How Did Alligator Work?
A Work in Progress from J. Christley

I had a meeting with a group at NSMRL (Naval Submarine Medical Research Lab) in which we discussed the atmosphere control operation, diver technology, and general operational tactical use.  The upshot of the meeting was a set of ideas about how the boat was used, atmosphere controlled and how the diver was deployed.  I am attempting to write a general tactical operational plan and diver deployment plan.  The purpose of this exercise is to refine our thinking about how Alligator was used.  I hope to have enough of this plan reviewed and a consensus reached so it can be presented as part of the October Symposium in Norfolk .

Here are some of the parts of the tactical operational plan.

A.  Initial preparations: Alongside
  1. The boat is trimmed out with the entire crew, all diver equipages aboard (including explosives and electrical cables) aboard and in place. The position of all the personnel within the boat is to remain fixed throughout the operation with the exception of the movements of the Commander, diver and one tender/compressor operator. 
  2. The diver lockout trunk is dry and closed.  The diver lockout trunk compensation tank is full.
  3.  With the ballast tanks empty and the buoyancy chambers housed, the trim is adjusted to a predetermined freeboard and longitudinal angle with the addition, subtraction and positioning of lead ballast bars.  This freeboard and angle must be experimentally determined and might change for each operation and crew modification.
  4. The air purification chamber is filled with caustic solution per the “De Villeroi plan”
  5.  The battery is filled with electrolyte and sealed.  (silver/zinc battery similar to telegraph batteries in use at the time – gassing not a problem because of low power consumption and no charging requirement)
B. Underway, transiting to target
    1. Operation to take place at night timed to use favorable tide. 
    2. It should be noted that for proper operation of the oar propulsion mode, a trained crew was required.  The training that was necessary was that normal in the Navy for a boat crew.  The group had to be able to row together and on command.  For screw propulsion, the level of training and coordination would be lessened. 
    3. Once underway the boat would be towed to a release point as close to the target as practicable given the target type, sea condition, tide forces and currents and enemy watchfulness.
    4. At the release point, the boat would release the tow and proceed on the surface with the conning hatch open.
C. Diver Deployment  
    1. Once the boat had proceeded on the surface as close as practicable to the target, it would be submerged by flooding water into the ballast tanks.  The tanks would be vented into the boat’s interior building an initial positive pressure inside the hull. Fasten shut the conning dome and commence pumping air using the air purification and circulation equipment.  It is felt (as a result of the meeting at NSMRL) that this was a hand crank operated compressor type device that had a three mode system of operation. 

a. It could draw air in from the outside via the air tube and discharge this air to the interior of the boat.

b. It could draw air from low in the boat, direct this air through the air purification chamber and thence to the interior of the boat (cleaning some of the CO2 from the air) and

c. A combination of a and b drawing some air from outside and from inside the boat discharging the air through the air purification chamber thence to boat’s interior.

      The normal method of operation would be to draw air through the air tube which would be above water (a).  If the air tube was not above water mode (b) would be used.  After a period of time the mode of operation would shift to (b) if the air tube was submerged or (c) if it was above water. That there was an air distribution system within the boat is pointed out in an Eakins report as to what was missing when he took over the boat. 


a. It is felt that the operation of the boat would be such that it would not have to submerge more than 10 to 15 feet (keel depth) and could maintain depth easily by deploying the buoyancy chambers as it dove and ran in to the target.  The slow speed, combined with operating on a relatively dark night (cloudy and/or moonless) and with a small chop in the water it would remain undetectable.  [Someone would have to know exactly what to look for then be able to find the bobbing chambers in the water – which would have been difficult]

b. {Keep in mind that the boat would have only to go a couple of miles completely submerged and would do so slowly.}

c. The deployment and recovery of the diver is assumed to be a ‘one shot’ operation and if further deployments are required the boat must leave the area, surface and renew preparations for attacking a target.


    1. The diver’s lockout trunk inner door would be unlatched and opened.  The diver would enter the space.  Once inside the tender (inside the boat) would commence pumping the air compressor and the diver (in the chamber) would verify air was flowing into the helmet.  This air compressor and its requisite air line into the lockout chamber are not shown in the De Villeroi design drawing. 
    2. When the diver is satisfied, the diver chamber door is shut and sealed.  The diver then opens the latches on the access hatch and raps on the bulkhead indicating he is ready to open the hatch.
    3. One of the oarsmen (the boat is at ‘All Stop’) starts to pump the contents of the lockout chamber compensation tank overboard.  The volume of this tank equals the volume of water that would enter the diver lockout chamber up to the level of the upper lip of the access hatch if the hatch were to be opened. 
    4. The diver opens the access hatch (it opens downward) and deploys a weighted knotted line out the hatch.  This line is a method for the diver to reenter the lockout chamber unaided.  He also deploys the air hose contained in the lockout chamber in such a manner as to ensure it won’t hang up in the chamber when he exits the boat.
    5. The diver exits the boat, proceeds to the target and deploys the explosive package or performs what ever other task he required.  If an explosive package is deployed, the detonation cable is laid out as the diver maneuvers from the boat to the target.  The explosive package can either be deployed from the diver lockout chamber (using insulated bulkhead stuffing tubes for the wire) or it could be propositioned with the wire reel outside the boat hull prior to the commencement of the operation.
    6. The diver proceeds back to the weighted knotted line and makes his way back into the lockout chamber.  He insures the connections for the electrical detonation wire is connected correctly and does not foul the hatch.  He pulls in any air hose and the knotted line which would foul the hatch. 
    7. The diver closes the hatch and the air still being pumped into the helmet blows most of the water in the lockout chamber out.  The remaining water is equal to the volume of the now empty compensating tank.  During diver deployment and reentry, small amounts of water can be pumped out of and flooded into the forward ballast tank to compensate for the diver leaving and reentering. 
    8. When the diver is recovered, the boat can leave the area.  When the boat is at what might be perceived as a safe distance the explosives can be detonated by electrical current from inside the boat.
If the boat can operate at a relatively shallow depth (keeping the air tube above the surface) it can maintain a positive pressure inside all during the diver deployment.  If not, an initial positive pressure would be enough to supply the diver (through a dedicated compressor) without drawing a vacuum in the boat.
Note: For an alternate method of making an attack with Alligator, see "Another Way to Use the Sub-marine Propeller?" (--CV)

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