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Rear Admiral Edwin A. Anderson, Commander, Squadron One, Patrol Force, Atlantic Fleet, Plan for the Protection of Shipping in the Florida Straits




25 May, 1918       

     Should German submarines be sent into the Western Atlantic, it is extremely probable that they would operate in the Florida Straits. About one and a third million tons of shipping passes through the Straits in each direction a month and as the Straits are only 50 miles wide for about 200 miles of their length, from Cay Sal Bank to the northern end of Little Bahama Bank, it would be difficult for shipping to avoid submarines lying in wait.

     It is therefore important that all the possible systems for protecting shipping be carefully examined and that a certain definite plan of action be adopted.

     In making this examination it will be assumed that we have the following craft available in the Straits in limited numbers:-

1. Submarine chasers and scout patrols, attached to the District Forces.

2. Yachts and gunboats, attached to the American Patrol Detachment.

3. Submarines, attached to Third Submarine Division.

4. Seaplanes, blimps and kite balloons, with vessels suitable for towing them, attached to the District Forces.

     It will also be assumed that there are equal periods of daylight and darkness, 12 hours each in length.

     In general there are three systems which may be used to protect shipping in the Florida Straits:

1. To patrol the entire Straits and to send shipping through singly and somewhat equally distributed over the entire zone. This will be called the “Zone System”.

2. To patrol a narrow lane—10 or 15 miles wide—and to send shipping singly through it. This will be called the “Lane System.”3. To collect all vessels into convoys and send them through under escort. This will be called the “Convoy System”.

     The “Zone System”—has the following advantages:

1. As the entire zone in which the enemy is operating is covered by patrols of surface craft, aircraft (including kite balloons) and submarines during daylight, and by surface craft and submarines during night, the enemy submarines will be given less time to lie on the surface for resting their crews, charging batteries and making repairs. During daylight they will be compelled to remain submerged while in the zone. During darkness they will run a risk of detection while on the surface, especially by our submarines and vessels equipped with hydrophones. A submarine charging batteries can be heard for a great distance by hydrophones.

2. As the merchant vessels steam singly they may run at their maximum speed through the danger zone.

3. As they will not have to wait for the formation of a convoy they will save time.

4. They may maneuver better to avoid submarine attack than vessels in a convoy, especially during darkness.

5. Being spread over a wider area they run less risk of collision during darkness.

6. As the operations of the naval forces are entirely independent of the movements of the shipping, and as the movements of the merchant vessels have no relation to each other, the zone system is far simpler and requires less expert control than the two other systems.

7. As the shipping in distributed over a wider area than it would be in the “Lane System”, individual vessels have a better chance of avoiding submarines, especially during darkness. The submarines must also be spread over a wider area, thus decreasing their effectiveness.

8. It would be possible for a single ship to run smokeless during daylight in the zone but it would be difficult to prevent each vessel of a convoy of 10-12 ships from making smoke, and the smoke of just one vessel would give the location of the entire convoy to the submarines.

9. As a single ship would run much less risk of being discovered during darkness than would a convoy, it would have a greater chance of avoiding submarines during that period than vessels in a convoy.

10. It would be possible to zigzag during moonlight, a procedure which would be dangerous in a convoy.

11. In attacking a single vessel a submarine would have to approach to a short range for firing torpedoes, while Browning shots could be fired from a considerable distance with excellent chances of hitting vessels of a convoy.

12. No lights need be carried at night for other vessels to keep position by, as would be necessary in a convoy.

13. There would be no occasion to make light signals during darkness; this is done frequently in convoys and allows them to be discovered by submarines.

14. The presence of aircraft, especially blimps and kite balloons, does not disclose the position of the convoy.

     The “Zone System” has, on the other hand, the following disadvantages:

1. As naval forces are not used in conjunction with shipping merchant vessels cannot count upon their assistance in discouraging enemy attack and their actually assistance while an attack is being made. Unarmed vessels in the coastwise trade would be exposed o gunfire attack. If naval vessels happen to be close at hand when an attack is being made and are able to render assistance it would be purely a matter of good luck.

2. During the period when the enemy submarines are actually hunting they would, of course, be attracted to the merchant vessels, and as our forces would not be disposed with any reference to these merchant vessels, the naval forces would have less opportunity, in general, of actually sighting and attacking enemy submarines.

3. As the patrolling craft would necessarily be spread over a wide area, whenever one of them sighted an enemy submarine, he could not depend upon the assistance of other naval vessels in attacking on account of the difficulty of communicating with them and the long time which must elapse before they could arrive upon the scene of action.

4. As much wider area must be covered the patrolling craft would have to proceed to a greater distance from ther bases, thus involving greater difficulty for them in case of breakdown. This applies especially to aircraft.

5. As our submarines must act independently of our other naval forces due to the possibility of their being mistaken for enemy submarines, it would not be possible to allot them an area in which there will be none of the other classes of our naval forces, as could be done if the lane or convoy systems were used. As the submarines will therefore be compelled to operate in an area patrolled by aircraft and surface vessels, they are liable to be mistaken for enemy submarines attacked by our forces.

6. As the Florida Straits comprise about 2000 square miles of water, to properly patrol this immense area would require a force altogether out of proportion to that required by the other systems, and beyond our present resources.

     The “Lane System” has the following advantages:

1. Merchant vessels, as they steam singly, may proceed at their highest speed.

2. They would not have to wait for the formation of convoys.

3. They could maneuver better to avoid submarine attack than vessels in a convoy, especially during darkness.

4. They would run less danger of collision than vessels in a convoy.

5. This system is not so complicated as the convoy system and does not require such efficient control.

6. Due to the fact that a single ship covers less area than a convoy it does not run such a chance of being discovered during darkness.

7. It would be possible to zigzag at night. Zigzagging would be effective on moonlight nights.

8. The enemy submarine, having but one ship as a target, must gain a closer firing position than when a large convoy is available as a target.

9. No lights need be carried at night for other vessels to keep position.

10. No signals need be exchanged with other vessels during darkness.

11. The presence of aircraft does not disclose the location of shipping to such an extent as it would in the “Convoy System”.

12. As the naval forces are concentrated for the protection of a narrow lane through which all the shipping passes, the latter could count upon the naval forces to discourage submarine attack and to give actual assistance in case of attack to a greater extent than if the “Zone System”were used.

13. A naval craft sighting a submarine could count upon greater and quicker assistance from other naval forces than in the case of the“Zone System.”

14. A naval craft breaking down has more chance of obtaining assistance than in the case of the “Zone System.”

15. Our submarines could operate outside the lane and thus will not run the risk of being mistaken for enemy submarines.

     The “Lane System” has, on the other hand, the following disadvantages:

1. Enemy submarines when outside the lane will not be subject to attack by any of our naval forces other than submarines. As we have but four submarines in the Straits and these are in poor material condition, this is not a real menace to the enemy. He would be able to remain on the surface even during daylight without appreciable danger,provided he goes a moderate distance outside the lane. Thus he would be able to charge batteries, make repairs and refresh his crew at will.

2. As the shipping is concentrated in a narrow lane there would be a greater risk of collision than when the “Zone System” is used.

3. This system would be more complicated than the “Zone System”.

4. As the lane cannot be changed frequently without great confusion the enemy would soon be able to locate it and would thus sight without difficulty all vessels passing through in daylight. A large number of vessels passing through at night would be seen. Once vessels are seen at night successful attacks can be carried out much easier than in daylight.

5. The presence of aircraft would disclose the lane.

6. Naval forces cannot be counted on for such immediate assistance as when the “Convoy System” is used.

7. When submarines are sighted by a naval craft others cannot be called to his assistance as soon as if the “Convoy System” were used.

8. The “Lane System” to be at all effective must have the lane established either along the Florida Reefs or Bahama Banks merchant vessels would not keep to any other lanes. The Bahama Banks can be eliminated as the distances from our bases are so great that aircraft patrol, the most efficient in the Straits, would be ineffective. If the lane was established along the Florida Reefs the enemy would soon locate it and, especially at night, have excellent chances of destroying shipping.

9. The “Lane System” to be effective would require much greater forces than the “Convoy System”, but less than the “Patrol System”.

     The “Convoy System” has the following advantages:

1. The total available naval forces, being massed around the positions of the convoys, will almost completely discourage submarine attack, except under very favorable conditions or when especially expert enemy commanders are present.

2. Whenever enemy submarines attack merchant vessels the greatest possible naval force will be actually on the spot so that a quick massed counter-strike can be carried out against them.

3. Any naval or merchant vessels injured by accident or enemy action will receive immediate assistance, their crews being rescued if the vessels are destroyed.

4. There will be practically no chance of conflict between our submarines and our other naval forces.

5. As the route of the convoy may be changed from day to day it cannot be determined by the enemy, as could the lane when that system is used.

6. Submarines would be unable to make gunfire attacks.

7. Smaller forces are required for this protection of shipping than by either the “Patrol System” or “Lane System”.

8. There is no record of a convoy being attacked while escorted by aircraft.

     The “Convoy System” has, on the other hand, the following disadvantages:

1. Vessels must proceed at a speed somewhat slower than that of the slowest ship in the convoy. This means a delay in the carrying of supplies and a longer period in the danger zone.

2. Vessels will have to slow down as to arrive at the designated rendezvous at the required times. If one convoy is formed a day this delay will not be important.

3. Ships cannot maneuver with such readiness to avoid submarine attack as when cruising singly, especially during darkness.

4. There is most danger of collision.

5. This system is more complicated than the other two.

6. A convoy runs a greater risk of being discovered than a single ship, especially during darkness.

7. A convoy cannot zigzag at night.

8. Torpedoes can be fired at long range at the entire convoy.

9. Stern lights must be shown for other ships to keep position by.

10. There is a tendency for the merchant ships to signal at night, thus disclosing the position of the convoy. Only one light on one ship will give the position of the whole convoy to the enemy.

11. A certain amount of energy must be expended by the watch on deck in keeping position in the convoy, which takes their attention away from the important duty of keeping an efficient lookout.

12. The presence of escorting aircraft disclose the position of the convoy.

13. Enemy submarines, as long as they keep clear of the convoys, may rest on the surface at will for charging batteries and making repairs.

     In general it is decided that in daylight the “Convoy System” is the best. During darkness the “Zone System” is the most efficient. A combination of these two systems would give the best results.

     It now remains to decide whether we will:

1. Send the individual vessels or convoys through the Straits on route selected at random, so that vessels will be spread somewhat equally over the entire zone of the Florida Straits,


2. Send them all close in to Florida Reefs and Coast.

     The first method of routing vessels has the following advantages:

1. Single ships have an excellent chance of avoiding submarines during a dark night, and good chances during a moonlight night and daylight. Convoys will have a good chance of avoiding submarines during dark nights and a slight chance during daylight.

2. Ships proceeding northward may take some advantage of the Gulf Stream, although it will be bad practice for many of them to keep near the axis, as this would cause enemy submarines to concentrate along it.

3. Ships and convoys will have room to maneuver to avoid torpedoe attack.

     The second method has the following advantages:

1. On the trip to the southward the Gulf Stream will be avoided to the greatest extent possible.

2.   Only one side need be protected against submarine attack. Thus all the lookouts and escorting vessels can be concentrated on one side, doubling the efficiency of the defense.

3.   Submarines would not like to operate near the reefs in the strong and constantly changing current events.

4.   For a part of the run the vessels would be in comparatively shoal water, which is decidedly unfavorable for submarine operations.

5. If vessels are badly damaged by torpedo hits they may be quickly beached.

6. Seaplanes and blimps would be more efficient as they would have little trouble in locating the convoy and would be able to remain with it longer, as less fuel would be expended in reaching it. In case of casualties seaplane would usually be able to reach the smooth water inside the reefs or in the inland water ways for landings. If forced landings are made in rough water the chances are very great that struts would be broken, thus preventing the machine from getting off the water again even if the engine trouble was repaired. The plane would have to be towed to the nearest air station, an operation which is nearly certain to damage it very badly. On the other hand, if a landing could be made in smooth water, the defect could be quickly corrected and the plane could be flown back to the air station.

7. Observers on the lighthouses could report enemy submarines and also the passing of our own craft and convoys.

     In general it is best to keep close to the Reefs during daylight and to spread vessels and convoys over the entire zone of the Florida Straits during darkness.

     Thus it is best:

1. During Daylight—To send vessels in convoys close to the Florida Reefs.

2.During Darkness—To send vessels singly on routes spread somewhat equally over the entire zone of the Straits. . . .

Source Note: Cy, DNA, RG 45, Entry 520, Box 414.