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196 ° THe Marine REVIEW REPULSION REPULSION UNITS OF RESISTANCE Dy I as bee oS SCALE FOR FORCE ! | \ Fia.5. FORCES, FORWARD AND AFT, ACTING UPON: -MODEL 858 WHEN PASSING MODEL 866 EXPRESSED AS FRACTIONS OF THE TOTAL RESISTANCE . DISTANCE OF CENTER LINES APART = 3.90 FEET. : di SPEED 2 TO 3 KNOTS 2 SCALE FOR FORCE IN UNITS OF RESISTANCE AITRACTION ------_ ATTRACTION : LH ie oO % fe INT "BSB L AFT OF BOW POINT "2071 AFT4 Bow 9 8 ? 6 5 4 ASTERN we shall have both at bow and stern forces pulling the vessel bodily to port, and probably this will be suffi- ciently great to bring the vessels to- gether regardless of the rudder ac- tion. Loe In the experiments the models were not allowed to obey the forces set 'up, being compelled to remain paral- lel to one another. In the case of a vessel actually overtaking another, the conditions would be _ different, since the vessel would always re- spond to the forces corresponding to its position, unless neutralized by the action of the rudder. Fig. 3 showing the: action upon model 858 when abreast of model 866 and at some distance from it indi- cates that the forces involved fall off quite rapidly as the vessels' get farther apart. The forces are, how- ever, quite appreciable when the ves- sels are as much as one-third of their length apart. In considering the application of these results to cases arising in prac- tice it has already been pointed out that in practice vessels approaching one another closely are usually in shallow channels, where the forces may be expected to be greater than in deep water. Another difference is the fact that actwal vessels are self- propelled and it is frequently sup- posed that the suction of the pro- peller has much to do with the pheno- mena produced. In this connection, however, it should be pointed out that we have some line upon what the suction of the propeller can do from our knowl- edge as to the thrust deduction on actual vessels. The thrust deduction of a vessel is simply the suction of 3 2 ; 0 . 2 fi SCALE FOR POSITION IN FRACTIONS OF LENGTH « its own propeller upon the after part. Now, except for very full single screw vessels, the thrust deduction will seldom amount to as much as twenty per cent of the resistance. This being the case, considering Figs. 2 34 and 5, it is dificult to see how the Suction of the propeller upon the vessel at an appreciable distance can be 45° much as four or five per cent of the propeller thrust or the resist- ance, while the forces found from the bare model reactions are very much farger, This, of. course, does. not apply to the' wash from the propel- ler, but the wash is restricted to a comparatively narrow belt immedi- ately astern and hence cannot /be said to play an important part in suction phenomena. In conclusion I desire to record that the experiments upon which this paper is based' were carried out under tae direct, charee:-of Mr. 1. F. He- wins assisted by Mr. George Thorne, both of the Model Basin staff. Discussion on Taylor's Paper. Capt. George A. White.--I have had some talk with Mr. Taylor on suction and I think it the beginning of a most important discussion. Along with the question of suction comes the question of wave reaction. Mr. Taylor does not seem to have considered the displacement wave. The water moves faster. between the ves- sels and this has a tendency to force them together. - Vessels have a ten- dency to yaw in shallow water. The difficulty of shoal water navigation cannot be too strongly dwelt upon by the naval architect. The greater part of our commerce is carried on in shal- low. water. = ae 4 5 6 we 8 9 1. AHEAD Mr. J. J. Lynn--This paper appar- ently has not taken in the question of currents. I have had 30 years' obser- vation of the action of vessels passing 'in. a strone current at Port Huron, There seems to be a tendency for the stern to go to port. and most of the collisions that occur are near the boilers. What occurs in actual prac- tice is certainly at variance with the conclusions of this paper. Commander W. P. White.--It seems a pity that the suction of the screw is not demonstrated by experiments. Mr. Hermon A. Kelley, general, coun- sel for the Pittsburg Steamship Co., thinks that. the screw. effect is much greater than it has been given credit for. Whatever the cause, we find by actual experience the suction is great- er on a steamer than if she is being towed. In certain parts of St. Mary's river the government will not permit vessels to pass even though the chan- nel is 600 ft. wide, the current is so strong. I would suggest that Mr. Kelley be invited to express his opin- ion on this subject. He has proved in court that rules which may be fol- lowed safely in deep water navigation cannot be practiced in. certain chan- nels of the lakes without causing trou- ble, even though fundamentally sound. John Craig--Harvey D, Goulder has tried more collision cases than any one else and is the father of suction on the lakes. Let us hear from him. Harvey D. Goulder--In practical experience we find that the force of suction comes into play as we say, the ship smells the bottom, When a screw steamer gets over near the bank her stern will sheer to the bank. We don't find that in marked degree except where she has a screw. When when, Pt aie ah ata