SALVAGE EQUIPMENT USED IN RAISING SUBMARINE F-4. 11 
6 inches. If there had been a gradual slope of hard bottom in other than the ship 
channel, from the resting place of the vessel to the dock, it might have been possible 
to use the customary pontoon method, whereby vessels are lifted 5 or 6 feet at a 
time and are towed to a new landing between lifts. This plan would have called 
for at least one landing directly in the channel. If trouble had been experienced in 
making the next lift promptly, the channel would have been blocked to navigation. 
In the harbor itself the bottom is soft, due to silting from streams and sewers. To 
have landed the vessel in mud would have been extremely hazardous, because mud 
suction practically doubles the weight to be lifted. Furthermore, the vessel would 
have filled completely with mud through the large aperture, as the canvas mats 
placed by the divers could not be counted on to keep the opening sealed. 
SUBMERSIBLE PONTOON METHOD. 
It was therefore necessary to resort to a salvage method which would provide 
for lifting the vessel from a depth of 50 feet to a depth of not more than 25 feet 
at one step. The use of submersible pontoons offered the best solution of the 
problem. The plan adopted called for the use of three pontoons on each side of the 
vessel connected in pairs by chains passing under the boat and up through hawse 
pipes in the pontoons. The disposition of the pontoons is shown in Plate 13. 
The principal considerations which governed the design of the apparatus and 
the plan of operations were :— 
(a) A sufficient factor of ae had to be provided in the gear and in the ca- 
pacity of the pontoons to insure freedom from breakdown at the final operation of un- 
watering the pontoons. Account had to be taken of the fact that the stresses coming 
on the apparatus would be violent and sudden at times. 
(6) A plan had to be followed which would reduce the underwater work to a 
minimum, and which would not unnecessarily endanger the lives of the divers. 
These considerations ruled out any method calling for actual connection of the lift- 
ing chains to the hull of the vessel. It also ruled out making shackle connections to 
the pontoons. Any attempt to make such connections would have been dangerous 
and probably altogether impracticable, as the divers would either have had to work 
between the submarine and the pontoons, or under the pontoons, while these were 
suspended from above and were consequently pumping up and down in the swell. 
This consideration led to the use of chains, which were merely rove under the 
boat and brought up through hawse pipes in the pontoons. The divers were there- 
fore able to work in comparative safety on top of the pontoons while attaching the 
clamps to the chains as described below. 
(c) It was necessary to provide means for reeving the chains under the boat 
with accuracy in predetermined locations, and it was also necessary to land the 
cylinders within a few feet of their lifting positions. This required the use of a 
flexibly moored wrecking barge over the submarine. 
(d) The entire plan had to be based on the proposition of completing the work 
