Two more proa rigs
Sketchup model with A-frame mast and sheetlet points somewhat forward
Why do you need the complicated junk style sheeting? As long as you make the bottom boom reasonably solid, you can control the twist with a simple downhaul.
Cheers,
Mal.
Hi Adam,
if you use the idea from Mal, you may find some useful hints there: http://www.multihull.de/proa/p_bolger.htm
Years ago I gave some thoughts to the Bolger-/Benze-Rig.
Cheers
Othmar
Reading an idea by a respected naval architect (Eric Sponberg) - on another forum - he stated that having a jib that could move the tack from side to side would gain in pointing - 1 deg for each deg moved to leeward.
This was for monohulls - but i wondered the use on a proa (or catamaran).
The best example being on the Russel Brown type rig.
Does it help pointing any - while i doubt 1 deg for each deg to leeward does it make a difference?
The way i see it it would make a power/speed gain but not necessarily any gain in angle into the wind…..
Obviously hull and foils would make a difference but as it was an idea for mono’s and a proa is a more ideal hull for a mono - is there anything to this?
Reading an idea by a respected naval architect (Eric Sponberg) - on another forum - he stated that having a jib that could move the tack from side to side would gain in pointing - 1 deg for each deg moved to leeward.
There may be some truth in this but I doubt that you would get 1 deg of pointing for 1 deg of jib tack moved to leeward and there are other factors to consider.
Moving the jib tack to leeward is an attempt to separate the sails and have them work more independantly, more like a pair of una rigs rather than a single aerofoil, so the slot effect is reduced, the total thrust may be reduced but the L/D ratio may improve. A jib and main in combination is a slotted aerofoil, which will provide more thrust but will have more drag and won’t point as high as a una rig, but may provide better VMG depending on the hull and foils.
As a multihull will generally have a better hull/foil combined L/D ratio than a monohull, there is probably more reason to separate a the sails on a multihull than on a monohull because a multihull will gain more of an advantage from a rig with a better L/D ratio. As a real world example, a C-Class catamaran with it’s ultra fine hulls uses a very low drag wing rig to advantage whereas a Thames barge has a spritsail yawl rig with multiple jibs. If you put a wing rig on a Thames barge, it will probably go slower.
Why do you need the complicated junk style sheeting? As long as you make the bottom boom reasonably solid, you can control the twist with a simple downhaul.
Mal.
Good point. It was drawn that way only to explore the idea of taking strain off the mast. I like the simplicity of a downhaul for control though.
Hi Adam,
if you use the idea from Mal, you may find some useful hints there: http://www.multihull.de/proa/p_bolger.htm
Years ago I gave some thoughts to the Bolger-/Benze-Rig.
Cheers
Othmar
Thank you for that link; I had not seen it before.
Reading an idea by a respected naval architect (Eric Sponberg) - on another forum - he stated that having a jib that could move the tack from side to side would gain in pointing - 1 deg for each deg moved to leeward.
In addition to what Mal said, there is the possibility that excessive healing on a mono and the associated downgrade in L/D of the sails can be partially remedied by the movable jib tack. Imagine a completely sideways jib and how effective that would be versus a completely vertical jib.
I went to a local reservoir a few days ago to test my model rigged as a biplane sloop. Norwegian blue is 120cm long, with a beam from centreline to centreline of 83cm. The beam was determined by the length of the broken ski poles I had lying around (location determines the availability to material to be re-used).
The first picture shows that the boat has no rudders. I would want them at crew-carrying size, even if only for fine control in harbour, or when sculling the boat, but I wanted to test whether the rig is balanced.
Results from the 15 minutes when I had a bit of wind are still unclear. I can sail, I can steer, I can shunt. I am not sure how precise the steering is because I couldn’t judge how steady the wind was at the boat’s location. It appeared less precise than I wanted. However, it does look promising for balancing the lift and drag vectors of the above waterline and below waterline portions of a proa. It also looks promising for allowing a much wider boat, though how that works in a seaway still remains to be seen.
The ama’s deck barely breaks the surface. I had hoped for about equal volume above and below the waterline. This loading should make the boat less vulnerable to breaking waves from the ama’s side because the crest should just run over the ama without lifting it, and if the crest hits the inclined bracing plate between the akas, the crest should be deflected up and should push the ama down. The drawback is that the boat should be more vulnerable to being taken aback. At crew-carrying size, the solution would be to adjust density through water ballast.
I will test some more, take a video if practical.
Looks interesting Robert. Am I right in guessing that the steering is done mainly with the wingsail?
You could hang a weight out to leeward to adjust the immersion of the ama.
Am I right in guessing that the steering is done mainly with the wingsail?
Yes. The blue jib is the driving the sail, the yellow wing the steering sail. I made it a wing (even though my first attempt ended up being extremely crude) because I expect that it must often operate at low and occasionally negative angles of attack. A symmetrical wing seemed most suited to the job.
You could hang a weight out to leeward to adjust the immersion of the ama.
I cut the bottom off a hull that didn’t quite work out. It could serve as a safety ama. I didn’t want to invest the time in attaching it until I had some idea whether the rig works.
Yes. The blue jib is the driving the sail, the yellow wing the steering sail. I made it a wing (even though my first attempt ended up being extremely crude) because I expect that it must often operate at low and occasionally negative angles of attack. A symmetrical wing seemed most suited to the job.
Makes sense to me. I’m looking forward to seeing some more test results. After all my experiments with lifting rigs, I now like the idea of a sail to windward which pushes downward as it can be designed as a dynamically stable system. Lifting rigs, on the other hand, tend to be dynamically unstable.
Robert,
Interesting concept, much on the lines of Fritz Roth?
I am skeptical about lifting sails. They were first thought to be a good thing on windsurfers, but later found to be not so. Keep the sail upright for maximum drive and minimum drag. Water is a far better medium for giving lift. Whether the same applies to a downforce rig, I do not know.
Should (is) the main hull be a flat section, all the lateral resistance from the dagger board, as this is to windward of the sail?
Cheers Mark
Interesting concept, much on the lines of Fritz Roth?
That was the source of the windward jib when I started thinking about very wide boats. Thinking about sailing up a narrow channel, I wondered whether the jibs at the bows were necessary.
Robert,
I am skeptical about lifting sails. They were first thought to be a good thing on windsurfers, but later found to be not so. Keep the sail upright for maximum drive and minimum drag. Water is a far better medium for giving lift. Whether the same applies to a downforce rig, I do not know.
It’s a downforce rig only by accident. I am guessing that the structure needed for a stay that can be allowed to sag is less than the structure needed for a second mast. On a wide boat with a low rig, the stay happens to incline if the mast is close to vertical. I wouldn’t want to incline the mast to weather because then the force from the leeward sail would increase with heel.
Should (is) the main hull be a flat section, all the lateral resistance from the dagger board, as this is to windward of the sail?
It’s tortured ply. Not very much, because the main hull is relatively fat at 1:12, and I wasn’t sure how much I could bend the plywood without splitting it.
For a crew-carrying version, I am thinking about a simple box hull to lee, deep V to weather, both with fixed low aspect ratio asymmetric keels with end plates that also serve as steps to get out of the water. Bolger-style shallow sharpie rudders with end plates under the lee hull, shallower draft than the keel, and with very shallow slope leading edge, say 2:1. The rudder shafts could be quite fat, with each rudder fitting into a slot in the shaft.
Robert,
It will be fascinating to see how you get on with your windward staysail rig on Norwegian Blue…...Attached below is my latest incarnation of a similar rig I am aiming to use on my boat.
Your comment about wave deflection over the ama is also interesting…...Why not flare out the ama a bit, at least to leeward, and cant the decks up on the lee side and get some extra buoyancy for being caught aback at the same time???
Rob
Your comment about wave deflection over the ama is also interesting…...Why not flare out the ama a bit, at least to leeward, and cant the decks up on the lee side and get some extra buoyancy for being caught aback at the same time???
Interesting idea. I think it works best with your reverse Bruce foil. Sven wrote about the ama slamming into waves when it was too heavily loaded. There seem to be several solutions to that, some of them with empirical backing, others not yet tested:
1) Fly the ama, or at least keep it as lightly loaded as possible. But if you don’t want to focus all the time on keeping the boat balanced on that knife edge, from where do you get your reserve stability?
1a) Russ Brown’s boats get reserve stability from the lee pod.
1b) You get reserve stability from having extra weight in the ama and lifting it with the reverse Bruce foil. That will be slower than a really attentive helm. However, if you don’t have that helm with an attention span of up to a few weeks, then the smoother ride provided by your foil may well be faster in a seaway than keeping the same weight in the ama and letting the ama contour the water’s surface.
1c) Keep the ama light and add downforce as required through a vector foil.
2) Keep the ama firmly in the water, but
2a) Make it short, so that both hulls’ bows meet the wave at the same time (that is Rob Denney’s solution), or
2b) Let it pierce the waves. That is helped by having a small waterplane area. That is what I am trying. Having the flare low down, as in your drawing, would interfere. If you want such reserve buoyancy, but also limited response to waves through small waterplane area, then the reserve buoyancy should be above the variation in waterline you can normally expect. That starts looking like a Harryproa with a small waterplane area windward hull. Should smooth the ride, though it would cramp the accommodation a bit. As designed, Harryproas have that deck extending leewards from the ama, and if a breaking crest is high enough, it should push down on that deck. If the volume of the hull below that bridgedeck is too high, I would expect the hull to be lifted before the crest has opportunity to push down on the deck.
I use a square section wave-piercing plywood “log” as ama. I think it behaves like it should when going through waves. It does not slam or vibrate much if hit by a breaking waves from the side. This one is made out of plywood and styrofoam.
Cheers,
Johannes