CMBs were 40ft long, had a crew of three, carried two Lewis machine guns and a single torpedo. They had hydroplane hulls, hence their nickname "skimmers," but were made of plywood so were almost defenceless against enemy fire.
The fastest naval vessels afloat, they were ideal for slipping past the huge array of defences in the Gulf of Finland - except for the deafening noise they made when they reached their top speed of 45mph.
Protecting the sea approach to Petrograd was the forbidding island fortress of Kronstadt and its 15 forts - nine to the north, six to the south - with enough guns to halt any enemy fleet.
Furthermore, the forts were connected by a hidden breakwater that MI6 told Agar was only three feet under the surface and which, since CMBs drew 2ft 9in of water, meant that his two vessels would have only three inches to spare at normal speed.
Although the Gulf of Finland is 250 miles long, it is only 30 miles wide, and with gunboat patrols, floating and fixed mines, searchlights, submarines and seaplanes, it seemed impassable to any but the most intrepid sailor.
Cumming explained the mission to Agar in his office in Whitehall, and ordered him to choose only unmarried men with no immediate dependents for his seven-man team; Agar himself had been orphaned at the age of 12, and although he had a sweetheart they were not then engaged.
Cumming also warned Agar that in the event of capture he could expect no help, or even official recognition, from the British Government.