I tell in these lines the experiences of four years of sailing with the biplane rig of our catamaran Cuoncio Cuoncio, the design expectations and the answers of the trial by sea. I intend so to answer to the curiosity and often to the perplexity of whom is going to purchase the plans and build such a boat. I hope as well that this could be a starting point for opening a small forum where to exchange experiences and info to enlarge the diffusion of a rig that I love so much for reasons that are, at the same time, irreducibly sentimental and deeply rational. Documentation available on this theme is quite poor or so specific to make any prediction unfounded at the even slightest variation of the parameters. Obviously, we do not have wind gallery or tank tests at our disposal. We will just tell of our direct experience with that minimum objectivity deriving from the observation of the tell tails and from the GPS wind and speed indications.

Why these …peculiar ideas?
Cuoncio Cuoncio has been conceived as a biplane unstayed rigged catamaran and this feature is a fundamental aspect of her concept. You will have already guessed the frequently asked question: is it possible to use a traditional main and jib rig? Yes, of course, but the price would be a long list of complications that are alien to the design approach of a boat that is absolutely simple. You could even…attach a second hand stayed rigging on it, but it would be not properly done. Indeed a stayed mast means much more stress on the hull (deriving from the compression load induced by shrouds) than an unstayed one. It would require local reinforcements and further stiffeners that are not part of the original design.

Before you start imagining alternatives, I invite you to think a bit upon the qualities of this kind of rigging, deeply understanding the pros and cons.
Let's start from the cons that are a few, nay the problem is just one: in a sector of about 15 degrees around 90 degrees to apparent wind the boat does not sail properly because lee sail stops lifting. In other words she can't stay on a broad reach. Is it a great defect? It depends. We have to consider that we are speaking of a pure cruiser that, what's more, doesn't make speed her strong point, privileging accommodation comfort. Whenever our waypoint is quite athwartships, we keep on a close reach for a while and then bear up till running to our waypoint.

Many times, while close reaching or running, I have surprised myself thinking about changing my beloved biplane for a nice reaching rush!
But, in the long run, the biplane always gets the better. Who really couldn't resist it, can always set a gennaker that will grant some knots in light winds from close reaching onwards. Well, let's now have a look at all the advantages of such a rig!

Above all, the structural matter: all that's missing requires no expense and, above all, doesn't break. No shrouds, no turn-buckles, no chain plates, stiffeners, stays… All the stresses transmitted by the rig to the hull concern only the mast hole, and are substantially moderate and inversely proportional to the distance between mast hole and step. Moreover, an unstayed rig tends to automatically bend under gusts, flatting the sail without any crew regulation. No shroud and backstay allows strong roach for full batten sail. It gives an extremely low centre of effort, also permitted by splitting the sail area between two equal and parallel sails. Moreover it consents achieving the best elliptical sail area distribution, minimising induced resistances. Last but not least, the effect of splitting the sail area between two surfaces, according to the biplane theory, presents a certain further reduction in induced resistances (synthetically, as induced resistance is proportional to the lift squared, according to lift line theory, half lift would mean one quarter of the original induced resistance) in the case of there being no interference. Anyway, a certain gain is obtained in spite of any interference there may be between the two sails.

Steering is quite easy with the two sheets on cam cleats. The two sails are, obviously, self-tacking. Anybody can choose between two carbon masts, very light but very expensive too, and two aluminium masts, heavier but quite cheap and easily constructed by any amateur builder. Personally, I have never been particularly infatuated with carbon: certainly it is an extraordinary material but, unfortunately it doesn't bend before breaking. This, in my opinion, is not a very nice defect as it means no warning before demasting. Aluminium will certainly weigh three times as much, but it is by far safer having a very long plastic deformation phase. Finally, everything in design, as in life, being relative, let's remember that Cuoncio Cuoncio (that in Naples's dialect means…slowly slowly) is certainly not a racing monster and its safety, easy building and low budget criteria are in this case far more important than extreme lightness and speed.

Can she sail close-hauled?
Yes, of course and even very well! Obviously, not any towing tank data is available for Cuoncio Cuoncio, but tests at sea don't lie! Close-hauling or reaching is often her best bet as for speed; leeway is moderate and best VMG is around 50° to the true wind. She can also be sailed up to 30° to the true wind but speed is reduced and leeway increased. It is not by chance that design teams with quite different budgets chose an unstayed biplane rig for mega-catamaran Team Philips at the first edition of The Race. Evidently, this configuration revealed itself so satisfactory to be proposed, a couple of years later, on the Yves Parlier's Hydroplaneur Mediatis Règion Aquitaine.

The last couple of pros is probably less rational: one is the sight of a biplane at sea while running wing and wing; the other, lastly, is in a sea full of boats all alike, all in white plastic and with the same rig, once you chose to build her yourself, why shouldn't you make your jolly, little, adorable boat a bit different?

How to sail a biplane?
Can a biplane, on top of it all without any kind of daggerboard, succeed in close-haul sailing? And tacking? Will her be easy to sail? They are common question looking to such a peculiar object! Nevertheless we have to note that, compared to a usual main-jib rig, differences are not substantial. They are the same kind of differences that we can experience if sailing a ketch rather than a yawl, a cutter rather than a schooner. The principle is the same: there is the wind and there are the sails, with their trim and interactions. We are always asked to observe and understand how one sail's trim influences the other. In headsail rigs we keep concentrated on the 'channel' trim: in one sense all the game is played there. In the biplane the channel has different position and dimension but the principle doesn't change: one sail feels the other and modifies its circulation phenomena.

The basic rule to easily understand how to sail this peculiar rig is that, while on a headsail rig the balance of the boat is achieved regulating the forces on the longitudinal plane (jib fore and main aft), on the biplane the same forces acts on the transversal plane: lee sail brings the boat to windward and windward one makes her bear up. It is very similar to the feeling of having two engines while manoeuvring in a harbour. Once you have understood this, the boat will tack, bear up or luff as you will ask for. These phenomena are by far more sensible coming out from a rest situation or from a tack: while sailing, when the circulation phenomena are firmly established, the two sails act as a single wing and trimming effects on balance are sensibly lighter.
Let's try to make these concepts more clear:

The windward sail makes the boat bear up, the leeward makes her luff.

Coming out from a lying to or lying athwart situation (1), first haul windward main and make the boat get headway (2); then, slowly haul lee too (3) and finally trim both looking to tell tails.

Coming out from a tack (1), make the boat get headway only under windward sail and let the leeward sail amain (2) bearing up to get speed. Only when the boat is firm at the helm we haul the leeward too and trim both(3).

When sailing close-hauled, the tell tails tell us to set leeward main slightly more hauled then the windward. Let's remember that a full batten main is near not to shiver in about every condition, telling us about nothing of its circulation phenomena. The only way to realise what's happening is to look at the tell tails. Anyway, there is nothing of the biplane configuration that penalise close-hauled sailing. It allows, on the contrary, for larger sail area due to lower centre of effort. Cuoncio Cuoncio has her best VMG round 50 degrees to the true wind. Nevertheless she can sail up to 30 degrees to true wind. These features are due to her multi-hull nature, to the relatively high cruising speed and to the absence of daggerboards rather than to her rig.

While reaching, in a nearly fifteen degrees sector (between 90 and 105 degrees to true wind) leeward main stops lifting at the point to be 'sucked' to windward by the suction of the leeward side of the windward sail (you will pardon the pun!). This so called 'back-wind' effect and can be investigated with the well known lift line theory. Here, without getting bored with too scientific treatment, it is enough to understand that in the indicated sector you will be able to sail with only your windward sail or, otherwise, to choose a close reaching and running tacking course.

Running setting is not different from any other main. Bringing the wind further aft until running dead, windward main will gybe. You are now running wing and wing. It is the slowest running but the most suggestive as well.

Tacking
There is no typical feature of this rig that can compromise the success of a tack. Don't forget that the position of the centre of effort relatively to the centre of lateral resistance is almost the same of any other sailing vessel. As explained, the role traditionally played by the jib in helping to come out from a tack is here done by the windward sail (windward after tack, obviously!). You will easily imagine that a catamaran without daggerboards will never tack like an America's Cup or a dinghy. This feature is common to all multi-hulls independently from their own rig and sail plan that will have to be well designed and balanced anyway.

Design observations
We remark that the design motivations for choosing a biplane rig concern the aim to lower the centre of effort and to use unstayed masts. As seen from these two aspects other advantages derive: splitting the sail area between two surfaces allows a certain reduction in induced resistances; no shrouds and backstay allow strong roach for full batten sail with optimal elliptical sail area distribution; no shrouds means no shroud resistance as well. Some aspects of design are very similar to the ones of any other kind of rig as, for instance, the balancing and the dimensioning of the sail plan related to the righting moment. Some other aspects are typical of such a rig: they are the height of luff, the aspect ratio and the distance between the two masts. They are the real heart of the matter: in fact, the interference between the two sail is mostly related to the length of their luff (properly to their draft as defined in lift line theory). In other words, as it happens for main and jib, the total lift is lower than the sum of the lift produced by each single sail if independently working in different flows. This decrement of total lift is due to the interference that pressure-suction side of each sail exerts on the other. Interference is, in its turn, proportional to the distance between the two sails and to their draft and hence to their aspect ratio, sail area being equal.

This is the conventional representation of what happens to a flow when it encounters a wing or, in this case, a sail. The straight dash dot line is hull centre line, the sail is darker. The curved dash dot line is the so called stagnation line and it is the border line between pressure and suction areas, respectively windward (+) and leeward (-). The other lines indicate with their proximity the flow speed: the closer the lines the faster the flow and then, according to Bernoulli, the lower the pressure. All is clear? I hope so!

Now, if another sail is put at a certain distance from the first, an interference is inevitably created. In detail, the suction zone leeward of the windward sail will tend to suck air from the higher pressure zone windward of leeward sail as shown by the arrow in the figure. Besides modifying the flow, this effect evidently cause a fall in pressure difference between the two sides of each sail and consequently a fall in lift. Don't be shocked: in every 'traditional' rig one sail (the main) sacrifice itself working in the header caused by the upwash of the jib. The matter is always to optimise the general efficiency of the whole, as if it was a single wing. The fundamental parameters of this game of compromise are, in the peculiar case of the biplane, the lift proportional to the aspect ratio or to the draft of the sails (sail area being equal) and the distance in between.

As a reference, we report here these parameters for the biplane catamarans mentioned in the text (Hydroplaneur and Team Philips data are approximate and deducted from available material):

Cuoncio Cuoncio S:
SA (each main) 15,3mq
Luff 7,5m
Foot 2,8m
Distance between masts 3,2m
Distance/luff ratio 0,4

Cuoncio Cuoncio M:
SA (each main) 23,5mq
Luff 8,2m
Foot 3,9m
Distance between masts 4,3m
Distance/luff ratio 0,52

Hydroplaneur:
SA (each main) 150mq
Luff 22m
Foot 9m
Distance between masts 11m
Distance/luff ratio 0,5

Team Philips:
SA (each main) 380mq
Luff 41m
Foot 12m
Distance between masts 21m
Distance/luff ratio 0,5

The present comparison of distance/luff ratios indicates that presumably (and not without a certain approximation) an optimal value is round 0,5 (as the two all round tested racer show). Cuoncio cuoncio S sacrifices something to be able to occasionally thrust herself in some harbour without taking the place of three boats! Racer monsters don't come to compromises and privilege efficiency above all. Moreover the longer the distance between masts, the larger the beam (distance between hulls centrelines) and the higher the righting moment and the larger the amount of sail the boat can carry. With reference to this, we have to note that Cuoncio Cuoncio S measures 6x3m, Cuoncio Cuoncio M 7,5x4,3m, the Hydroplaneur 18,30x15,05m and she is about as wide as she's long (!!!), Team Philips 37x21m (she doesn't joke too!). Also note that the Hydroplaneur only has 11m spaced masts, while she has 15m beam: it is surely due to the design choice to adopt stayed masts and main sheet rail, but it also denounce that 0,5 distance/luff ratio has been reputed the optimum for sail plan efficiency.