practical proa sailplans
Hey,
I agree with Johannes on the induced drag point. If you want good windward performance then you want a high lift to drag (L/D) ratio from your sailplan and foils (very similar principles apply to both). You’ll want sails with a comparatively high aspect ratio, and as few separate sails as possible. With a bermuda rig, having a large mainsail and a very small jib (mostly there to help keep the flow attached to the mainsail over a larger range of sheeting angles) seems to be the best way to go for windward performance. Marchaj presented some data in Sail Performance on that, and if you take a look modern high performance dinghies, which are all optimized for triangle courses where upwind performance is king (they have a spinnaker / gennaker for downwind after all), then you’ll see that almost all of them have that sail setup.
On a side note, a planform which approximates an elliptical planform tends to be quite a lot better than a simple triangle as well, so those square-topped mainsails really are a significant improvement over a triangle, due to significant reductions in tip vortex drag.
This is a personal opinion of mine, but it seems to me that us proa builders suffer a bit here: our mast is in the middle of the boat, so we need sail area (SA) up front to get our center of effort (CE) forward a bit, so we’re pretty much stuck with the larger jibs (anyone disagree here?—I’d be all ears! 😊 ) As soon as you are off the really close hauled courses though and get into a close reach, then more foresail area (relative to the main), gives you more driving force per unit of heeling moment, so if you’re not sailing triangle courses and not trying to get the absolute best velocity made good (VMG) directly upwind, then larger foresails are good for you.
When reaching or running on the other hand, the max lift coefficient (C_L) is pretty much all you care about (putting the limitations of longitudinal stability aside for a moment), so then you want sails which can be set at a high angle of attack (AoA) without stalling, so you produce the highest possible C_L. That means lower aspect ratio (AR) sails are good, or in particular crab claw sails which, thanks to their delta-wing planform and resulting leeward vortices, are extremely resistant to stalling at high AoAs. The lower CE of both of those kinds of sails also means you can get more sail area up for a given amount of longitudinal stability, but as was mentioned, the effect of wind gradient within the first 10m of the surface is extreme, there is a LOT more wind further up, unless you are really in a hard blow where the air is turbulently mixed.
So overall, the topic is extremely complicated, and it really depends on what kind of a boat you want and if/where you want to shine performance-wise.
Cheers,
Marco
As an addendum: I don’t know enough about what you want, to make any real suggestion, but I can tell you what I intend to do, maybe that’ll help you. 😉 I’m aiming for fairly high performance overall, so I’m looking at 7.5-8m mast for my 8m proa, AR of around 4 for the square-topped fully-battened deeply reefable mainsail. Out of what I believe to be necessity, I’m currently thinking I’ll put a bit more than 50% of the SA in the foretriangle in the form of a rather large jib, though if it wasn’t necessary from a CE standpoint, I’d prefer a smaller jib. The storm jib will double as a staysail for reaching in light airs.
Performance-wise I think that setup should be good upwind, and decent for reaching/downwind. At some point I’ll probably try to improve the latter by adding a another sail to the inventory. It seems to me that carrying a gennaker is the easiest solution because it packs and handles nicely, but I’d be curious to compare it to a crab claw as well, even if the latter is a bit trickier to handle.—Would the higher lift coefficient of the crab claw create a net performance gain over a gennaker, or would the gennaker profit so much from the stronger winds further up from the surface that it’s overall driving force is better? And crucially, which of the two provides more driving force per unit of bow-down moment? I really don’t know the answer to those questions, so the issue of reaching/downwind laundry is still wide open for me! 😉
One thing to keep in mind where upwind performance is concerned, is that what goes on underwater is every bit as important as what is above the water. A great sailplan together with shoddy boards is just a waste of money. When sailing downwind, your leeway isn’t as much of a concern, so you just want to get the boards up / ensure they don’t produce a lot of drag.
It’s a long post, but I hope the info helps! 😊
Marco
Marco
thanks for all this thought provoking information.
Though a lifelong sailor, the subtleties of sail design is new to me.
Could we have a glossary for all the acronyms and other terms?
L/D lift/drag
SA?
CE center of effort
VMG?
max lift coefficient (C_L)
AoA - angle of attack
prismatic coefficient…
but to your points -
“square-topped mainsails really are a significant improvement over a triangle,”
Interesting because I’ve been kicking around a design for a rotating teardrop mast with a kind of hard ply pocket at the peak extending aft which the sail head hauls up into. So its the triangular peak that causes voices?
“When reaching or running on the other hand,”
assuming a fast craft, there will be little sailing beyond a square reach anyway. However, I’m keen to try a ‘trade winds’ approach, which should be very feasible on a trimaran or proa without extra spars. In my case, flying the two jibs from the aft mast, with tacks fwd, to port and stbd creating a rogalo wing form - self stabilising 😊
” there is a LOT more wind further up, “
point taken
AR?
SA - sail area?
” I’m currently thinking I’ll put a bit more than 50% of the SA in the foretriangle in the form of a rather large jib, though if it wasn’t necessary from a CE standpoint, I’d prefer a smaller jib. “
you’ll have a job at both ends and drop/haul when shunting?
“I really don’t know the answer to those questions, so the issue of reaching/downwind laundry is still wide open for me! 😉”
like i said, the trade winds rig is attractive to me. I thought maybe you could make the two sails by cutting a conventional parachute spinnaker down the middle 😊
“One thing to keep in mind where upwind performance is concerned, is that what goes on underwater is every bit as important as what is above the water. A great sailplan together with shoddy boards is just a waste of money. “
I was going for deep V, no boards - until persuaded otherwise…
Simon
Hey,
I edited the post so all the acronyms can be clearly and easily found. 😊
you’ll have a jib at both ends and drop/haul when shunting?
Yes, and in the long term I’ll probably have two jibs on hand for that, but in the short term I’m going to be really cash-strapped for the start, so I may opt for only a single large jib to begin with, which means I’ll be parked a little longer when shunting. Either that or I try to get the boat going on the new tack with the storm jib and mainsail (there could be balance issues with the boat there), and then hoist the jib on the new bow once the boat is underway, just to reduce the amount of time spent parked a bit (which important if I’m racing someone for instance). In the long run just having two jibs is the desired configuration for me though! 😊
One thing to keep in mind where upwind performance is concerned, is that what goes on underwater is every bit as important as what is above the water. A great sailplan together with shoddy boards is just a waste of money.
I was going for deep V, no boards - until persuaded otherwise…
It’s something I’ve heard on the forum a couple of times, that without any boards a hull will go to windward, barely, with a lot of leeway, and it makes perfect sense because the hull is essentially a really low AR foil, providing a really poor hydrodynamic L/D ratio. You need that windward lift from the hull/boards, to avoid just drifting to leeward, and boards, with their higher ARs have much better L/D ratios, and thus make for much better windward performance overall.
How shallow are the waters where you intend to do most of your sailing? Side hung rudders and daggerboards, while not quite as efficient performance-wise as their through-hull counterparts (they have a bit of extra drag since they pierce the surface), are much easier to build, and since they can kick up, are great for shallow waters as well. So if you wanted to save the complexity of daggerboards altogether, then using a side-hung one is only a small step up in complexity/build time (mostly buildign the baord itself), but you’ll improve your windward performance significantly. That’s kind of an 80/20 measure; if you want more performance still, then you can go for through-hull boards in trunks, but that adds quite a lot to construction time, and they are not as well suited for sailing in really shallow waters as their side-hung counterparts.
Keep in mind that we are talking a lot about windward performance here. I’ll venture to claim that good windward performance is the hardest to achieve, good reaching and running performance are a bit easier to come by, so it’s worth thinking about where you invest you hard-earned money and free time. On fast boats though, the apparent wind shifts forward a lot though, so windward performance is more important overall than it is for a monohull.
The considerations for reaching / running are quite different, because leeway is not much of an issue (no boards at all would just fine for downwind sailing), and the requirements for the sails are quite different too, so I think it’s a good idea to look at what sort of conditions (wind speed and courses) you will face most frequently in the area where you intend to sail. A good place to get some hard data on that, is from the pilot charts for your region.
Also, are you going to be single-handing a lot, or will you have a second crew member around for sail changes / shunting?
Cheers,
Marco
P.S. - Just a little technical tip, when quoting, use the ‘quote’ feature which the forum provides (either you can quote the whole post, or just put [qu ote] [\qu ote] blocks (without the spaces) around it—it makes the posts a little neater and easier to read. 😊
So its the triangular peak that causes voices?
Not exactly.
First of all, we are talking about a vortex, or vortices as plural (if my English does not fail me…)
Theoretical aerodynamics studies of wings (or hydrodynamics, for that matter) assume first an infinite span. The wing has no end. So you can study the pressure distribution on both sides of the wing without “end effect”. The end effect is the vortex we are talking about here.
Imagine a plane wing. On the under side of the wing, you have high pressure, on the top side you have low pressure: the combination of both is is what create the lift force.
If that wing is of infinite length, there is no reason for the air flow on both sides of the wing to deviate to the left or the right. In other words, air particles travel around the wing in a vertical plane.
But the wing has an end…
At the end of the wing, the air that is under the wing (under high pressure) is trying to go where there is less pressure (don’t we all do?...); i.e. on the top side of the wing. Instead of meeting the low pressure air at the trailing edge of the wing, this high pressure air is trying to escape to the side, around the end of the wing, to get on the top side of the wing. By doing so, this air flow is no longer in a vertical plane; air particles are actually following a “corkscrew” trajectory. They start travelling under the wing, then turn around the tip of the wing and end up on the top side of the wing.
This corkscrew trajectory is the vortex we are talking about. And once triggered, this “corkscrew trajectory” goes on for a long time behind the wing.
And this vortex is generating a LOT of drag; this is why on all modern airliners, you can see those little “wingtips” that are small vertical sections, at the end of the wing. Their role is to “break up” this vortex, or more realistically limit its size and therefore reduce drag (which means less kerosene consumption for the plane and cash in the bank for the airline).
The shape of the tip of the wing also has an impact on the size of that vortex; as said earlier, ellipse sail plan shapes are better than rectangles… and much better than triangles.
This is why all modern high performance sail plans do not have a “conventional” mainsail ending in a triangle shape at the top of the mast, but rather a fully batten “square top”. It is not perfect, but it is much better…
Hope this helps,
Laurent
Laurent
thanks for the vortex explanation. Very clear
So its the triangular peak that causes voices?
Not exactly.
First of all, we are talking about a vortex, or vortices as plural (if my English does not fail me…)arrgh - stupid spellcheck!
” The shape of the tip of the wing also has an impact on the size of that vortex; as said earlier, ellipse sail plan shapes are better than rectangles… and much better than triangles.
This is why all modern high performance sail plans do not have a “conventional” mainsail ending in a triangle shape at the top of the mast, but rather a fully batten “square top”. It is not perfect, but it is much better…”
So how do you minimize masthead vortices? would some sort of a teardrop spade plate horizontal on the mast tip help?
thanks!
Simon
Marco
> I edited the post so all the acronyms can be clearly and easily found. 😊
thanks!
One thing to keep in mind where upwind performance is concerned, is that what goes on underwater is every bit as important as what is above the water. A great sailplan together with shoddy boards is just a waste of money.
I was going for deep V, no boards - until persuaded otherwise…
It’s something I’ve heard on the forum a couple of times, that without any boards a hull will go to windward, barely, with a lot of leeway, and it makes perfect sense because the hull is essentially a really low AR foil, providing a really poor hydrodynamic L/D ratio. You need that windward lift from the hull/boards, to avoid just drifting to leeward, and boards, with their higher ARs have much better L/D ratios, and thus make for much better windward performance overall.
If its the AR of the foil that is important, why not have six or seven narrow dagger boards - a kind of foil schooner idea? Or arrow of dagger boards in a line across the ‘trampoline’ between hull and ama?
I think reducing leeway and creating lift are different things. Any vertical plane in the water will resist going sideways, the lift aspect counteracts leeway, right?
thanks
Simon
Another point to keep in mind is that there is generally a lot of flow seperation around the mast (unless it has a really good profile and can rotate around its base), kind of like a wind shadow behidn the mast if you will, which means the leading edge of the sail makes little to no contribution to lift because there’s just no airflow there. Like a said, a rotating mast with a good section can do leaps and bounds to resolve that issue, but a triangular sail, ontop of being horrible for drag in terms of the tip vortex, gets very narrow at the top, meaning that the top 15% or so of the sail actually produce no lift whatsoever as well. You can cut the top 15% off, put a batten there, and leave the mast at it’s full previous height, and the performance will stay the same. If you shorten the mast and make a square-topped rig out of the whole thing, then you’ve got some pretty big gains in L/D ratio on hand, probably most of that from reductions in the tip vortex, but also a bit by just getting rid of unnecessary mast height (extra drag).
Marco
If its the AR of the foil that is important, why not have six or seven narrow dagger boards - a kind of foil schooner idea? Or arrow of dagger boards in a line across the ‘trampoline’ between hull and ama?
—because then you have six or seven tip vortices, and then you’re basically burning in hell drag-wise. 😉 Another reason is potential interferance between the foils. As far as I know, in general multi-element foils (basically several wings with slots between them) such as they are used in normal jet aircraft for takeoff and landing, the spoilers on F1 cars, the slotted wingsails on C-Class catamarans (or AC45/72s), or a jib-mainsail combination in a sailboat, are great for increasing the maximum lift coefficient of the foils, but are generally bad for the L/D ratio. As far as I know the wingsail boats sail with the slot closed when going to windward, and they open the slot up for reaching / downwind work.
Designing multi-element wings is pretty difficult; if the downwash from a foil hits the element behind it, then you get an absolute disaster in terms of overall L/D. Depending on how your boards are lined up, they may or may not be interferring with each other in this way.
I think reducing leeway and creating lift are different things. Any vertical plane in the water will resist going sideways, the lift aspect counteracts leeway, right?
Not really. Effectively the whole hull sliding thorugh the water at an angle acts as a big low aspect ratio foil. A flat plate set to the flow at an angle will develop lift the same way as a foil with a NACA section will, but it will do so at a considerably higher expense in drag, and worse stalling characteristics. From purely a performance point of view it thus makes the most sense to optimize the hull for minimum drag when travelling forward, and have daggerboards with asymmetrical foil sections, with high L/D ratios, to keep your leeway to a minimum at a comparatively low cost in drag.
Higher aspect ratio foils, be that daggerboards or sailing rigs, do have disadvantages though: they are structurally more fragile, generate more heeling moment (in the case of a rig), and have a smaller range of operating angles of attack before they stall, which can make handling them quite tricky. A stall is what happens when you set a wing at a too high angle of attack, and the flow is suddenly not able to follow the contour of the wing anymore, but seperates, and thus suddenly generates far less lift. This and other factors (some mentioned above) place practical limits on the AR for sails and foils on a sailboat, since neither the wind nor the flow which your boards and rudder experience is a nice clean laminar flow, but it’s turbulent and all mixed up, plus there’s some guy playing with the sheets and tiller all the time, and thus there’s always some variation in the AoA.
Low aspect ratio foils are very resistant to stalling, they need a really high AoA before they stall, so you would never have a rudder stall on a full-keeled boat like a bristol channel pilot cutter for instance, since the rudder is part of the really long full keel (you can think of it as one big foil). That’s good for seaworthiness, but bad for your L/D ratio.
In practical boating an example of a stall which you may have come across (I know I have on a dinghy 😊 ), is if you are sailing at speed on a fin-keel boat / dinghy, and very rapidly set the rudder to a large angle of attack, say >45°, because you really want to get around that buoy in a hurry, then the rudder will stall, essentially ceasing to generate lift. In that moment you’ll feel the rudder/tiller going all mushy on you, and then the boat starts spinning completely out of control, which goes on until either you’ve gone through a wild broach and then come to a stop after frantically dumping the sheets 😊 , or the rudder has a lower AoA and it’s had a moment for the flow to reattach again. You need something like 5-10 times the chord-length worth of fluid to flow over the foil, for the flow reattach, so if you’re sailing at 12 knots = 20 feet per second (thankfully I wasn’t!), and your rudder has a chord of 1 foot, then it takes ~0.25-0.50 secs at a low AoA for the flow to reattach (0.25s * 20 ft/s = 5ft = 5 * 1ft) so your rudder is working again. If you’re going slower, it obviously takes even longer.
So it’s always a tradeoff between the higher L/D ratio and cost + seaworthiness.
Cheers,
Marco
.....As soon as you are off the really close hauled courses though and get into a close reach, then more foresail area (relative to the main), gives you more driving force per unit of heeling moment, so if you’re not sailing triangle courses and not trying to get the absolute best velocity made good (VMG) directly upwind, then larger foresails are good for you.
Marco ,
I think you have it the wrong way round…... as a foresail is eased off from closehauled it is more difficult to keep a good shape due to excessive camber, lost leech control and increasingly being shadowed by the mainsail the further you go off the wind. The bigger the foresail, the bigger the problem. Mainsails are better off the wind than a foresail and fully battened square top mainsails definitely have to be the way to go. Not just because of tip vortex issues, but also because you get more sail area up higher, on a shorter mast. You also get some automatic gust response in that the top battens flex off in a gust, feathering the top, where you want to dump power the most….
Some squaretop images below FYGI
Simon,
A three masted schooner rig on a 34 ft boat is definitely one mast too many, because it gives a rig with a CE even further off centre (aft) than a 2 masted schooner rig plus a central mast gets in the way down below in a narrow boat. Nor do you need 3 crossbeams. The extra mast and aka are extra weight expense and complication for no tangible gain that I can see.
But you are right, if you do have a schooner rig, to have a tie between the boom ends seems a good way to go. Because it means in a gust or bearing away, you only have to release the aft sheet to spill wind in both sails and during a shunt, you only have to pull in the (new) aft sheet on the new tack to draw in both sails. If the tie is longer than the distance between the two masts, then the aft boom will oversheet automatically. Any adjustment you need in boom angles between the two sails can be dealt with by making the length of the tie adjustable. To pick up Kevin’s point about sheeting in the front sail first for controlability during a shunt, if you shorten the tie just before the shunt to something less than the distance between masts, then even if you are sheeting in the aft sail, the front one will fill and draw first. You then ease the tie off when you are properly under way on the new tack to get the best (relative) boom angles again. I think it is worth a try…..
Rob
Johannes
thanks for the war ram wingsail link, very informative.
Simon
Another point to keep in mind is that there is generally a lot of flow seperation around the mast (unless it has a really good profile and can rotate around its base), kind of like a wind shadow behidn the mast if you will, which means the leading edge of the sail makes little to no contribution to lift because there’s just no airflow there. Like a said, a rotating mast with a good section can do leaps and bounds to resolve that issue, but a triangular sail, ontop of being horrible for drag in terms of the tip vortex, gets very narrow at the top, meaning that the top 15% or so of the sail actually produce no lift whatsoever as well. You can cut the top 15% off, put a batten there, and leave the mast at it’s full previous height, and the performance will stay the same. If you shorten the mast and make a square-topped rig out of the whole thing, then you’ve got some pretty big gains in L/D ratio on hand, probably most of that from reductions in the tip vortex, but also a bit by just getting rid of unnecessary mast height (extra drag).
Marco
Interesting that the fine tip of the classic modern bermudan rig turns out to be a liability!
I’m playing around with the idea of a rotating teardrop mast. I hope to make one for a10’dinghy this summer, with an integral boom and - what to call it - short fixed gaff. Sail will be close to rectangular.
Marco
“That’s good for seaworthiness, but bad for your L/D ratio.”
right. its all a big compromise.
Thanks for the fact filled post!
Simon
[quote ] You also get some automatic gust response in that the top battens flex off in a gust, feathering the top, where you want to dump power the most….
right
Some squaretop images below FYGI
nice , that looks like Sydney harbor - 18’ skiffs?
that parasol for the jib is whacky, but laminar flow is king, obviously, no matter how you get it.
Remonds me of the (theory of) the Kort nozzle
A three masted schooner rig on a 34 ft boat is definitely one mast too many, because it gives a rig with a CE even further off centre (aft) than a 2 masted schooner rig plus a central mast gets in the way down below in a narrow boat. Nor do you need 3 crossbeams. The extra mast and aka are extra weight expense and complication for no tangible gain that I can see.
all good points, I had come to that conclusion myself.
But you are right, if you do have a schooner rig, to have a tie between the boom ends seems a good way to go. Because it means in a gust or bearing away, you only have to release the aft sheet to spill wind in both sails and during a shunt, you only have to pull in the (new) aft sheet on the new tack to draw in both sails. If the tie is longer than the distance between the two masts, then the aft boom will oversheet automatically. Any adjustment you need in boom angles between the two sails can be dealt with by making the length of the tie adjustable. To pick up Kevin’s point about sheeting in the front sail first for controlability during a shunt, if you shorten the tie just before the shunt to something less than the distance between masts, then even if you are sheeting in the aft sail, the front one will fill and draw first. You then ease the tie off when you are properly under way on the new tack to get the best (relative) boom angles again. I think it is worth a try…..
Again,all good points. thanks for your thoughts
Simon