The ONS-5 Convoy and U-boat battle

By Mike Downes
Paper presented to the Company of Master Mariners, Sydney Branch, 24 November 1993, reprinted with permission.

The chief ONS-5 adversary was the Type VII U-boat. The most common variant, the Type VIIC, was 67 x 5.85 x 4.37 metres (220 ft x 19 x 14 feet) and displaced 770/865 tons surfaced/submerged. Two supercharged four-stroke Germaniawerft six-cylinder diesels delivered 2800-3200 bhp to twin propellers and, with fully charged batteries, the two electric motors produced 750 SHP. Maximum speeds were17 knots surfaced or 7.6 knots submerged. Armament typically included six torpedo tubes with nine reloads, but some Type VIIs were specialist minelayers, flak-ships, milch-cows, etc.The Type VIIs usually carried one 88 mm deck gun and multiple variations of 27 mm and 20 mm AA guns. Crew numbers might vary from 44 to 60.

The fiercest convoy/U-boat battle of WW II, indeed one of the major turning points of the war, was the defence of convoy ONS-5. Until that point, U-boats had almost succeeded in starving Britain into submission, with all that that implies. This was the scrap that finally turned the corner (Syrett 1994, Roskill 1956).

Priority: U-boats

In 1939 Germany did not have very many U-boats, but they soon proved to be a very effective weapon. As the war went on, Admiral Döenitz, who had been in charge of the U-boat arm, was made operational head of the German navy in late 1942. The three big ships, Scharnhorst, Gneisenau and Prinz Eugen had made their successful dash up the English Channel in early 1942 and were now bottled up in German ports. Döenitz paid them off to provide the sailors to man the 17 U-boats that he was building every month. At the time of this battle there were some 400 U-boats in commission, building or working up.

These U-boats were fitted with torpedo tubes and a decent-sized gun for surface attack. To detect shipping they relied on visual lookouts and on good quality hydrophones. Their greatest asset was speed. They only did two knots under water with bursts to seven knots, but they usually travelled at between 14 and 16 knots on the surface.

Tactics

Once a convoy was detected, they would shadow it, reporting directly to Döenitz and to other submarines in the pack. Then they would try to get ahead of the convoy, where they would lie in wait. In daylight hours with good visibility they would tend to go deep and try to attack from inside the convoy itself. Their favourite tactic was to attack down-wind and down-sea on the surface at night, submerging as soon as the torpedoes had been fired.

They did not have radar but did have a radar detector geared to the 10 cm wavelength. Unfortunately for them, we were using a three cm wavelength radar, which they could not detect. Another weakness was that they had to report daily by radio to Döenitz, who closely controlled them, but their signals were detected by direction finders from shore and a few British ships. In some cases the signals could be decoded and read by Ultra, the secret British signal decoding section.

The British Escort Groups at this time comprised older destroyers with one boiler removed and replaced by additional fuel tanks to give greater range, as well as new frigates and corvettes.

Radar and Asdic limitations

We all had good radar but this did not have the modern display. We had only a single small cathode ray tube that gave a distance and a hand-trained aerial that indicated the bearing. Asdics, the underwater detection system, was similarly hand-trained and effective in most conditions out to about 1500 to 1800 yards. However, because of water noise it became pretty useless in really heavy weather and at any speed over 16 knots. The asdic beam worked basically in and just below the horizontal plane, with the result that a submarine echo could only be tracked down to some 100 to 150 yards in front of the ship. It was then guesswork as to what the submarine was doing until we were in position to drop our depth charges from the stern.

Hedgehog

So the Admiralty started fitting some ships with hedgehog. Hedgehog is an ahead-throwing weapon firing off 24 bombs with contact fuses. These formed a circle some 24 metres in diameter, about 220 metres in front of the ship. Any bomb that exploded against a submarine’s casing was powerful enough to blow a hole in the pressure hull.

Hedgehog was a 24-barrel mortar. Each round carried 13.6 kg (30 pounds) of TNT or 16.3 kg of torpex.

Finally, one or two ships of every Escort Group were also being fitted with a high frequency direction finder (H/F D/F). This was a rather inaccurate instrument but could detect submarine radio transmissions and gave an indication from which direction the attack was likely to come and the very approximate range (either groundwave or distance from the U-boat making the signal). Our own ship, Tay, was the only ship so fitted in the B7 Escort Group.

HMS Tay

So much for the protagonists. I was a lieutenant and the navigator of HMS Tay, which was the Number Two ship in the B7 Escort Group. My job was to plot the information coming from the asdics, radar, etc. on a track chart. My station was in a small hut next to the anti-submarine cabinet at the front of the bridge, where the skipper could look down from above and see the display. I also had the job of monitoring the signals from the other ships.

HMS Tay K-232, commissioned in 1942, was the fourth of 151 River class frigates built in WW II. The class displaced 1370 tons, on a 91.8 x 11 x 2.74 metres (301 x 36.5 x 9 feet) hull. Two boilers fed two four-cylinder VTE engines that developed 5500 ihp and could push the ships along at 20 knots. These specialist ASW ships carried two 102 mm (four-inch) and 10 x 20 mm guns, one hedgehog mount and 150 depth charges. Crew size might vary between 107 and 140.

Our boss in B7 Escort Group was CMDR Peter Gretton. He was a brilliant commander and worked up B7 to become the ace of the Close Escort Groups in the same way that CAPT Johnnie Walker was the ace of the Support Groups. The Close Escort Group operated as the close escort to the convoy, usually taking station one mile clear, whereas the Support Groups were fairly fast free-roaming ships whose function was to close any convoy under attack and provide additional support. When they detected a submarine, they could sit on it for perhaps 24 hours until it surfaced and could be killed. The Close Escorts had to remain with the convoy and while we did detach for single submarines when there were none others threatening, we usually only concentrated on putting the submarine down until the convoy had gone past.

CMDR Peter Gretton commanded HMS Duncan D-99, a D class destroyer leader, 1400 tons, 97 x 10 x 1.37 metres (318 x 33 x 4.5 feet) with four 120 mm (4.7 inch) one 77 mm (3 inch) guns and eight torpedo tubes. Commissioned in 1933, her 38,000 hp engines gave a handy speed of 36 knots (later 25 knots). In a 1940 modernisation refit Duncan lost one main gun and four torpedo tubes, but bolstered her ASW armament and range.

Convoy ONS-5

ONS-5 was a medium-size slow convoy, west-bound from Liverpool for Halifax and North American ports. The convoy speed was officially 7.5 knots, but in practice was nearer six knots and maybe half that in heavy weather. The 44 ships formed up in 12 columns of four or less, having a front of about 5.5 nautical miles. The escorts were zig-zagging approximately three-quarters of a mile further out.

There would be a fast ship sweeping across the front, the slower but very effective corvettes protecting each side, and one or two faster ships astern, ready to reinforce or take over an attack as necessary.

The battle started on 27 April when we beat off attacks by eight submarines during the night. This kept up and our first loss was during the following morning. This was to set the pattern for the next few days, with many attacks being made and a number of ships sunk. The battle lasted eight days, fought in gales with wind force between six and ten or more, with very heavy seas that frequently scattered the convoy. In fact, ten ships became detached and formed their own convoy escorted by the corvette Pink. Strangely enough, that group did not get attacked again. The rest of us battled on, beating off attacks, frequently in ice and constantly bitterly cold. During the heavy weather, our asdic transducer in Tay jumped its support bearings and became jammed, so that we were impotent against submerged submarines.

Four wolfpacks

German records suggest that four wolfpacks were involved in the ONS-5 battle: Wolfpack Star comprised 18 U-boats, Amsell I and II contributed 13 and Fink had 28.

Then, Peter Gretton in Duncan began to run out of fuel and the weather was so bad that he was unable to refuel from the tanker provided for this purpose. In those days, when we refuelled at sea, the tanker paid out perhaps 150 yards of hose, the end of which we picked up on the fo’c’sle head. The bight of this hose was always dragging in the water and the strain, with both ships pitching in heavy seas, would often prevent it being connected. If and when this was done, the hose could break, even before any oil was pumped. Eventually, Duncan had to leave to refuel in Greenland, and Tay, with our skipper Bob Sherwood as Acting Senior Officer of the escort, was left in charge for the final three nights.

Three ships hit almost simultaneously

I clearly remember being on afternoon watch on 5 May looking around towards the convoy just in time to see three ships hit almost simultaneously. The first was Selvestan, which sank stern first. Gharinda sank bow first and the little ship Bonde broke in two. All three sank within two minutes. It was a sight I’ll never forget. These were the last of our ships sunk and at this point the score was 11 ships to one U-boat. We had a rescue ship, the trawler Northern Spray, which was full, having picked up 146 survivors. Tay picked up some 143 other survivors from those three ships and this also made us very crowded, as our own crew was only 126.

The Admiralty had instructed the Third Support Group to come and assist us and they joined on 2 May, but most of them had to leave a few hours later because the weather was still too rough for them to refuel and they were running out. Only the destroyers Offa and Oribi remained until the final night. On the evening of 5 May, we received a signal from the Admiralty saying “25 U-boats in contact with your convoy, 40 in the immediate vicinity closing, 70 in general area.” In fact, we now know, from German records, that 51 submarines were deployed to trap ONS-5 and that Döenitz had called for an all-out effort. One of the U-boat skippers who survived the war has since said that the U-boats thought that they could wipe out the entire convoy during that night, but at last the weather changed in our favour with near-calm conditions and thick fog.

HMS Pink K-137, was one of the four Flower class corvettes working with the destroyers HMS Duncan and Vidette and the frigate HMS Tay in Escort Force B7. The group also included two designated rescue ships, the trawlers HMS Northern Gem and Northern Spray. Pink and sister ships Sunflower, Snowflake and Loosestrife displaced 940 tons, on a 62.5 x 10 x 3.5 metres (205 x 33 x 11.5 feet) hull. Two boilers served a four-cycle triple expansion reciprocating steam engine that delivered 2750 ihp to a single shaft, giving a maximum speed of about 16 knots. They carried one 101 mm (four-inch) gun, two depth charge throwers, two sets of depth charge rails and 40 depth charges.

This meant that the U-boats on the surface had trouble finding the convoy, whereas we could pick them up easily on radar. But now we only had Vidette, Loosestrife, Sunflower and Snowflake as effective close escort vessels. Tay was in control but had no asdic. Offa and Oribi were all that were left of the support group and they were stationed five miles further out on each bow. We were all running out of depth charges.

Oribi ramming

The signals coming in were exciting. Oribi: “Ramming.” Vidette: “Sank one U-boat with hedgehog.” Loosestrife: “Three contacts am engaging.” Then, “One dived. Dropped five-charge pattern.” Finally, “Third contact dived, dropped 10-charge pattern. U-boat surfaced alongside and blew up.” Snowflake signalled: “Three echoes bearing 185 degrees.” Then, “U-boat sighted, turned away.” Then, “Second U-boat sighted, engaged with four-inch gun until dived.” Then, “Third U-boat sighted, dived, dropped one charge. No charges left.”

Although this left Snowflake largely impotent, she was not yet finished. She came across the U-boat that had been rammed by Oribi, but which had not yet sunk, and both vessels had a brief gun duel. The U-boat eventually sank or scuttled herself but, in the confusion and fog, Snowflake turned to ram a radar echo which she thought was a submarine. She almost rammed Sunflower, who was coming to assist her.

Snowflake then made the signal, “Lights in water, interrogative save?” To which we had to reply “Negative, resume station,” because we were so short-handed. And that turned out to be the end of the battle as far as B7 was concerned. Four hours later the U-boats withdrew.

That night, we escorts claimed seven U-boats sunk, four very probably sunk, two probable and many damaged. In fact, the Germans later confirmed that they had lost 11 U-boats (one by aircraft attack) in that 24 hours. Döenitz called off the attack and thereafter the German skippers seemed to have lost their nerve. B7’s next convoy, eastbound, was attacked by 20 U-boats but the attacks were not pushed home. We did not lose a single ship, but we sank five of the attackers.

Harold Chesterman

Incidentally, many Master Mariners know the skipper of Snowflake. He was Harold Chesterman, who lived in Caloundra, and was for years in the Queensland Lighthouse Service ships. He sent me a photograph of his empty depth charge rails, with a notice, “Sold out”.

41 U-boats sunk in May 1943

At the time, Britain was the forward base for the second front and troops were pouring into the country. We lost 97 ships in the Atlantic in the first 20 days of March 1943. Britain was nearing starvation levels and the navy, while their ready-use tanks were full, had a reserve supply of only 30 days of oil fuel. Quite clearly, we could not have sustained these loss rates, but in May 1943, a total of 41 U-boats were sunk for the loss of only 17 ships.

Our own escorts were increasing in numbers and skill, whereas the U-boats had lost their ace skippers and their ability to attack on the surface, due to radar and H/F D/F. They lost the initiative and never regained it. The massed wolf packs, which were so nearly successful, had failed and were never really tried again. The main reasons for our success in the late spring of 1943 were the increased numbers of escorts being commissioned and, for the first time, the formation of the Support Groups together with very intensive training of all ships and crews. New tactics were being worked out and practised by the groups.

Air support

Hedgehog was being fitted to frigates and sloops and this made a big difference. H/F D/F was being carried in one or two ships of every escort group and that also helped, but the main help came from the air.

In 1940 and 1941, 35 CAM ships were fitted with a rocket-powered cradle on a 22.8 metres (75 feet) long track. The first successful Hurricane launch was from a similar-looking RN-manned “Fighter Catapult Ship”, HMS Maplin, when LEUT R. Everett RNVR shot down a shadower, 3 August 1941. The first CAM ship launch, 1 November 1941 from the SS Empire Foam “frightened the shadower away”. In two years’ service, CAM ships launched only eight aircraft in anger. They shot down six enemy and lost one RAF pilot. Unfortunately, the conspicuous CAM ships became prime targets and 12 of the 35 were lost to enemy action.

Escort carriers

Then there were the small escort carriers, (see: Emergence of the escort carriers) aircraft carriers built on merchant ship hulls, which initially carried four Swordfish aircraft, sufficient to force a submarine to submerge so that it could not keep up with the convoy. If a gale blew up quickly, when these 80 knot aircraft were astern, the carrier had to turn back to pick them up. And finally, there was the bridging of the air gap between the USA, Greenland, Iceland and Britain by Liberator and similar long-range aircraft, although some of those airfields were often closed by bad weather.

I knew Peter Gretton very well as he frequently sailed in Tay, before getting his own ship Duncan. Later he rose to become VADM Sir Peter Gretton, KCB, DSO, OBE, DSC and Chief of Naval Staff, but in those days he was the youngest confirmed Commander in the RN.

Kept in touch Bob Sherwood, my captain for three years in Tay, came from the Holyhead/Dublin Ferries and subsequently returned to the ferry service. He eventually became Marine Superintendent for the ferry services running out of Britain to the continent, Ireland, etc. Ray Hart in Vidette was a Canadian and Harold Chesterman in Snowflake an Australian. The other two skippers were British. I still keep in touch with four officers from the Tay, even after all these years. She was a very happy ship and B7 was an extremely efficient Escort Group. Those eight days were some of the most hectic of my life.

References:

Roskill, S.W. History of the Second World War: The war at sea, 1939-1945, Vol II. HMSO,1956.
Syrett, D. The defeat of the German U-boats: The Battle of the Atlantic. Chap III, The Battle for Convoy ONS-5. Columbia: University of South Carolina Press, 1994.
The Battle of the Atlantic: The official account of the fight against the U-boats, 1939-1945. HMSO, 1946.

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