CORRESPONDENCE.

THE ACTION OF SCREW-PROPELLERS.

To the Editor of the "Nautical Magazine." SIR,-As the action of screw-propellers is now attracting a good deal of attention among seamen, I venture to ask you to insert a remark on one point of your correspondent "W.C.'s" useful notes, viz., on the statement that "a right-handed propeller going continuously ahead will turn a vessel round faster under the starboard than under the port helm."

This is, I think, not so supported by evidence as to be acceptable as a general rule, and as I have served in a right-handed screw-steamer, where we carried starboard helm, I think it incorrect, but the fact that a difference of opinion exists shows the necessity for further experiment.

When a vessel has not attained uniform motion under the action of her engines, so that the screw blades above and below the axis of the propeller are working with great slip, doubtless, unless the upper blades are deeply immersed, the lower ones experience most resistance, and deliver most water sternward, hence, if calm and smooth, the vessel would turn from a stationary position fastest to port; but when uniform motion has been attained, it appears highly probable, from the writings of Professor Osborne Reynolds, and others, that the wake motion and dead water cause the blades above the axis to be working with considerable slip, while those below are working with very much less; hence, there would be a resultant lateral thrust transmitted to the screw shaft, in a direction opposite to that in which the upper blade was moving or tending to turn the bow to starboard.

It seems worth noticing that if the screw be reversed while the vessel have headway and wake motion, the same causes would transfer the superiority of thrust to the lower blade, hence, as in s.s. Hankow, when the engines were reversed at a speed of ten knots and helm amidships the ship turned to starboard, and showed your correspondent's second proposition to be true. Your obedient servant,

May, 1879.

S. L.

TIDE TABLES FOR JUNE, 1879.

Also Ports of Reference for the Constants in the next Table.

11 22 11 50 10 15 10 43 0 32 1 6 6 18 6 48 1 38 2 9 6

5 37 2 15 2 84 1 9 1. 57 6 17 2 53 3 12 1 50 21 37 6 57 3 31 3 50 2 29 2

8 20 8 46 3 46 4 8 42
9 9 9 32 4 44 5 10 9
22 4 45 9 54 10 16 5 35 5 5810
10 11 0 6 21 6 4210 491

7 17 7 36 4 9 4 27 3 6 8 24 6 22 6 41- 0 7 42 8 0

7 18 7 18 7 47 0 8 0 33 6 38 7

8 16 8 17 8 47 1 5 1 37 7 33 8 14 43 5 5 0 7 0 36 9 9 17 9 44 2

49 10 7 10 29 2 59 8 23 9 20 9 43 6 8 6 29 1 53 2 14 10 51 11 13 3 45 4

531

TIDAL CONSTANTS

FOR VARIOUS BRITISH, IRISH, AND EUROPEAN PORTS.

By applying the Tidal Constant of the place, according to its sign (+ add, sub.), to the time of high water on the given day at the port of reference, you have the time of high water at the place sought.

Beachy head & Rye bay

WEATHER FORECAST FOR JUNE, 1879.

THE CURRENTS OR TENDENCY OF THE AIR OVER THE BRITISH ISLANDS FOR THE MONTH OF JUNE, 1879.

NOTE.-Sun's gradients at first N. Easterly, then S. Easterly succeeded by S. Westerly about the beginning of July.

1. The Table indicates

REMARKS.

Strong N. Easterly tendency from the 1st to the

4th

Probable Winds.

N. Wly. to N. Ely. in the N.
S. Wly. to Sly. in the South.

Strong S. Easterly tendency from the 5th to the 9th, N.Ely. and S.Wly.

دو

S. Westerly

N. Westerly

10th 19th

N. Easterly

18th, light S.Wly. to S.Ely. 27th, N.Ely.backing to S.Wly. 30th, N.Wly. to Wly.

2. Moon going South from the 1st to the 4th.

3. Change from the Westerly to the Easterly tendency about the 1st.

1st to the 10th, Stormy: being period of Moon's Easterly tendency.
It would be noticed that the corresponding period
of Moon's Easterly tendency for April (8th to the
15th) and May (5th to the 14th) were stormy.
During this period the strongest gale will pro-
bably occur on the 1st or 2nd, from the N.W. to
the N.E., if pressure one or two days previously
relatively high to the S.W., or, what is the same
thing, if wind Westerly to North-Westerly, but
if pressure relatively high to the North and East
then gale will probably blow from S.E. to N.E.
19th, Generally fine with occasional thunderstorm.
27th, Changeable.

11th 20th

28th 30th, Stormy. Moderate Westerly to North-Westerly gale probable on 28th or 29th.

5. The gale predicted on the 6th of May blew from N.W to N.E. instead of from S.E. This was caused by pressure being previously relatively high to the South and West. It must be understood that the General Forecast is based upon an examination of past weather, and being necessarily imperfect, I would deprecate attention being paid to it to the exclusion of the Table of Currents which is really the more important.

D. D.

MARINE INVENTIONS.

Monthly List of Patents-Communicated by Messrs. Wm. P. Thompson & Co., British and International Patent and Trademark Agents and Consulting Engineers, 323, High Holborn, London, W.C., and 6, Lord Street, Liverpool.

ENGLISH (APPLICATIONS).

1517. Edward Lyon Berthon, Romsey, Hampshire. "Improvements in collapsing boats, canoes, and canoe-shaped boats covered with canvas."