On 15th August 1940 the Luftwaffe carried out its only major raid on northern England in which they suffered substantial losses. The losses were so high that the scale of this raid was never repeated although there were numerous smaller and single aircraft raids throughout the remainder of 1940 and beyond.
The targets of the raids were a number of RAF airfields in North Yorkshire and around the Newcastle area. The raid consisted of approximately 135 aircraft; 65 He111 and 35 Bf110 from Norway plus 50 Ju88 from Denmark. Very few of the aircraft from Norway, whose targets were close to Newcastle, got through however the Ju88s managed to do some considerable damage to RAF Driffield where 10 British bombers were destroyed on the ground and damage done to the base. In total RAF Fighter Command managed to put 9 Squadrons into the air although only 7 were Spitfire/Hurricane squadrons and shot down 8 He111, 7 Ju88 and 8 Bf110 for the loss of just two aircraft with another half a dozen damaged plus 3 pilots seriously wounded with no loss of life.
If we then move on exactly one month to the day that is now known as the Battle of Britain Day where the Luftwaffe flew more sorties in a single day than at any other time during the battle and consider what might have been if they had decided to strike in the north again. In the south east of England and in the skies above London the bombers of the Luftwaffe were being used as bait to draw fighter command into the skies where they would then become targets for the German Bf109 escorts. The bombers were regularly out number two or three to one in the German formations which meant there were often large numbers of bombers sitting at airfields in France and Denmark etc.
What if on 15th September, following intelligence reports and recon flights the Luftwaffe decided to launch an attack on the Rosyth naval base. Such a raid would consist of anything up to 300 He111 / Ju88 bombers escorted by 100 Bf110s and would have included the 24 He115 torpedo armed floatplanes available in Norway.
At Rosyth on 15th September the Home Fleet had gathered a vast array of warships ready to repulse the expected German invasion. This fleet included two battleships, one battle cruiser, three AA cruisers, three light cruisers and eighteen destroyers (I think this is right but some may have been out on patrol).
The fighter cover at this time was a little thinned down and there were only one full strength Spitfire and one full strength Hurricane squadron within 45 minutes flying time of Rosyth plus a squadron of Defiants. In addition to this there were two training units which could put up 12 Hurricanes between them about 20 - 25 minutes away and three understrength squadrons resting and reforming 25 - 30 minutes away which had a further 24 Spitfires and Hurricanes between them. So in total Fighter Command could put in the air the equivolent of five front line squadrons plus one of Defiants against 400 + aircraft.
The other thing to consider is that fighter command would get only 30 minutes warning IF the bombers flew directly to the target ... and the number of aircraft would have been an estimate until the observer corp could get a visual fix on the formation so the full threat would not be recognised until the Germans were 10-15 minutes from their targets. In the previous raid the initial estimates from the radar only were as low as 30-50 aircraft rather than the 135 or so that actually attacked. This meant that only one RAF squadron was put in the air until it was realised the scale of the attack introducing quite a delay in the RAF's response.
So taking all that into consideration, what kind of damage could this raid do to the Royal Navies capabilities at this crucial stage of the war?