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24 stated to be over ten times the value of the force which ‘would be exerted by the same wind on a sail of area equal to the projected area of the tower, that is, equal to an area represented by the height of the tower multiplied by its diameter, this sail being placed squarely in the path of the wind. Apparently the conditions giving the greatest value of F also give practically the lowest value of x, the result being that when the wind is 20 degrees, or DIRECTION OF wine FIG2 torr 108 . Towers rotating in a clockwise direction. FIG. 2—TOWER SHIP SAILS BEST WITH WIND TWO POINTS ABAFT THE BEAM about two points, abaft the beam, and the towers are rotating with a peri- pheral speed equal to about 3.5 times the wind speed, the resultant propulsive effect is a maximum. The resultant pres- sure from the towers acts along the middle line of the vessel and there is no tendency to drift and no need to use helm. This is illustrated in Fig. 2. Effect of Squalls The question now arises as to the effect on a tower of changes in wind speed, such as result from sudden squalls and so on. In the first place, we may as- sume the tower to be still. Experiments carried out in these circumstances by Herr Flettner, and also experiments made previously, indicate that the force on the tower due to a steady wind pressure is . less than the force on the mast and rig- ging which would be necessary to carry a sail area ten times the projected area of the tower. While the actual projected area of the mast, yards, rigging, etc., may be less than the projected area of the tower, when light flexible members are subjected to wind pressure they vi- brate, and their effective area of resist- ance increases considerably. The steady, smooth cylindrical shape of the tower, on the other hand, facilitates the passage of the air currents to some extent, and it is found that the total pressure on the tower is less than that on the masts and rigging. Experiments were made by Herr Flettner on a tower having a fluted surface, but it was found that while the propulsive effect was good, the resistance of the tower to direct wind pressures, when it was not rotating, was increased, and this idea was, therefore, dropped. When the smooth tower is rotating, the MARINE REVIEW experiments carried out showed that no increase in wind pressure, in the direction of the wind currents, resulted from an increase in the wind speed beyond about 46 feet a second. This is a matter of some importance, as it means that sud- den gusts of wind or sudden in- creases of wind speed, arising from a beam wind, would not create a cap- sizing effect greater than that due to a wind of about 40 feet a second, ob- viously a valuable safeguard when pre- determining the measure of stability nec- nessary for a new vessel. Using Higher Wind Speed Considering now the effect of increase in wind speed on the resultant force on the tower (we have so far only consid- ered the component of this force acting on the tower in the direction of the wind), we are faced with further inter- esting experimental facts. It has already been remarked that the maximum rfe- sultant force on the tower is experienced when the value of U is about 3.5 times the value of V. If now we have our tower rotating with a peripheral speed of, say, 70 feet a second, we shall be obtaining the best propulsive effect from a wind two points abaft the beam, and of a velocity of about 20 feet a second. An increase in wind speed to 35 feet a second, that is, a change from a mod- erate breeze to a fairly fresh breeze, would reduce the ratio of U to V from 3.5 to only 2. The resultant force é 2 r) stopped nn a Vv toeee WIND X REVERSED F ieee aa r January, 1925 rotating tower of nearly 50 per cent, the net alteration of propulsive effect due to the sudden change being hardly notice- able. To obtain the full effect of the increased wind speed the peripheral speed of the towers would need to be increased to about 120 feet a second. Preventing Loss of Efficiency So far we have considered the prob- lem as one of only two dimensions, and it is necessary to appreciate the fact that unless steps are taken in the practical design to maintain the correctness of this assumption, the results obtained will be unsatisfactory. Without going into the question of the actual scientific reasons for the effects created by the rotating tower, we know that there is a region of relatively low pressure on the side of the tower on which the direction of the wind and the direction of rotation of the tower are the same, and a region of relatively high pressure on the side of the tower on which these two motions are in opposition to one another. Briefly, loss of efficiency will occur if the ex- ternal air at the ends of the rotating tower is allowed access to this region of low pressure, or if air from the high pressure region is allowed to leak out into the external air existing at the ends of the tower. To prevent either of these losses occurring, end plates are fit- ted to the tower, these having an over- all diameter about 50 per cent greater than the diameter of the tower. With pevereeereses® De 3 Zz ° REVERSED nn se é tr rer etesenesSreoe STOPPED A Sequence of eight positions turning against wind without helm. Fd. = Forward. A = Aft. EF = Foree on Tower. Fig 3. FIG. 3—TURNING TOWER SHIP AGAINST WIND WITHOUT HELM would only be slightly altered for while the effective pressure of a wind of 35 feet a second is nearly double that of a wind of 20 feet a second, when it acts on a flat sail, with a rotating tower the drop in the ratio U to V, from 3.5 to 2 involves a loss of efficiency of the these, in position, it appears reasonable to assume that fairly accurate estimates can be obtained by considering the wind effects on towers as 2-dimensional. In applying the results of the experi- mental work to the case of the BucKau, two towers were erected, each about 48