148 NOTES ON LIEE-SAVING APPLIANCES. 



pounds effort on each crank was required to swing the boat in its most unfavorable 

 position, that is, going inboard. 



A clear deck with ample room to put up a standard davit is not always avail- 

 able. 



The tables and diagrams given by Mr. Forbes in his paper are not to be con- 

 fused with the Broady davit. This paper was a comparison of davits designed on 

 Mr. Norton's theory and the practical quadrant davit. Mr. Norton refuses to 

 accept the figures and data given by Mr. Forbes, but he will find it difficult to show 

 the data given incorrect. The diagrams given were arrived at in a scientific manner 

 and give exact theoretical comparative values. In the formula given, it should 

 be noticed that the values are given for friction in the screw, and other resistances, 

 tending to decrease the efficiency of the davits. 



Welin davits roll on the quadrant flanges and do not in any way sUde. The 

 weight is carried by this flange and the function of the rack is only to control the 

 swing of the arm. The friction in the rack is therefore only the ordinary spur-gear 

 friction. The arm of the Welin davit is supported sidewise by the tie-rod at about 

 one-third of its height from the lower end which is guided by the quadrant flange. 

 The Norton da\dt arm, on the contrary, is supported sidewise only at the lower end 

 and depends altogether for its steadiness on the length of pin and the location of 

 bearings. It is not supported in any way by the sleeve or screw in a sidewise direc- 

 tion. The advantage in having the arm supported at a high point as in quadrant 

 davits is obvious. The deflection in the arm is also lessened by this arrangement 

 on account of the comparatively short arm above the pivoting point. 



In the Welin davit it is impossible for the boat, in any position, to hit the screw, 

 it being protected by the top brace of the framework ; in the Norton davit screw, the 

 screw, for a considerable part of the boat's travel, is entirely unguarded against a 

 blow, and it may easily be put out of order by the slightest blow from the boat, which 

 is likely to occur in rough weather. 



In the Welin davit the screw has been designed with the utmost care. It is 

 supported by means of stationary bearings at each end, giving rigidity in all direc- 

 tions as against the screw in the Norton davit supported only at one end and then 

 by a swivel bearing. This would not cause any serious objection if the screw on 

 the Norton davit were subjected to tension only, but it is under compression through 

 about 45 per cent of the travel. It is impossible for working purposes to make the 

 nut fit absolutely snug on the screw, and the nut sleeve must be given considerable 

 clearance in the end supported by the arm. It is clear that this construction, 

 when under compression, must give a very dangerous blend in the connection 

 between the screw and the nut sleeve and this means an ever-increasing wear, bend 

 and friction. The importance of this cannot be too strongly emphasized. I main- 

 tain that all such types of davits would be out of order or have to be renewed in 

 less than five years by the general wear and tear of the numerous boat drills required. 



In the Welin davit the thrust is taken up by a soUd column, i. e., the screw, 

 and the thrust is the only stress taken up by the screw as all other stresses are taken 

 up in the tie-rod and in the rack arrangement, which are all stationary. 



