JETS.

I5 6

are 9 inelies in diameter, formed of copper pipes bent to aradius of 18 inches, and so pivoted that either end can bepresented to the discharge orifice in the side of the vessel.. Theamount of water passed through the pumps in fifteen seconds isequal to the whole displacement of the boat. The water is dis-charged at a velocity of 37'25 feet per second (about 1 ton persecond being discharged with a lift of 21-J feet), the speed ofthe boat being 2T4 feet per second (or 12'65 knots per hour).Careful experiments were made by means of a thin plate l^Vinch square, attached to the end of a lever and placed in the jet,just where it left the nozzle. The pressure on this plate wasrecorded by a dynamometer attached to the other end of thelever, and the lever was arranged so as to enable the plate tobe shifted about, and the pressure to be recorded over the wholejet. The mean pressure was found to be nine-tenths of thatin the centre. Professor Ilankine’s formula for the efficiency ofthe jet is as follows:—

w v s

Efficiency of jet =

9

W V S WS 2

~g~ + -%g~

fwv 22 9~

w = weight of water discharged in lbs. per second ;v = speed of vessel in feet per second ;s = slip or acceleration;g = 32'2 feet per second.

The efficiency of the jet was found to be *71, and of thepump 46. The efficiency of the jet and pump combined was•33, this being the useful work of the jet divided by the effec-tive HP. The total efficiency was ’25, this being the usefulwork in the jet divided by the indicated HP.

A comparison of the efficiency of a jet propeller with a screwpropeller was made by Mr. Barnaby as follows:—■

Screw Boat Efficiencies. Engine , '77 ; Screw Propeller , -65;total, - 5.

Hydraulic Boat Efficiencies. Engine , - 77; Jet Propeller , '71 ;Pump , - 46 ; total, - 254.