practical proa sailplans
Diazo
fascinating. complex.
Regarding the ‘following sail’. This discussion has been largely about cat schooner rigs, and people have ‘ignored the jib for the moment’. But isn’t a jib in a sloop doing the same thing as the leading mainsail in a schooner? And what of the ‘slot effect’ is that an outmoded concept?
Pretty much, though when the jib overlaps the mainsail there are some other factors in play. The “slot effect,” as it was believed by monohull sailors for decades, was pretty much nonsense, but there is an actual slot effect. These links will give you a good idea of how it works:
http://adg.stanford.edu/aa241/highlift/highliftintro.html
http://en.wikipedia.org/wiki/Leading_edge_slot
http://en.wikipedia.org/wiki/Leading-edge_slats
There is always a trade-off in aerodynamics of course, and in this case when you add slots or jibs or slats you are trading a higher lift coefficient for a lower lift/drag ratio. Whether or not this is advantageous depends on the speed capabilities of the boat. Upwind, the faster the boat relative to wind speed, the more important a high L/D becomes. As the boat gets heavier and draggier, the more important maximum lift coefficient becomes. Off wind, L-max pretty much always rules. Most of the proas we experiment with- short, made of plywood, not much attention to parasitic drag, crew weight similar to boat weight, etc., will be in a drag range where a jib will be a big help most of the time.
Something else needs to be mentioned, and that is the use of a small jib or “blade” to compensate for sail twist. In effect you are adding a slot to the lower half of the mainsail in order to keep it from stalling when the top half is correctly trimmed for the winds aloft. That function extends the value of the jib to boats that theoretically are fast enough to do without.
And how does the relationship between the two sails change as the distance apart increases? Is there sense in putting masts as far fore and aft as possible, rather than a boom-length apart?
At 8 chord lengths the effect mostly has disappeared. 😊 So theoretically this could work, if you were able to build a long-and-light enough hull to get the masts far apart and still have adequate sail area to drive the boat. As Rob mentioned, it’s really a practical problem; on a reasonable hull, if you move the masts too close to the ends, you greatly exaggerate pitching, which will slow the boat down much more than the added spacing between the sails will speed it up.
And what about the biplane rig,which is getting some attention lately. The arguments I’ve heard here and elsewhere are that on a fast craft with high apparent wind, blanketing of the lee sail will not be a problem. How does this jive with the ‘downwash’ argument?
There isn’t a whole lot of practical information available on biplane rigs, but once again you either (A) need space between them, the more the better, or (B) they need to be arranged to positively influence each other. Generally on biplanes the downwash is not a big factor except when one sail is blanketing the other. I think they would be OK on a fast hull. Since the area is carried lower down, there wouldn’t be as much overturning force as on a conventional sloop with the same sail area, so on a marginal platform that would be a good thing. Of course- again- there are no free lunches; because of the wind gradient, having two short masts puts the sail closer to the water and out of the faster winds that a taller mono-rig, of the same area, would extend into. So, in lighter winds that are the norm in most places, the tall sloop will out-perform the biplane- by a lot.
Biplane links:
http://aero.stanford.edu/reports/nonplanarwings/Multiplanes.html
http://cafefoundation.org/v2/pdf_tech/High.Lift/NACA.1938..high.lift.biplanes.pdf
Diazo et al
Sorry to have been out of touch - I took a short vacation, but continued to think and draw.
I am now the proud owner of two Marchaj books, the original sailing theory and practice 1964, and Sail Performance 1990. Gotta love Amazon for second hand books. He is very insistent on the crab claw - its truly amazing that so little R+D has happened since. Of course the short-handed shuntability factor is a major hurdle. Which is why I started thinking about the cat schooner rig in the first place. Two sails with a comparatively high AR, but shorter masts for reduced tipping, and more viable for unstayed masts.
My design puts the masts as close together as possible. 32’ hull, two ~ 32’ masts, 10’ apart, 9’ booms, held down to a flat deck on semicircular tracks. I was heartened to see Marhaj’s discussion of the under the boom turbulence, so this seems to argue for the wisdom of my track idea. Gap will be quite small - I think I can get it down to an inch easily.
Which make me wonder why deck-sweeper jibs went out of fashion…
Likewise the walking stick mast. Surely that gives you close to the desired elliptical shape? But Marchaj also says the generalized fish dorsal fin shape (swept back mast and leech) is more efficient than elliptical.
And the Park Avenue boom - that came and went too.
In any case, it seems that from an aerodynamic theory point of view, for the bermudan rig - square tops with appropriate battening make sense, and we see this. But why don’t you see end plates at both ends?
Its amusing that the tall vertical mast - without which the bermudan rig would not exist - is itself a major source of drag. If I pursue the Bermudan cat schooner approach, it will be with a rotating aerofoil section or round mast with Wharram pocket. Still the removability question is unresolved.
Thanks for input on the non-existent slot effect. I also found the small jib to counteract twisting of the lower main idea useful. And the links.
Something else needs to be mentioned, and that is the use of a small jib or “blade” to compensate for sail twist. In effect you are adding a slot to the lower half of the mainsail in order to keep it from stalling when the top half is correctly trimmed for the winds aloft. That function extends the value of the jib to boats that theoretically are fast enough to do without.
interesting
At 8 chord lengths the effect mostly has disappeared. 😊 So theoretically this could work, if you were able to build a long-and-light enough hull to get the masts far apart and still have adequate sail area to drive the boat.
or… wing masts 18” deep 😊. This look like a venetian blind standing on its side 😊
As Rob mentioned, … on a reasonable hull, if you move the masts too close to the ends, you greatly exaggerate pitching,
right.
There is always a trade-off in aerodynamics of course,
Aerodynamics, like politics, is the art of the possible. But as I recall from looking at hang-glider aerodynamics years ago, there is a huge difference between low speed aerodynamics and high speed aerodynamics. Which makes me skeptical of constant references to plane wing aerodynamics. Better to look at gliders and ultralights.
Simon
There’s some really great posts in here, I definitely learned a couple of new things too! 😊
I wanted to set the record straight on one matter though. I made the claim earlier in this topic that proas may need larger headsails than other types of sailboat to ensure that the CE is sufficiently far forward so the boat will balance reasonably well. I was wrong though, ‘Madness’ proves that is not the case: it reportedly has excellent helm balance, and it features a 241 sqft main and 126sqft jib in the racing setup, so the ratio of mainsail area to jib area to is 2:1, and it works fine. The much higher lift coefficients on the jib must be enough to move the overall CE forward sufficiently. Armed with that knowledge, that’s the sort of distribution of sail area I’m going to go with, since it’s better when going hard upwind (close hauled), and I can pack a larger mainsail (and corresponding mast) on the boat for my normal sailing conditions that way (more area in the main and less in the jib). That opens the door to better light-airs performance as well, since when it gets light, there’s now a lot of foretriangle area I can still fill with a larger jib if I need the extra sail area, so I get the best of both worlds, at the cost of an extra jib. 😊
Having a larger jib / genoa in your sail inventory can be a very good idea anyway, since for a close reach you get more driving force per amount of heeling moment, than you would with the full main and a small jib. This effect is pronounced enough that on some boats you get better performance by reefing the main and putting up a genoa / masthead jib, than you would with the unreefed main and a smaller jib. You get more dirving force per unit of heeling moment, for exactly the reason that Diazo mentioned when talkign about having multiple sails.
For close-hauled work, the smaller jib is definitely better though since you want a high L/D ratio.
Cheers,
Marco
Here is the rig which I have been developing for use on “Sidecar”:
It is a “biplane” rig using balanced club footed staysails. The two shown are fully battened and reefable, the third (light weather sail) is rolled up on the end of a bowsprit or prod. In heavy weather, the rolled LW sail can be taken down and stowed and the prod slides back into the windward staysail boom to reduce windage. As configured, it means I never have to change any sails during shunting. And when shortening as the wind blows up, I only have to reef them (or take them down). The chart also below gives a rough indication of the (flatwater) sail shortening windspeeds using Shuttleworth’s formula adjusted for dynamic displacement and centre of gravity for each combination of sails and boat trim. I have a pivoting foil on the ama hence the varying figures.
I see the advantages of this rig as:
1) Smallest, lightest mast section. No sail track, no rotating wing masts (and controls).
2) No mast interference on any sail.
3) Overall rig CE is more central, which should make balancing easier.
4) Mainsail is balanced, reducing sheet load and travel. Ditto when light weather jib is deployed on the weather staysail.
5) No need to duplicate sails or swap them from end to end for shunting.
6) Windward staysail(s) lose power automatically (sooner) when the boat heels too much (Pizzey heeled rig effect) and “hang” set in very light weather.
7) No wind shadowing deep downwind.
8) No need to change down to smaller (duplicate) sails, just reef them.
9) Booms are clear of both hatches and the cockpit.
10) Potential “lee bow” effect of windward staysail(s) on mainsail.
Disadvantages:
1) Complicated booms and sail controls. Everything has to pass through (or near) the pivots.
2) Potential wind shadowing around 80-100 degrees apparent wind.
3) Two booms
4) The mast has to be stiff….flexing won’t de-power the rig, more likely to increase power.
Ftitz Roth’s website I found interesting: http://proadesign.com/
In it, he shows that if a sail is small enough and far enough out to windward, there comes a point where the boat can’t be blown over by sail area alone. The Shuttleworth figures for Sidecar confirm it as well…Even rigged with one reef staysail only works that way.
That’s the theory, I will let you know how it works out in practice…...I have the carbon spar blanks for it already .
Rob
Please don’t forget to consider the ballestron rig. It has all of the the efficiency and commonality advantages of the sloop rig; it is very easy to shunt, probably the easiest of all rigs; it’s (arguably) more efficient downwind than a sloop rig; being taken aback is not a problem; it’s simple, only one mast and one boom.
Mal.
Hey Rob,
in those curves you drew, is the 200kg water ballast with of without the effect of the foil? If it gets rough out there, it might become hard to keep it in the water… The huge operating range you have there is great though. 😊
Cheers,
Marco
.....in those curves you drew, is the 200kg water ballast with of without the effect of the foil?
The water ballast windspeeds shown include foil down lift…....if you want to know the effect of water ballast without foils for any sail combination, subtract the relevant foil down lift wind speed from water ballast wind speed and then add it to the foil vertical / retracted wind speed. Bear in mind also, that Sidecar’s water ballast location is not ideal, so water ballast benefit could be greater.
Remember that these are flat water figures only for “Sidecar”, using a theoretical formula, calculated displacements, estimated centres of gravity and guestimated foil lifts for only one (upwind) boatspeed, so there is potentially a big margin for error…...I did the spreadsheet and chart to get a ball park feel of the likely / most effective sail shortening sequence. It all may be totally different in real life…...
Rob
In ‘Seaworthiness’ Marchaj brought up the point that in the very worst of seaways your RM can be reduced by 90% or more compared to the static value, because at the wave-crests you are nearly in freefall (which would make for a momentary stability of zero). Boats with a shallow draft are much more strongly affected these vertical accelerations and their accompanying changes in dynamic stability than deep full keel monohulls for instance, because the accelerations at the surface of the water are much more pronounced than deeper below the surface. Lower CB means lower accelerations, but that’s the price we pay for our fast light boats, it’s no different for the high performance flat-bottom monohulls. That said, I don’t know how large the vertical accelerations in the waves are which you’d typically encounter, so I really can’t say how big the effect is going to be, not to mention that there’ll be plenty of other factors like pitching of the sailplan, at play there as well.
Marco
Please don’t forget to consider the ballestron rig. It has all of the the efficiency and commonality advantages of the sloop rig; it is very easy to shunt, probably the easiest of all rigs; it’s (arguably) more efficient downwind than a sloop rig; being taken aback is not a problem; it’s simple, only one mast and one boom.
I agree with Mal, it is a good rig and I agree with all the advantages, especially shunting and being more efficient down wind. Plus reduced sheet loads and travel, plus no duplication of foresails. So why don’t we see more of them??
1) ANY cantilever mast using the same material as a stayed sloop rig mast is significantly larger in section (drag and mainsail interference) and heavier (pitching) than a stayed rig. It also flexes a lot more, which is good for the mainsail, bad for the jib.
2) The necessary “bury” below decks ruins accommodation and adds even more weight in a Pacific proa. In an Atlantic or Harryproa, if you put the rig in the lee hull, you also have to build a lot more lee hull (weight and windage) to get the requisite “bury”.
3) Booms: (Mal’s solutions excepted, the way to go! ) are HUGE and heavy, way above deck to get adequate headroom under the jib end (pitching and windage) Plus all the winches and controls that need to stay up there to make it all work.
4) Balance: The full rig and maybe the first reef are usually fine, but after that, unless you fancy standing on the front end of the boom to change headsails, you may have an overbalance situation. Big square headed mainsails help in this regard, but then that tends to spoil the initial balance.
You pay your money and take your choice…....
Rob
In ‘Seaworthiness’ Marchaj brought up the point that in the very worst of seaways your RM can be reduced by 90% or more compared to the static value, because at the wave-crests you are nearly in freefall (which would make for a momentary stability of zero). Boats with a shallow draft are much more strongly affected these vertical accelerations and their accompanying changes in dynamic stability than deep full keel monohulls for instance, because the accelerations at the surface of the water are much more pronounced than deeper below the surface.
I have a feeling that this applies differently to multi’s, which have a different kind of stability (not form stability related to hull shape). The rate of rotation around the COG depends on the lever arm and mass of outrigger, which is independent of the buoyancy. I could be wrong on this of course, don’t have the book to check.
1) ANY cantilever mast using the same material as a stayed sloop rig mast is significantly larger in section (drag and mainsail interference) and heavier (pitching) than a stayed rig. It also flexes a lot more, which is good for the mainsail, bad for the jib.
2) The necessary “bury” below decks ruins accommodation and adds even more weight in a Pacific proa. In an Atlantic or Harryproa, if you put the rig in the lee hull, you also have to build a lot more lee hull (weight and windage) to get the requisite “bury”.
3) Booms: (Mal’s solutions excepted, the way to go! ) are HUGE and heavy, way above deck to get adequate headroom under the jib end (pitching and windage) Plus all the winches and controls that need to stay up there to make it all work.
4) Balance: The full rig and maybe the first reef are usually fine, but after that, unless you fancy standing on the front end of the boom to change headsails, you may have an overbalance situation. Big square headed mainsails help in this regard, but then that tends to spoil the initial balance.You pay your money and take your choice…....
Rob
1) As I’m sure I have been at pains to point out in other threads, there is no reason why you can’t design a ballestron rig which uses an identical, or even smaller section mast for most of it’s length, compared to a conventional sloop. Only a short section of mast just above the deck needs to be large enough to take the full cantilever load. above that you can use shrouds to support a small section mast. I have built a full sized ballestron rig this way (Mi6 proa) and many model yacht rigs. If you use shrouds for the upper mast, forestay tension is not a problem.
2) With a deck stepped mast you still require some structure under the deck to take the rig compression loads. The watertight trunk which may be required for a ballestron mast may take up a bit more volume than a conventional structure, but with a proa the mast doesn’t have to be on centreline (or even in the hull at all), so you can put it where it will have the least impact on accommodation.
The weight of a ballestron rig is not significantly higher than a conventional rig. With a conventional rig the weight is dispersed more over the boat, but it is still there somewhere. Ultimately, the total loads are the same for any rig, it just depends on how the loads are transmitted into the hull stucture.
3) There is no reason why a ballestron boom section needs to be any larger than for a conventional boom arrangement. I don’t know why other ballestron rig designers use such massive booms, nor why they they have them so far off the deck (ugly). If anything, with a ballestron rig you can get the boom to be almost sitting on the deck.
4) To take full advantage of the self balancing aspect of the ballestron rig, the jib should be designed reefed at the same time as the mainsail. For simplicity you can use a roller furling jib which could be operated from the cockpit, but slab reefing a jib on a bellestron boom should be no more difficult than slab reefing a mainsail.
Unfortunately, there are few examples of ballestron rigs, and most of them seem to follow the design principles that Rob mentions above. This, I think, has led to misconceptions about the limitations of ballestron rigs and has stifled their development. For proas particularly, the ballestron rig is a really good solution, but it is often overlooked.
Mal.