| Navy | The Royal Navy |
| Type | Submarine |
| Class | S |
| Pennant | 98 S |
| Mod | First Group |
| Built by | Chatham Dockyard (Chatham, U.K.) |
| Ordered | 16 Mar 1931 |
| Laid down | 14 Sep 1931 |
| Launched | 15 Nov 1932 |
| Commissioned | 2 Oct 1933 |
| Lost | 7 Jan 1940 |
| Loss position | 54° 19'N, 7° 30'E |
| History | Seahorse (Lt. Dennis Staunton Massy-Dawson, RN) sailed from Rosyth to patrol off Heligoland on 26 December 1939. She was most likely sunk after being attacked and depth charged by ships from the German 1st Minesweeping Flotilla on 7 January 1940 about 15 nautical miles north-west of Heligoland in position 54º19'N, 07º30'E. It is unlikely that she was sunk by the German Sperrbrecher IV/Oakland south-east of Helgoland on 29 December 1939. This vessel opened fire on a presumed periscope with a 20mm gun and though an oil slick was seen, the water was only 24-metres deep and no depth charges were dropped. HMS Seahorse was never heard from after sailing from Blyth on 26 December 1939 for patrol off western Jutland. Her operational area was to be initially off Heligoland then move to the mouth of the Elbe on 30 December and return to Blyth on 9 January 1940. At 1435 hours on 6 January, in Quadrat AN 9567 (left upper corner) [also reported as 54°19.8'N, 07°30'E], the German minesweeper M 5 (KL Dierksen) detected a submarine and carried out depth charge attacks until 0947 hours the next morning. At 1318 hours, after dropping a buoy in Quadrat AN 9591, she abandoned the search to join the 12th minesweeping flotilla engaged in sinking HMS Undine. On 11 January, M 5 returned to the area and carried out seven depth charges attacks, observing an oil slick in the process. The submarine chaser UJ 121 was ordered to stand on guard in the area while a diving vessel was sent but no wreck was reported to have been found. HMS Seahorse had disappeared without a trace and it is likely that she was probably mined. |
Commands listed for HMS Seahorse (98 S)
Please note that we're still working on this section
and that we only list Commanding Officers for the duration of the Second World War.
| Commander | From | To | |
| 1 | Lt. Dennis Staunton Massy-Dawson, RN | 15 Apr 1939 | Jan 1940 (+) |
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Notable events involving Seahorse include:
The history of HMS Seahorse as compiled on this page is extracted from the patrol reports and logbooks of HMS Seahorse. Corrections and details regarding information from the enemy's side are kindly provided by Mr. Platon Alexiades, a naval researcher from Canada.
This page was last updated in January 2015.
24 Aug 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) departed Dundee to take up a patrol position to the south-west of Stavanger, Norway. When the war with Germany commenced this became her 1st war patrol.
For the daily positions of HMS Seahorse during this patrol see the map below.
(1)
5 Sep 1939
While on the return passage to Dundee HMS Seahorse (Lt. D.S. Massy-Dawson, RN) was bombed in error by a British aircraft at 2007 hours in position 56º40'N, 01º04'W. This was Anson K8845 of 233 Squadron (P.O. Yorke and Sgt. D.T. Muir) which dropped two 100lb A/S bombs. She sustained some damage which required her to dock.
Seahorse had dived when the aircraft was sighted but her after hydroplanes jammed and depth control was lost. In the meantime two depth charges were dropped by the aircraft but these caused no damage. Seahorse then broached and her bow came out of the water. A third depth charge was then dropped but this also caused no damage. Before Seahorse was able to submerge again a fourth depth charge was dropped very close which shook the submarine severely but it caused no damage. Seahorse then went down again and hit the bottom heavily at 220 feet causing damage to her A/S dome. The aircraft had dived so low that the explosions of the bombs damaged it seriously and it had to ditch but the two airmen were rescued. (1)
6 Sep 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) ended her 1st war patrol at Dundee.
She shifted from Dundee to Rosyth later the same day. (2)
7 Sep 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) is docked in the floating dock at the Rosyth Dockyard. (2)
11 Sep 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) is undocked. (2)
12 Sep 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) shifted from Rosyth to Dundee. (2)
16 Sep 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) departed Dundee for her 2nd war patrol. She was ordered to patrol off the south-west coast of Norway.
For the daily and attack positions of HMS Seahorse during this patrol see the map below.
(1)
17 Sep 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) fired three torpedoes against German U-boat U-36 in the North Sea in position 56°42'N, 00°52'E. No hits were obtained. The U-boat was attacking the Danish N.J. Ohlsen (819 GRT, 1925).
1140 hours - Sighted the conning tower of a submarine bearing 305°. Range was 11000 yards. Started attack.
1204 hours - Observed the submarine to fire two rounds with her gun, apparently at a merchant ship to the south.
1210 hours - In position 56°42'N, 00°52'E fired three torpedoes at the enemy submarine from 7000 yards. Went deep on firing and turned to the northward.
1219 hours - Heard a single heavy explosion. Remained deep.
1239 hours - Returned to periscope depth. No submarine in sight. (1)
2 Oct 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) ended her 2nd war patrol at Dundee. (1)
4 Oct 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) proceeded from Dundee to Rosyth where she was immediately docked in the floating dock at the Rosyth Dockyard. (3)
8 Oct 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) is undocked at Rosyth. She then returned to Dundee. (3)
14 Oct 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) shifted from Dundee to Rosyth. (3)
17 Oct 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) departed Rosyth for her 3rd war patrol. She was ordered to patrol off the south coast of Norway / Skagerrak.
For the daily positions of HMS Seahorse during this patrol see the map below.
(1)
30 Oct 1939
At 2330 hours, in position 56°29'N, 01°10'W, HMS Seahorse (Lt. D.S. Massy-Dawson, RN) sighted a submarine. An attack was started but before torpedoes could be fired the submarine dived. This was most likely U-21 (KL Frauenheim) which was patrolling in the same area but the U-boat did not observe the British submarine. (1)
31 Oct 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) ended her 3rd war patrol at Rosyth. (1)
9 Nov 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) departed Rosyth for Blyth. (4)
10 Nov 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) arrived at Blyth. (4)
12 Nov 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) departed Blyth for her 4th war patrol. She was ordered to patrol in the North Sea to the north-west of The Netherlands.
For the daily positions of HMS Seahorse during this patrol see the map below.
(1)
18 Nov 1939
During the morning HMS Seahorse (Lt. D.S. Massy-Dawson, RN) operating off Terschelling, sighted two Roeder-class destroyers but could not get in an attacking position. These were almost certainly Z 21 / Wilhelm Heidkamp and Z 19 / Hermann Künne. (1)
28 Nov 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) ended her 4th war patrol at Blyth. (1)
29 Nov 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) was docked at Blyth. (4)
12 Dec 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) was undocked. (5)
13 Dec 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) departed Blyth for her 5th war patrol. She was ordered to patrol off the British east coast. (5)
15 Dec 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) ended her 5th war patrol at Blyth after having been recalled. (5)
26 Dec 1939
HMS Seahorse (Lt. D.S. Massy-Dawson, RN) departed Blyth for her 6th war patrol. She was ordered to patrol in the Heligoland Bight. (1)
Sources
- ADM 199/1840
- ADM 173/16033
- ADM 173/16034
- ADM 173/16035
- ADM 199/373
ADM numbers indicate documents at the British National Archives at Kew, London.