My stepfather is of the opinion that the reason Britain won the war with Germany is solely down to the superior range finder on the HMS Warspite and its affect in the Battle of Narvik. (He also coincidentally owns this rangefinder)
He says the Battle of Britain was irrelevant in terms of stopping a German invasion, and that the destruction of 10 German destroyers, which was all down to this magical rangefinder, is the reason Germany couldnt invade Britan and win the war.
I find it hard to believe, in a war over so many years with numerous countries involved, that this is the one reason Germany lost, so i thought id ask you to see he is full of shit. He also says that the only reason Britain won in the Falklands is because the USA lost in Vietnam but thats another question.
Your stepfather is right about a few things, but there's some quite considerable gaps in his analysis of the battle.
He's certainly right that the Battle of Britain was essentially irrelevant as to determining whether or not a German invasion of Britain was feasible or not. Amphibious assaults require naval superiority (or ideally supremacy) to be successful. Otherwise the troop and supply ships will be exposed and will take losses. With the losses suffered at Narvik, this task became all but impossible for the Germans. Following the losses, the RN had more cruisers in home waters than the Kriegsmarine had destroyers, while the Nore Command, the RN HQ covering the likely invasion zone, had three times as many destroyers as rhe entire Kriegsmarine.
However, the task was already exceedingly difficult, to the point where the battle at Narvik made little difference. The Royal Navy greatly outnumbered the German Kriegsmarine, in all classes of ships. This was only exacerbated by losses in the Norwegian campaign. Ignoring the losses at Narvik, the Germans had lost a heavy cruiser and two light cruisers. Three more ships took heavy damage: a third light cruiser, the pocket battleship Lutzow and the battlecruiser Scharnhorst. This gave the Kriegsmarine a total force available of one battlecruiser, one pocket battleship (albeit one undergoing refit), one heavy cruiser (with another nearing completion), and three light cruisers. Against this, the RN had in home waters three battleships, two battlecruisers, 11 cruisers and 80 destroyers, plus countless minesweepers, corvettes and other small craft. Even without the losses at Narvik, the Kriegsmarine was outnumbered in destroyers by a ratio of four to one. While the losses represented half the Kriegsmarine's destroyer force, this force was already tiny in comparison to the RN.
Another misconception held by your stepfather is that the German losses at Narvik were caused by Warspite alone. Two naval battles were fought at Narvik during the campaign, one on the 10th April 1940, and the second three days later. During the first, the RN's 2nd Destroyer Flotilla, with 5 H-class destroyers, carried out a surprise raid on the German destroyers unloading and replenishing at Narvik. Two German destroyers were sunk, and another seriously damaged, before the Germans were able to bring their numbers to bear, sinking two British ships and forcing the remainder to retreat. However, the Germans did not pursue, allowing the British to sink a newly arrived supply ship, carrying ammunition for the German destroyers. This left the German ships critically low on ammunition. They were also low on fuel, with one of the two tankers assigned to support them having been lost during its trip to Narvik. During the night of the 11th-12th, two German destroyers ran aground, with one being so heavily damaged it was decided to turn her into a stationary defence battery. On the 13th, Warspite arrived at Narvik, accompanied by nine destroyers. Warspite's Swordfish aircraft bombed and sunk a U-boat immediately prior to the engagement. The aircraft would then continue to spot for the British force, assisting with their accuracy. It warned the British force of the arrival of the first German destroyer, the one which had run aground the night before, and which was quickly sunk by the two leading British destroyers. The British force advanced down the fjord, and fought a short, sharp engagement against three German destroyers. The British Punjabi took heavy damage, but the German ships had exhausted their ammunition. One ran aground and was torpedoed by the British ship Eskimo, while another was sunk by gunfire, mainly from the British destroyers. The remaining German ships, with no ammo or fuel, hugely outnumbered and outgunned, and with no hope of reinforcements, were scuttled by their crews. Warspite's role in the battle was limited. British success in the battle was more the result of destroying German supplies, and of outnumbering the Germans than of any superior rangefinder aboard Warspite. As an aside, I'm not sure what rangefinder that would be - her 1934-37 refit did not significantly alter her rangefinder fit, only replaced the fire control table they fed into. She was still using the 30 foot turret rangefinders installed in 1918 as well as 15ft rangefinders on her director control towers.
While I've written enough here for one day, I'd be interested in hearing how he justifies his statements about the Falklands - I really don't see much connection with the Vietnam War.