English: Step-Aside was a tactic by which warships could attack a U-boat armed with acoustic torpedoes.

From "Conduct of anti-U-boat operations, part 2, Chapter IV (ADM 239/298)

Anti-Gnat Tactics

380. (a) Ships capable of more than 24 knots.

Close U-Boat as fast as possible without using Foxer. The U-Boat may be kept right ahead while the range is greater than 6,000 yards, but when the range is less than 6,000 yards the U-Boat should be kept not less than 20° on the bow.

When the U-Boat dives the ship should close the diving position as fast as possible until it is necessary to reduce speed to commence the Asdic search. Speed should then be reduced to revolutions for 8 knots. Where speed is reduced, course should be altered not less than 20° and held for 1 minute. One depth charge should be dropped while in the danger area. The search should be conducted at 8 knots.

(b) Ships not capable of more than 24 knots.

Close U-Boat as fast as possible, operating Foxer. The U-Boat may be kept right ahead while the range is greater than 6,000 yards, but when the range is less than 6,000 yards the U-Boat should be kept not less than 20° on the bow.

When the U-Boat dives the ship should close the diving position as fast as possible until it is necessary to reduce speed to commence the Asdic search. Speed should then be reduced to revolutions for 8 knots. Foxer may be tripped when speed is reduced, one depth charge being dropped at the same time to shake off any Gnat that may have been captured by the Foxer.

The search should be conducted at 8 knots.

(c) Ships not capable of more than 24 knots and not fitted with Foxer. These ships must carry out "Step Aside Procedure". This procedure is applicable to all speeds up to 25 knots. Ships shoud carry it out at their highest practicable speed.

Step Aside Procedure

381. (1) If contact is obtained at more than 6,000 yards range:—

(a) Steer directly towards till the range is 6,000 yards. This is Zero time. Continue to steer towards for 2 minutes.

At Zero +2—
(b) Alter course 60° to port or starboard. If two ships are chasing they should alter outwards. Hold course for 3 minutes.

At Zero +5—
(c) Alter course to the present bearing of the U-Boat. If the range is now greater than 6,000 yards, close until it is 6,000 yards and then repeat the whole process. Continue doing this until the U-Boat dives. If the range is less than 6,000 yards, steer directly towards the U-Boat for two minutes and then alter course the same way as it was altered at (b) and repeat (b) and (c). Continue doing this until the U-Boat dives.

(2) If contact is obtained at less than 6,000 yards range:—

Carry out the procedure in (b) and (c) above. The first alteration should be made so as to put the U-Boat on the opposite bow to the original contact.