On the one hand, there were steps that could be taken towards this. The US Navy (and to a lesser extent, the Royal Navy) operated ships called Seaplane Tenders. These carried the equipment necessary to support, refuel and rearm large seaplanes like the American PBY Catalina or British Shorts Sunderland. During the First World War, the RN had experimented with using seaplane-carrying lighters towed behind destroyers, as a way of extending the ranges of its seaplanes in the North Sea. In theory, these could have been used to extend the range of seaplanes into the middle of the North Atlantic. In practice, this was impossible. The main reason for this was weather. Seaplanes could not land, nor take off, if the sea was too rough. Even a relatively moderate sea could make a take-off or landing too dangerous to reliably carry out. Aircraft sitting on the sea's surface could easily be damaged and sunk by waves. As the convoys were travelling in all weathers, in all seasons, seaplanes would only be useful for a small fraction of the convoys; those that could guarantee good weather and smooth seas throughout. The North Atlantic was rarely so cooperative. Another issue was that the PBYs, Sunderlands and other flying boats were not the only maritime patrol aircraft that operated over the North Atlantic. There were plently of landplanes that made patrols; Vickers Wellingtons, B-24 Liberators, Lockheed Hudsons and so on. These could not land on the surface of the sea, and needed land bases. Until the Portuguese leased bases on the Azores to the Allies, these were not present, resulting in the 'air gap'.