Two more proa rigs
Using Johannes’ ama cross section as a diagrammatic example (1) which has little reserve buoyancy, for practically no extra wind or wave resistance (from left hand side) you can double the amount of reserve buoyancy / additional load carrying capacity (3). There would also be more resistance to a wave pushing the ama under whilst breaking over it and there is a possibility of a little additional dynamic lift pushing the ama up and out to windward. Or if you are happy with the amount of reserve buoyancy, you could reduce the above waterline profile to further reduce wind and wave resistance.
Proas are unique in their potential to develop asymmetric above waterline hull sections to minimise drag and wave impact whilst maintaining / increasing cross sectional area. You can see that most of the large offshore racing trimarans have (roughly) diamond shaped forebodies like (1), but only a proa can do anything like (2) or (3).
You could apply the same principle to the vaka, to provide the same bow reserve buoyancy in a lower forebody profile and also provide a long “pod” which at 30 degrees plus in my design does the same as a Jzerro pod, without any potential for tripping. It should also provide some dynamic righting lift as well.
You could also apply the same principle to any deck house structure, to get more usable volume below and increase buoyancy / righting arm from a 90 degree knockdown.
With regard to the lifting Bruce foil shown on my design, it will also pivot back under the aka wing to provide (controllable amounts of) decreasing lifting moment then increased righting moment through to complete retraction under the aka wing, out of the way hidden from wind and wave by the ama.
I like this good idea
TINK
Second test. There was some video being recorded, but I have no idea yet what is on it. Probably not worth uploading anyway, because mostly very little was happening, and the few interesting moments were either too brief or too far from the camera.
First, the problem: the boat doesn’t go where I want it to, but I may have worked out why. I noticed the boat still turning into the wind when the steering sail (the yellow wing) was athwartships and the driving sail (the blue jib) pulling. My intention was that this combination should make the boat fall off. It seems the centre of effort of the driving sail is to the lee of the total centre of lift and drag of all the underwater parts. The cure would be to move some of the drag to lee, specifically shorten or cut off the board and replace it with one on the lee hull. That would need a bit of surgery. The alternative is to move the driving sail to weather of the ama. That needs a bow sprit, and longer beams, so that I have somewhere for the sheets to attach. That is easier.
Second, an interesting observation: the boat was never in danger of capsizing sideways. It never flew the ama. Once a gust hit when the boat was very close, the driving sail was sheeted in and the steering sail was pretty much in neutral. The gust didn’t lift the ama, it pushed it down. I don’t think I would have gotten that if the steering sail had been pulling as well, but I didn’t expect to see that even from the driving sail alone unless I reefed. This looks good for a larger version sailing off a lee shore in strong winds, especially in combination with an ama that lets wave crests run over it.
The bow did dive twice, once to about 45 degrees, and once I thought I’d have to swim to retrieve the boat. The model clearly has more transverse than longitudinal stability, even with a fairly skinny ama. It’s solid wood, though. And the driving sail doesn’t add to stability at all on a broad reach when it is at a right angle to the course. The pushing down increases the closer to the wind you sail.
Robert,
Your driving (windward) jib’s centre of effort looks as if it will always be well aft of the centre line / CLR and therefore moving the rig further out to windward may not work as you expect…... Have you considered a counter balanced jib boom along the foot, which will bring the CE further forward???
Your observation re downforce is interesting and seems to confirm Fritz Roth’s own experiences.
Rob
Your driving (windward) jib’s centre of effort looks as if it will always be well aft of the centre line / CLR [snip] Have you considered a counter balanced jib boom along the foot, which will bring the CE further forward???
It’s not that much aft because the sail is already balanced. Have a look at the luff in the lower of my two pictures. The boom is a Bierig camber spar. Like a wishbone boom, it is not at the foot of the sail.
and therefore moving the rig further out to windward may not work as you expect…...
Think of the force vectors as seen from above. So long as the sail’s total force vector has a forward component, moving the sail further to windward also moves the line along which the force acts further forward.