Like Bourne's design, the Drebbel boat relied on oarsmen for both surface and sub-surface propulsion, with leather gaskets preserving the integrity of a greased leathcr-sheathed hull constructed of wood on iron frames. Unlike Bourne, Drebbel's craft was actually built between 1620-1626, and was tested on London's River Thames, reportedly at depths between 12 and 15 feet; although it is more likely that it operated only partially awash. During the first test, it leaked badly. Although there is controversy as to its true nature - was it really a kind of diving bell or a true submarine? - there is little doubt that the boat undertook at least one successful partially submerged trial lasting around three hours, with twelve oarsmen and a number of foolhardy passengers on board. Its true significance was the development of an air supply, akin to the modern snorkel, first seen operationally on the German submarine U539 in January 1944, although Drebbel is also said to have created a chemical that purified air within the submerged boat. As he had discovered how to produce oxygen from healing potassium nitrate (saltpetre), this may be plausible. If used, the process makes the courage of the 17th Century pioneer submariners even more breathtaking.