The reader has by now learned to distinguish the different types of windmills and will be able to recognize them from a distance.
We shall now look a little more closely at the various important parts which at once strike us on the outside of a windmill.
The principal components of a windmill are of course the SAILS.
Indeed, it is the sails which transmit the wind power to all those parts which together form the windmill. Without sails a mill is a mill no more; sails are essential to it. It is obvious that the shape and the construction of the sails are of primary importance, for they determine the proportion of the energy which can be transmitted from the wind to the mill. It is the same thing as with a sailing-vessel, where the shape, position, and size of the sail determine in the first place the propulsion and the speed of the vessel. This is not the only point on which an obvious analogy exists between windmills and sailing-vessels, both being wind-driven mechanisms.
Just as the sail is spread out as a wing-shaped surface behind the mast on a vessel, so behind the stock of a windmill sail there is a surface slightly inclined to the common plane, consisting in this case of a sail-cloth covering the frame. This frame is a system of bars mortised into the stock and connected together with laths or uplongs. The bars in the transverse direction project slightly through the stock and are connected in the longitudinal direction by the uplongs. Attached to the stock are the leading boards, a set of boards which may be compared to some extent to a foresail before the mast. The wind, blowing on the sails, gives a sideways force component which makes the sails turn.
The sails of a windmill in many respects resemble the wings of a bird. If we examine which birds are the fastest fliers, we find these to be birds with long, pointed, relatively flat wings, such as swallows, gulls, and many others. Short and blunt, bulging wings belong to the rather poor fliers, such as sparrows and finches, and to the very clumsy fliers, such as chickens and the like.
Now how did the sails of yachts develop? From the bulging sails of the Old Dutch round and flat-bottomed vessels, such as boyers and barges (at first even with the primitive spritsail), yachts gradually became equipped with flatter and taller riggings with large, later very upright gaffs, the houary rigging, and from the early twentieth century, as a consistent improvement, Bermudian rigging has been used. A modern yacht is rigged with tall, pointed sails.
Entirely on the same lines we find that the most primitive windmills, as we know them from the region of the Near East and around the Mediterranean, had sails which were short and bulging. Later on, sails grew longer, at first with a sail-cloth projecting both in front of and behind the stock:
compare the square-rigged sea-going vessel, so familiar to us from illustrations, but now hardly ever found on the oceans unfortunately. It seems that the renowned Leeghwater, in the first half of the seventeenth century, was the first to suggest that it would be better to replace this sail by one entirely behind the stock. It had happened that when the storm changed its direction, the gusts began to blow on the back of the sails, causing them to turn in the opposite direction. In such a case the brake on the wind shaft has no effect, for instead of contracting on the brake wheel, it will be loosened through the reversal of the direction of rotation, so that the sails will turn faster and faster. This may cause them to break or the bearing to run hot, finally resulting in the mill being set on fire. Such an unfortunate event seems to have induced Leeghwater to introduce the sails in the form in which we know them to this day. The trouble of the flapping of the sail-cloths as they pass the mill body was also much reduced.
Types of sail:
A. Oldest type, double-sided (about 1600)
B. Normal old-fashioned Dutch type (one leading board taken away)
C. Shuttered type, with air brake
D. Shuttered type, with sky scraper
The sail is a wing-shaped surface, integral with the stock and placed on its driving side. As you will no doubt have gathered, the stock is the long timber which is mortised through the poll end and tapers towards its ends. The stock may have a length of as much as 75 to 95 feet.
Formerly the stocks were always made of pitch-pine, but in the second half of the nineteenth century iron stocks came to be used with increasing frequency.
The plane of the sail has a slight twist. On the side of the shaft the first bar is mortised into the stock at an angle of about fifteen degrees, the last bar at an angle which is much smaller or almost zero (relative to the plane of rotation of the sails). The reason of this is as follows.
Any sailing-man knows that when a ship gathers speed with a given force of the wind and sailing direction, the sheets have to be hauled upon, the sails have to be tightened. This is because the resultant of the wind and the component due to the speed of the ship comes in more forward. In the case of the sails of a windmill, when revolving, the part near the shaft has a relatively low speed, but the speed with which the stock cleaves the air becomes greater for each point that is further away from the shaft. The speed at the tip of the sail accordingly is many times greater than at points nearer to the shaft.
Consequently the sail will have at the tip a much smaller angle than nearer to the shaft. Our ancestors knew this quite well from experience, although theoretically they may not have been able to fathom this problem. Thus the sails had developed in a form which could not be improved upon very much during the past few centuries and which therefore was not modified or improved in any way.
Until - in the early years of the twentieth century - technology began to concern itself with aviation!
Before 1910 the various pioneers surprised us with their attempts to rise in the air like birds and to cover a certain distance in flight. The illustrations show that according to our present-day notions these small planes were primitively constructed and made a more or less clumsy impression.
However, the interest in aerodynamical problems had been aroused and was stimulated enormously. Experiments in wind tunnels were conducted, all sorts of aerodynamical profiles and constructions were tested, and everything was put on a more scientific basis. From about 1910 junior Delft graduates began to specialize in this field. Yacht-sailing also was studied from the scientific point of view: in 1925 appeared the well-known work by Manfred Curry on the aerodynamical principles of sailing, which on the one hand was the result and on the other hand the precursor of a somewhat revolutionary change in current conceptions about this.
In Germany the engineer K. Bilau for a considerable time had been studying aerodynamical problems, including that of the effect of the wind on the sails of a windmill. His merit lay primarily in the theoretical sphere. He hit upon the idea of giving the sails of a windmill a solid profile, without any framework and sail-cloths, and to provide them with air brakes.
In our youth the fine, short casting-angles of to-day did not yet exist; we carried the long bamboo angling rods across our shoulders. None of us will have failed to observe what great force was required to move even so thin a rod at all rapidly through the air. This struck us as a strange thing, and it goes to show what enormous resistance has to be overcome when a thick, square beam - for after all that is what a stock amounts to - is to turn rapidly through the air.
The millwright A. J. DEKKER was among the first to recognize this difficulty and to meet it in a very simple way. He did so by lining the stock with sheet metal passing round the leading part of the sail and tapering towards the back on both sides, so as to meet the framework. Thus the whole sail is given an aerodynamically improved shape, resembling the profile of the wing of an aircraft, now so familiar to us. To our view, at the present day, when even prams and fountain pens are 'streamlined', there is nothing extraordinary in this, but in the twenties the principle was regarded as rather revolutionary.
The improvements in the sails suggested by the millwright VAN BUSSEL, of Weert, were based on similar applications.
TEN HAVE, the millwright of Vorden, and VAN RIET, of Goes, fitted sails with wooden boards instead of sail-cloths; these wooden boards are adapted to pivot about an axis parallel to the stock. These sails accordingly are self-reefing under the influence of centrifugal weights. It is possible to influence the position of the boards also at will at the tail by means of a transmission mechanism, even to such an extent that the boards will act as a brake, so that one can stop the sails in a 'noiseless' way before putting on the brake.
Apart from sails with sail-cloths and the later sails with a solid profile, there are the sails with hinged shutters like those of a venetian blind, already in use in England since 1772, where they were called 'spring sails'.
The surface of such a sail consists of interconnected shutters, mounted at right angles to the stock and adapted to pivot about their own axes against spring action. These sails therefore are self-reefing and present the great advantage that the miller does not have to set or reef them, which involves a considerable saving of work. In ordinary circumstances the shutters form one continuous surface, which, like the 'normal' sail, is somewhat twisted. When the wind gathers strength, the shutters are opened a little by the pressure, thus spilling the wind. This gives rise to an automatic adjustment, so that - at least theoretically - the mill will perform a more or less constant number of revolutions in a light as well as a stronger wind.
Shutter-sails were very popular in England and modifications of them are found in Groningen Province, Germany, and Denmark. Yet it seems as if their very obvious advantages do not quite outweigh the drawbacks of this automatic system; in other parts of the Netherlands, apart from Groningen and Friesland, they are scarcely found.
The 'patent sail' was invented in England in 1807 by Sir WILLIAM CUBITT. In this, all the shutters of all the sails were connected by a spider coupling in front of the poll end. This was fastened to a rod which passed right through a bore along the wind shaft, which was controlled at the tail by adding weights on a chain attached to a lever.
Air brakes were first made by CATCHPOLE in England in 1860. They consisted of two longitudinal shutters in the leading edge of the tip of a 'patent sail'. When the sails turn too fast, the air brakes turn outwards, disturb the proper profile, and consequently act as powerful and very smoothly operating brakes, so that the racing of the mill is prevented.
There are shutter-sails of more recent design working in combination with an air brake (millwright BREMER, of Adorp), so that the shutters and air brake are not only controlled by the centrifugal weights fitted on the sails, but at will also at the tail.
In general it may be said that outwardly the shutter-sails are not so handsome as the ordinary sails, whose latticework, when seen from afar, stands out beautifully like a gossamer spider's web against the sky.
Manfred Curry has also drawn special attention to the fact that the mainsail of a yacht is pulled by the incomplete vacuum on the lee-side rather than thrust on the windward side. Further, that the formation of this extremely important incomplete vacuum behind the mast is greatly promoted by the slot effect due to the presence of a foresail. The influence of the foresail on the speed of the vessel is disproportionately greater than corresponds to the increase of sail area which it involves.
This slot effect was applied by Ir. FAUEL to the sails of a windmill. The leading board is set in such a way that a slot is formed between this board and the sail itself; these improved sails as a rule are called 'jib-sails'. The consequence of the slot effect is that the sails develop a much greater pull. In particular in a light wind this is quite evident: the mill will work in the slightest breath of wind. On the other hand the sails must not turn too fast; this has several technical disadvantages, and as a rule therefore an air brake is fitted in the leading board and is opened by a centrifugal weight via a system of cranks and levers when the speed becomes too great.
Improvements in windmill sails according to the various systems in the course of the years have been an important factor in the struggle for survival of the windmills. In a good many cases windmills have been preserved in working order because of such improvements.
The plane in which the sails rotate is not exactly vertical, as one would expect at first sight, since the wind brushes horizontally over the earth's surface. It was, however, found empirically at a very early date that it presented certain advantages to give the plane of the sails a slightly inclined position and that the effect of the wind was thus greater.
The curved lines of air flow (figures at the left give the wind velocity at different heights above the ground)
Our ancestors probably did not bother about the theoretical explanation of this. The reason is that the air flow, brushing across the land, will meet with resistance from the ground, the buildings on it, and the trees, so that the lower strata are arrested more than are the higher strata. Later measurements of the wind have shown that at a level of 43 feet the velocity of the wind is about 10 per cent greater than at 20 feet above ground-level. Consequently the lines of air flow tend to curve slightly downwards. The wind therefore is 'caught' better when the plane of the sails is inclined a little backwards.
The plane of the sails also has to be inclined - and this is perhaps the original reason - because the sails must be able to move past the mill body. For reasons of stability the body is considerably broader at the base than at the top, and without such an inclined position of the sails the shaft would have to project exceptionally far beyond the cap.
The wind shaft, which is of course at right angles to the plane of the sails, thus also has an inclined position in the cap, reckoned from the front to the rear. Its rear end is supported in bearings, which are secured on a heavy wooden structure and thus prevent the shaft from slipping backwards.
It need hardly be said that in smock mills the movable CAP forms the top of the mill body. It is as it were the bonnet on the head of the mill. The cap is high in front and low at the back.
The heavy wind shaft, which is hidden in the cap, is slightly inclined from front to rear. In front its end projects through the cap; the two stocks are mortised into it. The straight front wall of the cap is extended in the downward direction beneath the poll end, i.e. the part of the shaft projecting in front from the mill, in the form of a wooden board which affords protection from rain and which is usually beautifully ornamented. This is the baard (beard) of the mill, the graceful part which was eminently suited for the application of a name, dates, and symbolical representations; it gives a personal note to the mill and serves as a decoration. Quite often handsome carvings are found, and the beard is usually painted in bright colours: green, red, white, gilt. It is the counterpart of the decorations that used to be made on the stems of the Old Dutch ships and which impart such a gay note to them.
Baard (beard or date board)
On a great many windmills beautiful beards are still to be seen and in museums several samples can be admired, originating from mills that have long disappeared. For obvious reasons millers used to vie with each other in having their mills fitted with a fine, skilfully carved beard, the outward sign of prosperity. The beard is the striking part of a windmill, by which it is distinguished from its fellows. A fine piece of hard and durable timber was generally chosen for it.
The end of the wind shaft, projecting in front from the cap above the beard, is also painted in bright colours and decorated with a star, which enlivens the whole and as an ornament harmonizes with the bright-coloured beard.
The cap itself is covered with wooden boarding or thatched; the latter is far and away the most beautiful covering.
The cap of the polder mill, South-Holland type
Front elevation with poll end and beard
The BRAKE HANDLE projects at the back of the cap; at its end is fastened a long rope, which hangs down almost to the tail. By means of this BRAKE ROPE the brake lever is pulled, so as to put the BRAKE on or take it off, as desired.
The cap of the polder mill, South-Holland type
Rear elevation with the weather boards and the brake handle
In the case of wip mills the upright makelaar (finial) is found at the back of the cap, often finely carved and sometimes carrying a wind vane on the top. In front the underside of the body of a wip mill is often delicately curved; in the middle of the breast there is a borstnaald (prick post), which in turn ends in a finial in the form of a drop-shaped knob, ball, acorn, or the like at the bottom.
The STAGE is naturally found only on the so-called stellingmolens (tower mills with a stage). These are the mills which, in order to get a good wind, have to tower above the surrounding buildings of the town or above the trees in rural districts. The great height of these very tall corn mills is utilized at the same time to accommodate a number of floors, on which are performed the various operations which have been discussed above.
To support the stage, a large number of horizontal tie beams project radially from the tower; their ends are secured to braces extending obliquely downwards, their lower ends likewise supported in the tower. Horizontal tie beams carry a platform of boards, and round the platform is a fence, which forms a handrail for the stage.
The TAIL. Just as many caps of peasant women used to have a 'tail', one might imagine the cap of a windmill extended with what is called the 'tail' of the mill. This is the system of poles which extend downwards at the back of the mill and meet more or less at one point. At that point is also found the WINDING GEAR: a capstan wheel, sometimes with a platform.
Passing through the front part of the cap is a transverse beam which projects some distance beyond the cap on either side. From the ends of this FRONT TIE BEAM two other beams extend downwards: the LONG BRACES. At the rear a shorter beam passes through the cap: the REAR TIE BEAM, from whose ends again two beams extend downwards: the SHORT BRACES. Finally in the middle of the rear of the cap there is the heavy TAIL POLE, also extending from the top downwards.
Tail and winding mechanism of a stone towermill
Both the long and the short braces are firmly joined at the bottom to the tail pole so as to form one unit with it, and this whole system forms the tail of the mill. The tail thus has the cap firmly in its grip, and when the tail is turned round the mill, the cap is turned round as well.
All this applies to the corn mills of the smock-mill type and to the drainage mills that are found in South Holland. The cap of the drainage mills of the smock-mill type in North Holland is turned round inside the top of the mill; these mills are the so-called inner winders. They lack the characteristic tail.
The turning of the cap by means of the tail takes place on the ground, or on the stage in the case of tower mills with a stage, with the aid of the CAPSTAN WHEEL, i.e. the big wheel with a number of spokes serving as handles. The turning is necessary in order to make the sails face the wind squarely, from whatever direction it blows. The capstan has a drum round which a chain is wound. One end of the chain is hitched to one of the ANCHOR POSTS, and when the capstan wheel is turned, the tail moves round as the chain is wound up. The wheel can be fixed in any desired position about one of the spokes with the aid of a fixed end of chain with an eye.
The chain can be shifted each time from one anchor post to the next and be hitched to it, after which the same operations are, if necessary, repeated.
The wheel that turns the cap and sails to face the wind
Corn mill (Groningen type) with automatic winding by means
of a fantail on the roof of the cap.
One lever for the brake, one lever for striking the shutters
The anchor posts are sturdy and heavy posts which are sunk fairly deep into the ground, from which their round heads project a short distance. They have more or less the same function as mooring posts for mooring ships in the water or near the water-front, and they look rather like them.
As a rule twelve of these posts, and sometimes more, are grouped at regular intervals round the mill. Their heads are finished in a simple and neat manner, and of course are properly painted, usually white, so that they are easily visible, even in the dark, and one does not run the risk of stumbling over them.
The capstan wheel is also painted in gay and striking colours, and thus enhances the graceful appearance of the mill. A star painted on the end of the axle provides a pleasant note.
When we say that the wheel has to be turned to wind the mill, this should not be imagined to be easy going. Winding is a heavy job, in spite of the great length of the spokes of the wheel. As a rule the miller will not turn the wheel by hand alone, but he will simply step on to the spokes, forcing them down by his own weight: he treads the capstan wheel as in a treadmill.
Sometimes a fantail was mounted on the cap of the mill for turning the mill automatically into the wind through gearing and a rack round the top of the tower. This automatic winding device never became very popular in the Netherlands; it was only in the province of Groningen that some mills were equipped with it.
A more modern form of automatic winding has been designed for windmills generating electricity; this will be discussed in the relative chapter.
We have now described all the exterior features of the windmill. But in addition to these a good many other things associated with the windmill are to be seen in the near vicinity, thus forming an attractive complex.
The corn mill with the yard, the living quarters of the miller and his family, the drive with the entrance gate, the stores and other annexes, all these together form a small world apart.
By the side of the drainage mills we see, in the immediate neighbourhood, the simple 'summer house' of the miller; near the mill there are the scoop wheel or the Archimedean screw with the automatic sluice door, the head race which feeds the water to be pumped, and the tail race which discharges it.
Floating waterplants, duck-weed, and bits of wood drawn in with the polder water are held back by the barred gate. Alongside this gate is the plank bridge which enables the miller regularly to remove the dirt and thus to keep the gate clean.
Usually the owner of a drainage mill is also a keen fisherman, and the drying hoop-nets and other nets on the stakes, in the midst of the polder landscape suffused with light, with its water scenery, its aquatic flora, its flying birds, and the inevitable goat on its tether in the yard, present a picture of rural peacefulness and beauty. A sluice with some water seeping through the old wooden doors, the barking dog that comes to meet the visitor, these furnish the musical accompaniment to the pastoral picture which has such a beneficial effect on the hurried townsman.
In the base of the mill the windows with the small panes, the racks with the scoured clogs and on one of the numerous gates and rails the rapidly whirring toy windmill: these complete the charming scene.
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