- running directly downwind without leeway
- too when drifting sideways without forward movement
We could explain, that the vessel is "stretched out" between CE and CLR in the wind direction
We have to regret, but we don't know much around what type of sails the original Inca Rafts used. But we know they were made by cotton cloth. The Spanish chroniclers laconic say us "the same type of sails as on our "navios". Navio means big ship.
And what means that? is the question now 500 years after.
We know two types of common sails from the Mediterranean Sea, as we can demonstrate with this picture:
The lateen sail was used on the Caravels. The square sail was main sail on the Carracs - but with a Lateen sail as spanker. The carracs were the ocean going trade ships.
But which type the Spaniards had build on the Pacific side of Panama isthmus, we have no clear evidence.
Nevertheless, some hundreds of years later we got drawings and later on photos as showed the square sail, and never a latin sail - even if we can't refuse the choice of John Haslett using two lateen sails on his last replica rafts. In all cases the lateen sail is considered beating better to the wind - mainly because of the slanted rigid yard to cut up against the wind. Therefore beating against the wind wasn't Haslett's problem - that was the missing wind in the Calm Belt near Equator - and finally the shipworm Teredo Navalis.
An other advantage could be the weight, as it seems possible to reduce to well under the half of a classic stif yard of pine. That lightweight construction could show up very rational, specially in the time before arrival of the Spaniards, when they still didn't know the use of a block nor a sheave in top of the mast. With a lightweight yard, it could be much easier to hoist the sail.
One of the core points for wind sailing is to minimize side drift, what means a high lateral hydraulic resistence and that can be obtained either by employ a long slim hull or by giving your floating item an effective keel, that can keep a steady course - and don't forget, that it is the straight side trunk on the raft, as work as a keel.
Upon this floating hull we mount a rig with sail as can catch the wind from nearly any direction and transform wind-force to a force pushing the craft along in its sail-direction. Both Square rig and 'For and Aft' rig as the Lateen rig can that.
Until now raft expeditions have focused upon the sailing and less upon experiment with the function of a Guara-system, often relying on the theory, that if they plunge sufficient Guaras down between the trunks, then they will obtain sufficient keel to permit beating against the wind. That is now by the Kontiki2 expedition verified, not to be the case. That is the hull as has to give that stability. Guaras are mainly for steering.
Two masts instead of one will add more steering options by balancing the wind press, but that is not decisive - if we have Guaras.
With the knowledge, that we can create our hold in water, where we want it on the raft, the core point of the rule is now: "if the sail is adjusted - - - "
The task of any sail is to change the crude forces of wind into a forward directed force as provide propulsion the longitudinal direction, where we hold lowest Hydraulic Resistance + plus a force across the hull as can be opposed by a HIGH Lateral Water Resistance of keel and sideline.
The pointing of the raft is as explained a result of the Center of Wind forces in connection with the Guaras - but some leeway we will always have - except when sailing directly downwind.
By Beam reach the wind begin to be able to sweep along the cambered canvas and create what the airplane engineers call a lift across the sail, but together with a drag along the canvas, and the size and direction of those two forces are clearly dynamic and dependent of the wind speed.
The size of drag and lift is too dependent of camber, and where a "for and aft" sail can have a rather asymmetrical profile just as an airplane wing, a square sail hold a symmetrical profile, due to its need for changing of leech from "leading edge" to "trailing edge" and viceversa, when coming-about in a tack. This symmetry has as consequence, that the attack-point of the dynamic forces with sufficient accuracy can be calculated as the center of canvas just as the static Center of Wind.
Drag and lift is a theme for much discussion, and I will therefor limit my part to say, that the leech of our square sail has to be able to cut up in the wind and split it in two:
one part to go to back of sail and blow up the canvas profile - - - and another part to sweep along the front-side of the canvas where controlled by the COANDA EFFECT, it will keep along all the curved surface and create both a drag along the canvas and as named, a lift perpendicular to the sail.
The camber has neither to be too high nor too low, to keep the coanda adhesion. Beeing a square sail with the needed symmetry, it is neither possible to calculate anything from a foil-table for standard air-plane wings. So take it as it is, and counteract with the Guaras.
This make me memorize my first homemade sail (sewn by an unexperienced 15 year schoolboy on his mother's Singer machine). Result: a too abrupt leading edge and a too abrupt trailing edge to let the air flow, and this phenomeno had as consequence, that I only could sail with wind pressing on backside of canvas and in no way against the wind.
Such sail manoeuvres as pictured are not possible with vessels as have one mast and one sail only - and that whatever we are talking vikingship, Inca-raft, catboat, optimist with whatever type of lonely sail.
When Thor Heyerdahl in 1947 prepared his raft Kon-tiki he knew rather well the original South American balsa-hull shape, but around the sail he only knew that it should be a square sail. Fortunately square sail was and still is an integrated part of the Norwegian sail tradition, so he transfered a norse rigging and mounted it on his A-mast. Square sail with a little strange mounted topsail. Furthermore he of unknown reasons placed a minor square sail on a mizzen mast located well aft of the main mast - as a ketch - but that mizzen was only seen in the start from Callao - it disappeared on later photos, probably because a mizzen is difficult for the stability when running downwind.
That is a great advantage to employ sails, as you can handle and reef from deck - without need to climb masts nor balance along yards for reefing.
Lateen sail belong to the family of 'for and aft' sails. Lateen rig - Bermuda rig - spritsail and gaff-sail are all considered as 'for and aft' sail; and 'for & aft' sails are known to beat higher to the wind than a square sail. This probably is due to the fact that windward boltrope is laced to a yard or tied around a mast as can sustain a cut-up against the wind without flutter in sail. But nothing is gratis, and the price is, that operating behind a mast or yard induce some turbulence as render ineffective the first strip of the canvas.
A stay-sail too is a 'for and aft' sail. It is very effective and can work higher to the wind due to the fact, than a slim forstay is rigid but doesn't create the same wind-turbulence problem as a thicker wooden boom.
Tension of the leading edge
That indicate, that IF we can create and maintain the windward leech on square sail with a sharp cutting edge, as doesn't crimple nor flutter, then we could have a nearly perfect sail for tacking against the wind. But exactly that is still rarely seen.
Square sail is often square - or perhaps trapezoid - or something else.
The experience from mono-masted square sailers is, that any square sailer should be able to beat 80 degrees to the wind.
With well trimmed sail the common is 70˚ - but the best I have heard is 58˚.
square sail with bowline hauled tight
- the wind-cutting leech is sharp
- and camber is rather flat
- no deformation of canvas
The lower sail corners hold each a CLEW with a rope to tie
When a clew is tied directly to the boat in wind side, it is called "TACK" - else "clew".
Clew is fastened to a place near helmsman with a long rope called SHEET
A square sail transfer the winds force to the hul via 3 fastenings only:
1): The parrel, as tie the yard by seizing around the mast top
2): The tack, as tie the lower wind side corner to front end of hull
3): The clew (the corner on the canvas as is counterpart to the tack), as via a rope (the sheet) tie the aft corner to the lee side - near helmsman.
That is those three fastenings as transfer all the winds force from the sail to the boat. The braces only is used for adjusting the sail and not for transfer of any winds force.
Mono-masted sailers with one sail only have not many options to make change in a spread of canvas, and are therefor limited in any balancing with their Center of Wind.
All what such a square sailer can do is to move her CE in a half-circle centered in the parrel around the mast - limited by the chocking of yard against stay. Of course skipper can change the radius a little by tie the tack to another cleat or sheet-in /sheet-out - but nothing else.
Nevertheless such crafts can be good sailers, as steer with the dynamic and not the static part of the CLR: the streaming of water around the underwater hull - because they have their rudder, whether sidemounted steer-oar or rudder with gudgeon and pintle. Furthermore they are born rather balanced in relation to CE and CLR.
On an Inca-raft we have no rudder, but but we can by our Guaras work with the static part of the hydraulic forces and place CLR where we want all over the raft. That means, that on a raft we have to adjust everything related to course by our Guaras. We have no rudder and we don't need to rely on the limited options of our square sail !
The eminent force of the square sailer with her lonely square sail is demonstrated here by this replica of an 1000 year old oceangoing longship - a swift beach-landing troop-transporter from the time of the Norman conquest of England 1066. The "Seastallion of Glendalough" is here showing her extreme fine sail-qualities going 58 degrees to the wind.
Note her perfect trim of the technical points:
1): The mast is placed central in the vessel.
2): The centre of Sail Ce is hanging outside the lee gunwale - to give a stable pointing.
3): The same Centre of Sail has moved ahead of mast to comply with the CLR, as with the speed of boat has moved against stem.
And that is more or less the same conditions we have on an inca-raft.
The small differences:
An inca-raft doesn't need any "beitaas" because there is enough width in the hull to fasten the tack on a cleat inboard. The incas seems not to use a bowline to the forestay. The more than one hundred year old glassplate photo shown first on this site indicate a sprit /spar /pole to sustain the windward leech in the same cringle on the boltrope and the sprit perhaps held in position by two toplines. That gives more spatial options for sail-adjustments, as the Norwegians wasn't able to do - and can too stretch out and flatten the camber. But we miss practical experience from the sailor people.
One point more to the Incas ;o)
Clear display of graduated ropebands along the yard >>>
A sail can work together with the wind in more ways as:
A): catch the ram-presure of wind directly on the backside of a sail, as then push the boat ahead.
B): lead a flow of wind along the curved frontside of the canvas as will add an aerodynamic lift to the force on the backside.
The square sail and the spinnaker (as too is a square sail) are very fine for roomy downwind sailing, making use of the ram-force of the wind - just as the Spanish galleons and caravels did passing over the Atlantic Ocean
By downwind sailing both the square sail and of course the spinnaker work best with a "baggy" shape, but inside the "bag" there are normally a "cushion" of stagnant air
Taking the wind from more broad-side bring other options as reflection of the wind
A wind as reflect from the back side of the sail will give a push - just as a simple billard ball give a push against its cushion when reflected
Creating conditions so a wind is sweeping over a cambered surface (sail, wall or hilltop), then is given options for the coanda effect.
The coanda phenomeno is that fact, that a laminar flow along a curved surface will keep sticked to the same curved surface. Such a deviation of a flow from a straight line take its force from the canvas all along the cambered surface, and the result is a perpendicular force to the sail.
If the flow is broken by turbulence it may escape the canvas, and the lift on sail is lost. Deformation of the curved surface such as a change too steep in camber - or a stay deforming the profile - will favour a turbulence.
- Try the coanda effect in your kitchen with a spoon
- as shown here with the animation right, not only the jet is bend, too the spoon is attracted
When an airflow is split up in such a way that the front flow is passing over a courved surface, before unite again with the other flow as pass behind as a chord - then the speedy flow on the convex front side will lower its internal pressure in relation to the other.
As result the pressure difference create a "lift" perpendicular on the sail - and that is just in same way as an airplane wing work.
A difference: the Coanda effect will bend a flow along a curved wall without access for wind on backside.
Bernoulli work with the pressure difference between two flows.
A square sail don't have this facility, it has only a leech probably reinforced by a bolt rope. The square sail trick is to stretch this foreleech most possible - on one-masted square-rigging by tying the foreleech with a bowline to the forestay or the bow and at same time haul down in aft brace.
The classic more-masted square-sailers had to fasten where they could, and because of that, it was generally known, that a square-rigged vessel on her best could go 7 point (70-80 degree) to the wind. In hundreds of years the western tall ships with square-sails had to wait for favourable winds before sailing off - or follow the trade winds on their way around the world.
A square sail is more fitted for running than lateen sails, specially those of more rounded shape like a spinnaker. Of that reason many oldtimer Atlantic sailers of caravel type (lateen rigged) often got changed their rigging on the islands west of Africa, for then the next month or two to follow the trade wind across the Atlantic ocean to the Caribean sea equipped with square sails.
Downwind sailing was the purpose, when the first sail in pre-historic time entered in the boats - to easy the rowing, as a first step. And boats depicted from ancient time was sailing downwind.
Sailing downwind we can only make use of the ram-force of wind and the speed of a boat running downwind will never get higher than the windspeed itself. But the general rule around the need to place the CE downwind of CLR are still valid.
The Mediterranean galleys could be driven both by sail and oars, but in situations of broad reach they could lower their sails to avoid a difficult heeling and move ahead alone by their effective slave-powered oars - or they could accept the heeling and use the sail to stabilize the wave movements when waves were crossing their course.
Even if they could employ both oars and sail they probably were very fine high speed runners for sail alone. To stabilize the running their two steer oars worked as a pair of aft mounted Guaras moving the CLR of the underwater body aft against stern, but many of them could move the CE further ahead by hoisting a foresail on a raked mast.
By downwind the risk for capsize is minor, due to minimal broadside wind forces and because we have a rather big longitudinal stability. Therefor normal running for a reasonable wind we se crafts make use of this and set all what they have of sails - but fore sails to keep their windcenter CE ahead.
lifted up by a high and abrupt wave and then plunging bowsprit and bow into next wave
The real difficulty for downwind sailing arise when the CE = Center of Wind is too near or even worse: aft of the watercenter CLR. That is physically dangerous situation, where a gust easily can overturn a boat, because the streaming water yield a force on the craft, as can move the actual CLR too far ahead.
That is well enough as long as the sailing is right downwind - at least until the water stream against the bow change or shift side - perhaps because a wave only hit the one bow-side. That situation is not more dangerous than a quick helmsman could oppose, but doing that, the inerty of the ship probably will swing her over to sail on the other bow - and he again has to counteract.
In that way he will slalom ahead - first on one bow, and then the other - and each time with all risk for capsizing.
As told later, the pharaohnic boats on Nile river 5000 years ago could obtain the same by dipping deeper their steer-oars.
The dynamic part of both water and wind will oscillate with gust and waves and that make exact calculations of center of wind and center of water resistance impossible with any degree of
But we neither need to know that - we only need to know their nature to be able to counteract the actual situation by adjustments of sail and Guaras.
With the two theories at hand: #1): balanced side-sliding - and #2): the weathercock-principle, skipper know something about how his raft will react, when he plunge-in or lift-up a Guara or two somewhere on his raft, and that is what skipper has to do.
The task of a prow is to stabilize the sailing by cleve and split up the in-comming flow of water in two - going left and right - passing port and starboard sides.
If the shape of the stem make the CLR move too much forward - more than you can adapt with your actual sail setting - you can get in troubles, running the risk that the raft will luff too much up, so the sail get into the dead area of no-go-zone, and therefor can't work. The solution in this case is of course to steer more leeward.
Sailing on, your sailing depend of how your sail will transform the wind force into a forward force and a broadside force, and the propagation in those two directions will depend of that hydraulic resistance, as the vessel meet in the two directions, and respectively define the headway and the leeway.
Both forward and lateral forces will accelerate the craft until the speed in the two directions have created a hydraulic resistance of same size as the forces.
And because a underwater-hull normaly has a low forward resistance we will sail speedily forward - but even with a high lateral resistance it can't escape a slow sidewart movement = leeway.
Both hydraulic resistances are attacking in the same Hydraulic Center of the underwater hull - in the CLR.
We could explain, that the vessel is "stretched out" between CE and CLR in the wind direction