\ | Theory / | \ | 54.00 | 45.00 | | | | --------------+--------------------+----------+----------+---------- In each case the unit of pressure is ¼ oz.

In the first trial there was a distance of 1½ in. between the jet and point of its contact with the plate, while in the second trial this space was diminished to ½ in. It will be noticed that as this distance increases we have augmented pressures, and these are not due, as might be supposed, to increase of head, which is practically nothing, but they are due to the recoil of a portion of the stream, which occurs increasingly as it becomes more and more broken up. These alterations in pressure can only be eliminated when care is taken to measure that only due to impact, without at the same time adding the effect of an imperfect reaction. Any stream that can run off at all points from a smooth surface gives the minimum of pressure thereon, for then the least resistance is offered to the destruction of the vertical element of its velocity, but this freedom becomes lost when a stream is diverted into a confined channel. As pressure is an indication and measure of lost velocity, we may then reasonably look for greater pressure on the scale when a stream is confined after impact than when it discharges freely in every direction.

Experimentally this is shown to be the case, for when the same oblong jet, discharged under the same conditions, impinged vertically upon a smooth plate, and gave a pressure of 71 units, gave 87 units when discharged into a confined right-angled channel. This result emphasizes the necessity for confining streams of water whenever it is desired to receive the greatest pressure by arresting their velocity. Such streams will always endeavor to escape in the directions of least resistance, and, therefore, in a turbine means should be provided to prevent any lateral deviation of the streams while passing through their buckets. So with screw propellers the great mass of surrounding water may be regarded as acting like a channel with elastic sides, which permits the area enlarging as the velocity of a current passing diminishes. The experiments thus far described have been made with jets of an oblong shape, and they give results differing in some degree from those obtained with circular jets. Yet as the general conclusions from both are found the same, it will avoid unnecessary prolixity by using the data from experiments made with a circular jet of 0.05 square inch area, discharging a stream at the rate of 40 ft. per second.

This amounts to 52 lb. of water per minute with an available head of 25 ft., or 1,300 foot-pounds per minute. The tubes which received and directed the course of this jet were generally of lead, having a perfectly smooth internal surface, for it was found that with a rougher surface the flow of water is retarded, and changes occur in the data obtained. Any stream having its course changed presses against the body causing such change, this pressure increasing in proportion to the angle through which the change is made, and also according to the radius of a curve around which it flows. This fact has long been known to hydraulic engineers, and formulae exist by which such pressures can be determined; nevertheless, it will be useful to study these relations from a somewhat different point of view than has been hitherto adopted, more particularly as they bear upon the construction of screw propellers and turbines; and by directing the stream, AB, Fig. 3, vertically into a tube 3/8 in. internal diameter and bent so as to turn the jet horizontally, and placing the whole arrangement upon a compound weighing machine, it is easy to ascertain the downward pressure, AB, due to impact, and the horizontal pressures, CB, due to reaction.

In theoretical investigations it may be convenient to assume both these pressures exactly equal, and this has been done in the paper "On Screw Propellers" already referred to; but this brings in an error of no importance so far as general principles are involved, but one which destroys much of the value such researches might, otherwise possess for those who are engaged in the practical construction of screw propellers or turbines. The downward impact pressure, AB, is always somewhat greater than the horizontal reaction, BC, and any proportions between these two can only be accurately ascertained by trials. In these particular experiments the jet of water flowed 40 ft. per second through an orifice of 0.05 square inch area, and in every case its course was bent to a right angle. The pressures for impact and reaction were weighed coincidently, with results given by columns 1 and 2, Table II.

FIG. 3

FIG. 4

 Table II. - Impact and Reaction in Confined Channels. -----------------------------+-------+---------+----------+-------

Number of column. | 1 | 2 | 3 | 4

-----------------------------+-------+---------+----------+-------

Description of experiments. |Impact.|Reaction.|Resultant.| Angles

| | | | ABS.

-----------------------------+-------+---------+----------+-------

Smooth London tube, 1¾ in. | 71 | 62 | 94.25 | 49°

mean radius. | | | |

| | | |

Rough wrought iron tube, | 78 | 52 | 98.75 | 56.5°

1¾ in. | | | |

| | | |

Smooth leaden tube bent to a | 71 | 40 | 81.5 | 60

sharp right angle. | | | |

-----------------------------+-------+---------+----------+------ 

The third column is obtained by constructing a parallelogram of forces, where impact and reaction form the measures of opposing sides, and it furnishes the resultant due to both forces. The fourth column gives the inclination ABS, at which the line of impact must incline toward a plane surface RS, Fig. 3, so as to produce this maximum resultant perpendicularly upon it; as the resultant given in column 3 indicates the full practical effect of impact and reaction. When a stream has its direction changed to one at right angles to its original course, and as such a changed direction is all that can be hoped for by ordinary screw propellers, the figures in column 3 should bear some relationship to such cases. Therefore, it becomes an inquiry of some interest as to what angle of impact has been found best in those screw propellers which have given the best results in practical work. Taking one of the most improved propellers made by the late Mr. Robert Griffiths, its blades do not conform to the lines of a true screw, but it is an oblique paddle, where the acting portions of its blades were set at 48 deg. to the keel of the ship or 42 deg. to the plane of rotation.