Why is there one big sail and one small sail on a sail boat?

Time to expand my answer the question is 'why" not how it works

The big sail does most of the work and the smaller sail, the one in the front greatly enhances handling it helps keep the bow pointed in the desired Direction of travel somewhat like a weather vane and makes it possible to travel into the wind.

The jib is first known to have been used on one-masted vessels. Its use began to spread about 1600 and extended to larger war vessels about 1700. Jibs proved handy in helping to steer and were much valued—e.g., on the square-rigger, as a means of better close-hauled sailing and of setting extra sail with comparatively little labor demand. In some ships the number of jibs reached five or more, and often the jibboom itself required an extension, the flying jibboom, is an extension of the bowsprit
http://www.britannica.com

1.It was not until the 1920 and the advent of the wind tunnel that the full advantage of the jib was discovered, and could be explained since the 1920’s the jib has been improved to maximize the Bernoulli effect (same principal that generates lift on an airplane wing) the Genoa also takes advantage of another one of Bernoulli’s Theorems it states that as a subsonic fluid passes through a converging opening, its velocity increases

The space between the mainsail and the jib forces the air to accelerate on the low pressure side of the mainsail thereby multiplying the effect of the Mainsail and allowing the boat to travel faster then the velocity of the wind. In fact a sailing vessel can reach 12 knots in a 10 knot wind

There are many versions of this sail plan - Marconi, Bermudan, Jib headed, Leg of Mutton or sliding Gunter if I understand the question correctly. You may be referring to a twin unstayed cat rig (as seen on Norwalk Island Sharpies for example) or a Junk rig - they typically have a large fore sail and a smaller mizzen. The 'mainsail' is not always the biggest sail. I am sorry for this long winded and rambling answer, but indulge me, I do answer the question in here somewhere. :)

Lets imagine a simple single mast rigged in the conventional modern Bermudan manner, we'll say a full masthead rig fore and aft with a number 2 jib (100%)

The reason you see the simple triangular Bermudan rig so commonly is the efficient upwind performance of this highly tuned rig design. (Very much the common use of this rig is as a result of the market wanting the same thing as the ultra-high performance racing machines, just like we want cars for the street with 'race car' features) Generally a jib that extends back past the mast will be called a Genoa, but the term jib to describe a triangular sail hanked to a forestay is very old and universal - hence the early versions of this rig often being called 'Jib Headed", it just refers to the triangular shape in a vertical setup. ( Lanteen rigs are also triangular, but laid right back and on a huge yards.) The term jib was applied to the foremost sail(s), the foresails of course, the innermost one would be a fore staysail on a ketch for example. Bowsprits were chiefly fitted to enable larger and larger areas of sail to be carried, however a vessels rig is dynamic and there is ample example of this in the Royal Navy of the seventeenth and early eighteenth century (U.K.). Rigs were altered to effect the job required and the materials available - it was not uncommon to have the same small boat remove it's bowsprit in heavily congested waters to enable her to maneuver within restricted areas. The term "Jib" today is irretrievably applied to a foresail hanked to a leading stay of some sort.

The origin of the Bermudan rig occurred primarily in these waters in the 1600's period, (although similar rigs can be found in Holland in this period), and was particularly driven by the islands lying roughly West to East which caused a great necessity in efficient upwind performance due to the prevailing westerly winds. The wind could be gusty and unpredictable near islands, and the Dutch were very influenced by the Lanteen rigs of the Moors introduced to them whilst under Spanish occupation and rule in earlier times. The Lanteen yards were stepped into thwarts and lifted to a more vertical angle, the eventual descendants of this first type were the main transport type well into the twentieth century in the Bermudan archipelago.

These boats became in time famous for their unrivaled performance upwind, and incredible ability downwind due to the huge sail areas they could present to the wind. They were the cargo and transport medium of the time, and they absolutely dominated any other craft at that time. The skill levels to handle such a high powered rig is no different then as would be needed for any modern high performance sailboat. Most had at least two masts or more, and it was only by the nineteenth century that the gaf rig was dispensed with, and any square topsails and the modern triangular pattern that would be so familiar to us appears. The rig so ubiquitous in the twentieth century is a much simplified and far smaller one than the ones that produced such performance in times past. The Bermudan Sloop was always a light, fast vessel, and at least a part of it's success was this lightness of structure. While other vessels may have been able to hold them on some courses they were able to perform at all points of the wind, so they could run away from competing vessels at will, a big trade advantage. The rig was also renowned for its ability to rapidly cope should conditions suddenly change.

Classic boats of traditional types are preserved and raced to this day in Bermuda and require Olympic level training to handle, there has always been an appreciation of this rig for any educated sailor. Chile has a similar traditional racing yacht by the way.

Jibs are commonly labeled by their percentage of area compared to the mainsail - you will notice I referred to a no. 2 jib at 100% , which would be the same area as the main of our fantasy rig. Jibs may be 125% or even 150% of the mainsail area, but equally 80%, 60%, 40% and so on - A numbering system that most sailors will recognize globally will label the jibs from 1 (largest) to 4, 5, 6 etc. (smallest) - sort of depends on your budget, and if you are racing and need every possible combination for the ultimate of tuning potential - or cruising and just require the most basic of selection.

Now what does this mean?

The jib gives a performance boost to the main by accelerating the airflow around the back of the mainsail, this acts to dramatically improve the low pressure zone at the rear of this sail and thus the drive i.e. speed - also this allows the yacht to be pointed higher without much loss of boat speed and in turn this is a very important factor if the situation is a race - as this is one of THE ways to win a race. The first link below will show a book preview that has some very well explained diagrams regarding this process. The ultimate speed rigs in many classes do not use an overlapping jib, tests have proven the most efficient rigs are not the most overlapped. The more vertical the leading edge of the jib the more efficient it will be.

Once we are not pointing as high as possible this rig rapidly loses it's advantages, in fact it can be less efficient than some other very well proven older designs, I particularly like the look of a well balanced Gaf Ketch rig - and really am impressed with Junk rigs - you can reef a junk with one hand and keep drinking your cuppa with the other! Some of this is to do with the limitations of sail area that can be held upwind compared to off the wind with a modern Bermudan rig, the spinnaker can exceed the standard sail area in order to redress this.

So, the simple answer to the question is that the most popular sail-plan in the current era is a version of the high performance racing rig as seen on the super-maxis etc. and that has a linage stretching back centuries. It is a highly tuned rig under considerable strain, it is expensive and often requires construction methods unlikely to be within the average persons ability thus needing professional riggers and equipment for a high performance rig. Continuous maintenance is required to remain in tip top shape, it is very good upwind, and is a very well understood and powerful rig that perfectly serves its design function. An acknowledged draw-back is the way this rig will pull the boat over on it's side compared to more traditional rigs, which will not tend to put so much thrust high up the sail-plan and so not lay the vessel over as much.

I have added some interesting links below, it's a very big question you have asked really!! :)

I have seen photo's from the very early 1900's and late 1800's showing fast vessels for the day with their jibs pulled to windward of the main and resting against it to make effectively one giant sail. This was the hot racing tip of that era in some ways, and obviously pre-dates knowledge of the function of the slot. (refer my first link.) These boats were tremendously over-canvassed in any case so they would have had astonishing performance in any case. Often the rig from boom end to bowsprit tip measured twice as long as the boat hull!! They went real hard though - when asking the oldtimers that sailed these boats they say they absolutely ripped through the water at full song.

My Gunter rigged Heron class dinghy will virtually not point at all without it's jib, which while only a fraction of the area of the main has a benefit to the boat in speed and balance far exceeding it's proportion of area. (designed by Jack Holt around 1954 for a parent/child and general knockabout, the 11' 3" Heron fights always to screw into the wind if rigged wrong and/or sailed carelessly, and teaches good seamanship at all points of wind - you can get wet if you don't pay attention! Racing is very close with this class.) Popular class in U.K. and Australia

the jib (and it is called a jib in a dinghy) adds both speed and pointing ability (travelling close to the wind) to the boat, improving the boats performance. Also it balances out the forces acting on the boat and improves the boats natural trim in the water while sailing.

Also it means alot of boats are alot more fun for racing in =)

Edit: I now agree with Kilo, except for the Bernoulli principle being what causes airplane wings and sails to work. Although there is some Bernoulli involved, read what NASA has to say about the production of lift.

http://www.grc.nasa.gov/WWW/K-12/airplan...

Other wise, I couldn't agree more with your updated answer.



The main sail (the big one in back) is the primary mover and also acts as a weathervane since it is behind the mast.

The forward sail or headsail, (wrongly called a jib and more accurately called a forestay sail) adds power to the main sail when sailing upwind (close hauled) or even with the wind coming from the side (beam reach).

Here's how it works. The main sail is shaped like an airplane wing that is standing straight up. As the wind flows across the sail, the sail produces lift or power to move the boat, just like an airplane wing lifts an airplane. The force that is applied by the wind to the sail is transferred to the mast and rigging and to the hull of the boat and the boat moves in the water.

The small sail up front directs more wind to flow over the outside (leeward) of the main sail and this causes an even greater amount of power in the main sail. Some of the wind striking the headsail also provides power to move the boat too, but the biggest advantage is the force multiplier to the main sail.

Here's a link that might help:

http://www.boatsafe.com/kids/033199kidsq...

And another more technical one:

http://www.physclips.unsw.edu.au/jw/sail...