Breaking the blockade for ball bearings

Motor Gun Boats (MGBs) that were modified during WWII succeeding in bringing vital components from Sweden to the UK and contributed to the development of the Paxman diesel. 

During a five-month period from October 1943 to March 1944 one of the most amazing operations of the Second World War was undertaken. This entailed the running of high-speed merchant launches from the UK into Sweden to bring back ball bearings, vital to Great Britain’s war effort and only available from the latter country. A second operation between September 1944 and February 1945 took various weaponry from the UK to Sweden for onward supply to the Danish resistance but was not a success due to severe weather conditions.

Apart from the almost swashbuckling aspects of the voyages themselves, the story encompasses the development of an advanced high-speed marine diesel in the UK and a class of Motor Gun Boats (MGBs), one of which would later become the world’s first vessel with gas turbine machinery, MGB 2009.

Cross section of an early Paxman VRB engine (Credit: Pages from the past)

A Turkish order

In the beginning, just before the Second World War broke out, the Turkish Navy ordered eight MGBs from Camper and Nicholson in the UK. Larger than the contemporary Royal Navy units being built by Vosper and the British Power Boat Co., these vessels were unusual in being diesel powered by three 16-cylinder Paxman VRB design units rated at 1,000 bhp each at 1,750 rpm.

However, before completion of the class, war had broken out and all eight were taken over by the British Admiralty. Three immediately entered Royal Navy service in their intended roles while the other five were modified for the blockade running operation into Sweden, avoiding German naval patrols based in Holland and Norway, to transport the ball bearings back to the UK. This was already being carried out by civilian aircraft but the loads were very small. The MGB conversion entailed converting the hull into a cargo space with a capacity of around 40 tons and fitting a new bridge structure. They would be manned and operated as merchant ships. 

The operational plan

The loading port in Sweden was Lysekil and the plan was to sail on the evening of arrival and be back in the Humber two days later in the morning. In practice such a schedule was rarely achieved for a variety of reasons. 

The first run was scheduled to commence on 23 September 1943 but was postponed for a month because of engine bearing problems and commenced with all five vessels sailing on the evening of 26 October. Only Gay Viking reached Sweden, the others turning back because of bad weather and mechanical problems. By October 31 Gay Viking had returned with 40 tons of cargo.

Over the next five months, eight further successful round trips were completed, bringing the total cargo carried over the period to 347.5 tons against the planned 400 tons. During the same period aircraft flights carried just 88 tons to the UK. However, the operations were not completed without loss, Master Standfast being captured by German forces on 2 November without ever completing a round trip. Separately, Nonsuch only managed one round trip due to breaking two crankshafts. Hopewell completed two trips but broke its centre engine crankshaft on 6 March, while the two Gay boats carried out three voyages, but both also suffered crank problems.

Nevertheless, against all adversities the objectives were met and most of the required cargo shipped. The blockade runners proved to be successful with their low silhouettes and relatively high speeds (certainly amongst the fastest merchant coasters ever built) and if Paxmans had had more time to test and develop their VRB engines, over 400 tons could have been shipped.

Paxman’s VRB engine

The VRB design originated in the late 1930s as a 600 bhp diesel intended for propelling tanks. It was of 60° vee form with a 7in bore and a 7.75in stroke. The first marine application was in the experimental motor torpedo boat Tarret of 1939, equipped with two 16 VRBs, each rated at 1000 bhp.

That the early engines suffered several mechanical breakages including crankshafts, was due primarily to the speed with which they were put into production and the fact that they operated at the then high speed for such an engine of 1,600 rpm. Once sorted though, the design would be developed into a family of high-speed engines culminating in the successful Ventura and Valenta ranges, proving the soundness of the original VRB design. 

John Barnes is a journalist and author and former editor of Marine Engineers Review.