I don't know exactly but i think that 300 submarines would be equal to 2-3 Panzer Divisions(anyone that knows more can say if that is correct?)
So,if he manages to convince Hitler that they need to win the BoB in the water,what would be the effects of such decision?I think that IF they won in Britain the subs can be melted to tanks in 6 months-1 year
Given the devastation caused by a relative handful of U Boats had 300 been available which triples or Quadruples the number of boats at sea at any given time over OTL - and this will suddenly massively up the number of ships being sunk beyond a sustainable number month on month which means that the British have to react and react fast.
The quickest thing the Brits can do is retask Bomber command to coastal command (which is what they should have done OTL) given the impact a relative handful of aircraft had on the BOA suddenly having scores if not hundreds operating across the western approaches and as far as possible covering convoys is goign to cause the Uboat fleet many problems.
While it would not be the Sailing to their deaths of May 43+ as many of the technologies necessary for that victory still had to align for the Allies - it would still result in many U-boat losses and
Historically the losses in 1940-43 while heavy proved too be low enough for the combined voices of bomber commands Linderman, Harris and Portal to drown out any attempt to wrest suitable aircraft away from the Strategic bombing of Germany.
For example between 1941 and early 1943 the RAF only had one Squadron of VLR Liberators tasked to Coastal command - a squadron that had to scrounge for spares from BC and many of the 'few' bombers assigned earlier in the war were 2nd class bombers.
If the Germans can keep 4x as many Uboats at sea over OTL those voices will no longer be loud enough and the resources of BC will be transferred to Coastal Command
Then of course curtail any large warship construction and concentrate on building as many escorts as possible and training crews accordingly.
Also I do not believe that the Germans building 300 U Boats by 1940 is achievable - recall that they had only started making them again in 1936
Secondly if Germany is not making Panzer divisions then it might not beat France in 1940 making the whole BOA Academic anyway.
Germany had IIRC 9 Pz Divisions involved in the Battle of France - and they were a critical component in the success of that battle - start removing several of these Pz Divisions and well - maybe France can hold on?
Also the majority of Steel used by Germany in WW2 was overwhelmingly in the construction of buildings and fortifications - not in the building of tanks and warships etc.
What prevented Germany from having built several times more tanks, Aircraft and U-boats etc was the same restriction as everyone else - the lack of factories / trained workers.
Building armament industries takes years of planning and investment - German tank production for example reached its highest output in 1944 despite the attentions of Bomber command and the USAAF - this is because many sites of the German war industry vital for the construction of tank components etc were still expanding between 1939 and 1943
So it is impossible for a large part of the Armaments industry to switch from Tank production to U-boat construction and then back to Tank production in the space of a few years.
Germany cannot win on land without a massive investment in Tank/AFV production and cannot possibly 'Win' the war without a large number of U-boats.