THE DIPOD

FROM THE MASTS OF THE SAILING SHIPS OF THE ANCIENT EMPIRE
TO THE TOOL TO LIFT AND HANDLE STONE BLOCKS



Bulletin de la Société Française d'Egyptologie, No. 140, Octobre 1997

Dr François CORRARD (1)(4)

Note of the translator:
This site is about the dipod, a two legged mast in Egypt Ancient Empire sailboats also a lift tool which may have been used in buildind pyramids.
This site reproduces a paper of Doctor François Corrard in the Bulletin de la Société Française d'Egyptologie no. 140, Octobre 1997. The original paper is also presented in French on this site and is the base reference with pictures and bibliographic notes. This is just an attempt to translate the French paper. The translator requests your understanding.
To access the French version in a separate window, kindly press the shift key and, without releasing the shift key, click on the French flag top right.

At the beginning of the Old Empire, Egypt, already the sailing birthplace in the Mediterranean (2), adopts for its sailing ships the dipod (3), this particular mast, which will make it possible to multiply by ten the functions of river and maritime transportation. In same time, the advent of the stone, which supplants brick, and its control will generate major buildings of the history of this country. In these two fields, mankind pushed its technical options until their limits. What were the interactions between them? It is known already that shipping was essential for the provisioning of the large building sites. How about the dipod? Has it been landed and used on earth?

I - THE DIPOD, MAST OF SAILING SHIP

1 - Its characteristics

fig 1
Fig. 1. Boat Vth dynasty - Tomb of Ty
Formed of two wood trunks assembled at the top, the two feet drawn aside in a triangle, it is the highest mast of its time (5), (11 meters according to figure 1, in proportion of the characters). It is movable by a rotation around an axis joining its two feet, either lying on the boat, or set up. The feet are then attached to the same cross beam, sometimes resting against wooden squares, by large twisted and tended bonds. It forms thus, a particularly robust structure. Contemporary of the Old Empire, it appears with the III th Dynastie and disappears with the VI th that is a 500 years life (from 2700 years to 2200 years B.C.). Its development was such on the Nile, given the abundance of drawn testimonies, that it is practically specific of this Egyptian epoch (only some examples scattered of dipods are found elsewhere in the history of sailing,).

2 - This mast corresponds to a particular hull

Mast and hull are interdependent. To assemble a hull, there are two options:

This choice relies on the characteristics of the vegetable environment and of a faithful knowhow transmission.

Indeed, the first boats consist of papyrus bales, laid out at the periphery, in superimposed rows and firmly bound one with the others at regular intervals and at the two ends (6) (Fig.2).

fig 2
Fig. 2. Boat made with reed bales, used on lake Titicaca (Museum of Ethnology Cambridge), probably very close to the ones used on the Nile
When the hulls are built with wood, the tree used is the Niloticum Acacia, described by Herodote (7), still of use at the beginning of this century (8). It is dense, will not putrefy, is filled with knots thus breakable and impossible to curve. It is cut off in thick boards, coarsely rectilinear of short length which are assembled one with the others like “bricks of one wall” using bonds and pegs, then worked to the adze (Fig.3). This assembly, like the boat of reed, has no or little internal frame.
fig 3
Fig. 3. Assembly of the wooden edges as a brick wall - XIIth dynasty (L., D., II, 126).
The large funerary boat of Kheops (40 m long), made of cedar wood however easier to work, are assembled in the same way, each piece of wood with its immediate neighbors. The few cross pieces at the bottom of the hull do not go up until the top of the edges and do not have a relationship with the beams (Fig.4 and 5).
fig 4
Fig. 4. Inside of the Kheops hull, being reconstructed. (photo G. Goyon)
The solidity of the hull is ensured by the only thickness of the edges (13 to 14 cm) on which these beams lay (9). Boats of Dahchour (10 m long), built according to same principles in the Middle Empire, have also thick edges (7 to 9 cm). These hulls, thus built without central skeleton, are flexible and deformable. To rigidify them, the Egyptians use the longitudinal tension of a large cable fixed at the two ends of the boat. The edges, while deviating, tighten the bonds (pegs and ropes) which maintain the assembly of each board.
This system is used
fig 5
Fig. 5. Structure of the Kheops hull - IVth dynasty. (B. Landström, Ships of the Pharaohs).
fig 6
Fig. 6. Curving the edges before setting the beams - XIIth dynasty. (L., D., II, 126)
fig 7
Fig. 7. Seagoing boat - Vth dynasty. (Sahuré, II, pl. XIII)
Some authors have thought that this longitudinal tension tended to curve the hull, or even to fold it, or even more to break it by the bottom, consequently justifying the need for the presence of a keel and couples. In fact, this longitudinal traction respects the bottom of the hull and curve the edges (Fig.8). Moreover, this curve of the walls of the boat is opposed to the pressure external of the water which tends to crush the hull. This deformation fulfills exactly the function of the couples. This technique compensates for the the internal structure weakness.

fig 8
Fig. 8. Change in shape caused by the lengthwise cable tension.

In this flexible environment, the dipod has two assets:

3 - To sail with a dipod

Before embarking, here are some simple navigation principles:

fig 9
Fig. 9. Force distribution. Mystic boat Vth dynasty. (Deshasheh, pl. VI)

Let us embark now for a voyage towards the south. Some blows of oars take us away from the bank. The sailors, at the back of the boat, orient with their oars the boat in the axis of the river. The north wind is present. We will be able to use the sail, and, first of all, to set up the dipod. This delicate operation, is usually accomplished in two steps (Fig.10) according to this only graphic testimony; to raise the dipod then to hoist the heavy sail. Another simultaneous method mast-sail could use the force of the wind (Fig.11). Indeed, the dipod is set up only when there is wind blowing forward from the back of the boat, therefore in the direction of gyration of the mast. Here is practically the operation; at the beginning, the sail is in place on the mast lying. The men by raising the end allow the wind to engulf itself in the sail and to finish the rotation. At the end, it is necessary to slacken a seil to soften the jolt then to tight it again to sail. Once the sail is set up, the back wind gives a very pleasant pace. The boat sails effortless (Fig.12). Indeed, without a keel, the least yaw spontaneously brings it back in the wind axis. There is no side force which may make it to lean or even to capsize. All is easy when the axis of the river is superimposed on that of the wind, which is frequent. When a light variation occurs, a correction by the oars rudders makes navigation still possible. But over 15°, it is necessary to bring down the sail and to use the oars. It is there, the limit of this navigation: 30° out of the 360° possible.

fig 10
Fig. 10. Setting up the dipod - IV dynasty. (Tomb of Abibi at Saqqarah, Cairo Museum)
fig 11
Fig. 11. Hypothesis to set up simultaneously the mast and the sail.
fig 12
Fig. 12. Automatic adjustement to a small change of the back wind.

4 - Disappearance of the dipod to the profit of the single mast

Major changes appear (Fig.13): How this change occurred? Is this the result of external contributions, conquests on ethnic groups of the South which would have used such riggings? Admittedly, the first representations of a single mast, before the Old Empire, come from Middle Egypt (Fig.14), Sudan (Fig.15) and Nubia but the clear offsetting of these masts of small size testifies of an exclusive use of back wind and thus does not constitute an innovation. In addition, conquests should have had brought a sudden and total appropriation of all the improvements. However, quite to the contrary, progressive and disparate modifications testify of a gradual research, of a constant inventiveness to be put to the credit of the Egyptians:
fig 13
Fig. 13. Force distribution - New Empire. (B. Landstöm, Ships of the Pharaohs)
fig 14
Fig. 14. Gerzean boat, about 3200 B.C. (vase of the British Museum n° 36326)
fig 15
Fig. 15. Rock engraving - Sudan, 10 km south of the Egyptian border. (G.J. Verwers, 1962)

fig 16
Fig. 16. Vth dynasty (B. Landstöm, Ships of the Pharaohs)

fig 17
Fig. 17. VIth dynasty (Deir El Gebrawi, II, pl. VII, lower register)

Why this transition? Which is the stake?
With this modified rigging, the sail functions differently (Fig.18). It is not perpendicular any more to the axis of the wind but only tilted compared to this one. The airflows which are presented at the edge of attack will pass on both sides from the sail. Those which run out behind will accelerate because the distance to be traversed is a little longer than for those which run in front. Space between the molecules increases, the air is less dense generating a depression, whereas in front of the sail, the force of the wind generates a pressure. (18)
The resultant of all these pressures applied to the sail creates, (Fig.19)

This wind control increases considerably the driving freedom. The mast does not need more to be dismounted, to be so high and so robust. The dipod doesn’t have any more raison to be present. It has represented the quintessence of the transposition on board of the observation of the dead leaf carried by the wind. The human genius, in this case Egyptian, has intuitively transcended this natural fact. The disappearance of the dipod opens the way of the modern sail, that which we practise today.

fig 18
Fig. 18. Forces on the sail.
fig 19
Fig. 19. Forces on the hull.

II - THE DIPOD AS A TOOL TO LIFT AND HANDLE STONE BLOCKS

1 - Transport of the stone blocks on ground and on the Nile

Shipping transformed the Nile quiet force into an economic, political but also architectural power. Expeditions were numerous to the mines and careers of the South to supply the building sites of Pharaoh. These are the same men who transport the stone blocks from the extraction area to the boat, then who transport them on the river as sailors. The study of the dignitaries titles reveals that some of them associate nautical responsibilities - Chancellor of God, Lieutenant, Ordering crews, Leader pilot or Chief of the crews of the two boats - with the title of Director for great work for the king, the person in charge of a building site of Pharaoh. These two statutes can be simultaneous for some or successive for others. Ship management and building are thus much related. Couldn't a technique used on board have been used on ground? Could the dipod, with its so well controlled rocking movement, have been used on ground for lifting and handling of the stone blocks?

2 - The function of the dipod is identical, as a mast or as a lift tool

In the two situations (Fig.20 and 21), the system has two trunks of wood assembled in a triangle and the same rotational movement. On board, the tensions of bastaques behind, balance the force applied by the wind to the sail. On ground, the traction on the rope is opposed to the weight of the block. In both cases, the forces are exerted at its top and act in a perpendicular plan to the rotation axis.

fig 20
fig 21
Fig. 20. Dipod aboard.
Fig. 21. Landed dipod.

3 - How to use the dipod

The system provides a lift function the importance of which varies with the initial slope of the dipod compared to the ground.
Sustentation function

Sustentation

At start, the angle is of 70°. The block is maintained in sustentation close to the ground. A retaining cord opposed to the traction cord makes it possible to keep this position of balance. The block, guided by hand, can be moved for a few tens of centimeters, positioned with a high degree of precision, this with great facility.


Lift function
elevation

The initial angle is 50 to 55°. Lifting is more important (approximately 30 cm) for example, to load or unload a block on a sledge. The required effort is more important. The addition of a second dipod decreases the tractive effort.
In both cases, the muscular effort is short, maximum at the beginning. More the dipod turns, more the tractive effort decreases and is null after a 30° to 40° rotation. The displacement of the men is short, of 1 meter to 1.50 meter.
Lastly, the dipod can:

Use of two dipods
the addition of a second dipod decreases the effort required by at least one third, if it is (see Trigonometrical Appendix)

4 - Experiments

We used as material:

With only one dipod, we have, with much ease, moved, positioned (Fig.22) and set up the limestone block (Fig.23).
With two dipods, the same block was deposited (Fig.24) on a 15 cm thick pallet - supposed to represent a sledge - and unloaded (Fig.25). Eleven men mobilized this mass of a 1.5 metric ton.
fig 22
Fig. 22. Sustentation. Positioning precisely the block by hand. On the left, the retaining cord. The traction cord is on the right. (single dipod)
fig 23
Fig. 23. Setting up the block vertically (single dipod).
fig 24
Fig. 24. Loading the block on a 15 cm thick pallet. (double isometric dipod).
fig 25
Fig. 25. Unloading the block from the pallet. (double isometric dipod)

Use of a dipod chain (Fig.26)
Instead of two, we can align several dipods, in particular on the side of a pyramid, the leg in the angle formed by two bases, one row out of two. The rope goes in the higher angle of each intersection. It is hauled by the men placed on the higher base. On the steps, other workmen, two by dipod, maintain them horizontal, at the beginning.
The cable traction causes the the lower dipod rotation with the load. The rope, approximately parallel to the stone side, during this movement, deviates then approaches the second dipod, rests on the intersection of its arms and move it at its turn. The two wood parts of the first dipod, which is no more in function, are separated to make it flat and let pass the load which goes up without discontinuity.

5 - Place of the dipod on a building site

What are its specificities? Its extremely simplicity, its lightness, its great handiness, its ease of use and its interesting yield. These assets confer it a possible place besides the already indexed handling tools which are levers, wooden logs, brick scaffoldings, ropes and sledges.
As regards its lift use on the side of a pyramid, Herodote mentioned during its voyage - approximately 4 centuries B.C. - the use of machines made of short pieces of wood, laid out of step in step and making it possible to hoist the stones. Admittedly, for very heavy loads, the dipod does not compete with the interest of the sledge hauled on silt coated slopes. But, for stones of more modest size, it keeps all its interest to contribute to the construction.

a b
c d
Fig. 26. Lifting a block with a chain of aligned dipods. Model of the first steps of the Kheops pyramid at the same scale as the characters (54 0/00).

6 - Assumption or reality?

We do not have any element, this day, validating the use of the dipod on ground. Can one find traces of use, print of the two feet on the ground or notches of the rope on the edges of the blocks? Those of the colossus of Djéhoutihétep are protected during its transport. We can think that the stones of quality profited from this attention. As regards the more ordinary blocks, their unsmoothed faces and thus their nonrectilinear edges do not facilitate finding such traces.
Perhaps a day, we will have confirmation of the place of the dipod in the large building sites. Its assets confers it a place today from a practical and theoretical point of view.

TRIGONOMETRIC APPENDIX

We shall evaluate the traction force necessary to lift the block in three dipod configurations.
At each moment, the traction force and the weight can be decomposed in forces which are:

1 - Use of a single dipod


Calculations are made in the dipod mediator plane, that is the plane perpendicular to the dipode which contains its symetry axis.
We project in the mediator plane the two forces applied to the dipod - the weight and the traction force -.
We note respectively P et F the projected vectors norms.
The two angles a and b défined below are measured in the dipod mediator plane.
a is the angle between the dipod symetry axis and a horizontal line.
b is the complement (to 90°) of the angle between the projection in the mediator plane of the traction force and the dipod symetry axis.
h is the dipod height.
The moment vector caused by the weight relatively to the dipod rotation axis has a norm of:
Phcosa
The moment vector caused by the traction force relatively to the dipod rotation axis has a norm of:
Fhcosb
The instantaneous equilibrium is obtained by equaling the norms of these two moment vectors, that is:
Phcosa = Fhcosb
From this, we deduce:
F = P cosa/cosb

This formula can be extended when two dipods of same height are placed in series with P replaced by a force.

2 - Use of two dipods, called ( I ) et ( I I )

At the equilibrium:
Pcosa = P1cosb
F1cosd = Fcosc
F1 = P1

From this, we deduce:
F = P x (cosa/cosb) x (cosd/cosc)

Two positions of the second dipod ( I I ) improve the yield - that is decrease F for a same weight P -

3 - When the system rotates,

The angle a increases, F decreases in the three configurations:

4 - Theoretical study of the effort to provide,

(depending on the weight P, the angle a and of the use of one or two dipods).

These angles are theoretical. They correspond to a position at which the dipod becomes really functionnal. In practice, the angle at start must be smaller than the optimal angle a to take into account the lengthening of the cord and making turns at the top of each piece of wood.

Notes