NOTES ON LIFE-SAVING APPLIANCES. 139 



particular advantage in this respect, since in its case the boat will strike against the 

 cast-steel quadrant or the davit arm itself, the blow being transmitted directly to 

 the screw and guide shaft. The blow may also occur when the guide sleeve is in 

 the middle between the ends of the screw and guide shaft, a position in which they 

 are least capable of sustaining it without injury. However, both constructions 

 are so substantial that there is really no serious danger of injury from any blow 

 likely to be received in this way. 



The next point discussed is the comparative effort on the crank for the quadrant 

 and fixed-pin types. It is a fairly simple proposition to figure static forces on an 

 apparatus of this kind if the forces are in one plane, but when it comes to figuring 

 frictional forces and making proper assumptions for coefficients of friction, the 

 problem is much more difficult, and the expressions given in the paper appear quite 

 too simple to be true. Probably they are not true of the turning effort required 

 on the crank of a sheath-screw davit, for all the sets installed have been tested and 

 found satisfactory, the empty boat or equivalent weight being readily swung out 

 with one man on each crank, in a little over a minute's time. It is quite true 

 that the quadrant-type is somewhat easier to start outboard than the fixed-pin type, 

 on account of the more nearly downward direction of the boat from the start, but 

 this is accomplished at the sacrifice of the feature that the boat shall be naturally 

 lifted from, and over the chocks, and makes some special form of tumbling chocks 

 always necessary with the quadrant type. Theoretically this type of davit ought 

 almost to roll out of itself, if the ship is on an even keel, but the fact that some effort 

 is required on the crank all of the way, is due to the introduction of numerous forces 

 and frictional resistances, many of which are not mentioned in the paper or included 

 in the formula. The forces are not in one plane, but on the contrary have several 

 different planes and lines of action. By examining the construction of the quadrant 

 davit illustrated in the paper, it will be noted that the davit arm is carried on an 

 overhung pin cast in one piece with the sleeve embracing the guide shaft and the 

 screw, the center line of the pin being about on the same level as the center line of 

 the guide shaft, and the center line of the screw being some inches below and about 

 vertically under the guide shaft. The pressure of the davit arm on the pin there- 

 fore exerts a considerable moment on the guide sleeve in one direction, which in 

 turn being transmitted to the screw produces a considerable moment in another 

 direction. None of the forces are in line with each other, and even the pressure 

 transmitted by the davit to the pin is not a simple pressure, as there is quite a 

 twisting moment introduced by the fore-and-aft overhang of the davit. All these 

 forces must produce considerable frictional resistance, not to mention the friction 

 of the rough rolling surface and the gear teeth where the quadrant meshes with the 

 rack on the deck. 



The forces on the screw-operated pin davit are somewhat simpler, being more 

 nearly in line, but even in this case figures are practically worthless, and seem useless 

 in the face of an actual trial with the loaded boat or an equivalent weight. The 

 paper estimates that the quadrant type is also slightly easier to rig in, but it is very 



