Inca's Balsa Log Raft
The Power of Hull-shape - and Sails
Directional stability of a craft depends of hull shape.
Sail Ship development
A sail powered ship need a 'clear axis' to keep a steady course sailing ahead with wind across - but not necessarily a symmetric hull. A heeling sail-ship always has an asymmetric underwater hull shape. The elegant shaped Venetian gondola, too hold an asymmetric shape to compensate the one-sided propulsion from her only oar. And despite their mounted "sidecar", neither the Pacific sail-powered out-rigged canoe nor the proa show any sail-problem.
Such a clear sail axis of course passes geometrically through CLR = Center of Lateral Resistance
To use a sail as propulsion is rather more problematic, because we have forces from the wind crossing our sail-direction.
When we in pre-historic time tried to mount a sail on our boat to catch the wind, we first drifted downwind, but nevertheless we had got a propulsion, as in some way was superior to our rowing. But not each time we hoisted our sail, the wind blew to where we wanted to go with our vessel, and we had therefor to get something else, as could push along and keep a pointed course - even with wind across.
That could our rowing boats, so we didn't meet difficulties in that, and we soon learned to manage the sail sailing along in directions other than the wind blew.
But the old steer-oar remained many centuries on its place aft starboard side, where it was landed as paddle from our canoes and rowing boats.
The hidden keel-effect
Keel we call the central backbone of the hull - even if there are no outer fin to see.
In historic time very few sail ships have had a typical outer keel, because all ships were build on the shore - or in the larger cases, on a slipway. Nevertheless, the shape of a hull gave the needed sail-stability. Long and slim for speed - or more wide for better transport capacity - but all crafts were sailing well in the pointed sail-direction.
Streamlined the hulls were, and with only one clear axis - and not square nor circular, as these forms have more than one sail axis.
by hands and feet, poles, paddles and oars
- or too with a motor of some kind -
The family with round bodies
On an extreme left of the scale we show circular shape as in no way are able to sail by the wind other than drifting. Coracles and Basket Boats are examples of round boats. Basket boats we find in Iraq, India, Tibet, Vietnam etc but too in Ireland and Wales under names as 'Thung Chai', Kufa or other - but in the English world she is known as Coracle - sometimes 'a Moses'.
Basket boats exist in all sizes - for personal use, for many people and for cargo. All are build light-weight as a basket woven of willow-withies or bamboo-split or whatever they have, and covered by a hide or weaving as is sealed watertight by tar or varnish. Light weight construction, least wetted surface with largest carrying capacity - and too possible to ROLL on land - as they always have done.
Circular Baskets are lightweight boats
In Vietnam, as seen on the photo, they have removed that "rotation problem" by mounting a fin /rudder
For the experimenting people: how to build a coracle:
A little bit better is the cousin to Coracle - the short and broad type of rowing-boat
Thousand years ago the shown type of cargo ship (knar) verified their seaworthiness serving the sail route from North of Europa to Iceland, Greenland and Canada.
The few words to say is, that with longer hull relative to beam (high length /beam ratio) we gained speed, and the Norlandsboat could be a lovely example of this. This type was in common use until around 70 years ago.
The Floating Square Hulls >>> RAFTS
Shape#1 on extreme left is shown a rectangular body moving across its broad side. That too exists, but we use more this principle in the cases, where we want to stop a sailing by - 'heave-to' = turning the broad side against wind, to have a minimum drift - caused too litle room for drift? - or to drift for landing along a quay.
Shape#2 and #3 - A square hull shape is not much better than a round basket-boat. It seems as a square craft /raft will be able to move equal easy in whatever direction - centerline as well as along one of its diagonals.
Shape#4 and 5 - That bring us to the rectangular raft = a floating "box", as is missing a typical bow. That hull-shape is fine as pontoon or lighter hauled or pushed but with sail it is more problematic. It can sail with wind from aft, but with wind going abeam she show the tendency to use one of the front corners as stem, and she sails happily along a diagonal. Prolonging the rectangle with higher length /beam ratio, the diagonal goes nearer to the center line and the sides take over as guidance, and the raft behave again as a craft - sailing ahead. An example of rectangular raft can be found at page #8.
Shape#6 and #7 The testimonies from The Spaniards all tell us, that the oceangoing South American rafts without visible difficulty could beat against the wind. The common shape as reported was with a long trunk in the middle and some smaller along side - as the shape of a hand. That means that the South American rafts had a dedicated stem, and that construction eliminated both fore corners as prow.
Bottom shape of Hull:
The underneath of an Inca-raft is not known from any chroniclers. The only indication we have from ancient time is, that the central trunk is larger than those along side. Probably larger in both length and diameter. Shaped as a hand. But with hundred of years of raft development in South America this raft design must have been optimized, and the hull shape therefor useful for balsa rafts: A pointed stem and a long sideline - and a steering controlled by Guaras /daggerboards.
If that is the general style of rafts known since Inca time, it probably has a reason, and my guess is, that this central trunk outside being a stem, has a function as low aspect long-keel - to stabilize the course under sail. The Guaras itself are to consider as adjustable fins to balance the side-drift, as all sail-ships have, with wind abeam. That means to keep a pointed course by balance the side-sliding between for- and aft-end.
In connection with this it could be interesting to find out, if this long central trunk as the Incas used - had a function as the modern bulbous bow -
1): reducing water-resistance -
2): change a bow wave before the bow -
3): or it by its larger dimention (diameter) had a function in cross wind as a stabilizing long-keel -
Theoretically we get a different result by the building-methods:
A): Building on a slipway, laying up the trunks on an even plane as a steel frame, give a smooth underside, but the price could be the cuts in trunks to fit a junction with next layer of crossbeams -
B): Building floating in water as Heyerdahl did or laying the trunks on a flexible support - can give a smoother topside together with the opportunity to get a profiled underneath as for example a fat keel trunk - and too a chance to make the junctions with the overlaying crossbeams with minimum cuts and wounds in the surface.
Cuts in the buoyancy-trunks open up wounds for easy penetration of water and quicker water-logging - and perhaps an easy access for Teredo Navalis, the ship-worm.
between the craft and the two elements: WIND and WATER.
One of the core points for wind sailing is to minimize side drift, what can be done either by employ a long slim hull or by giving your floating item an effective keel, that can keep a steady course. 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, For and Aft rig and 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 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.
- as indicated here -
Ship shape - a study of Prows
Thor Heyerdahl mounted a "snow plough" in the stem
(Tangaroa converted for Film)
- raft Rahiti Tane -
The pointed stem seems a need for shape of flat-bottomed hulls, and that theme is the main objective for this site.
The photo of Kontiki is from the book: 'American Indians in the Pacific' by Heyerdahl. On his raft Thor Heyerdahl mounted a "snow plough" in the stem to pass better through the water - or perhaps to reduce the splash.
The Kontiki-raid showed us an awful seamanship - but was a wonderful adventure to tell about - and this gave impulse to many later adventurous raids.
The 3 balsas were build in Guayaquil in the classic Ecuadorian shape. Later on in 1973 they were sent over the Pacific Ocean, where they after a hard passing landed with the most of the rafts in Australia.
The 4 brave old men on the An-tiki-raft crossed the Atlantic in the wake of Colunbus. They reduced the water-resistance by employing only 4 plastic tubes as floaters for buoyancy, so we have doubt about it was a four 'bodied raft' - or a 'double catamaran'
The limitation for square Sail on square Hull
The geometry of sails:
The wind shall fill the sail - and the sail shall push (or pull) the vessel forward.
For beating, a sail need a tight and rigid fore leech to cut-up against wind as neither flap nor flutter. That we have when leech is sustained by a mast or a yard as at the 'fore and aft' or lateen rigs.
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. The one-masted ships by tying the foreleech with a bowline to a forestay 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 result is, that a square-rigged vessel on their 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 - and on their way around the world: follow the trade winds.
classic stable hull
unstable sail direction
lee corner as stem
luff corner as stem
fore and aft sails
Fig.#2 - The sketch demonstrates, how we would like to sail a square rigged barge /raft. But that we can't! The wind abeam will together with the water force the barge to choose one of its front corners as prow.
Fig.#3 - The sketch is showing, that with lee corner as stem, we can sail on. The pointed course show around 70 deg. but the real course is along the diagonal, what means +20 deg. = 90deg, as NOT is against any wind.
Fig.#4 - The sketch is showing an impossible situation for a square sailor, and not because of the hull - but mainly because of the sail. To sail with this luff corner as stem a square sail will get difficulties, because we have nothing solid to fasten any bowline as is hanging hanging outside hull in port side. To fasten the tack outside the hull we could use the Norwegian boom 'Beiteås' and/or we too could use the 'South American square-sail trick' as is shown on both the first and the second photo of Brüning.
With a sprit, a spear, a pole attached to an eye on the bow of this luff leech, it should be possible to sustain and stabilize the leech against the wind outside beam of the hull.
This trick-theory we would like to have verified by some active sailors.
Fig.#5 - If we instead of square sail had mounted a 'for and aft' rig, she theoretically would have done better.
Take care! - it neither is so. Sailing ahead, a hull as is pressing the front end into the water - whereas the water is slipping aft.
That has as consequence, that sailing with a wind abeam - then the wind as will try to move ALL the craft sideways mostly will move the stern, because the prow is kept fixed by the press of water.
vv A square rigged raft, as in some way has obtained to sail with luff corner as prow, will experience her aft end will be pressed over for to sail-on with lee corner as stem - and there she will stay, because there her balance is stable. And that is what this Norwegian raft-photo is showing.
In the Norwegian balsa raft case it wouldn't have been any improvement to change rigging, the best would have been to cut the balsa trunks in shape of a pointed bow.
If we calculate the side drift as something constant.
The real course is the result of steered course added to leeway
(whatever this i due to wind or current)
But on the rectangular raft as use the one corner as stem we could have doubt around where the vessel is pointing - the centerline or the diagonal?
The captain on the cruise liner know it, and that is why his ship is supplied with trusters for and aft to compensate. Too the freight skipper know it. He has only one propeller and one rudder, and therefor he is very careful, when he is entering in a narrow harbor entrance with the wind across. He probably will not do that without a tug-boat for assistance.
Sail-ships generally are trimmed for wind abeam, but they can't escape leeway. In some special situations we directly make use of this sidewards drift - as for example the eel-drifter as the name indicates work drifting, or when a square sailor 'heave to' - with sail hoisted wait drifting.
When we are navigating a sail boat, we have to calculate with leeway, and the side-drift depends alone of the force of wind and the area of sail.
The stronger the wind, the more leeway.
If we check up the drift, it is rather constant per hour or minute and it is rather independent if we are sailing along or not. The wind generated leeway we can't change much, unless we have a centerboard or lee-board, as can increase the lateral (sideway) water resistence.
What is now our task is to sail as far ahead in relation to this permanent drift - that means to make the leeway to something relative smaller.
For a raft, as not is dependent of streaming water, we plunge in our Guaras, so the raft point up more against the wind - what means that the drift of for-end go in balance with the drift of aft-end. Then subsequently we adjust our sails for this course.
Fig-2): Accepting that the drift is near the same all the time, then it is only the headway as can improve the total performance - what will relatively reduce the leeway.
That we do by adjusting the sails. But as said, sails have some limits, as among other things depend of what type of rigging. 'For and aft' rigging for example is known to go higher to the wind than a square-sail, but that only have importance, when we are beating against wind. With wind abeam both types of sails more or less do alike.
Fig-3): On the other hand, when we reef a sail, we get relative more leeway, and that we always have known.
What happens is, that when we reduce the sail, then we reduce the headway together with the drift coming from sail, but we don't reduce the drift coming from wind press on the hull - so therefor the drift altogether gain more relative to the headway, and we can't go so high to the wind.
Some play with numbers: If the wind on our vessel (boat or raft) gives a sideway drift of 200 meters an hour, (and that we can measure nearly statically), then the task is to advance most possible in this hour. If we move our craft with 1 knot ahead (as a Kontiki-raft), that will bring us about 2000 meters ahead, while sailing the same ship with 6 knots, we will reach 10 kilometers. When we are calculating our drift in degrees, we will get 6 degrees respectively 1 degree leeway.
- the problem is either the hull, the sail or missing seamanship.
A raft is as a flat-bottomed sail-craft, just as every one of the sail-ships with lee-boards depicted earlier at page #6.
The vessels shown are all flat-bottomed sail crafts without keel, but nevertheless they without greater problems are able to beat against wind and keep a stable course - but of course, a lee-board of some type will make her beat even higher to the wind.