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Two more proa rigs

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I am interested in getting comments on two proa rigs (one of them has to wait until I have made the drawings).

The first is simply the Russell Brown rig with jibs that have rigid, curved battens.  The inspiration comes from junk rigs and from AYRS/Bolger rigs: http://proafile.com/archive/article/phil_bolger_1927_2009

A report by someone who tried the latter stated that the sail is powerful, but gives trouble when shunting in strong wind.  Then it may be necessary to drop the sail for each shunt.  And if the sail has enough roach to approach a semi-elliptical planform, then the top twists the wrong way because there the centre of pressure is in front of the line between the foot and the head of the sail. 

I think both problems can be solved by having two asymmetric sails, one at each end.  Then each batten can slide up and down a stay, attached just in front of the centre of pressure, so that the sail is almost balanced.  I drew a shared halyard and downhaul.  The halyard determines how much sail you have up, and the downhaul lets you swap from one sail to the other, pulling only one line. 

The stay or luff does not need to be taught.  That is necessary for conventional jibs, genoas and staysails because the sheet pulls along the cloth and flattens the camber.  If the luff sags, camber comes back in.  The jibs I have drawn get their shape from rigid battens.  The sheet can pull at a right angle to the cloth.  Wind pressure then would make the sail sag not aft, but leewards, and that would only try to flatten the sail, not increase camber (because the deepest part of the camber has to stretch most if the sail sags leeward).  Abolishing the need for tight luffs should take a lot of load out of the rig and hull, as in the junk rig.

I tried drawing junk rig style sheeting, but because of the roughly triangular planform, the upper sheetlets would move forward relative to the sheeting point, and the angle would change too much.  The sail needs slab reefing, which I have not drawn in.

There is still an issue known from the junk rig, namely whether the batten angle and spacing allows the sail to stack properly when lowered.  Draw a diagonal from the aft end of one batten to the point where the next higher batten attaches to the stay.  Then draw an arc of the same radius back down to the lower batten.  If that arc touches the lower batten aft of the stay, then the sail needs to stretch to allow the higher batten to be lowered that much (unless the next higher batten is attached so that it can slide aft).  This is the case for the lower two panels of the sail I have drawn.  If the sail can’t stretch that far, or if the stretching wears out or distorts the sail, change batten spacing or angle.  The upper panels I have drawn are fine.

The rigid battens don’t allow an overlapping sail, but on the other hand, the sail projects forward of the stay.  Seeing that proas usually need to move the centre of pressure forward, this rig should do the job, using less heavily loaded components.  Because the sail shape comes primarily from the battens, broad seeming may not be necessary, so this could be a home-made sail.  It would also have a better shape when sheeted out than a more conventionally sheeted jib.

The lowest batten should be fixed in place as far as movement up and down the stay is concerned. The simplest way would be to crimp to the stay two of those soft metal slugs used to make eyes in wire, one just above and one just below of where you want the lowest batten. So that the sail and battens can be removed from the stay, either use small U-bolts through the battens (reinforce the central section some more than you would do anyway, to compensate for the weakening caused by the holes), or else put a shackle around the stay, then lash it to the batten.  Then attach the batten. In that case, you must attach the sail to the battens after attaching the battens to the stay. That means you can have batten pockets only on one side of the stay. On the other side, the sail must be lashed to the batten.

If the spacing between battens imposed by stacking becomes so wide that the luff starts fluttering, insert short, rigid battens only for the first 20% or so of the chord. That cured fluttering of the luff in David Tyler’s soft wing junk
sails. Being junk sails, they don’t have high luff tension either. I think they’ve got 20000 nm on them now, which seems reasonably extensive testing.

These jibs lend themselves well to be made into soft wing sails, with all downhaul and slab reefing lines inside the sail. That would make them more forgiving of unsteady flow and changes in angle of attack, so should be faster.  The cost is more complication, because you may have to start thinking about the shape of the sail panels. Unless, I suppose, you go for a perfectly triangular planform and treat the sail cloth as a cone, which is a perfectly flat plate rolled up. Or two cones, seeing that I drew a second triangle below the lowest batten. Given that no slots would be needed, and no way to reverse camber from one tack to the other, this might be a relatively cheap way to make an efficient wing.

I’ll describe the other idea for a rig when I’ve had time to make drawings.

      [ Edited: 30 April 2012 02:17 PM by Robert Biegler ]

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Biplane Sloop

I once asked on the Yahoo proafile group why Pacific proas were not built to be very wide, say about as wide as long.  The theoretical advantages are greater righting moment for a given weight, and greater rotational inertia, resisting wave-induced capsize.  Take a Pacific proa half as wide as long.  Double the beam.  Neglecting the weight of the beams for a first approximation, the same righting moment can be had with an ama half the weight.  If we treat both hulls as point masses, and take into account that rotational inertia is proportional to both weight and the square of its distance from the point of rotation, we find that even though stability is the same, rotational inertia is twice as much as before.  If the ama also has low volume, breaking crests should roll right over without lifting it much, and lifting only the lee hull.

The problem, I was told, is that the drag so far to windward would give wildly unbalanced steering.  I realized that merely moving the centre of effort of the rig forward wouldn’t do much good.  The boat would have a balanced helm only on one course, when the sail force vector on the lee side and the combined lift and drag of both hull further to windward would happen to line up.  On any other course, the boat would again be unbalanced.  The centre of effort must be moved to windward.

Adding a windward staysail to Russ Brown’s two-way sloop, like Fritz Roth does on his vector foil proa, would go some way towards achieving this aim, without the structure needed to support another mast.  Then I thought about how many sails would need to be trimmed, raised and lowered during each shunt.  When short-shunting up a channel, perhaps only the windward staysail and the main would be needed.  Also, when going downwind, main and staysail would blanket the forward jib.  The logical next step was to reduce the rig to main and windward staysail only.  That gives a sloop rig turned by 90º.  If the mast is engineered to be unstayed, then the stays to either end can be omitted, and the mainsail can have a big roach without causing trouble when caught aback.

Such a boat may achieve self-steering by sail trim only.  On all courses from hard on the wind to a broad reach, the mainsail would be trimmed to have a smaller angle of attack than the staysail.  It is mostly the stay sail drives that the boat.  If the boat falls off, the staysail would not gain much lift, and eventually begins to stall.  Meanwhile, the mainsail powers up and moves the centre of pressure leeward, and turns the boat back into the wind.  If the boat luffs, the mainsail should lose drive faster than the staysail.  The centre of pressure moves to windward and makes the boat fall off.  I think this should work for apparent wind angles from hard on the wind to at least 120º.  When sailing dead downwind, the two sails could be sheeted to a V-shape, like trade wind sails.  All this should work better the wider the boat is.

Because the mainsail needs to operate at small angles of attack, I have drawn a symmetrical soft wing sail.  A wing because it should not flutter at small angles of attack, and symmetrical because asymmetry induced by wind pressure depends on larger angles of attack, and mechanisms to bend the wing would be too complicated to bother with on a cruiser.

Another feature of the rig is that the windward staysail pulls the windward hull down.  Depending on the angle of the stay to the horizontal, the sail may need a bit of reefing before it reaches the point where it does not heel the boat.  Of course, the mainsail and drag on the structure exposed to the wind would still generate a heeling moment, so non-heeling may only be achieved when the staysail is deeply reefed.

I have drawn in junk rig style sheeting, because it makes reefing so much easier and reduces loads.  I am not sure that will work for the staysail.  The greater the stay’s angle to the vertical, the more the wind would tend to blow the reefed sail panels up the stay.  If the rig needs reefing lines to keep them down, the junk-style sheetlets may no longer offer any advantage.

The battens in the staysail would be rigid and permanently curved.  The batten pockets need to be wide enough to allow the battens to rotate around the profile’s chord line, but only about 45º either way from the plane of the sail.  Then wind pressure and weight will make the flip over so that the sail’s camber is always down and on the lee side.  Because the battens take care of the sail profile, the sheets do not need to stretch the sail into the right profile.  That means the stay does not need to be taught.  In fact, if it sags off to lee, that will tend to flatten the sail.

Should the boat be caught aback, the staysail will tend to lift the ama, reducing the problem of having a small volume hull on the lee side.

If beating upwind in breaking seas, it would be good not to have much lateral resistance on the lee side.  A keel or dagger in the ama would be an option, though I am rather temped by a hinged vector foil between the hulls.  If everything works as I hope, such a boat would use its rudders only for precise control in port, or when moving under power.

Regards

Robert Biegler

     

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I realy like your “junk-jib”. A very clever way to get more sailarea far forward on a proa. Very good with the angle the wire has. It will probably help lift the bow out of the water, and counteract some of the turning effect when beating hard upwind.

I will see if i can test it on my small advanced sharpie proa.

Regards.
Johannes.

     

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Robert,

Without the addition of the battens, the jib looks like those in use nearly a centure ago called a “balance jib”  The jib would be boomed and the boom would be secured to the deck with a line placing a percentage forward of the line.  It’s a bit like the balanced rudder effect, but the trouble is keeping it taught enough for good shape when the wind is up.  Hobies and other cats with jibs an loose jibs deal with the luff sag by designing it into the sail shape.  some cats also have self tacking jibs and battens as well.  By combining all the elements into a single sail (especially if you use a wire luffed sail or a modern no stretch equavilent) you might get some pretty good performance with low sheet loads in addition to your self steering potential.

Dan

     
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dstgean - 07 May 2012 12:06 PM

Without the addition of the battens, the jib looks like those in use nearly a centure ago called a “balance jib”

I have seen pictures.  The balance jib may have been part of the inspiration, but the sails I positively remember leading me to this idea are the junk rig and the AYRS/Bolger rig.

dstgean - 07 May 2012 12:06 PM

the trouble is keeping it taught enough for good shape when the wind is up.

I am hoping that the rigid battens take care of the shape without having to resort to a taught luff.  The rigidity of the battens is key.  Flexible battens like in the Hobie 16 jib don’t take away the need for a taught luff.  And at least on the Hobie 16, the three part halyard is used to tighten the wire luff quite a bit, then the main sheet puts more load on.

Regards

Robert Biegler

     
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Robert,

Let me just say that I am impressed with the junk/jib concept and keep coming back to it for all the reasons you mention. I also wonder if they could be used in conjuction with an A-frame mast so that each is run up and down one side of the A-frame. This would allow for the leach to extend further aft and yet still swing through if caught aback, hence allowing a larger sail area in the junk/jib and therefore a single sail, yet it’s COE is where it is needed for balanced rudder force. If workable this single sail is attractive to me for ease of operation/reefing/efficiency of a single sail.

Best regards,

Adam

     
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I think the A frame mast with junk jibs idea is really interesting. The disadvantages are that you need two sails (cost and weight) and shunting may be a little slow compared to, say, a ballestron rig. Otherwise it makes sense.

Cheers,

Mal.

     
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I’m sitting here staring at the stay sail, and I keep thinking about just eliminating the main.  The CE is moved so far to windward, the sail actually needs to move AFT to achieve balance, which it does naturally with this configuration.  I also haven’t analyzed a full vector diagram of the forces, but it seems that a gust would actually DEPRESS the Ama instead of try to lift it.  As mentioned above, if/when the rig goes aback, the ama would actually tend to be lifted.  I wonder how efficient the drive component would be.  It seems that in light winds the weight of the sail would naturally try to make it “luff”.

I’ve been noodling schemes to get an unstayed mast for a split junk far enough to windward to leave both rudders down all of the time for completely simplistic shunting without adding a bunch of weight, windage or awkward struts that mess up the cockpit area.  This allows the mast to be easily supported in the hull.

Cool concept.

Tom

     

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I followed the discussions in this forum about various ideas of rigs for proas. I don’t wont calling into question, what the cause of it. Humans are always dissatisfied. At last I asked me - back to the roots - is there a signifying improvement to the good old crab claw sail? If I compare the effort, material, and costs, I am in doubt. Two bamboo spars, a short mast, some fabric - ready to sail. I know, shunting a crab claw is sophisticated work, like proa sailing itself. IMHO it’s better to change the type of boat, as to rape the simple basics. This reffers only to ‘small’ proas until 30 ft. For bigger boats are always standard rigs like sloop or schooner used, because in our times there’s rarely large crew to handle a big crab claw available.
No harm meant!
Othmar

     
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Othmar,

I don’t blame you at all for expressing your feelings about crab claws sails. They are beautiful and classic, and classics become classics for a reason.

I’m guilty though of being greedy and wanting a sail that can reef and dosn’t care if it gets caught aback but also dosn’t drag its feet if the sailor is single handing. I think the reason proa rigs evoke so much discussion is because that combination arguably does not exist yet. I suppose one of my old teachers had it right in saying humans are “satisficers”, that is, that if humans can’t have perfection they want the nearest thing to it before they are satisfied. I will say though that if physics only allowed crab claw sails I wouldn’t be disappointed. But since physics allows for other rigs, I feel a drive to carry on trying to innovate and dream of another rig that satisfies my greed, perhaps a greed similar to the greed felt by the fellows who came up with and improved upon the crab claw rig. Who knows, I might even come full circle and join you in the crab claw proa club after I journey into the unknown.

Best,

Adam

     
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Tom - 24 July 2012 07:45 AM

I’m sitting here staring at the stay sail, and I keep thinking about just eliminating the main.  The CE is moved so far to windward, the sail actually needs to move AFT to achieve balance, which it does naturally with this configuration.

At which point it really just becomes an Atlantic-styled proa, no?  There is nothing wrong with that of course, but it does indeed change the way that many other aspects of the design should be addressed.

     
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Adam,

thanks for your comment.

First - if you want to see a reefable crab claw, look at: http://www.multihull.de/forum/viewtopic.php?f=6&t=158&p=512#p512
Text by Reto written in German, but pictures say all.

Second - caught aback. During my sailing at the Aegean coast I think every 20th shunt results in that situation. Where’s the problem? Open the halyard, the sail falls into the lazy jacks, turn the boat, try again.

Third - if I see the designs with two sails, and also the lot of blocks and sheets ... I believe the crab claw is more comfortable single handing (sorry Robert).

I am really not against progress, but I allow myself to question about the sense grin

Best
Othmar

     
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MTP - 24 July 2012 10:32 AM
Tom - 24 July 2012 07:45 AM

I’m sitting here staring at the stay sail, and I keep thinking about just eliminating the main.  The CE is moved so far to windward, the sail actually needs to move AFT to achieve balance, which it does naturally with this configuration.

At which point it really just becomes an Atlantic-styled proa, no?  There is nothing wrong with that of course, but it does indeed change the way that many other aspects of the design should be addressed.

Actually, with the Atlantic the mast is mounted in the windward hull, most of the displacement is also in the windward.  With a Harry, mastbisnto leeward and most of the displacement to windward.  Here we’re still talking mast too lee and weight to lee, so I’m thinking it’s more pacific than anything else, though it’s certainly a mutation.

Othmar,
If I was sailing on a bay, ocean or large lake, I’d probably be very inclined towards a crab claw.  my waters are a large river where I’m either running dead down wind against a current or short tacking/shunting against the wind with the current.  I’ve also got wildly variable winds.  my priorities are excellent downwind manners and super fast and simple shunting/ tacking against the wind along with easy/ adjustable reefing.  Thus, my interest in a junk version wink.  Many of a proas virtues have nothing to do with the rig.

Tom

     

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Let me be really greedy and ask for camber control of the junk/jib. Four potential ways I see:

1) compromise for simplicity sake and just have less camber in the upper battens which would be used in high wind/ reef situations, leaving the more cambered lower panels reefed

2) ditch the battens and use a wishbone club boom for camber control of a “balance jib”.

3) replace the bottom batten with a rigid boom. All panels above this boom would gently curve into the head, such that tensioning the luff and leach would try to straighten the curved luff and leach thereby adding camber. Something tells me this would not be practical due to the tension required given battens need to be reasonably stiff.

4) Add “bowstrings” to each batten when more camber is desired. A pain, but adjustable, simple, and cheap.  Might even have two sets of bowstrings i.e. for 5Kt, 15Kt wind respectively

Edit: I forgot that Robert has already addressed camber and the potential for self-adjusting camber. Very intriguing. Perhaps that could be helped along by using stretchy fabric at the luff and leach.

 

      [ Edited: 24 July 2012 06:57 PM by Adam ]
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Adam - 23 July 2012 08:20 PM

I also wonder if they could be used in conjuction with an A-frame mast so that each is run up and down one side of the A-frame. This would allow for the leach to extend further aft and yet still swing through if caught aback, hence allowing a larger sail area in the junk/jib

That way, you can avoid a triangular planform, and you can use junk-style sheeting again because you don’t have the problem of the sheet attachment points moving forward when you reef.

I had to think a bit about how you would raise such a mast.  I suppose a tabernacle at one end, a hinge where the two legs meet, and pushing up the other end.

Scantlings would be much like two unstayed masts, and windage, too.  So I thought why not use a schooner rig?  You could link the halyards on deck if both wound onto drums and you had a way of linking the drums.  a pair of cogs, a bicycle chain and a clutch would do.  Then I thought even that is not necessary.  A wing profile is supposed to have a lot less drag than a round mast.  So set an asymmetric wing sail with junk rig sheeting on each mast, use only the forward mast, let the aft sail swing freely.

Adam - 24 July 2012 06:36 PM

Let me be really greedy and ask for camber control of the junk/jib. Four potential ways I see:

1) compromise for simplicity sake and just have less camber in the upper battens which would be used in high wind/ reef situations, leaving the more cambered lower panels reefed

On an asymmetric wing you might get automatic camber control by sheeting not from the end of a batten, but somewhere further forward, and making the batten flexible.  When sheeting from the batten end, flexible battens on a junk rig only give you more camber in stronger wind.  If you sheeted from the middle of a straight and flexible batten, you would get the end flexing away, but you would also have camber the wrong way.  If you can use a sail that can be asymmetric because you use it only on one tack, that is not a big problem.  However, you could tune this automatic adjustment only for a particular wing loading.  When you reef, you get the same heeling moment at greater wing loading, and your automatic adjustment will not be tuned right.

If you are content with angle of attack adjustment, the rigid battens on the jib I drew in rig 1 let you adjust where the rotation point is relative to the balance point.  At the boom (lowest batten) you might have the rotation point at 25% of chord.  Add a bit of roach and/or make the luff a bit concave and at the top batten, the rotation point might be at 10% of chord.  Halyard tension should minimise twist in light wind, but the upper part will still twist off in stronger wind.
 
I am not sure to what extent you can do the same with a double-sheeted junk sail.  You can’t really play around with halyard tension unless you add something like slab reefing.  What I outlined above depends on whatever reduces twist not changing with wind speed, and whatever induces twist growing with wind speed.

Adam - 24 July 2012 06:36 PM

2) ditch the battens and use a wishbone club boom for camber control of a “balance jib”.

Then you need a taught luff again.  If you don’t have rigid battens keeping luff and leach apart, luff tension has to do the job.

Adam - 24 July 2012 06:36 PM

3) replace the bottom batten with a rigid boom.

That’s what all the battens are supposed to be anyway.  Call them curved booms if that makes the point clearer.

If I were building such a rig and were willing to make it more complicated to improve the aerodynamics, my intuition is to go to a soft wing sail with self-adjusting twist and perhaps self adjusting camber, rather than a single surface with adjustable camber.  The round leading edge of a soft wing is more forgiving of variation in angle of attack.  But that preference is only intuition.  What with both rigs still being vapourware, I have no experience to justify the intuition.

Regards

Robert Biegler

     
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Robert Biegler - 26 July 2012 11:06 AM

Scantlings would be much like two unstayed masts, and windage, too.

 

The situation might not be so dire as this; one leg of the mast gets to lean on the other, so strength can be something like half of a single unstayed mast. Thus, windage and scantlings can be reduced accordingly. Further, a single stay to windward is permissible with certain geometries. Finally, the sail being able to swing through when caught aback, along with sheets cleated to windward means no significant forces will come from the unstayed direction. This will be true even with no intervention from the sailor.

Robert Biegler - 26 July 2012 11:06 AM

On an asymmetric wing you might get automatic camber control by sheeting not from the end of a batten, but somewhere further forward, and making the batten flexible.

 

This has the effect of increasing sheet loads but taken to completion this also reduces loads on the mast and the sail starts to look like a kite that is stabilized and made straight by a mast. Very interesting.

Best,

Adam

      [ Edited: 26 July 2012 07:50 PM by Adam ]