Rotor Ship

ROTOR SHIP. Wind propulsion for navigational purposes, in the commonly accepted sense, although suited for some par ticular trades, is practically obsolete for cargo carrying; and the sailing ship, pure and simple, cannot be said to have a definite future. Many sailing vessels are fitted with auxiliary propelling machinery, hut, excluding fishing craft, these vessels have not proved an unqualified success.

Anton Flettner, the inventor of the rotor ship, originally in tended to construct ships with metal sails, being convinced that the effect of metal sails is much greater than that of canvas sails. The idea was to build the metal sails with sections similar to those used in the construction of aircraft planes. It was intended that the sails should revolve freely around a pivot mast, and then be put by a special rudder blade in such a position that the wind would drive the ship ahead.

Experiments were carried out at the University of Gottingen with canvas sails, metal sails and model ships. The result of these experiments showed that the effect of metal sails could be made approximately double that of canvas sails, a necessary condition being, however, that a third part of the sail area should be turna ble. Designs were got out, but the plans did not materialize.

In the case of the rotor ship the inventor states that it is not intended to drive ships solely by wind rotors, but that they shall serve as an auxiliary power upon steam and motor vessels. In the vessel under discussion, the power of the wind is not made use of by sails, but by means of large metal cylinders.

Revolving Cylinders.

In 1922 experiments were carried out at Gottingen with revolving cylinders, and it was then dis covered that the pressure exerted upon a cylinder revolving in an air current was considerably greater than had been supposed. Actually, the power exerted on a normal cylinder was about four or five times as large as that on a normal sail. When, however, discs of a larger diameter were provided at the ends of the cylin ders, it was found possible to increase the effect to nine or ten times the amount of wind effect in the normal sail. One condition for this, however, is that the revolving speed of the cylinders is about 3 to 41 times as great as that of the wind. An ordinary sailing vessel requires to take down all her canvas in a hurricane, but the rotor ship could continue sailing, with more stability for manoeuvring.

The vessel selected for the first tests was the three-masted schooner "Buckau." She had a displacement of 96o tons, and was fitted with an auxiliary motor of some 200 horse-power. The canvas rig of the vessel was dismantled, and in place of the fore and third masts, two very strong masts were erected. The new masts were shorter, being 42ft. in height. These masts were pro vided with bearings at the upper and lower ends to allow for the free rotation of the cylinders, which were placed over the masts.

The cylinders were fitted with discs at either end, the discs being of greater diameter than the cylinders, but built as a part of them. In this particular case the cylinders are of sheet steel of o•o4in. thickness.

Naturally, the whole structure is suitably stiffened. The cylinders are rotated by means of electromotors, which will give the towers a speed of 125rev. per minute. Circumferential speed is approximately 6oft. per sec., and the power required to rotate the towers is nine horse-power. For working the plant one man only at the switch-board is required. By altering the circumfer ential speed of one or other of the cylinders the operator can correspondingly change the pressure exerted by the wind upon this, and so alter the vessel's course. When cruising, changing the wind side can be effected solely by the towers, when the ship can be stopped and driven astern.

The Magnus-effect.

The explanation of the phenomenon of the rotor ship may be traced to the so-called Magnus-effect, ex plained in 1853 by Prof. Magnus of Berlin, who found that a special power is exerted by an air current upon a revolving cylin der. The explanation of the reason for this effect was found, after more than 20 years of investigation, to be briefly as follows : When a cylinder revolves, the nearest stratum of air revolves with the cylinder, owing to the friction of the cylinder being much greater than the friction of the air molecules against each other. The nearest stratum induces the next one also to revolve, but, naturally, this is done at a much slower speed. In the same manner the strata lying more distantly from the cylinder are moved more and more slowly, until at a certain distance the in fluence stops. If such a rotating cylinder is impinged upon by an air current, the speed of which is slower than the circumferential speed of the cylinder, the streamlines are directed, so that at one side the air is rarefied by the frictional effect of the cylinder, and at the other side it is compressed. These changes of pressure are the causes of the Magnus-effect, and they create a power in a direction away from the side of the rarefied air, and through the centre of the densest air patch, or side in which the streamlines are compressed. Actually the real direction of the power is not always at right angles to the wind direction, but diverges in a measure which is dependent on the speed ratio of the wind cur rent to that of the circumferential speed of the cylinder.

The sea-going trial of the rotor ship "Buckau," from Germany to the Forth, was claimed to be successful, but little has since been heard of the subject.

See Marine Engineer Officers' Magazine (Jan. 1925).

(F. J. D.)