From the Commanding Officer of HMS Porpoise
To The Captain (S), 3rd Submarine Flotilla, HMS Titania
Dated May 8-1941

Apr. 19:
15:15 - Sailed from Halifax.

Apr. 20:
11:30 - Dived.
12:00 - Position 43 15N 60 35W.
Weather - SSE 2, c.8, 01, 019.
21:30 - Dense fog. Station keeping by Asdic approximately one mile on starboard wing of convoy.

Apr. 21:
12:00 - Position 43 41N 56 57W.
Weather - S.?. 2, o.2, 13, 1014.

Apr. 22:
12:00 - Position 42 14N 55 04W.
Weather - SW 3, o.2, 13, 1012?.

Apr. 23:
02:00 - Fog lifted. Five ships in sight to port.
04:00 - Fog cleared. Seven ships in sight. Ordered them to steer 099 at 6 knots. Proceeded to westward.
06:00 - Met SS Cleodora and ordered her to rejoin.
10:30 - Sighted convoy bearing 130 degrees, 12 miles.
11:40 - In station on convoy.
12:00 - Position 42 10N 49 30W.
Weather - W 2, b.8, 11, 1020.

Apr. 24:
09:15 - Dived.
12:00 - Position 43 16N 46 57W.
Weather - NW 2, b.c.7, 21, 1030.
23:00 - Clocks advanced one hour to zone plus 3.

Apr. 25:
05:00 - Dived.
12:00 - Position 44 53N 44 32W.
Weather - NW 2, o.c.8, 13, 1030.
? - Received C.-in-C. W.A.'s 1531/25 routing Porpoise home.

Apr. 26:
10:15 - Sighted HMS Alaunia and convoy HX 122.
12:00 - Position 47 12N 43 14W.
Weather - N 2, b.8, 23, 1023.

Apr. 27:
12:00 - Position 49 45N 42 13W.
Weather - NNW 3, b.c.8, 23, 1028.
16:00 - Dived.

Apr. 28:
10:00 - Dived.
12:00 - Position 52 04N 40 48W.
Weather - N 2, b.8, 22, 1025.

Apr. 29:
12:00 - Position 54 29N 38 22W.
Weather - E 1, b.c.8, 02, 1016.
15:30 - Parted company with convoy SC 29. Carried out surface sweep to southwestward until 17:00, then to eastward.
18:00 - Dived on course 045 degrees.
20:00 - Surfaced and proceeded in accordance with C.-in-C. W.A.'s 1531/25.
23:00 - Clocks advanced one hour to zone plus 2.

Apr. 30:
12:00 - Position 55 55N 33 35W.
Weather - ENE 3, b.c.8, 33, 1008.
23:00 - Clocks advanced one hour to zone plus 1.

May 1:
12:00 - Position 56 41N 26 42W.
Weather - ENE 3, o.c.4, 24, 1024.
17:25 - Dived in position 56 42N 25 00W.
212:20 - Surfaced.

May 2:
03:30 - Dived.
12:00 - Position 56 47N 22 58W.
Weather - SE 4, c.6, 35, 1016.
21:45 - Surfaced.

May 3:
03:10 - Dived.
12:00 - Position 56 48N 20 47W.
Weather - NE 2, o.5, 13, 1019.
21:30 - Surfaced.

May 4:
02:35 - Passed my 0903/4 reporting late arrival.
03:10 - Dived.
03:25 - Heard hammering noise*.
12:00 - Position 56 49N 18 16W.
Weather - E 1, c.6, 02, 1020.
21:30 - Surfaced.

May 5:
0?:45 - Dived.
05:00 - Clocks advanced one hour to GMT.
12:00 - Position 56 50N ?W (looks like 5 00W).
Weather - E 2, o.c.7, 12, 1024.
22:45 - Surfaced.

May 6:
03:40 - Dived.
12:00 - Position 56 45N ?W (looks like 12 23W).
Weather - ENE 8?, b.c.7, 12, 1022?
18:00 - Heard hammering noise again.
22:30 - Surfaced.

May 7:
03:25 - Dived.
12:00 - Position 56 44N ?W.
Weather - NE 1, c.8, ? ?
22:00 - Surfaced. Sighted convoy bearing ? 3? miles.
22:45 - Altered course to westward? to let convoy get clear.
23:45 - Altered course for rendezvous.
23:59 - Clocks advanced one hour to zone minus 1.

Porpoise's report ends here, but there's a document discussing the mysterious hammering noise, as follows:

*No theories about these mysterious transmissions can be advanced.

Noted with interest. D. of P. would be interested to know if D.A/S.W. thinks there is anything in C.O. Porpoise's suggestion that these mysterious transmissions might be connected with the homing of U-boats on to a convoy.

An advance copy of this Report of Proceedings was forwarded to D. A/S W. who has been investigating the "sonic" echo sounder transmissions reported.

A somewhat similar phenomenon was reported by Camelia on the 21st March 1941, following the second of a series of 4 depth charge attacks on a U-boat which is believed to have been an Italian. It was described by Camelia as a "very rapid tapping or hammering" which continued for "a good five minutes". The explanation advanced by the U-boat Assessment Committee was the accidental switching on of some form of echo sounding gear.

A further possible case has now been reported by St. Francis in her signal timed 1307B/20/5 reporting her attack on a U-boat at 0510 on that date. She reports "At 0235 heard noise very similar to echo sounding machine. Noise varied as from deep to shallow setting and was not recognised as coming from submarine". It is believed that the U-boat which attacked the convoy was Italian but there is no evidence to prove that the U-boat was in the immediate vicinity when the noise was heard (St. Francis investigated without result) and the attack did not develop for another 90 minutes.

No positive conclusion can be drawn from this evidence but it is proposed to issue the enclosed signal to draw attention to the subject and to collect further evidence. (Copy of the enclosure mentioned is not available).

(This is signed "Director of Anti-Submarine Warfare, 24th May, 1941", but signature is illegible).