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Reply to the remarks made by B. F. Isherwood, Esq., Chief Engineer, U. S. Navy, on nystrom's screw propeller
138
.Mechanics, Physics, and Chemistry.
¢retion, and we can only judge from the havoc and devastation that ensues as to the immediate cause of the event. From this it follows that, in many of the explosions on record, few if any, of the real circumstances of the case are made known, and we are left to draw conclusions from the appearance of the ruptured parts, and the terrific consequences which too frequently follow as a result. This want of evidence as to the precise condition of a boiler with all its valves and mountings, preceding an explosion, is much to be -regretted, as it causes a degree of mystery to surround the whole transaction; and the vague and sometimes inaccurate testimony of witnesses, but too often baffles all attempts at research, and creates additional cause of alarm to all those exposed to the occurrence of similar dangers. To be Continued. For the Journal of the Franklin Institute.
Reply to the Remarks made by B. F. Isherwood, Esq., C/~iefEnglneer, U. ~q. 3Wavy, on 3Wystrom's Screw Propeller. In the July number of the Journal, I find a long article, headed "Remarks on Nystrom's Screw Propeller," written by B. F. Ish~rwood, Esq., Chief Eng. U. S. Navy. With the permission of the Editor, I will claim 1he privilege of an answer--which self defence and common justice demand. J . W . NYSTROM. Mr. Isherwood asks, in his first remark, "What would be the nature of that Joss of effect, if any existed? It would be shown in the increased slip of ltte screw, for the following reason: the water being thrown off r~tdiatly in all directions from the axis, by the centrifugal force communicated to it by the revolving screw, there would he a vacuumabout the axis, provided the ceIitrifugal force forming the vac~tum exceeded the force with which the surrounding water would flow in to ill] i t , and the. resistance to the screw would be decreased in proportion to the extent of this vacuum; that is, the slip of the screw would be increased, The loss
of effect, therefore, due to this cenb~fugal force, would be furnished by the increased slip of the screw. But if the water flowed into the vacuum as fast as it was formed, the resistance to t l l screw would evidently remain the same as though there were no ceatt'ifugal force in action; and this is what actually occurs in practice." This may be the fact with straight-bladed propellers,,but not when the propeller is centripetal. The water flowing into the vacuum as fast as it forms, and immediately thrown out by the centrifugal force, it will act as if there were no centrifugal force. Let us look at it a little closer. Before the water comes in contact with the Propeller, it has no movement; but as far as the propeller gives it movement, the effect is delivered to the water, no matter in what direction it may be. The helicoid surface of the propeller blades acts to throw the water backa'ards in direction parallel to the centre line of the propeller; let its magnitude and direction be represented by the line e b, plate IX.; the oentrifugal force of the water at the same time acts outwards in the • ,n of the radii; let the line a c represent its magnitude and direction, ie line a b will represent the magnitude and directionof the total
Reply to Remarks on 2¢'ystrom's Screw Propeller.
139
effect delivered from the propeller. The line c b represents the useful effect acting to propel the vesseI, and'the line a c represents the useless effect caused by the Centrifugal force. If this is not clear,enough., I would ask, Is it possible that the propeller can give the water a mouon without expending some of its effect? And if that motion of the water is useless in propelling the' vessel, the effect expended on it tnust he lost~ it may belitttle or much; this is in regard to the straight-bladed or common propellers. I stated, in the specification of the centripetal propeller (Vol.xxb p. 350,) that the effect which is gained by counteracting the centrifugal force will be found by the angle th_e centrifugal force is acting upon. It seems Mr. Isherwood has not examined if there exists such an angle; now the question is, whether the centrifugal force acts to propel the vessel~ where the propeller is centripetal. Let a plane cut the propeller through its centre line, the section of the blades will be a curved line of the same nature as the spiral which forms the eneratrix for the screw; that is, the angles v are proportionable at the same dxstance from the centre, and dept.nds on the centrifugal force (in straight bladed propellers this section will be a straight line in the direction of the radii). When the propeller is revolving, the water between the blades is acted by the centrifugal force upon an inclined plane, represented by the section of the blades (see plate); let the line a c represent the magnitude and direction of the centrifilgal force in the point a, draw through the point a the tangent t b, draw from the point c a line e d at right angles to t b, then the line e d represents the magnitude of a force acting at right angles on the inclined plane at the point a; from d draw a line d e at right angles to a c, then'the line d e i'epresents the magnitu~le of the force useful to Tropel the vesse2'~ and caused by the centrifugal force. Were it necessary, ~'ords could be multiplied and pages filled in the elucidation of the subjedt; and the consequent pressure of water about it; and then the slips found in practice, ranging from 15 to 30 per cent. for maximum, and it will be seen how erroneously the effect of any centrifugal force must be exaggerated, to make it productive of a vacuum at the axis of the's(rrew." We will look at this a moment. Suppose a propeller of 13 feeLin diameter (whick Mr. Isherwood calls a mean size) making 65 revOlii'iibiis per minute, and a mass of water weighing 50 pounds revolve with the propeller in the periphery, the centrifugal forcewill be C = 4 × 3'142 x 652× 6"5 × 50=470 pounds; 32"2. × 602 the upward pressure of that 50 lbs. of water at the depth of 13 feet will be ~ 50 2 0.785 × 62"5] x 0"785 × 62-5 x 6"5= 322 pounds. consequently, the centrifugal force is sufficient to make a vacuum in the centre of the propeller, because the centrifugal force is 148 pounds more than the upward pressure; here the slip is not taken into consideration;
140
dllechanics, Physics, and Chemistry.
then the centrifilgal force will be less. But instead of that~ the centrifugal force is multiplied within the entire extremity of the periphery of the propeller; the flowing in of the water to fill up the vacuum occurs only m the cefitre~ or around the hub~ and there the stern post occupies considerable space. But if the centrifugal force is not sufficient to cause a perfect vacuum in the hub of the propeller, it is sufficient to give the water a corresponding irreffalar and useless motion, which is proved to be a loss of effect. Mr. Isherwood says: "With a straight generatrix touchir~g the axis~ the lateral component of the oblique surface of the screw is tangential to a cylinder~ ~ &c. What has this to do with the centrifugal force ? But if that is the reason why the centrifugal force acts as if there was no centrifugal forc% the Chief Engineer's philosophy is certainly above the comprehension of common minds. Again, Mr. Isherwood says: " T h e same slip, or, in other words, the same resistance is obtained from the water, with screws of equal diameter~ pitch, and length~ whether they have straight, inclined~ or curved generatrices; for the curved generatrix is obviously but a modification of the inclined generatrix2 ~ Only a mod~cation of the generatrix. That is no reason at all why the propeller should act equal in the water~ if making the screw of iron and the nut also of iron~ when screwed together, it will act equal with any generatrix of the screw. But the action of the propeller in the water is different; the resistance I have proved, in the foregoing, that by counteracting the centrifugal foroe, an additional resistance---- d e will be gained (see plate,)and acts direct inpropelling the vessel. Mr. Isherwood further states: " I t is an inclined generatrix, whose inclination momentarily changes. It is therefore governed by the same principles." Of course it is; I did not say it was on any other principle. Mr. I. speaks of "the practical disadvantage and loss of labor attending the use o[ a curved generatrix~ though none theoretically." Is it the manual labor required in making the propeller ? or is it the manual labor to use the propellgr when it is in the water? or is it the labor of the .engine which is lost? On what principle.can it be proved that the friction Is greater in curved bladed propellers; and if it is so, does it not prove that the resistance is greater in propelling the vessel? Mr. Isherwood says: "Believing that sufficient has been written to put this subject in a true light~" &c. We are obliged' to the U. S. Chief Engineer for the clearness of his light, amt sincerely hope its brilliancy will not serve only to make darkness visible; aml as he finds it needful for the fnrther enlightenment of the readers of the Journal, to notice only a few of our statementsj we will listen patiently to the conclusion of the whole matter. First, that the "water forced backwards by the blades of the screw is in directions at right angles to the surface of those blades." I did not say any such thing in my specification. I did say in direction parallel to the azis of the propeller; not in direction at right angles to the surface of those blades; but if I had~ that does not change the shape or operation of the propeller. Mr. Isherwood quotes my statements in regard to ocean steamers; he
.Mechanics, Physics, and Chemistry.
¢retion, and we can only judge from the havoc and devastation that ensues as to the immediate cause of the event. From this it follows that, in many of the explosions on record, few if any, of the real circumstances of the case are made known, and we are left to draw conclusions from the appearance of the ruptured parts, and the terrific consequences which too frequently follow as a result. This want of evidence as to the precise condition of a boiler with all its valves and mountings, preceding an explosion, is much to be -regretted, as it causes a degree of mystery to surround the whole transaction; and the vague and sometimes inaccurate testimony of witnesses, but too often baffles all attempts at research, and creates additional cause of alarm to all those exposed to the occurrence of similar dangers. To be Continued. For the Journal of the Franklin Institute.
Reply to the Remarks made by B. F. Isherwood, Esq., C/~iefEnglneer, U. ~q. 3Wavy, on 3Wystrom's Screw Propeller. In the July number of the Journal, I find a long article, headed "Remarks on Nystrom's Screw Propeller," written by B. F. Ish~rwood, Esq., Chief Eng. U. S. Navy. With the permission of the Editor, I will claim 1he privilege of an answer--which self defence and common justice demand. J . W . NYSTROM. Mr. Isherwood asks, in his first remark, "What would be the nature of that Joss of effect, if any existed? It would be shown in the increased slip of ltte screw, for the following reason: the water being thrown off r~tdiatly in all directions from the axis, by the centrifugal force communicated to it by the revolving screw, there would he a vacuumabout the axis, provided the ceIitrifugal force forming the vac~tum exceeded the force with which the surrounding water would flow in to ill] i t , and the. resistance to the screw would be decreased in proportion to the extent of this vacuum; that is, the slip of the screw would be increased, The loss
of effect, therefore, due to this cenb~fugal force, would be furnished by the increased slip of the screw. But if the water flowed into the vacuum as fast as it was formed, the resistance to t l l screw would evidently remain the same as though there were no ceatt'ifugal force in action; and this is what actually occurs in practice." This may be the fact with straight-bladed propellers,,but not when the propeller is centripetal. The water flowing into the vacuum as fast as it forms, and immediately thrown out by the centrifugal force, it will act as if there were no centrifugal force. Let us look at it a little closer. Before the water comes in contact with the Propeller, it has no movement; but as far as the propeller gives it movement, the effect is delivered to the water, no matter in what direction it may be. The helicoid surface of the propeller blades acts to throw the water backa'ards in direction parallel to the centre line of the propeller; let its magnitude and direction be represented by the line e b, plate IX.; the oentrifugal force of the water at the same time acts outwards in the • ,n of the radii; let the line a c represent its magnitude and direction, ie line a b will represent the magnitude and directionof the total
Reply to Remarks on 2¢'ystrom's Screw Propeller.
139
effect delivered from the propeller. The line c b represents the useful effect acting to propel the vesseI, and'the line a c represents the useless effect caused by the Centrifugal force. If this is not clear,enough., I would ask, Is it possible that the propeller can give the water a mouon without expending some of its effect? And if that motion of the water is useless in propelling the' vessel, the effect expended on it tnust he lost~ it may belitttle or much; this is in regard to the straight-bladed or common propellers. I stated, in the specification of the centripetal propeller (Vol.xxb p. 350,) that the effect which is gained by counteracting the centrifugal force will be found by the angle th_e centrifugal force is acting upon. It seems Mr. Isherwood has not examined if there exists such an angle; now the question is, whether the centrifugal force acts to propel the vessel~ where the propeller is centripetal. Let a plane cut the propeller through its centre line, the section of the blades will be a curved line of the same nature as the spiral which forms the eneratrix for the screw; that is, the angles v are proportionable at the same dxstance from the centre, and dept.nds on the centrifugal force (in straight bladed propellers this section will be a straight line in the direction of the radii). When the propeller is revolving, the water between the blades is acted by the centrifugal force upon an inclined plane, represented by the section of the blades (see plate); let the line a c represent the magnitude and direction of the centrifilgal force in the point a, draw through the point a the tangent t b, draw from the point c a line e d at right angles to t b, then the line e d represents the magnitude of a force acting at right angles on the inclined plane at the point a; from d draw a line d e at right angles to a c, then'the line d e i'epresents the magnitu~le of the force useful to Tropel the vesse2'~ and caused by the centrifugal force. Were it necessary, ~'ords could be multiplied and pages filled in the elucidation of the subjedt; and the consequent pressure of water about it; and then the slips found in practice, ranging from 15 to 30 per cent. for maximum, and it will be seen how erroneously the effect of any centrifugal force must be exaggerated, to make it productive of a vacuum at the axis of the's(rrew." We will look at this a moment. Suppose a propeller of 13 feeLin diameter (whick Mr. Isherwood calls a mean size) making 65 revOlii'iibiis per minute, and a mass of water weighing 50 pounds revolve with the propeller in the periphery, the centrifugal forcewill be C = 4 × 3'142 x 652× 6"5 × 50=470 pounds; 32"2. × 602 the upward pressure of that 50 lbs. of water at the depth of 13 feet will be ~ 50 2 0.785 × 62"5] x 0"785 × 62-5 x 6"5= 322 pounds. consequently, the centrifugal force is sufficient to make a vacuum in the centre of the propeller, because the centrifugal force is 148 pounds more than the upward pressure; here the slip is not taken into consideration;
140
dllechanics, Physics, and Chemistry.
then the centrifilgal force will be less. But instead of that~ the centrifugal force is multiplied within the entire extremity of the periphery of the propeller; the flowing in of the water to fill up the vacuum occurs only m the cefitre~ or around the hub~ and there the stern post occupies considerable space. But if the centrifugal force is not sufficient to cause a perfect vacuum in the hub of the propeller, it is sufficient to give the water a corresponding irreffalar and useless motion, which is proved to be a loss of effect. Mr. Isherwood says: "With a straight generatrix touchir~g the axis~ the lateral component of the oblique surface of the screw is tangential to a cylinder~ ~ &c. What has this to do with the centrifugal force ? But if that is the reason why the centrifugal force acts as if there was no centrifugal forc% the Chief Engineer's philosophy is certainly above the comprehension of common minds. Again, Mr. Isherwood says: " T h e same slip, or, in other words, the same resistance is obtained from the water, with screws of equal diameter~ pitch, and length~ whether they have straight, inclined~ or curved generatrices; for the curved generatrix is obviously but a modification of the inclined generatrix2 ~ Only a mod~cation of the generatrix. That is no reason at all why the propeller should act equal in the water~ if making the screw of iron and the nut also of iron~ when screwed together, it will act equal with any generatrix of the screw. But the action of the propeller in the water is different; the resistance I have proved, in the foregoing, that by counteracting the centrifugal foroe, an additional resistance---- d e will be gained (see plate,)and acts direct inpropelling the vessel. Mr. Isherwood further states: " I t is an inclined generatrix, whose inclination momentarily changes. It is therefore governed by the same principles." Of course it is; I did not say it was on any other principle. Mr. I. speaks of "the practical disadvantage and loss of labor attending the use o[ a curved generatrix~ though none theoretically." Is it the manual labor required in making the propeller ? or is it the manual labor to use the propellgr when it is in the water? or is it the labor of the .engine which is lost? On what principle.can it be proved that the friction Is greater in curved bladed propellers; and if it is so, does it not prove that the resistance is greater in propelling the vessel? Mr. Isherwood says: "Believing that sufficient has been written to put this subject in a true light~" &c. We are obliged' to the U. S. Chief Engineer for the clearness of his light, amt sincerely hope its brilliancy will not serve only to make darkness visible; aml as he finds it needful for the fnrther enlightenment of the readers of the Journal, to notice only a few of our statementsj we will listen patiently to the conclusion of the whole matter. First, that the "water forced backwards by the blades of the screw is in directions at right angles to the surface of those blades." I did not say any such thing in my specification. I did say in direction parallel to the azis of the propeller; not in direction at right angles to the surface of those blades; but if I had~ that does not change the shape or operation of the propeller. Mr. Isherwood quotes my statements in regard to ocean steamers; he