<<a bi-directional square rig similar to what is used on
Maltese-Falcon. It is a very promising design as it dramatically
simplifies shunting.>>
Rick,
I agree that the rig is attractive from a performance perspective
-- it could be optimized as an airfoil in both directions and
probably use a fixed mast without bearings (even simpler!).
However, I don't yet see how one could design away the risk of
putting the full sail area to the wind when shunting in a huge
blow. This was a bit of a testy discussion a while ago when Todd
was doing great work with the models.
The models work well, even when overloaded, because they are
lightweight, stronger than a full boat for their size, and the sail
change happens quickly.
But let's say it's a 48' proa and the wind doubles in strength as
you're headed towards a shoal, an island, or pinched by a coast or
in an inlet. Or maybe the wind triples -- say a squall rolls in
surprisingly quickly from windward, with the sun to leeward, and the
leading winds didn't announce themselves.
With the una rig, we dump the sheets and then figure out what to
do. Drop the sails? No problem. Reef? Okay. Get some tea?
Sure. Shunt then reef? Sit in neutral? Reef then shunt? Any
options works, and you can take as long as you want implementing it,
even singlehanded. Even if the sheets jam, a knife solves the
worst-case scenario.
Putting that full sail area broadside to the now-huge wind would
scare the pogies out of me. Can the boat take it? The sail
frames? Will we fly a hull or capsize? How quickly can I move that
sail through the window by myself? What if it's at night? What if
the mast or sail rotation system jams? Will the sail go effectively
to neutral if you cut the sheets? Will it oscillate, causing
dynamic stress, and/or periodically going broadside to the wind
again?
(the attached PDF illustrates the issue if my words don't do the
trick)
If the boat does capsize, and the sealed mast keeps it from going
turtle, the dynari / square rig would make it harder and more
dangerous to right than a sail that just blows to leeward (or hangs
down towards the water while righting).
Not only would I like to be free of the fear of that risk, I'd
just like to be free of the risk.
The Maltese Falcon could handle the stress of the side-change
because it's a monohull that's going to dump the force by heeling.
And if the wind is really too high for the sail area, they'll press
buttons to hydraulically roll up as many panels as needed before
tacking. We can also assume that they have a great maintenance
schedule and the crew and budget to make sure those hyraulics are
working all the time.
---
I don't ask because I'm trying to beat a dead horse -- I'm curious
about options to cut out the risk. I'm sold on the bidirectional
rudders, even though they are imperfect; so it would be neat to have
a bidirectional sail as well.
I had almost committed to buying a used Drangonfly 920 that had
been grounded, and putting in the time to repair it, when I came
upon the harryproas. I wouldn't have thought that all those
features that contribute to weight and risk/failure could have been
designed away, but rob did it, and I can never look at that huge
stayed rotating mast the same way again. Maybe there's a solution
for the square rig, too.
- Mike
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: Arto Hakkarainen <ahakkara@yahoo.com>
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