ventilating, and if the resulting design could be competitive with
the underwater foils. My guess is that the plate would definitely
help, but that it also wouldn't quite equal Rick's planned design.
But I don't have the software to prove this hunch.
- Mike
Doug Haines wrote:
could I please
get the layman's definition for what yer'all are
talkin' about please.
It is either onee of the theoretical non issue type
things that gets brought up here by certain
academia, or it is the actual rudder problem that is
happening to a few harry's.
I don't exactly understand what those bigger boats
are having happen, but it does sound like my
troubles on sidecar.
perhaps rudolf would chime in again - just to help
clarify things up for me.
I cleard up every rudder issue i had including
massive "ventilation?" and no bite at all/
unsteerable.
sidecar has had no dilemas whatsoever since i
increased the rudder blade size.
Doug
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