In the 1920's there was a commonly held belief that German First World War submarines were superior to their British counterparts in both design and construction. In some ways they were superior to the British boats, but their speeds were less than their British counterparts. This was surprising as the German designers had the great advantage of being able to install engines of about 300 b.h.p. per cylinder in lieu of the British limit of 100 b.h.p. There was some uncertainty as to the actual number of U-Boats built, but Sir Arthur Johns thought it to be about 378 with a further 183 under construction at the time of the Armistice. Over half the boats that had been built were destroyed by allied forces or by accidents.