Rick:
A fully balanced sail such as the
Maltese Falcon pretty much needs bidirectional sheeting to control
it.....or would that be a preventer? That is a sheet from each
end of the boom, or the battens also. The split junk rigs such
as the original split camber panel junk rig designed by Slieve
McGalliard, or Paul McKays Aerojunk, which while it may not be as
attractive to the eye, to me offers many advantages, have been
pushed out as far as a full third of the sail forward of the
boom. Beyond that they are a problem. Unlike the falcon, they
end up with two airfoil shapes with a slot. Arne Kverneland, who more or less pioneered the camber
panel junk rig where the camber is sewn into each panel, has
interestingly found that the "bad tack" where the sail lays
against the mast which deforms the sail as a result is pretty
much a myth. There is no observable loss in performance or
efficiency from one tack to the other. Something I found
interesting. The camber panel rigs are more powerful and close
winded than the HM type rigs as Blondie Hassler introduced us
to.
Looking at Paul's
Aerojunk, I can envision a modification in which a third sail
panel would be used between the fore section and the aft
section. That third panel would actually be two panels,
connected at the leech in such a way to provide an exhaust slot,
and the luff of one would lay to one side of the mast near it's
mid point, and the other on the opposite side creating two inlet
slots where air passing the round mast section would be captured
between these two panels, eliminating the turbulence it would
normally have that causes the drag, and the "tamed air" would
then be exhausted strategically over the main sail. This
makes the round mast section the leading edge of an airfoil for
all intents and purposes, without the fabric being wrapped
around the mast as on a wingsail. Done properly the sail would
deploy and reef as effortlessly as the sail without it would.
These two panels would attach to each section of the batten cage
in a way that would allow them to scoop the air off the mast,
and for the outlet end to shift one way or the other through a
tack. I can see this in my mind's eye ( yes there is such an
eye ), but describing it clearly is something else. Soft
wing sails have it seems always been a problem when reefing,
wanting to cling the mast. I haven't ever handled one so this
is based entirely on reading accounts. Pete Hill quickly
abandoned the ones he made for Oryx, and that is pretty telling,
though unfortunately he doesn't tell the story, and Carley wrote
extensively but wrote travelogue, not tech stuff. The idea of
the two panels is to eliminate this problem, the air that passes
through them would inflate them like a modern parachute.
I fear my
communication skills are not quite up to the job of keeping pace
with my imagination skills ;-)
I wish there had
been more footage of your model proa.....
H.W.
On 3/1/19 4:34 PM, Rick Willoughby
rickwill@bigpond.net.au [harryproa] wrote:
The reducing heeling moment when pressed is a safety feature. I am not inclined to test any boat on the limit unless it's mine and the consequences are not dire. However any boat that spends a long time at sea has a high probability that the limits of control will get tested.
I have not seen that Aerojunk rig but I did do some calculations on a bi-directional square rig similar to what is used on Maltese-Falcon:It is a very promising design as it dramatically simplifies shunting. The sail only needs to be rotated about 10 degrees during a windward shunt. You get some idea of that on this model where I was playing with a canted bi-directional sail:In the first few seconds you see how the boat shunts just by swinging the sail a few degrees. There is also a shunt about 25m from the shore but the gust caught me out on the return leg and I was too slow to shunt and ended up beached. We actually got this boat to fly with just rudders in the water but never got a video of that. We only had two controls, steering and sail rotation. We needed to have some means of controlling the cant angle to depower in gusts but never got that far.
It appears Paul McKay’s Aerojunk rig has addressed some of the issues I identified as we got into detail of the bi-directional square rigger - note the three sheeting points. For a bi-directional square rig each bottom corner of the panels would require sheets and the trailing sheets would need to be released to depower. One of the issues when the sail is not directly attached to the mast is the high drag of the circular section.. The drag coefficient is often quoted at 0.5 but that is at high Re#.. At low Re# the drag coefficient is around 1.2 and for a freestanding circular mast it forms the majority of the drag on the rig. With square rig I found it was beneficial to include a fairing panel for the mast to lower the drag of the rig. The yards were curved to achieve the camber and each fairing panel would be secured to yards at each corner; in fact, the fairing panels may have their own small yards that help form the shape and also hold the main yards close to the mast.
This idea was explored as an alternative to the schooner rig to simplify shunting compared with swinging the Aerorig through almost 180 degrees on a windward shunt. It does not improve the balance much. The issue with going to schooner rig was the high cost involved in making two new mast bearings and associated hull stiffening plus the second mast. The schooner rig should be the preferred option for a new build although I have not sailed on a HP with that rig.
Rick
On 2 Mar 2019, at 1:42 am, StoneTool owly@ttc-cmc.net [harryproa] <harryproa@yahoogroups.com.au> wrote:
Rick:The heeling aspect was discussed in an earlier thread........My single question with regard to this is how relevant is it in real life sailing... when one is not pushing the boat to it's limits as you might in racing??? It's one of the ingenious aspects of Rob's designs. A lot of details had to come together in the HP that make it very very different from other proas so that it would be a stable safe ocean going platform that can complete with other blue water boats and acquit itself well, even excel. I'm beginning to understand the value of the slenderness ratio.... I forget the proper term.It has seemed to me since I "discovered" the HPs that one of the split junk rigs would be a pretty ideal rig on an HP, not requiring the extremely heavy boom and pivoting mast. The aerojunk by Paul McKay that flies a flat sail inside of a batten cage which imposes camber on the sail by virtue of it's shape and the lines connecting the sail to the cage... which can be adjusted fairly easily to change camber, is my favorite rig. It is rugged, and it is simple, and eliminates things like a track and cars, and expensive commercially made sails with huge amounts of tension across the entire sail. It would be particularly appropriate where two masts are used.The HP really falls closer to a trimaran than a catamaran in terms of payload and accommodations per foot LOA. Those are my most heavily weighted criteria after safety.
__._,_.___
Posted by: StoneTool <owly@ttc-cmc.net>
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