- too it is for drifting sideways without forward movement
we could explain, that the hull 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 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 a straight side trunk on a raft 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 analyzing 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 true - 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 in that forward direction, where we have 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 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 split up the wind 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 dangerous for the stability when running downwind.
Square sail is - square !
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.
That indicate, that if we can create and maintain the the windward leech on square sail with a sharp cutting edge as doesn't crimple nor flutter, then we could have a perfect sail - but exactly that is still not seen.
Beating against the wind sailing mono-masted vessel with one square sail only:
the windcutting edge on the sail - the windward - is tightened up and tensioned most possible between tack and the yard controlled by downhaul on the lee brace and sustained with either a bowline or a sprit. The tension from bowline or sprit too will lower the camber and thus permit higher beat.
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˚.
1): Dead run: yard and sail are set across the vessel and both sheets and braces are used to control the sail.
2): Broad reach: Port yardarm is braced aft and the port clew is sheeted aft, starboard brace and sheet are loosened.
3): Beam reach: Port yardarm is braced further aft and port clew is sheeted in further, starboard clew is secured forward of the mast is become starboard tack.
4): Close hauled: The sail is set close to the centerline of the vessel as possible, port clew is sheeted in as far as possible and port yardarm is braced around as much as possible. Starboard tack is secured further forward and the starboard bowline may be tensioned to help maintain the tension in the luff of the sail.
square sail with bowline tight
- wind-cutting leech sharp
- rather flat camber
- no deformation of canvas leech
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.
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 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.
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.
One point more to the Incas ;o)
A sail can work together with the wind in two ways:
- 1): catch the ram-presure of wind directly on the backside of a perpendicular sail, pushing the boat ahead.
- 2): 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) 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 "pillow" 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
Ref. Newton's third law about Action and Reaction
If the flow is broken by turbulence it may leave 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
- not only the jet is bend, too the spoon is attracted
Ref. Coanda effect = the sticking flow Ref. Coanda effect says, that a flow of gas or liquid will follow a convex surface, when it enter as a tangent to this surface.
Ref. The aerodynamic lift - according to the teorem of Bernoulli
Note the decreasing ropebands on picture >>>
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. But even so, the general was, 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 spinakker. 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 probably lowered their sails to avoid a difficult heeling and moved ahead alone by their effective slave-powered oars.
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 they 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.
lifted up by a high and abrupt wave and then plunging bowsprit and bow into next wave
Move ahead a Center of Wind CE in general could be done by setting studding sails on a foremast only, hoist a spinakker or lower the sail on the mizzen - or perhaps move backward CLR flinging out from stern any type of drouge.
An alternative, as I personally have experienced on a catamaran as grew unstable and started slalom ahead, is to lower her aft centerboards, as brought the CLR abaft - and that too could be the solution for a stable running with a balsa raft: to plunge down more Guaras in the stern, moving backwards the CLR = the hold in water.
The sails, if adjusted for sailing, will transform the force of wind to a forward force and a lateral force.
Both forward and lateral forces will accelerate the craft until the speed in the two directions have created a 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 hull is "stretched out" between CE and CLR in the wind direction