Maritime History of the Great Lakes

Marine Record (Cleveland, OH), July 18, 1895, p. 7

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_ STOPPING, STEERING AND RESISTANCE OF FLOATING VESSELS. BY JOSEPH R. OLDHAM, N. A. AND M. E. Referring to the steering of screw steamers, Sir Na- thaniel Barnaby states that when a screw is suddenly reversed, and before the headway is off the vessel, the action of the rudder is not to be depended upon. The following is an extract from the report of the committee appointed by the British Association to investigate the effect of propellers on the steering of vessels. The committee endeavored to ascertain how far the reversing of the screw in order to stop a ship did, or did not, interfere with the action of the rudder during the interval of stopping. ‘The report states: It is found an invariable rule that during the interval in which a ship is stopping herself by the reversal of her screw, the rudder produces none ofits usual effect to turn the ship, but that under these circumstances the effect of the rudder, such as it is, isto turn the shipin an opposite direction from that in which she would turn if the screw were going ahead. The magnitude of this reverse effect is always feeble and is different for dif- ferent ships, and even for the same ship under different conditions of loading. It also appears that, owing to the feeble influence of the rudder over the ship during the interval in which she is stopping, she is then at the mercy of any other influences that may act upon her. Thus, the wind, which always exerts an influence to turn the stem or forward end of the ship into the wind, but which in- fluence is usually well under control of the rudder, may when the screw is reversed, become paramount, and cause the ship to turn in a direction the very opposite of that which is desired. Also the reversed screw will exercise an influence which increases as the ship’s way is diminished, to turn the ship to starboard or port, according as it isright or left handed, this being particularly the case when the ship is in light draught. These several influences, the reversed effect of the rudder, the effect of the wind, and the action of the screw, will determine the course the ship takes during the interval of stopping. They may balance, in which case the ship will go straight on, or any one of the three may predominate and determine the course of the ship. The pressure on the screw blade which is moving in the same direction as that in which the stern of the ves- sel is turning is increased, while on the blade moving in the opposite direction the pressure is reduted, that is to say, if the screw is right-handed and the vessel is under port helm, the stern, consequently, traveling to port, the resistance of the lower blade which is moving towards the port side will be increased, and the resist- ance of the upper blade, which will be moving towards the starboard side, will be diminished, because the one is meeting the water, and the other is receding from it. The change of pressure will be proportional to the square of the angular velocity of the stern. The irreg- ular pressure causes the vibration frequently noticed when a screw vessel is rapidly turning. I think the opinion, or I may say the conviction, is not tuncommon amongst those connected with the working of high speed, fine lined steamers, that the great power of their engines will generally enable them to arrest the forward motion of the hull in less time and space than is required by comparatively low speed vessels, and reasoning thus it is sometimes assumed that their in- ability for rapid maneuvering through high efficiency of the rudder power alone is not of so great importance. This assumption, however, is not borne out by some careful experiments made under the directions of Prof. Osborne Reynolds, to discover the best rules for the guidance of ship captains in endeavoring to avoid col- lisions. From this it appears that the distance required by a screw steamer to bring herself to rest from full speed by the reversal of her screw, is independent, or nearly so, of the power of her engines, but depends upon the size and build of the ship, and generally lies between four and six times the ship’s length. As to the effect of shallow water on the steering of vessels, Mr. White states that it is a matter of common experience that ships which are perfectly under control in deep water, steer wildly and require careful watching when they are navigating in very shallow waters, and have their keels only a few feet from the bottom. This is well understood by lake shipmasters and others, and as our most intricate navigation is in shallow waters, it would seem to require no further argument to show that high efficiency in steering by a rudder is of the utmost importance on these rivers and lakes. ; As regards the form or contour of rudders, it may assist in arriving at sound conclusions on this head if I touch briefly on the resistance offered by water to the motion of floating bodies. The effect of hydrostatic rudder. THE MARINE RECORD. pressure on floating bodies appears to be very imper- fectly understood. For instance, with regard to sur- face friction of a ship moving through the water (and of course the rudder is part of a ship) some authorities ‘maintain that a square foot of surface, say on the side of the keel of a deep ship, meets with no greater fric- tional resistance than an immersed square foot near the load water line, whilst others assert that the friction augments in the same ratio as the hydrostatic pressure. Mr. White says: ‘‘ If there be no surface disturbance, the resistance at any speed is independent of the depth, and this is equally true of the resistance to direct and oblique motion of a body through water.” Col. Beaufoy ascertained the resistance of a plate moving normally to itself, when submerged to depths of three, six and nine feet below the surface, and found them practically identical at all the depths. On the other hand, Mr. Seaton maintains that the friction of the water per square foot of surface will depend on the pressure di- rectly, so that the resistance from a square foot near the water line is very different from one twenty feet below it. The success of torpedo boats depends almost wholly on their lightness of both hull and machinery, enabling them to do with so small a displacement that they liter- ally skim the water, and the pressuré per square foot of wetted skin is consequently very small. Unless small boats are made to float at a.very light draft they cannot be driven at high speeds, and.all experiments with fast river steamers on the Clyde and elsewhere, have shown this. From this and other experiments it would seem that the smaller the surface of the rudder at and near the keel the better. Some persons have confused hy- drostatical pressure with the dynamical conditions inci- dental to motion. The hydrostatical pressure sustained by the sides of the rudder, if held at an angle, balance one another and are quite distinct from the reaction due to change of momentum in streams having motion rel- atively to the rudder. Without motion of a ship through the water or of the water past the rudder, it can have no steering power. The check put upon streams by the rudder must produce a reaction not merely upon the rudder itself, but upon the portion of the stern post or dead-wood above the propeller aperture. This addi- tional pressure on the dead-wood willbe delivered on the side towards which the rudder is put’over it and must considerably assist the rudder in steering a steamer. When a rudder is placed at an angle with the keel line of a steamer and the streams of water impinge upon its surface, in consequence of the motion of the ship, or of the action of her propeller, the motion of these streams must be checked or diverted, and a change of momen- tum is produced which reacts upon the rudder and causes a normal pressure upon its surface and also upon the dead-wood of the screw aperture (except when the aperture is open at the top). ‘he large aperture or vacant space before the rudder, in which the screw is working, must also divert and disturb the symmetry of the stream lines and cause eddies and broken water to impinge on the rudder, thus causing the steering of the ship to be irregular and comparatively slow, or as sailors say, it causes the ship to steer wildly. For reg- ular and quick steering the smaller the screw aperture the better. The upper surface of the rudder is not more effective than the lower surface when the water flows uninterruptedly to the rudder over its total immersed depth, as in a sailing vessel, but in a screw steamer nearly the whole of the immersed surface of the rudder when the vessel is still, meets with broken water and irregular eddies when she is'at'sea, if the screw aper- ture be large and open for a considerable height above the top of the propeller blades, but when this aperture is reduced in breadth and closed in immediately above the screw propeller, the upper surface of the rudder then becomes much more efficient than the lower sur- face, because the stream lines of the water in motion flow symmetrically on until they come into actwal im- pact with the rudder, and in consequence the impulse of the water acts directly and uniformly on either side over the upper surface of the rudder, according to the direction in which it is turned; hence, it appears that the upper surface is more effective than the lower sur- face, which is directly affected by the screw propeller. BALANCED RUDDERS. By a balanced rudder I mean a rudder having a por- tion of its surface (usually about one-third) before the axis about which it rotates. Such rudders are not un- common on these lakes, indeed they may be be the common type of rudder on the inland seas; but not- withstanding the fact that such rudders are more effi- cient, as I shall presently demonstrate, they have occa- sionally and even recently been superceded by the com- mon tramp type of rudder and by a modification of that Mr. White tells us that the balanced type of rudder has been long known; Karl Stanhope proposed its adoption in 1790, and Capt. Shuldham fitted one to a ship about seventy years ago. It was also fitted in the Great Britain in 1845 and it has been frequently fitted to government ships during the last thirty years. Various rules have been used for determining the areas of rud- ders for steamers. For lake vessels having lengths from seven to eight times the beam, the immersed sur- face of the rudder should equal to about one-fiftieth of the immersed middle line longitudinal section of the ship, the extreme breadth being not greater than one sixth of the breadth of the beam plus1 foot. This is a slight modification of Mr. Scott Russell’s rule. Mr. Macrow says: Four features chiefly affect the readiness of a ship to answer helm: 1, Time occupied in putting helm hard over; 2, rudder pressure corresponding to would be 296.3 square feet. that position ; 3, moment of inertia of ship about vertical — axis passing through the center of gravity; 4, moment. of resistance to rotation. The diameter of circle turned in has been found to vary between six and eight times the length of the ship with ordinary rudders, and from four to five times with balanced rudders, using mannal power only. With steam power the circle turned in with ordinary rudders about four times the length of the ship, and with balanced rudders less than three times the length. : Rudder areas may be proportioned as follows: At and A?’ — middle line planes of two similar ships, a! and a? the rudder areas, 11 and 12 the lengths; then al. Al ji 3 a2 A2 | 12 \ As an example, suppose a lake steamer 300 feet long and 16 feet mean draft of water has a rudder sur- face of 100 square feet, the middle line plane being 4,800 square feet. Then fora steamer 400 by 20 feet draft, having a middle line plane equal to 8,000 square feet, the rudder area would be as follows: A 8,000 1 400 ) a a 100 4,800 300 \ by which we see that the area for the large steamer This simple and accurate formula for ocean steamers does not conform to our lake practice, indeed it is just as far in excess of the surface provided by our best shipbuilders as the diam- eter of rudder heads corresponding to the hand book formula, for high speed ocean steamers, is in excess of the. diameter found to be sufficient for high power lake steamers. Let me now just say a few words about the turning of steamships and the efficiency of balanced rudders. Prof. Rankine, when investigating the point of the instantaneous axis about which a ship should begin to turn when the rudder was first put over, regarded the first action of the rudder as an impulse, and experienced seamen declare that when a steamer has headway and the helm is put over, the head turns comparatively slowly, while the stern swerves suddenly to the right or the left. As an example of the superiority of balanced rudders, consider the trials of the Minotaur and Bellerophen. The Minotaur, with eighteen men at the wheels and sixty at the relieving tackles, turned a circle in 7% minutes, 1% minutes being occupied in putting the helm over to the very moderate angle of 23 degrees. ‘The Bellerophen hada balanced rudder with 25 per cent. greater area than that of the Minotaur, but it was put over atan angle of 37 degrees, and in, about twenty seconds by eight men when the ship was: steaming at about the same speed the Minotaur had attained. By means of either hydraulic or steam power the largest rudders can be worked and put hard over in a few seconds by one man or by the master himself when ships are moving at the highest speed. It still remains true, however, that the adoption of the balanced rudders minimizes the work to be done in steering, and it has been proved repeatedly by experi- mental trials, that large, high-speed ships fitted with balanced rudders, can be safely steered by hand, which could not be so steered with an ordinary rudder. SAULT TRAFFIC FOR JUNE. Following is.the official statement of the traffic through the St. Mary’s Falls canal for the month of June, 1895, as compared with the corresponding period for the season of 1894: EAST BOUND. ITEM DESIGNATION.| June ’94, | June 795. Copperinc.. ace. Net tons 12,754 17,288 Corleone on ee Bushels..... 469,489 3g ee Building stone ....| Net tons 3,770 5,868 Miourissis 2) beast Barrels..... 1,142,965 | 1,216,472 Tron: OLeic oe. aos Net tons.... 1,217,365 | 1,489,573 Tron, Pigs sons ot Net tons.... 2,070. 2,473 Wumber M. ft. b. m, 123,968 135,286 Silver ore......... Net tones oi seer epee Wiheaticcehi sae fe Bushels..... 2,934,890 | 1,584,571 Unclassified freig’t; Net tons 21,064 30,287 Passengers... Number.... ‘| 1,899 2,480 WEST BOUND. ITEM. DESIGNATION.| June 794. | June 95. Coal, anthracite...| Net tons.... 108,220 33,166 ‘¢ bituminous..} Net tons.... 12,300 185,719 WlOur.). sas cee Barrels... .|-. sure oes ho ee eee Grain}. oss Fie Bushels 3; 20s)524748 ee 8,000 Manufactured iron} Net tons 4,391 9,311 Salt.c wis Sind atawes Barrels...... 43,107 30,888 Unclassified freig’t| Net tons.... 32,204 38,365 Passengers........| Number..... 1,828 2,134 East bound freight, net toms .-................ 1,937,526 West bound freight, net tons. -:....,..0,%. 020%; 271,520 Total. 0.06. J ae ea 2,209,046

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