multiple sails

Looking at designs with multiple sails such as Crossbow II, LongShot and the Hobie Trifoiler makes me wonder:
Why stop at two sails? Why not 3 or 10? How closely can you space multiple sails (or vertical wings) so that they are all still generating lift and not getting in eachother's way aerodynamically?

 

Though it might not be so obvious, less can sometimes mean more when it comes to sails.
- Less sails you have, less drag you will have too. It means that you can point higher to the wind with a single sail than with multiple sails of the same total area and same aspect ratio.
- For the same total sail area and same geometrical shape, multiple sails will be less tall, thus catching slower wind close to the ground.
- But for the same total sail area, multiple sails will also give a smaller heeling moment, so you can have lighter structures supporting them.
- and so on.
It all depends on the goal you want to obtain, as usual.

Cheers

 

Great, thanks for the answer! You mention differing wind speeds at different heights. How about that? I've often wondered.
I assume airflow is turbulent near the water's surface due to waves?
How high up must you go over the waves before you get laminar airflow? Is it a simple function of wave height?

 

Rigging creates drag. A single sail rig will therefore have less overall drag. Also, multiple sails create interference and more turbulence which makes them be less efficient. On the other hand, multiple sails can help balance a boat which makes them desirable in some conditions.

 

Well, a vertical wind gradient exists because the airflow in contact with ground or with water has zero speed, due to friction. At some height above the ground it will have a non-zero speed (if there is some wind) which means that the wind speed will have an increasing value in the upwards direction. It can be expressed mathematically by an logarithmic function, which you can find explained very well in this site: http://www.onemetre.net/design/Gradient/Gradient.htm . But there are other ways of expressing the variation of wind with height, like exponential or power-law functions. See here, for example: http://en.wikipedia.org/wiki/Wind_gradient .
Therefore, a taller sail will be working in a higher average airflow, thus producing more lift than two or more smaller sails of the same shape and total area.

As about turbulence, it is true that chopped sea will create more air turbulence than a flat sea surface, but the turbulence level will also strongly depend on the overall configuration of the geographical area being condidered (nearby ground with trees, buildings, hills - for example). However, I am pretty sure that a laminar-flow wind would be a real rarity to find, so you can safely assume the entire air flow field to be turbulent, imho.
Unfortunately, I am not aware of any mathematical relationship between the wave height and the turbulence level of the airflow above it. Maybe someone else here will know better.
Cheers!

 

There is a relationship with turbulence and surface roughness (wave height in this case), the boundary layer thickness is related to surface roughness, free stream speed, and viscosity.

One large sail will always be more efficient (lift to drag ratio) than multiple smaller sails. That is, in the same wind you will get more thrust from one large sail than with multiple sails of the same area, even with theoretically perfect designs. In reality, there will also be more turbulence and flow interference with multiple rigs.

Consider that early aircraft were often bi-plane and tri-plane (and even more at times), but all current designs, from small sport planes and ultralights to advanced fighters, are all monoplanes. There is a reason for that.

 

As others have noted a taller sail can tap into a faster wind stream, and a higher aspect ratio sail has better lift/drag ratio (but less thrust) than a wider sail which will help a boat point better, hull permitting, since sail drag adds to leeway. Which explains why an old-fashioned long-keeled draggy hull that is never going to point up well gets a low-aspect ratio sail with lots of area whereas a sleek fin-keeled racing hull gets a tall high-aspect sail of proportionately less area. On a large sailing ship the use of multiple masts reduces the size of individual sails so they can be handled with limited manpower, and provides more drive than a single, taller mast with proprtionately less heeling moment.

As far as the closeness of mast spacing is concerned, one can get some hints from biplane practice in multi-wing aircraft. Typical biplane design practice is to space the wings spacing not less than the wing chord. At that spacing the second wing's efficiency is reduced about 25% - an acceptable price to pay for the agility and compactness of a biplane - or a triplane. That translates to a mast-to-mast spacing at least equal to the sail foot dimension for a fore-and-aft rigged vessel, which is typical of schooner practice.

Obviously that minimum mast spacing essential in a vessel that is tacked into the wind, if the sails have booms, in order for the sail to move from side to side without hitting the following mast. However, a casual glance at some sailboats and sailing ships will reveal that there are exceptions. Free-footed sails such as the foresail of a modern cruising sailboat cheerfully overlap the mainsail, sometimes by a great deal as in the case of a genoa. This is to take advantage of the slot effect, similar to a slotted wing of an aircraft.

Square-rigged vessels that do not tack into the wind, but instead must bear off downwind and wear ship to change tack, can and often do have masts spaced closer than the width of the spars. However, really large square rigs rely more on brute force than aerodynamic efficiency - they’re not called windjammers for nothing.

If you are thinking in terms of an efficient, fast-moving vessel with the ability to sail fast and close to the wind, then the mast spacing needs to be increased. The spacing would be likely be greater for wings than for sails, since interaction between rigid wings is likely to be deleterious, as in the case of the biplane or triplane, whereas it can be beneficial with sails which can curve and take advantage of the slot effect. An example of an efficient square rigger is the Maltese Falcon where the mast-to-mast spacing is also rather more than usual. Because it is a square rigger it must still back its sails briefly while changing tack, of course, something that does not afflict a fore-and-aft rig, but it's about as close as anything to a large multi-masted ship with wingsails.

Hope I didn’t complicate it hopelessly. It is not a simple subject and there are folk on the forum with a great deal more knowledge than I have.